Carbed Fuel Delivery Issue

[tulowd]

Hey All:

Since I am now a year wiser, I thought I would ask for some help in rectifying the ongoing fuel delivery saga on my 333 carbed Fox vert. Car put down 388 hp at DaSilva; but was running at 14:1 AFR; so it was lean with some more power available. Locked out distributor, 37* timing; marks on plug electrode indicate perfect timing.

Full AN8 feed, return and vent setup; sumped OEM tank; Pro Systems 780 road race carb; ported Victor Jr Intake Mallory billet return regulator; Comp 140 Mallory pump

Pump started getting noisy once warmed up; even after I relocated it parallel to the sump output lines. Idle and fuel pressure appear to be stable, however the throttle response and cleanliness of the idle varies. Makes no difference if tank is empty or full or anything in between.

Burned out a Holley billet HP150 last year during the heatwave - Holley blamed the ethanol content; been only running Shell 91 since. Replaced the Comp pump yesterday with known good Edelbrock and same thing continues to happen:

Car has killer throttle response and power like a raped ape when engine runs the first 10 minutes.
AFR looks good, a little rich, meaning I need slightly smaller primary jetting; but it's good to dump fuel on a locked out distributor to keep it from pinging/ detonating.

Once warmed up, AFR goes full lean and only shows acceptable numbers while decelerating with no throttle. At highway speeds it sometimes does not even do that unless down a hill. Throttle response and power disappear. Plugs look great other than a couple have the soot ring around the outside, due to the overly large primary jets. Once it does this, no top end power and for sure a lack of fuel. Yesterday she definitely ran out of fuel on a 3rd gear 6000 rpm on ramp honk; so is it not getting the fuel volume or is the fuel aerated or is there more air going into the motor?

About to delete the return setup and try running the pump with out a regulator and then with my old Paxton deadhead reg.

Could it be:
fuel is simply being aerated from being dumped at the top of the tank thru the return line (no tube; simple splash down).
a vacuum leak (between the intake and heads or ?)
the carb is doing something goofy? Float levels etc are fine

Water temps always between 170-180 even on scorching days, other than when stuck in traffic.


Any suggestions or comments most appreciated - the difference in power and drivability is huge and I'm running out of patience.


Thx boys and girls

[TurboFox]

What does your fuel system look like between the regulator and the carb? Are you running a fuel log?

[tulowd]

-6An lines from regulator.

http://i1245.photobucket.com/albums/gg582/tulowd/87%20Vert%20TU%20LOWD2/IMG_5224_zpsec5be082.jpg


UPDATE: deleted regulator, now only running pump with adjustable pressure. It gets better when I run at 8 psi.

It appears the fuel bowls are very very hot, never noticed that before.

Makes sense, as the problem is most obvious in hot weather; the higher pressure compresses the air bubbles somewhat, and would explain the inconsistent AFR readings.

Gonna look for the phenolic spacer and a heat shield now. Then going to put the return regulator back on and see how it functions at the higher pressure. <Might even try the Comp pump back on too?

[Slope]

Do you have a pic handy of your fuel system near the carb?

I'd be hesitant to ditch the return-style system especially if fuel heating could be present.

What do you have for jetting/power valves?

[Stephen06GT]

If the fuel in the bowls is hot, is there a chance you are getting some form of vapour lock?

[tulowd]

Do you have a pic handy of your fuel system near the carb?

I'd be hesitant to ditch the return-style system especially if fuel heating could be present.

What do you have for jetting/power valves?


http://i1245.photobucket.com/albums/gg582/tulowd/87%20Vert%20TU%20LOWD2/MotorShotsFall20129.jpg

http://i1245.photobucket.com/albums/gg582/tulowd/87%20Vert%20TU%20LOWD2/IMG_3282_zpse64016d9.jpg

6.5 PV's front and rear; 78/80 jetting which is a little too rich, with proper fuel delivery; so likely back them down a little once this little dilemma is solved

[Slope]

Excellent shots. Everything on the top side appears to be out of harm's way.

