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Horsepwer/torque gain with rear gear change ?

Posted By: Mopardude440

Horsepwer/torque gain with rear gear change ? - 07/19/12 04:46 AM

was curious how much gain i could expect changing rear gears to 3.55 or 3.91 i have 3.23 now 69 roadrunner 4-speed 28in tire 440 510 hp/ 540 tq flywheel 310 hp/ 415 tq rear wheel 4400, redline thought would have done better but dyno crew says my rear gear is killing it,
Posted By: RoadRunner

Re: Horsepwer/torque gain with rear gear change ? - 07/19/12 05:07 AM

I don't think there should be any change.
Posted By: west

Re: Horsepwer/torque gain with rear gear change ? - 07/19/12 05:20 AM

It cant change H.P. but you will see a performance gain. I would recommend the 3:55
Posted By: DirectSubjection

Re: Horsepwer/torque gain with rear gear change ? - 07/19/12 05:35 AM



It will change the horsepower and torque level your system operates in at rpm levels.
Posted By: RSNOMO

Re: Horsepwer/torque gain with rear gear change ? - 07/19/12 06:54 AM

3:91...
Posted By: 340SIX

Re: Horsepwer/torque gain with rear gear change ? - 07/19/12 09:19 AM

Quote:

It cant change H.P. but you will see a performance gain. I would recommend the 3:55


I agree you will not get more hp gain but the steaper gear set will tune your trans/ engine combo so it will perform better showing more power as a matched set
I[ie} it it a matched set engine with right cam, trans rear
Hope I making censer here
changing from 323 to 355 is so little it will not help mubh at all and just mess up the speedo reading lol
I have done it my self when I broke the 323 and had some 355 laying around and did not notice much if at all other than the speedo being off
the big change was 323 to 391 and I liked that as with my cam and compression it when fine on 1-10 interstates well
Posted By: 70Cuda383

Re: Horsepwer/torque gain with rear gear change ? - 07/19/12 01:29 PM

a gearset won't change the Hp/Tq that the chassis dyno actually sees? I always heard that it would, as gears "multiply torque"

if it doesn't matter what the gear ratios are, then why do they always do the dyno pull in the 1:1 gear?
Posted By: jcc

Re: Horsepwer/torque gain with rear gear change ? - 07/19/12 01:35 PM

I would change the dyno crew first
Posted By: 471Magnum

Re: Horsepwer/torque gain with rear gear change ? - 07/19/12 02:26 PM

Quote:


if it doesn't matter what the gear ratios are, then why do they always do the dyno pull in the 1:1 gear?


Actually, it does and they don't.
Typically, the higher the gear (numerically lower), the lower the parasitic loss. Your 1:1 gear will give the least loss since everything is locked together inside the transmission. No gear friction.
That being said, a qualified dyno operator should put the car in the safest gear. That would be one that won't exceed a tire speed rating, or allow an unintended down/up shift during the pull. In the case of a three speed auto, they often put it in second.
A chassis dyno factors out gear ratio.
Posted By: DconD100

Re: Horsepwer/torque gain with rear gear change ? - 07/19/12 02:29 PM

So...if you made a dyno pull in second gear, wouldn't be the same as pulling it in high gear with 4.68 gears? I always wondered if the final gear made a difference on a wheel dyno. Do they have to input final gear ratio and tire diameter before they start?
Posted By: Anonymous

Re: Horsepwer/torque gain with rear gear change ? - 07/19/12 02:42 PM

Quote:

So...if you made a dyno pull in second gear, wouldn't be the same as pulling it in high gear with 4.68 gears? I always wondered if the final gear made a difference on a wheel dyno. Do they have to input final gear ratio and tire diameter before they start?



The dyno knows the engine rpm and the tire speed, so the overall gear ratio (factoring in tire size) is a pretty simple mathematical computation.

No one has addressed the OP's real concern which is that he has a 510 horsepower engine which is only putting 310 horsepower to the rear wheels. That does seem like a pretty huge power loss through the drivetrain.
Posted By: DconD100

Re: Horsepwer/torque gain with rear gear change ? - 07/19/12 03:02 PM



I realize that most of Kansas doesn't have a lot of elevation, but if the flywheel horsepower is a corrected number and the wheel horsepower is observed, that could explain some of the huge loss.
Posted By: 471Magnum

Re: Horsepwer/torque gain with rear gear change ? - 07/19/12 03:56 PM

3.91s will theoretically give more parasitic than would 3.23s, all else being equal. Maybe 5%. Probably less.

