New to the GM side of the street with a 2008 C6 LS3 VY M6 coup, former 05 Mustang GT owner. I've always heard that manual transmissions had a 15% hp loss and auto had a 18-20% lost of hp from the over all crank hp rating. However, I've seen several post where it seemed that there was a 10% loss on a C6 with a manual transmission and 12% for a auto.

So which is the correct figure? Thanks, Dewman

 

I can tell you that when i had my 06 A6 dynoed stock it came out to 332rwftlbs and 331rwhp. That comes out to a 17% loss. This was on a dynojet at Cartek`s shop.

Not to sure about the manuals but i have seen them dyno stock anywhere`s between 340-350rwhp.

Hope this helps.

 

Just for argument sake I us 12% with my manual

Numbers proved to be real close on with stock dyno.

375.8 rwhp for an 430 rated motor. 12% loss =378.

 

In the C6 it is closer to 10% for the manual and 12% for the auto. Something close to those numbers.

 

I believe the transmission loss for a manual is about 60HP and the automatic is about 70HP. Don't think of the losses as a percentage of engine HP, it's just a straight loss no matter what HP your engine has.

For example: a 400HP engine with an auto is about 330HP at the rear wheels (400 - 70 = 330). Now, you modify your engine to put out, let's say, 500HP which would yield 430HP at the rear wheels (500 - 70 = 430). The transmission didn't change it's power loss simply because you changed the engine HP.....

If you calculated it as a percentage, let's say 17%, then you would have the following: 400 x .17 = 68, 400 - 68 = 332HP at the rear wheels......the 17% is basically valid. However, after you modified your engine to 500HP with the same 17%: 500 x .17 = 85, 500 - 85 = 415HP at the rear wheels. See, how would the trany lose more HP simply because you changed the engine HP......doesn't make sense.

Again, don't think of the trany losses as a percentage of anything, just use the 60 and 70HP loss, manual and automatic, respectively.

 

You're on the right track. Here's my post from a thread last year about the same question:
http://forums.corvetteforum.com/show...8&postcount=28

"If we're talking about RWHP on a dyno, it depends on the type of dyno used. A Mustang dyno can put a load on a car and control the rate of engine acceleration (250 RPM/sec, 500 RPM/sec or whatever) or it can hold the RPM constant. That way, HP loss due to the inertia of engine components, drivetrain components and wheels/tires/rotors are held constant. On a Dynojet dyno, there is only the inertia of the drum to resist the torque at the wheels so when we add more HP or dyno in a lower gear, the time for the dyno run is less which means more HP loss to inertia.

Drivetrain HP loss on a chassis dyno is composed of the following:
1) second order or exponential losses like friction
2) first order or linear losses like inertia and gears
3) constants like windage of gears through the oil in the transmission/differential, shafts rotating through seals, and sidewall flex all at a particular RPM. In other words, those losses are independent of HP transmitted.
That leaves us with the equation HP (loss)= ax^2+bx+c where a, b, and c are constants and x is FWHP. "c" is the predominate term meaning most of the HP loss is a constant.

Other variables to consider is when a dyno run is done in a lower gear, the tire, transmission shafts/gears, and differential shafts/gears RPM is less reducing the HP loss to sidewall flex/windage and giving more RWHP. On a Dynojet, this is countered by a shorter dyno run increasing loss to inertia, not a factor on a Mustang dyno though since it keeps the time of the dyno run constant. Conversely, a dyno run in 5th or 6th on a Dynojet will have less loss to inertia (longer dyno run time) but more loss to windage/tires...but there's also a speed limitation on chassis dynos which prevents a run in 6th (unless you have some 4.56s in the rear). On the road, quicker acceleration in the lower gears results in more loss (as a percentage) to inertia, the same effect as a Dynojet.

As far as the 1:1 ratio of 4th gear being the best to dyno in, there is fact as a basis to that statement. Quality involute gears have a HP loss of ~1%, so when we dyno in 1st, 2nd, 3rd, 5th, or 6th, part of the drivetrain loss is ~2% of the HP transmitted through any of those gears (it's 2% because power flows through one set of gears from the input shaft to the countershaft then through another set of gears from the countershaft to the output shaft). You may notice the distinct absence of 4th gear in the list above...that's because there are no "gears" for 4th gear, something a lot of people don't know. When you make the shift to 4th gear, the synchro mechanism mechanically couples/locks the input shaft to the output shaft and power flows straight through the transmission making 4th gear the most efficient "gear" to dyno a car in as far as the transmission is concerned. It doesn't have anything to do with the fact the "ratio" is 1:1 since we could also put the power through gears at a 1:1 ratio and achieve the same output RPM.

Hopefully this shines a little light on the subject. You guys are on the right track, it's neither a percentage or a constant. But if you had to pick one, you'll be closer to FWHP if you use a constant. I.E. if you dyno a stock Z06 and come up with 450 RWHP for a 55 HP/10.9% loss, then mod it to 600 RWHP on the same dyno, you'll be much closer to the true FWHP if you add the 55 HP loss from stock rather than back calculate using the percentage number. And the guys mod their cars to 600+RWHP that just pull a percentage number out of their *** (like 15%) to back calculate their FWHP are just looking to stroke their ego with a big number and impress people. The only way to know the percentage is to remove the engine and put it on an engine dyno to get the FWHP. But then the first time you mod the engine, that number is not valid anymore.

And this statement you made is absolutely correct:

"So, back to square one, let's forget about accurate flywheel estimates and focus only on what a given dyno reads at the wheels!" "

 

After the mods i had installed i ended up with 441 rwhp and what i did was keep the 17% loss and came up with 530hp at the motor...So am i wrong in thinking this way since i had the work done at the same shop on the same dyno?

Or do i just add 70hp to 441rwhp?

 

You rarely see a dyno numbers at the crank because the engine has to be out of the car. Most LS 2 M6 dyno around 340 and auto's around 330. I do know of one test where GM High Tech Magazine did a LS 2 mod test at Livernois Motor Sports with the engine on a stand which gives crank HP. The engine had Livernois Stage 3 heads, stage 2 cam, CAI and Kooks 1 7/8 headers with straight exaust. It produced 544.8 HP and 483.3 TQ. A car that Livernois did with the same equipment and Corsa Sport exaust dyno'd at 472 RWHP and 412 RWTQ. This is on the same dyno.544/472=87%. In this case it looks like a 13% loss on a M6 .This is the only one I know of that would be a good example. Hope this helps.

 

In Apr 08 I had my stock 05 C6 A4 dynoded at 355 rwhp and 357rwtq. I had some mods done and redynoded about four days later and ended up with 437 rwhp and 418 rwtq. Do I add the 70hp or 45hp to get the FWHP?

Thanks
Mike

 

This has been discussed many times on this forum.....

I've seen a few engines run on a engine dyno and then installed in the car. This is what I've seen:
MN6=12% loss (Dynojet)
A6=17% loss (Dynojet)

I've heard all kinds of numbers thrown around, and while this may not be exact, I think it's pretty close.

 

Yes, to be fair (if you have an automatic), it's really close to 441 + 70 = 511HP at the crankshaft.

 

Yes, you would have about 437 + 70 = 507HP at the crankshaft. Keep in mind, the 70HP loss seems to be pretty accurate for an A6, but not exact; it could be 65HP loss on one Vette and 75HP loss on another. Your A4 is more than likely different (how much, only other A4 owners would know) than our A6's. So, the 70 is just a fun rule of thumb!

 

That was a great explanation.....I love equations, mathematics is a universal language! Thanks!