I take it that your under car lines are isolated from exhaust heat?

Also: plug the power valve in the rear metering plate and re-tune.

[Ponyryd]

^Was gonna suggest the same thing about the exhaust, very common of efi applications.
What I'd do is put a sight glass in line just before the carb, then you can see if its being aerated once it warms up.
Also, the return fuel should be running down the side of the tank, not directly into the fuel. Guess it doesn't matter now that you changed it though.
I'd also be checking the temp of the pump when its acting up. Where is the pump in relation to the tank?

[tulowd]

Excellent shots. Everything on the top side appears to be out of harm's way.

I take it that your under car lines are isolated from exhaust heat?

Also: plug the power valve in the rear metering plate and re-tune.


The main problem is in the primary circuit, as it is noticeably rougher and inconsistent at 1/4 throttle or less. The problem seems to be fuel temp / aeration related. Likely the latent heat in the float bowls is making it worse when the return style system is in use, since the fuel already has a higher percentage of air in it once it makes it to the bowls. Then as it boils, because it contains more air, it becomes worse than the simple non regulator setup now. Running at 8 psi reduces the symptom, but does not address the root problem of temperature; which seems to be coming from the motor, not the pump or system.
Fuel lines are run far away (other side of the frame rails) from exhaust except near the tank; the return line was installed after this photo was taken.

http://i1245.photobucket.com/albums/gg582/tulowd/87%20Vert%20TU%20LOWD2/IMG_3282_zpse64016d9.jpg


Carb is setup almost perfectly, but it cant function when the fuel is inconsistent; both temperature wise and air content.

[tulowd]

^Was gonna suggest the same thing about the exhaust, very common of efi applications.
What I'd do is put a sight glass in line just before the carb, then you can see if its being aerated once it warms up.
Also, the return fuel should be running down the side of the tank, not directly into the fuel. Guess it doesn't matter now that you changed it though.
I'd also be checking the temp of the pump when its acting up. Where is the pump in relation to the tank?


The amount of heat in the bowls is amazing. The fuel lines are always cool near the regulator, leaving me to believe the heat is coming from the motor, not somewhere in the fuel path.

http://i1245.photobucket.com/albums/gg582/tulowd/87%20Vert%20TU%20LOWD2/IMG_6129_zps9c1eeffc.jpg

Edelbrock pump is mounted similar, except it has the big filter in front instead of after.

[Ponyryd]

In that case I'd start by pulling the hood and going for a drive. Alternately you could try grabbing some bags of ice and tucking them around the carb, then going for a good drive. If the ice is still there when the engine is hot and it normally acts up, you know your problem is in the carb bowls. In which case you can get a spacer as mentioned earlier, or you can make up some sort of insulation system to help isolate the carb from the engine better.

[Stephen06GT]

Can you fab up some sort of heat shield to isolate the bowls from the heat source?

Edit: Should have read Ponyrd's post a little more carefully.

[ZR]

I'm also a fan of heat being your most likely culprit. More common in carb'd rides that run an electric fan. Old school mondo flex fan not only pulls tons of air through the rad but also keeps lots of air running up past the carb n helping it to keep it's cool.
Ice + hood off idea seems solid to me as well.

[TurboFox]

Toss in a phenolic spacer because it cheap and take it from there

[Ponyryd]

I'm also a fan of heat being your most likely culprit. More common in carb'd rides that run an electric fan. Old school mondo flex fan not only pulls tons of air through the rad but also keeps lots of air running up past the carb n helping it to keep it's cool.

Agreed, also the bay has been smoothed so you're losing more areas of cool air underhood. Also the fan you have doesn't look very big, and the shroud doesn't cover the whole rad.

[tulowd]

Agreed, also the bay has been smoothed so you're losing more areas of cool air underhood. Also the fan you have doesn't look very big, and the shroud doesn't cover the whole rad.