What is type/brand of chassis dyno? Mustang dynos give notoriously conservative, but arguably more accurate numbers than a Dynojet.

Are those engine numbers verified on an engine dyno? If so, those chassis numbers are low regardless of dyno type or gear ratio. I would expect to see 400+. You've got some serious parasitic loss in the drivetrain. Bad bearings, brake drag, driveline misalignment, etc. Is AFR being monitored during the pulls and is it good?
Posted By: DPelletier

Re: Horsepwer/torque gain with rear gear change ? - 07/19/12 04:34 PM

Quote:

I would change the dyno crew first






Dave
Posted By: DPelletier

Re: Horsepwer/torque gain with rear gear change ? - 07/19/12 04:34 PM

...and of course, the correct answer is zero.


Dave
Posted By: terzmo

Re: Horsepwer/torque gain with rear gear change ? - 07/19/12 05:48 PM

how do you change an engines potential by adjusting the rearend ratio ?? You'll change speed/rpm with different ratio in all the gears but the horsepower potential remains the same.

Different coverters(for auto guys/gals) will only enhance or decrease the "potential" EXCHANGE from engine to tranny to rearend.

It's like taking a full 16 oz glass of water and putting it in a 32 ounce glass...You still have 16 oz.
Posted By: 471Magnum

Re: Horsepwer/torque gain with rear gear change ? - 07/19/12 05:54 PM

Quote:

It's like taking a full 16 oz glass of water and putting it in a 32 ounce glass...You still have 16 oz.



Unless you spill some in the transfer process.
Posted By: DconD100

Re: Horsepwer/torque gain with rear gear change ? - 07/19/12 06:07 PM

Also, the type of air filter, headers, and mufflers on the car compared to the engine dyno could cause some loss...especially if the air cleaner lid was so close to the carb it was choking it.
Posted By: RUNCHARGER

Re: Horsepwer/torque gain with rear gear change ? - 07/19/12 06:37 PM

So they didn't take it past 4400? that would explain the okay torque differential but bad HP differential.
BTW that dyno operator probably failed math class.

Sheldon
Posted By: 70Cuda383

Re: Horsepwer/torque gain with rear gear change ? - 07/19/12 06:38 PM

and maybe it's a combo of a happy engine dyno and a stingy chassis dyno. if the engine dyno was reading 20-30 high, and the chassis is 20-30 low, that would also account for the variance.

If you really ARE losing that much Hp through the chassis, then start looking for problems...as mentioned, brakes dragging, driveline mis-aligned, bad bearings, maybe a bad torque converter, etc.

also, make sure nothing in the motor has gone bad from engine dyno to chassis dyno. if you are starting to wipe some lobes and have less lift now, or the the intake/exhaust is too restrictive, etc.

I know that how my engine was tested on the dyno, it'll NEVER see that kind of set-up in the vehicle unless I turned it into an all out race car. on the engine dyno there was no air cleaner whatsoever, no turbulance of intake air because there was no hood, no moving air over the carb, etc. just sitting static in a room. on the engine dyno it had the same headers as in the car, but they were open, with a vacuum system over the collector to take the fumes outside, vs a full exhaust in the car. it also had an electric water pump, with no other accessories, and the fuel pump, ignition, etc. was all driven by a stand-alone power source, so the flywheel hp was a pure, free number with NO loss anywhere.

add in a full exhaust, air cleaner, underhood turbulance, alternator, water pump, PS drag, and it wouldn't surprise me if suddenly my flywheel hp was down by 30, and I haven't even begun to account for parasitic loss through the transmission, U-joints on drive shaft, rear end bearings, inertia of the parts, etc.
Posted By: GTX MATT

Re: Horsepwer/torque gain with rear gear change ? - 07/19/12 11:31 PM

Gears multiply torque. Torque x RPM / 5252 = Horsepower.

The dyno measures torque. The gear, as well as multiplying torque, is dividing RPM by the same amount. So horsepower will remain the same no matter what the gear.

I've always assumed that since the dyno measures the torque and it knows the rpm of the tires it just takes the torque measurement and then reduced it to a 1:1 ratio.

Remember that even in direct drive the differential is multiplying torque and the height of the tires is dividing the torque. I'd be interested to hear from someone who knows for sure how the dyno is figuring it out.
Posted By: BSB67

Re: Horsepwer/torque gain with rear gear change ? - 07/20/12 12:28 AM

Quote:

I would change the dyno crew first



One or both of the dynos is/are wrong. If the chassis dyno results are not corrected to std conditions that would explain some or possibly all of the difference. All of the chassis dynos that I'm familiar with correct to std conditions.

