HP loss with gears on Dyno

[Russ M]

Besides owning a viper I also own a supra which is a race car, and as such gets alot of dyno time. What do I mean by alot, well lets say that at the dyno jet where the viper club was there is at least 150 pulls on file made by my supra. During all these pulls there have been many different gear selections and none have made any noticible differences. The only thing that has made a difference is the transmition gear which would be about 40 hp from 3rd to 4th.

 

[JonB]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by 1bad gts:
[B} ...............Dyno jet(when pressed) will unofficially admit there is a problem in not having a correction factor for rear end gears [/B]<HR></BLOCKQUOTE>

The original question was "HP Loss w/ gears on dyno"

The answer is a possible DynoJet problem, not a rear-gear cause-effect problem. Driveline loss has been debated at length between PartsRack and Unitrax, a renowned rear end / gear / drag equipment CA vendor......we have agreed that the extra driveline loss due to gears alone is likley "under 1.5 %" additional loss between 3.07 and 3.55. Some swear "NO DIFFERENCE"

The SOTP GAIN is real! The equivalent SOTP feel of a 68HP gain at the flywheel.....60 at the rear wheels.

 

[Tom Welch]

Final drive on the viper is found in 4th gear. The dyno does not care or know what rear end gears are in ANY car. To think that a rear end gear change especially higher in numerical value will hurt horsepower, is thinking in error.

Should you dyno at a facility that gives different values for different rear end gears, I suggest you find another facility.

Remember, make all pulls in 4th gear as that is 1 to 1 driveline gearing for the viper.

To take things one step further, to rest your laurels on a dyno sheet may surely get you devistated at the track. Dynos are only as accurate as the user, and more times then not(especially lately) their numbers have saddened the vehicle owner. Use them as a tool to determine if modifications have made a difference in power. Use 1/4 mile MPH for REAL horsepower numbers. An example; things like changes to valvesprings and reducing valve gear weight which result in quicker acceleration may not show on a dyno pull but will show a substantial gain on the dragstrip.

One final note. It is no secret that a dyno pull is equivelant to several 1/4 mile runs as your engine and drivetrain have to spin a 6000 lb drum! I've seen clutches smoke, tires fly apart and drivetrain parts stuck through dyno shop walls!

Want real results, get em from the track. A timeslip is worth a thousand dyno pulls!, and a video of your viper exceeding 135 MPH in the 1/4 is priceless!

Tom
Http://btrviper.com

"If we had some HAM, we could have some HAM and EGGS, IF we had some eggs!!"

 

[Russ M]

Tom,

You are correct that dyno #'s are just that, however a dyno is a priceless tunning tool without which most would do way more damage at the track.

 

[LTHL VPR]

I had changed gears a few times on another car, going from 3.08 to 3.73. I saw no measurable difference on the dyno, and I had over 40 runs.

I have definitely seen differences in cases where the transmission gears have changed.

 

[Venom Lover]

JNS,

Here's one data point for you.

Last December, my GTS dyno'd at the same shop at 456 rwhp and 490 ft-lbs. The only mod I have since then is the 3.55 rear end, and Saturday I dyno'd at 448 rwhp and 480 ft-lbs. So, for me, assuming good SAE correction factors, consistency of Darren's dyno, etc., the 3.55 rear end caused an apparent loss of 8 rwhp at 10 ft-lbs.

I defer to Cragin's wisdom, of course, but at 525 rwhp, you have about 15% on me, from which I would guess the gear is costing your an apparent 10 rwhp on the Dynojet (about 15% above my 8 rwhp).

All that being said, I agree with Tom Welch. Bring that beast out to Fontana this weekend and see what she pulls in the 1/4!!!

 

[JonB]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by 1bad gts:
John,both Dan Cragin and John Hennessey have told me they have seen 20 plus to the wheel loss on their 550 plus cars by switching gears.. I have a EMAIL(somewhere) from Dynojet in regards to this.<HR></BLOCKQUOTE>

This "loss" appears to be a time calibration factor, unique to Dynojet. When you add either a 3.55 or an aluminum flywheel, the "Time-to-peak-HP" is significanly reduced. Seldom do owners compare this factor....just HP! The HP, as noted by Tom Welch and others, should remain within 1%.....ie no factor.

But you may achieve peak HP 2-3 seconds FASTER with the mods..
This accounts for the SOTP feel of faster acceleration. When you speed up the process, DynoJet seems to stumble, and show up to 20 less HP that simply CANNOT be "gone" due to rear gearing!

The problem is with the instrument..not the gears

 

[Mike Brunton]

I'm not buying that there is a "correction factor" with gears. The dyno does NOT KNOW AND DOES NOT CARE what your gears are.

It knows the mass of the drum. It knows the RPM you are running. It calculates the acceleration of the drum, and that's ALL it needs to know to determine horsepower. Changing your tires, wheels, gears, tranny does NOT affect what the dynojet sees at all.