2200cfm Flexalite

hood has rear cowl openings for air flow from scoop and thru rad - hood doesn't bulge or grow at highway speeds, so airflow thru there is decent.
Only two aero changes to the car have been the brake ducts and the Mach 1 chin spoiler.

The heat issue plagued me last summer as well; but I was too stupid to check the float bowl temps, duhhhhh!! lol

[Scott]

If the problem is the fuel being too hot in the fuel bows there could be two causes. Firstly as other have pointed out radiated heat from the engine. Secondly rising fuel heat in general caused by heat generated by your fuel pump in the return style system. This is a common problem with multiple pumps in the S197 platform. Solution there is to use a controller that stages the 2nd / 3rd pumps on based on boost. Fore's FC3 controller is an example when used with a Hobs switch.

From you pictures you are feeding both bowls on your carb from the regulator to the passenger side bowls. I would expect if you ran returns from both bowls from the driver's side back to the regulator your fuel temperature would go down as fuel would not be in the bowls long enough to absorb heat from the engine. Aeromotive makes a 4-Port Carbureted Bypass Regulator, part number A1000. To see if this is the issue I guess you could put a couple of Y-Blocks in with your existing regulator. Basically fuel would not dead head in the bowls and there would be a continual flow through the fuel system. Paul, hope this makes sense? If not just shoot me a PM.

[tulowd]

If the problem is the fuel being too hot in the fuel bows there could be two causes. Firstly as other have pointed out radiated heat from the engine. Secondly rising fuel heat in general caused by heat generated by your fuel pump in the return style system. This is a common problem with multiple pumps in the S197 platform. Solution there is to use a controller that stages the 2nd / 3rd pumps on based on boost. Fore's FC3 controller is an example when used with a Hobs switch.

From you pictures you are feeding both bowls on your carb from the regulator to the passenger side bowls. I would expect if you ran returns from both bowls from the driver's side back to the regulator your fuel temperature would go down as fuel would not be in the bowls long enough to absorb heat from the engine. Aeromotive makes a 4-Port Carbureted Bypass Regulator, part number A1000. To see if this is the issue I guess you could put a couple of Y-Blocks in with your existing regulator. Basically fuel would not dead head in the bowls and there would be a continual flow through the fuel system. Paul, hope this makes sense? If not just shoot me a PM.

Thx Scott: That does make sense - in effect using the bowls as part of a fuel log / return portion as opposed to the dead head current setup. I think some of the new EFI rails run the return from the back not the front in a similar arrangement.

I am convinced the heat is from the motor not the pumps, as yesterday the new Edelbrock pump was still cool when the pressure started dropping and the bowls were hot. The fuel lines are never hot, just warm to the touch.

Going to try a heat shield and phenolic spacer first; then figure out if I have a secondary issue with the return feed into the tank aerating the fuel.

[tulowd]

After installing the heat shield and the phenolic spacer, it is better, but I found that both -8AN fuel lines are really hot front to back, including the disconnected return line. Pump is warm; tank is cool.
Will have to look at where the fuel lines are both run, as they were pulled off the car recently to drop the tank. Perhaps a heat shield is in order - the lines run close to the passenger side muffler for about a foot; everywhere else they are on the other side of the frame rails. Hmmmmmmmm

[Ponyryd]

How close to the muffler? any less than 2" is too close IMO.

[tulowd]

Found an old catalytic converter heatshield from my F150; went to PI and picked up some of that heat wrap.
Hopefully get the stuff installed in the morning.

I think its about 4 inches away, but my feeling is being braided line means it picks up heat from the frame rail and floor pan nearby.

Thx for everyone's input - will report back.