It is unlikely that the apparent loss of 100+ hp is due to parasitic losses. There would be a bunch of noise, heat and broken parts by now.

The only thing that makes horsepower is the engine.
Posted By: Dunnuck Racing

Re: Horsepwer/torque gain with rear gear change ? - 07/20/12 01:02 AM

Quote:

Gears multiply torque. Torque x RPM / 5252 = Horsepower.

The dyno measures torque. The gear, as well as multiplying torque, is dividing RPM by the same amount. So horsepower will remain the same no matter what the gear.

I've always assumed that since the dyno measures the torque and it knows the rpm of the tires it just takes the torque measurement and then reduced it to a 1:1 ratio.

Remember that even in direct drive the differential is multiplying torque and the height of the tires is dividing the torque. I'd be interested to hear from someone who knows for sure how the dyno is figuring it out.



Chassis dynos are good for comparing changes while tuning. The horsepower numbers they give are usually not very accurate,but are useful if you have a good dyno and operator to keep things repeatable.
Different dyno manufacturers have different ways of "figuring" what the numbers are. I have some pretty good paperwork comparing some of them. There is one brand of chassis dyno (would have to look up which one) that came up with a formula by back figuring based on what a vehichles manufacturer claimed the flywheel horsepower was.
If you ask some of them how they calculate things,some will tell you and others will tell you it is proprietary.
There are some that use a generic percentage for drivetrain losses too. We know some vehichles are more efficient then others though.
Back to the OP's questions though,tire and gear changes can change the numbers the dyno shows. The engine makes the same power. You can change tire pressure,or tighten the tie downs down more and see a difference due to effective gear ratio.
Get a good operator,use it for tuning changes,and you will be fine.
If you want real flywheel numbers,stick to an engine dyno.

Keith
Posted By: Rick_Ehrenberg

Re: Horsepwer/torque gain with rear gear change ? - 07/20/12 01:03 AM

Quote:

...and of course, the correct answer is zero.

Dave



For sure!...But the cheapo inertia type dynos (w/o a brake, e.g., DynoJet) may show some variation. EG: if you swap to an aluminum flywheel, the engine obviously makes no additional HP, but the D.J. will show a gain. Yet, the car will, in all likelihood, be quicker in the quarter mile. So an inertia dyno can be useful -- think of it more like a stationary drag strip than a true dynamometer.

If that car were mine, I'd sure leave the 3.23s...or go 2.94:1! Just my two cents...

Rick
Posted By: AndyF

Re: Horsepwer/torque gain with rear gear change ? - 07/20/12 01:46 AM

Well it is possible for the rear end ratio to kill the dyno pull, but I'd think with 500 ft-lbs on tap you should be able to pull 3.23 gears.

If the gear ratio is too high then the engine ends up lugging down and you never hit the rpm/sec increase where the engine really makes power.

But that engine should pull 3.23 gears. Anything deeper than 3.23 gears with a big stroker is in danger of just smoking the tires off.

I have 3.55 gears behind my 470 inch big block and it makes 475 at the rear tires on the chassis dyno. The power has to be put on carefully or else the run goes up in smoke. My car would probably make more power with 3.23 gears since we could accelerate harder for the full pull.
Posted By: Mopardude440

Re: Horsepwer/torque gain with rear gear change ? - 07/20/12 02:02 AM

engine dyno was a superflow, rear wheel dyno was a dynojet. Run condtions was 98.5 degrees 28.97 in-HG Humidity was 15% SAE 1.05 I know that the outside air temp hurts hp. but i didnt think it would drop that much lol dyno guy did a great job with how he set it up and was very professional he just told me it was my gears that was making the driveline percentage so big. Engine stops makeing power around 4,500 and both dyno sheets prove that, and Yes i agree that when the engine was dynoed there was ideal condtions with no drag on it, same headers, same carb, same dizzy only thing that was different was it was set up with MSD crank trigger and race gas at the time. Just trying to find out about if the dyno is accurate or if i can gain some numbers somewhere thanks all
Posted By: AndyF

Re: Horsepwer/torque gain with rear gear change ? - 07/20/12 02:09 AM

Does the engine sound like it is pulling hard on the chassis dyno? My guess is that the ignition curve is wrong. You can make killer power on the engine dyno at WOT with a locked distributor but it won't necessarily work very well on a chassis dyno.

