632C2

I have a 632" BBC w/Big Chief heads in a '67 Corvette street car. The motor made 1060 hp on an engine dyno and makes 900+ rwhp and 760+ rwtq on my DynoJet. This is with full exhaust connected. The car is not NHRA legal (no rollcage) and therefore I haven't raced it. I am probably the truest example of a "dyno racer".

I have had a G-Force GF-5R clutch assisted 5 speed manual trans in the car since 2002 and decided to try a PowerGlide for something different. This is the first time I have run a PowerGlide and the first time with an automatic of any kind in 20+ years.

After a considerable amount of research, I purchased an Ultimate "Big Dawg" Super Glide, Super Mega 10" bolt together (appx. 4500 rpm stall) converter, and Reactor Flexplate from Coan Racing. I explained on numerous occasions to them that I wanted their best trans/converter that would give me the least amount of power loss on the DynoJet. I was told that the torque would be higher with the PG vs. Manual and the horsepower would remain roughly the same.

I have made 6 runs with the PowerGlide and the best numbers are 830 rwhp and 747 rwtq. Disappointing to say the least. Based on 1060 flywheel horsepower, the manual trans lost appx. 15% at the rear wheels while the PowerGlide lost appx. 24% at the rear wheels.

I have included a DynoJet graph to illustrate what is going on. The horsepower loss is a definite concern but the thing that confuses me even more is the relationship between the engine rpm and the driveshaft rpm. Up to appx. 6200 rpm the engine is turning more rpm than the driveshaft – that I understand. However, after 6200 rpm the driveshaft spins faster than the engine. I have included a graph from my Racepak that shows this.

Notice that on the DynoJet graph the “rpm/mph” drops below that of the manual transmission at that same 6200 rpm. Also, the horsepower starts heading downhill at that point.

To show that this is a true A-B-A test, prior to the swap I made a few runs with the GF-5R 5 speed manual and recorded a best of 905.12 rwhp / 762.10 rwtq. Next, I installed the PowerGlide and recorded a best of 809.46 rwhp / 728.47 rwtq. Finally, I installed a Tremec TKO-600 5 speed manual (I sold the GF-5R) and recorded a best of 905.83 rwhp / 762.51 rwtq.

I am wondering if anyone has any ideas as to what is happening, especially with regards to the engine vs. driveshaft rpm? Coan has no explanation for the power loss and says the reason for the driveshaft spinning faster than the engine after 6200 rpm is that their converters are super efficient and solidly lock up.





Steve

427Hotrod

Let's see what everyone comes up with Steve.....


Man, I can't even imagine the motor I'd have to build to make up for that much power loss....

Yes I can...a Big Chief headed 632!!!


Sad part is even when you're running the auto you're up a hundred or so on me probably!!

Now I'm depressed!



JIM

gkull

I really don't see how the ouput could exceed the input in a 1-1 ratio top gear. I never would have guessed that a PG would suck up 100 hp. they must have put an awfully big pump in that PG. My racing 700R4 pump normally ran over 250 PSI and could really bang the gears, but just switching over to my TKO600 gave my smaller motors a kick in the pants with all the freed up power.

632C2

Ah, come on Jim! At least you can race yours on a regular basis. I can't wait until you put a Big Chief headed 632 in your car and race it. It will give me an idea of what I can expect.

Steve

PolishMike

Sounds just about right to me. typical auto tranny drivetrain is about 20-25% for RWD.

Its just a matter of physics, more parts, more heat spread, more discs to slip, more power lose.

Also may want to look into what else could have changed between dyno times. Ambient temps, fans, ect, and what gear were you running before/now?

PS- i used to live in sumner back in the day

632C2

George, this one really baffles me. Although I specifically didn't ask about the pump, Coan told me that there was nothing in their trans that would suck up that much power. I will ask them about the pump next time I talk to them.