[tulowd]

Did some more needless work, swapped out the pump, rerouted the fuel lines, sleeved them and made a heat shield and finally got rid of the regulator it still ran like crap.
Ran pump with return regulator for almost 2 hrs on Sat morning, cool as a cucumber.
A sponsor pm'd me and said my build has a vacuum leak from having a breather and pcv valve hooked up.
Ran it disconnected, and returned the carb to exactly how it was delivered from Pro Systems. Car ran great, lots more jam; smoother idle and throttle tip in. Also installed carb heat shield and phenolic spacer and got a new air cleaner base which unshrouded the carb and put the filter smack into the cold airstream from the hoodscoop. Bowls and lines are way cooler than before, so this is a good thing regardless.
http://i1245.photobucket.com/albums/gg582/tulowd/87%20Vert%20TU%20LOWD2/IMG_6185_zpscc09aa09.jpg
AFR looked good - until it warmed up and went lean again! BTW - idle speed and AFR down low stay the same under both conditions. Oil pressure needle moves a little while driving too when its real hot and lean.
Finally took the intake off, hoping to find a bad intake gasket - no such luck. Did find the brake booster take off fitting that had been plugged was not as tight as it should have been.
http://i1245.photobucket.com/albums/gg582/tulowd/87%20Vert%20TU%20LOWD2/IMG_6203_zps5763ccb3.jpg
http://i1245.photobucket.com/albums/gg582/tulowd/87%20Vert%20TU%20LOWD2/IMG_6189-Copy_zps86f24f49.jpg
Hopefully once that is sealed up and everything is back together it will be the end of this little challenge.
I'll report back once I get it back together later this week. Thx for everyone's help.

[Scott]

If the problem is the fuel being too hot in the fuel bows there could be two causes. Firstly as other have pointed out radiated heat from the engine. Secondly rising fuel heat in general caused by heat generated by your fuel pump in the return style system. This is a common problem with multiple pumps in the S197 platform. Solution there is to use a controller that stages the 2nd / 3rd pumps on based on boost. Fore's FC3 controller is an example when used with a Hobs switch.

From you pictures you are feeding both bowls on your carb from the regulator to the passenger side bowls. I would expect if you ran returns from both bowls from the driver's side back to the regulator your fuel temperature would go down as fuel would not be in the bowls long enough to absorb heat from the engine. Aeromotive makes a 4-Port Carbureted Bypass Regulator, part number A1000. To see if this is the issue I guess you could put a couple of Y-Blocks in with your existing regulator. Basically fuel would not dead head in the bowls and there would be a continual flow through the fuel system. Paul, hope this makes sense? If not just shoot me a PM.

Gave you some bad advise in the second paragraph, not sure what I was smoking, but it must have been good. You definitely do NOT need a 4-Port Bypass Regulator. The 2-port one you have is all that is needed. Simply feed both in ports of the carb form a Y-Fitting and route the carb out ports from each bowl to the 2-Port regulator. Sorry about that!

[tulowd]

Gave you some bad advise in the second paragraph, not sure what I was smoking, but it must have been good. You definitely do NOT need a 4-Port Bypass Regulator. The 2-port one you have is all that is needed. Simply feed both in ports of the carb form a Y-Fitting and route the carb out ports from each bowl to the 2-Port regulator. Sorry about that!

lol...you get a gold star for an excellent idea - the execution is my job. Pretty sure all that is needed is the float bowls modded and some more line. Basically carb would go before the regulator as part of the regulated flow; kinda like the fuel logs that run the regulator at the very end / last component before the return.

[Ponyryd]

Were the intake gaskets crushed at the bottom? See it often that the lower portion of the intake isn't sealing causing a vacuum leak inside the valley.

[tulowd]

Were the intake gaskets crushed at the bottom? See it often that the lower portion of the intake isn't sealing causing a vacuum leak inside the valley.

nope, they looked perfect. Hope it was really that NPT fitting....fingers crossed

[Scott]

lol...you get a gold star for an excellent idea - the execution is my job. Pretty sure all that is needed is the float bowls modded and some more line. Basically carb would go before the regulator as part of the regulated flow; kinda like the fuel logs that run the regulator at the very end / last component before the return.