You can also kill a dyno run by starting at too low of a speed. Another thing to look at is the carb. Accel pump makes a difference when doing a chassis dyno pull but it doesn't necessarily matter on a WOT engine dyno.

There can be tons of stuff that changed between the engine dyno and the chassis dyno. A plugged muffler or a weak fuel pump in the car will kill a lot of power. I did a dyno pull once with my e-brake on. That killed a bunch of power!

Put on your detective hat and figure it out. The power is in there somewhere.
Posted By: PHJ426

Re: Horsepwer/torque gain with rear gear change ? - 07/20/12 02:14 AM

Quote:

Quote:

...and of course, the correct answer is zero.

Dave



For sure!...But the cheapo inertia type dynos (w/o a brake, e.g., DynoJet) may show some variation. EG: if you swap to an aluminum flywheel, the engine obviously makes no additional HP, but the D.J. will show a gain. Yet, the car will, in all likelihood, be quicker in the quarter mile. So an inertia dyno can be useful -- think of it more like a stationary drag strip than a true dynamometer.

If that car were mine, I'd sure leave the 3.23s...or go 2.94:1! Just my two cents...

Rick



I dumped the 4:30's in my 72 Roadrunner and went with a 2.94 suregrip unit.....

Did my RB stroker have a HP loss?........

Not at all it just increased my cars ability to be driven past the 1/4 mile. There is a big world out there no need to travel it 1/4 mile at a time.....
Posted By: Mopardude440

Re: Horsepwer/torque gain with rear gear change ? - 07/20/12 02:19 AM

The engine is a 440 .484 cam eddy heads. Engine sounds great under the pull. 3" exhaust dynomax mufflers, all restored all new parts through out. The dist is recurved, On the rear wheel dyno the power curve is all most dead on on every run 290-308 hp torque 388-401
Posted By: Anonymous

Re: Horsepwer/torque gain with rear gear change ? - 07/20/12 02:53 AM

Quote:

Engine stops making power around 4,500



You have a 440 with a .484 cam that stops making power at 4,500 rpm? And it supposedly made 510 horsepower on an engine dyno?

Something not adding up here.
Posted By: Rick_Ehrenberg

Re: Horsepwer/torque gain with rear gear change ? - 07/20/12 02:58 AM

Quote:

was curious how much gain i could expect changing rear gears to 3.55 or 3.91 i have 3.23 now 69 roadrunner 4-speed 28in tire 440 510 hp/ 540 tq flywheel 310 hp/ 415 tq rear wheel 4400, redline thought would have done better but dyno crew says my rear gear is killing it,



Might be interesting to see what the 1/4-mile trap MPH is, then plug the numbers into an online calculator. As Larry Shepard used to say: "Drag strip dyno".

Rick
Posted By: bboogieart

Re: Horsepwer/torque gain with rear gear change ? - 07/20/12 03:21 AM

Quote:

Quote:

Engine stops making power around 4,500



You have a 440 with a .484 cam that stops making power at 4,500 rpm? And it supposedly made 510 horsepower on an engine dyno?

Something not adding up here.



This is what I was thinking too.
Posted By: Mopardude440

Re: Horsepwer/torque gain with rear gear change ? - 07/20/12 03:32 AM

I just pulled the engine dyno sheet out and at 4500 rpm its atmax hp and torque @ 4800 it starts dropping. it made 498hp not 510 like i thought torque was 540 "engine builder is a NHRA record holder and knows engines, and has his own dyno room so no problem there" He called the engine a Torque Monster LOL www.stinnettracing.com
Posted By: RSNOMO

Re: Horsepwer/torque gain with rear gear change ? - 07/20/12 05:57 AM

Legitimate points...

BUT, the first time you get spanked with the 3:23's...

After you get that engine sorted, if you want to GET IT ON...

And I say, IF...

3:91's...


(After all, you got a healthy 440...

If you want to cruise the interstate, get a mini-van)...
Posted By: PHJ426

Re: Horsepwer/torque gain with rear gear change ? - 07/20/12 07:23 PM

It all depends on your perspective on what a car should do well.

IIRC Rick's Road Runner runs in the 11's with a set of 3:23 gears in the car. Torque rules.....


Interstates are not just for mini vans.....LOL

Never imply we will be running the speed limit on the interstate..........
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