Maybe there is more driveline loss with shorter gears? I also don't think that there is a time correction factor going from 3.07's to 3.55's. A good friend of mine is an engineer who did some work at Dynojet - when I asked him about this, he said it was absolute bullpucky. His point (and I agree) is that (and not to be insulting), but a Viper putting down even 600hp ain't exactly hitting the limits of what a Dynojet can measure. They run 800hp Supras, 900HP Vipers, 1000hp cars, it's all the same to the DynoJet.

Are we so naieve to think that Dynojet's cant accurately measure horsepower on a 600hp Viper with 3.55's or 3.73's? I guess the thousands of measurements they do on drag cars with 4.10's and 800hp are all wrong too? &lt;sarcasm folks&gt;

I've heard of "rumours" to this effect - where do they come from? I've asked DynoJet and they said it is BS! I know an engineer who worked there who also said it was BS. The ONLY person I have heard this from is a certain tuner - and let's remember folks it would be in this tuners best interests to have people thinking their cars are making more power than they are measured at.

I want a name at DynoJet who will corroborate this story - because everyone I've ever talked to says it's a ridiculous assertion.

Name?

 

[Tom Welch]

Once again, I see a thread deteriorating due to myths and misconceptions. First of all, dynos are ok for determining if a given modification has increased power, BUT NOT ALL MODIFICATIONS. Modifications to air induction will show their true gains on the dragstrip when 120+ mph air is ramming through them, lighter weight valve gear will show an acceleration gain at the track more so than on the machine.

Use the dyno for what it is, a tuning machine, and not a very accurate one at that. We were dynoing last week and with the humidity at 87.6 F and the temperature at 88 F the dyno computer told us that our correction factor was 1.00. Rediculous! Anyone with a little knowledge of the weather knows that when the temp and dew point/humidity are equal in value, you get visible moisture-rain-fog, etc. These are the worst conditions(visible moisture is taking place of oxygen which the engine likes.......your engine does not burn water worth a crap!). A 1.00 correction factor should be set to meterological STANDARD DAY which is 29.92 baro with 59 F temp at sea level(the 29.92 number is then calculated for feet above sea level that the facility is at, hence the correction) Just like an airplane altimeter. All reciprocating and turbine aircraft engines are tested and certified in the above manner.

In any event, changing to a higher numerical gear will speed up the time of a dyno pull, at the same time it makes it easier for the drivetrain to move the mass of the drum. So, again, higher numerical gear should not make a change in power to a negative value, if anything your numbers should be better with a 3.55 gear through a 4th gear pull.

I suggest that if you see your numbers decay, attempt another pull but through more of the rpm band(start at a lower rpm)to allow the dyno to have time to see your complete pull.

Or you can always buy a G-Tech!!LOL.....

Tom

 

[Mark Young]

Ok, the engineer in me is forcing me to throw in my $.02 (actually its probably worth less than that).

I can't believe there is a problem with the Dynojet. The test is simply to accelerate a known mass over a given period of time. 4th gear is used on the Viper because it represents a 1:1 tranny ratio. Would anyone here expect to see the same HP #s if they used a different tranny gear but same rear-end ratio? How is that technically any different than using the same tranny gear but a different rear-end ratio? Maybe it would be interesting to do runs in 3rd and 5th gears to see what the effect is (just guessing here).

My point is that no matter how you change the gearing, the speed at which the drum is rotating at a given engine rpm is different. Given that, I would most certainly expect the dyno curves to be different as the drum mass will accelerate at a different rate if its speed is changed, and thus its not really surprising that the peak #s are off by some amount.

Anyway, thats the way I see it. I'm only an 'armchair tuner' though so I could be completely wrong

 

[RoyV101621]

If MPH in the Quarter is the key to HP then let create a hp vs. mph chart for Vipers.

 

[Venom Lover]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Mark Young:
4th gear is used on the Viper because it represents a 1:1 tranny ratio. Would anyone here expect to see the same HP #s if they used a different tranny gear but same rear-end ratio? How is that technically any different than using the same tranny gear but a different rear-end ratio? Maybe it would be interesting to do runs in 3rd and 5th gears to see what the effect is (just guessing here).

<HR></BLOCKQUOTE>

Mark,

A truly excellent observation, with which I agree totally! Unfortunately, I'm having problems with getting images uploaded to my web page, but I'm looking at a dyno sheet in which I did one third gear run and then two fourth gear runs (back when the car was stock), and sure enough, there's about 20-25 hp difference between the two curves (the 3rd gear run being lower). I agree, the effect on a Dynojet of a higher numerical rear end should be of the same nature to a pull in a lower gear. And, despite Brunton's eloquent outburst !!

, the evidence is irrefutable that the gear matters. I'll upload the dyno sheet when I can.