Exactly!

[tulowd]

drivers side intake surface
http://i1245.photobucket.com/albums/gg582/tulowd/87%20Vert%20TU%20LOWD2/Driversside_zps43062e3e.jpg
passenger side intake surface
http://i1245.photobucket.com/albums/gg582/tulowd/87%20Vert%20TU%20LOWD2/Passside_zps2af24a08.jpg
#1 cylinder intake valve is golden yellow and clean; other 7 valves have some crud buildup. Means coolant was leaking slighting into that cylinder. Also appears the gaskets were not sealing around several ports.
Motor had 1000 km when fresh in Sep 2010; then sat in unheated garage until it ran again in May 2013.
Any comments ?

[ZR]

Regardless, intake needed to come off for clean up and reseal.

[Ponyryd]

What kind of gaskets are you using? also, was the intake new or used when it went on the engine? same question for heads. Sitting in the cold for 3 years should not effect the gaskets.
You may want to check how that intake fit on the heads with no gaskets while it's off.

[ZR]

Agree on double checking fit with no gaskets, they just lay gaskets + intake on and take another look.
Seeing some of the hi perf intake gasket deteriorate after a few years. Suggesting to customers to replace once they start to look poochy where they extend out past the head / intake. Rest of it is always not far behind.
In your case, almost looks like corrosion may be lifting the intake a tic. Gasket is not leaving a good impression around every port.

[tulowd]

Manifold went back on after cleaning, along with new valve covers. Changed the open breather to suck air from inside the K+N air filter and put the PCV back on.
Problem still persists. Car rocks when engine cold, slowly gets weak up top and goes lean at all throttle positions incl at idle. Float bowls are cool to the touch; fuel pressure is perfect while driving, while its doing it.
http://i1245.photobucket.com/albums/gg582/tulowd/87%20Vert%20TU%20LOWD2/IMG_6291_zpsbfa086ef.jpg

Had a gent at Mustangs at Mosport play with the carb and timing for several hrs; no avail.
Plugs all look the same - a little sooty around outside, centre is tan and a little lean for my liking; main deal is AFR goes lean and no power. Timing stripe on plug electrode is centred - motor likes the 37* initial and locked out timing.
Temperature related - drove about an hr last night, started doing it within 20 minutes. Car sat for 45 minutes and was fine for about 5 min of driving; then did it again but not as bad.
Fuel pressure gage and vacuum gage visible while driving; 10 " HG at idle; looks to be ok while driving with various throttle positions.
Trying out another carb this morning to see if it something inside the Pro Systems one, no idea what it could be tho.

Only other thing it could be is the lack of air flow underneath the car from the Mach1 air dam. The fuel lines both get quite warm after running for a while, despite the return style and heat shielding etc. Even if the bowls are cool; if the fuel is getting heated in the lines, perhaps that makes it less dense and full of air so the carb cant meter it properly?

Starting to run out of ideas.

[ZR]

What is vac at WOT?

[tulowd]

app 2", I think. will check again with new carb on later today.

[ZR]

What carb you trying on?

[tulowd]

750 DP Holley known to be good.

[tulowd]

Looks like all the plugs were the same tho, lean everywhere on electrode, rich around outside. I also think the intake was only leaking a little bit of coolant into #1 intake; no air leaks.

Will also check vacuum gage again while driving. App 2" Hg at full throttle methinks; fuel pressure steady under all other conditions, app 0.5 psi movement for a second when I hit full throttle; then steady as rpm rise.

will also use the infrared temp gun on the header tubes to see if there is a big diff between any of them

[tulowd]

Swapped out to 750 DP. Same deal; goes lean after running a few minutes, besides running like crap. The Pro Systems carb is way smoother and tuned.

Starting to think it may be headgasket?

Going to do compression check, header pipe temp test and picking up a leakdown tester next week.