Steve

632C2

Thanks for the input. According to most experts I have talked to, a drag racing PowerGlide consumes appx. 18 horsepower. That's quite a ways away from 100 rwhp! Absolutely nothing changed between runs except for the temp, ABP, and humidity. The SAE correction factor takes that into account. But even with that, the conditions for the runs with the PowerGlide were better than when I ran with the manual. This is precisely the reason why I did a A-B-A test!

Steve

TimAT

Not trying to start anything, but I'm curious too. I worked for GM DEALERS for 15+ years building trannys in an earlier life, and I always thought that convertors had SOME slip, just by design. But 100HP? That's a bunch of loss. Are there any signs of heat? Slip = heat.

I'd be interested in learning about this one.

632C2

No heat problems here. According to the Racepak, the trans temp was between 130* and 135* throughout the run. Plus the fluid looks fresh and no metal shavings - which is always a good thing.

Steve

PolishMike

There is no way someone can tell you it will take 18 hp from you. Drivetrain lose will ALWAYS be a percantage of crank hp. So if the more power you run the more you will lose.


Like i asked before, what were the differences in outside temp (in the garage the dyno was in) between the two dyno times? Were the same fans used both times? You would be surprised how much outside temp changes dyno readings

enkeivette

Although I'm out of my league here, I'd be willing to bet that the car would still run faster at the track with the powerglide. With that much power wouldn't it be a waste to bang through 5 gears?

I've always been under the impression that a manual trans will eat 10-15% and an auto will eat 20-25% This is true in your case, and the hp loss is proportional to that. It just seems shocking as you're running on a much larger scale than the rest of us.

632C2

Mike I appreciate your feedback. My DynoJet is at my house and I have made close to 1000 runs with my various cars. I make sure that everything is as identical as possible. I know very well about how temps can effect dyno readings. For the record, the PG run I displayed had a temp of 71.4* while the MT run had a temp of 72.0*.

Steve

632C2

I agree with you on the car being faster at the track. I am just wanting a logical answer to my dilemma so I can sleep at night.

Steve

PolishMike

Sorry, didnt mean to insult you by asking basic question, wasnt aware you have your own dyno That is amazing, i wish I could do that some day. But like i said, i wasnt aware of your dyno expertise.

Other then that 24% is slightly high, but nothing out of the ordinary. You might be able to get it down to say 22-20 but that would take a huge amount of experimenting with different setups and $$$.

Little Mouse

They waste time with a five speed in a 1400 hp pro stocker they
don't waste any time with a two speed power glide.

632C2

No offense taken. A typical manual trans loss of 15% on a 400 horsepower motor amounts to 60 horsepower. Now if you change to an automatic and lose 24% that amount to 96 horsepower. This is a difference between the manual and automatic of 36 horsepower. I could live with that.

Now you take a manual trans loss of 15% on a 1060 horsepower motor and you come up with 159 horsepower. Changing to an automatic with a 24% loss amounts to 254.4 horsepower. That is a difference between the manual and automatic of 95.4 horsepower. I am having a hard time living with that. Sure it's easy to say you could never use that much power anyway but that's not me.

Now that being said, if this is as good as it gets then so be it. I just need to get to the bottom of the engine vs. driveshaft rpm situation.

Steve

Little Mouse

With all the power you have and no way to hook it up the two
speed is probably the best bet.

Z-man

Yeah - but think of the *convenience* of not having to do all that shifting...

S489

what about the torque converter? i did a lot of research before purchasing one recently learning many new things. the mfg should have put it on a dyno to measure the efficiency and stall. however, their machine likely is not able to produce the hp you have and therefore, could have missed badly on the design.
the disparity in rpm's could be a clue to this. there is no way you can have an "overdrive" without an overdrive! there has to be some correlation between the loose coupling (torque convverter) and the physics of the dyno.

pef427

Steve,
Herein lies your answer to the driveshaft RPM ???. Have the stall tested. We changed out a Turbo 400 in a friends drag car to a new identical brand stall and tranny and he lost 3/10ths at the track. This was a very consistent bracket car.
-Patrick