Grrrrrr..... lol

[ZR]

So it goes lean regardless of jetting?

[tulowd]

I would think there would not be a lot of vacuum lines still on your car to worry about. You could plug most and test drive. I would put temp gun on headers, but thought you had them all wrapped which may be a pain. If so than find trustworthy compression tester for quick test and if anything is off than move onto leakdown tester. Also assuming running temp is normal?

Only vac line is for PCV - when I run it open and disconnected no change.

Headers are ceramic coated not wrapped.



So it goes lean regardless of jetting?

jetting has no effect on lean condition - makes it go lean at idle and above, even on decelleration - running 76/76 front and rear with 6.5 power valves front and rear. 37* locked out timing.

[ZR]

What is your vac reading at idle?

[tulowd]

10 in HG @ 1000 rpm...rumpety rumpety so it does vary a bit

[ZR]

I'd be experimenting with a 5.5 or even 4.5.

[Ponyryd]

What is your base ign timing at?
I really don't see a head gasket causing a lean condition.

[Slope]

jetting has no effect on lean condition - makes it go lean at idle and above, even on decelleration - running 76/76 front and rear with 6.5 power valves front and rear. 37* locked out timing.

I'd be putting this carb back to stock jetting (even though in *MY* opinion it is oversized), removing and plugging the rear power valve and starting from scratch. The square jetting is bizarre for a road race application...

[ZR]

jetting has no effect on lean condition - makes it go lean at idle and above, even on decelleration - running 76/76 front and rear with 6.5 power valves front and rear. 37* locked out timing.

Can't get my head around your jetting either. Would also suggest what Ryan did. Remove and plug secondary PV and stagger the jetting to something like low 70's up front and 80 in the rear and see how it responds. How aggressive did you go with jetting before determining that regardless of size it would not go rich?

[tulowd]

Aug 27 Update:

At Revin High with Darren and Stan the carb guy.
Arrived at 4.
Left after midnite....
Flat tire on dyno....
Kooks slip on header collectors leak, throwing off all attempts at AFR readings
good news is my AEM gage is fine, bad news is gotta try and fix that, or replace the headers.

.... the challenges were many, lol
Did at least 20 pulls for driveability; had carb off car 10x, and at least another 10 on car changes

ended up with 10.5 PV and plugged rear PV
front squirter cam = green
front squirter to 45!!
Jetting is now 73 P and 84S (?)
changed all air bleeds
increased front PV passages
locked out timing reduced to 34* now
H20 Temps steady at 170
Oil temp 180


did at least 10 power pulls playing with jetting, squirters and timing
final number on his Mustang dyno 315 hp, 278 tq (reads way lower than the Dynojets)
motor made consistent power and is now part throttle driveable, idles nice and pulls hard from 4000 all the way into the rev limiter at 6600.
Plugs look good other than a little outer soot (rich idle?) after driving 50 km home. Electrode/strap timing mark looks good and the porcelain and centre show good ignition and fuel.

TMP Sept 10 is a go!


Big Thanks to Darren and Stan for not giving up as they were scratching their heads with a bunch of odd fuel curve challenges. Heard several times "never seen this before"; so I don't feel bad I was unable to get it right by myself.

[ZR]

Good to see your fixed up, had faith Darren would get you to the end zone.

[TurboFox]

Gotta love the 25%? lower dyno numbers

[tulowd]

17% and 19% lower actually. Darren warned me it would be way lower than the others I've been on (Redlines, Steedas and DaSilvas all were 374/379/388 respectively)

I don't really care; what will be reality is the trap speed in the quarter. That, along with the weight will verify whatever power the motor really makes. Torque peak was real high, hp peak only about 100 rpm apart - so technically a powerband like a 2 stroke.

Car drives nice and will be fast on the road course.

[TurboFox]

Sorry I didnt log your exact previous HP number in a spreadsheet. Its a much more realistic number and something that I can compare to because I've only ever dynoed my car on mustang dynos.

[tulowd]

what kind of power is the LS making now?

[TurboFox]

Still need to dyno it. Getting a bit anxious

[tulowd]

keep us posted - when where, etc

[Slope]

So, where does this leave you with your fuel delivery/lean when hot issue?

[TurboFox]

Sounds like its sorted? What size primaries on those kooks? 1 7/8?

[tulowd]

So, where does this leave you with your fuel delivery/lean when hot issue?
headers leak at the slip on collector, mainly from them hanging low and flattening out; so the O2 sensor is affected by that. Doesn't affect driveability, but would wreak havoc on a EFI car with O2 sensor input(s). Will try to fix that up and then fine tune the carb setup, especially at idle and lower rpm/throttle positions using the dynos 02 sensor.


Sounds like its sorted? What size primaries on those kooks? 1 7/8?
Yup; car drives pretty nice, especially when you get on it. Likes throttle rolling in; pulls like a freight train right into the 6600 rpm rev limiter. Power comes on a little earlier and a lot smoother, so it doesn't light up the tires in second anymore when it comes on the cam about 4000.

Good eyes, lol; Kooks 1-7/8 primaries, ceramic coated inside and out, full length.

[Slope]

headers leak at the slip on collector, mainly from them hanging low and flattening out; so the O2 sensor is affected by that. Doesn't affect driveability, but would wreak havoc on a EFI car with O2 sensor input(s). Will try to fix that up and then fine tune the carb setup, especially at idle and lower rpm/throttle positions using the dynos 02 sensor.

So the car WAS NOT lean, it was an O2 sensor issue?

[tulowd]

yes. was running rich....carb also had a huge lean spike upon throttle opening - didn't do that before - so that was also fixed. Never really found the problem other than the leak, but the car runs nicely. Once the header leak is fixed another dyno fine tune with AFR meter.

[tulowd]

Tuner@Straightline (http://www.corvetteforum.com/forums/members/243494-tuner-straightline.html)


From a Corvette Forum

http://www.corvetteforum.com/forums/customavatars/avatar243494_1.gif (http://www.corvetteforum.com/forums/members/243494-tuner-straightline.html)

My Corvette Photos (http://www.corvetteforum.com/photopost/showgallery.php?ppuser=243494)
Member Since: Feb 2007
Location: Frankfort Il



http://www.corvetteforum.com/forums/images/icons/icon1.gif
Here's a great explanation on how the two most common dynos function and why the difference in readings is a variable, not a standard percentage difference:


DynoJets are inertia dynos, and have been around for years, much longer than any type of load cell dyno. Inertia dyno's work on the principle of the acceleration of a known mass over time. Their rollers are the known mass. Weighing in at over 2500lbs or so. Your car gets strapped down to the machine, and the dyno collects it's data. It is able to calculate horsepower by measuring the acceleration in rpm of the rollers in regards to RPM. This is why gearing can affect the dyno results, more on that in a bit. Now that the dyno has recorded the horsepower curve, it can take the integral of that curve and get the torque curve. Since the dyno’s power calculations are based on the acceleration of mass over time in regards to RPM, gearing is very important. Since a vehicle with a lower gear ratio can accelerate the mass to a higher speed using less engine RPM, it will show a higher horsepower number than a car with a higher gear ratio. If a car is able to accelerate the dyno’s rollers from 200rpm (roller) to 300rpm (roller)in 1500rpm (engine), then the dyno is going to record more power than a car that did that in 2000rpm (engine).

Now we go to Mustang dyno’s and other loaded dyno’s. Our Mustang MD-1100SE dyno’s rollers weigh 2560lbs. That is the actual mass of the rollers, much like the DynoJet. That’s about where all the similarities end. When we get a car on our dyno, we enter two constants for the dyno’s algorithms. One being the vehicle weight, the other being what’s called “Horsepower At 50mph”. This is a number that represents how much horsepower it takes for the vehicle to push the air to maintain 50mph. This is used as the aerodynamic force. Mustang dyno’s are also equipped with a eddy currant load cell. Think of a magnetic brake from a freight train. This magnetic brake can apply enough resistance to stall a big rig. Off one side of the eddy currant load cell, there is a cantilever with a 5volt reference load sensor (strain gage). As the rollers are spinning this load sensor is measuring the actual torque being applied. So as the rollers spin, the load sensor is measuring the force being applied, sending that information to the dyno computer, taking into account the two constants entered earlier, computing the amount of resistance needed to be applied to the rollers to load the car so that the force of the rollers resistance is as close to the force the car sees on the street. The dyno is then able to calculate the total force being applied to the rollers in torque, and then taking the derivative of that torque curve to arrive at the horsepower curve. Since torque is an actual force of nature, like gravity and electricity, it can be directly measured. Horsepower is an idea that was thought up by man, and cannot be directly measured, only calculated.

I like to state it like this. . . I start by asking how much your car weighs, lets say 3500lbs. Now you take your car and you make a make a WOT rip in your tallest non overdrive gear, how much mass is your engine working against? 3500lbs right? Now you strap your car on a DynoJet and you make a WOT in the same gear, how much mass is your engine working against? 2500lbs right? Now you strap your car on a Mustang dyno, how much mass is your engine working against? 2500lbs. Plus the resistance being applied by the eddy current generator. We’ve seen anywhere for 470lbs of resistance to over 700lbs of resistance as measured in PAU force in the data logs. So which one is more accurate? Well they their both accurate. If a DynoJet dyno says you made 460rwhp, then you made 460rwhp. If a Mustang dyno says you made 460rwhp, you also made 460rwhp. Now which one of those numbers best represents what your car is doing when its on the street. That’s a different question.

The most important thing to remember is that a dyno is a testing tool. If the numbers keep increasing, then you’re doing the right thing. We try to look over at NET gain, instead of Peak HP numbers. A 30rwhp increase is a 30rwhp increase regardless of what dyno it is on.

Now I can address how to calculate the difference between one type of dyno and another. Simply put, you can’t. Because Mustang dyno’s have so many more variables, it’s not a simple percentage difference. We’ve had cars that made 422rwhp on our Dyno, two days later make 458rwhp on a DynoJet the next day. We’ve also had cars that made 550rwhp on our dyno, make 650+rwhp on a DynoJet a few days later at another shops Dyno Day. For instance, my 2002 Z28 with a forged internal LS6 Heads/Cam/Intake, makes 460rwhp on our dyno. I thought that was a little low, since I’ve had cam only LS6 Z06 vettes make 450rwhp. So I overlaid the dyno graphs. Guess what, the PAU force for my car was almost 200lbs more than the C5Z06 that made 450rwhp with cam only. So I entered the weight and horsepower at 50 number for a C5Z06 and did another horsepower rip with my car. The only reason I did that was to compare Apples to Apples. This time my car made 490rwhp, no other changes. Now I don’t go around saying my car made 490rwhp, I say what it actually did with the correct information entered into the computer. It made 460rwhp. Now if I ever get a chance to take it on a DynoJet (which I plan to in the spring), I have no doubts it’ll be over 500rwhp. I know this based on airflow and fuel consumption on the data logs.

But since we’re asked this question constantly we're fairly conservative, and hence tell our customers that the difference is closer to 6-7%, but as you make more power, and the more your car weighs, the difference increases as well. You must remember, Dyno's regardless of the type are tuning tools, and are in no means meant to tell people how fast their car is. Now which one is more "real world" is a totally different question. I like to explain it like this..... If you drive your car in a situation in which you have no mass and you're in a vacuum, so basically if you do intergalactic racing in space, use a DynoJet. If your car sees gravity, and has an aerodynamic coefficient, and you race on a planet called Earth, then use a Mustang Dyno