Can gears overcome H.P. deficit?

[M.Rad.]

I'm pretty sure that the answer is yes, but how much gearing to make up for how much power?

For example (all else being equal) how would the acceleration of a car with say, 350 hp and 3.70 gears compare to the same car with 450hp and 3.00 gears ?

Which would be the better combination on the street (assuming the 3.70 car had overdrive for cruise)?

Regards,
m.r.

[Schwartz Performance]

It's more of a torque deficit overcoming than hp.
Yes you're right though. Guys put taller gears in their cars to run faster quarter mile times.

The rest is all in the transmission gearing and tire sizes. I would prefer a 3.70 in either case as long as there was an overdrive Trans.

What exactly are you asking? Get to the point! ;)

-Dale

[M.Rad.]

Hi Dale, thanks for the response!
I'm trying to achieve a particular "feel" and level of performance. I'm trying to figure out if that can be done by means other than just adding power output from engine, which is expensive. Every one is always asking about producing more power to go faster, but I suspect that similar results can be achieved by jugling things like gear ratios, converter stall speed, suspension, etc. Gears are cheaper than adding a hundred H.P. ( or 100 lb/ft torque), and if I can get what I want by going that route it may be both simpler and cheaper.

Hopefully, others have done something similar and can lend some advice.

Regards,
M.R.

[Bonehead]

Gear changes are what the Jeep and off-road folks do to their limp hp vehicles to create the grunt they need to maneuver. They rarely do much outside of bolt ons to increase power. The common thing is a gear change and they do well with it.

[Ron Sutton]

I'm pretty sure that the answer is yes, but how much gearing to make up for how much power?

For example (all else being equal) how would the acceleration of a car with say, 350 hp and 3.70 gears compare to the same car with 450hp and 3.00 gears ?

Which would be the better combination on the street (assuming the 3.70 car had overdrive for cruise)?

Regards,
m.r.

Hi m.r.,

There is a formula we use in racing to compare different engine & gearing combinations ... but we need to know the power output at specific rpms of the two engines. If the two engines rev the same (same powerband range) its a moot point. But if one engine makes less power than the other, but that power is in a higher rpm range ... then it's worth calculating & comparing.

For example, on a road course, let's say our tightest corner brings our mid-corner speed down to 48 mph ... and our fastest speed at the end of the longest straight is 136 mph. Let's say we have two engines to compare ... one a 408" that makes 880 hp at 8000 rpm ... and the second is a stroked engine of the same bore with 454", making 890hp at 7000 rpm. As you can see from the torque curves in the simulated dyno sheet below, the 454 has more power all the way until the top.

104708

Legend
Red line: 454" HP Curve
Light Blue line: 454" Torque Curve
Green line: 408" HP Curve
Dark Blue line: 408" Torque Curve


We'll keep the details the same in our sample car.
Tires: 335/30R18 (25.9" tall)
4-speed trans with gear ratios of:
4th - 1.00
3rd - 1.28
2nd - 1.64
1st - 2.11

I prefer to work from the top down. In this case we want to pull strong to 136 mph on the long straight. Let’s use this formula:
Tire height ÷ 2 = A x .006 = B x RPM = C ÷ MPH = Gear Ratio

To run 136 mph with the 454, in 4th gear, the math looks like this:
25.9” ÷ 2 = 12.95 x .006 = 0.0777 x 7000 rpm = 543.9 ÷ 136 mph = 4.00 gear ratio
At 7000 hp, the engine makes 655# torque x 4.00 torque multiplier = 2620#

To run 136 mph with the 408, in 4th gear, the math looks like this:
25.9” ÷ 2 = 12.95 x .006 = 0.0777 x 8000 rpm = 621.6 ÷ 136 mph = 4.57 gear ratio
At 8000 hp, the engine makes 635# torque x 4.57 torque multiplier = 2901# (10.7% more torque)

So the 408” engine in this example would be significantly stronger at this point of the track, frankly getting to the 136 mph quicker & easier, being capable of more speed if we geared it accordingly.

--------

But let’s go to our slowest corner.

For this, let’s switch the formula around to find our rpm:
MPH x rear gear ratio x trans gear Ratio = D
Tire height ÷ 2 = A x .006 = B (which in our example is always 0.0777)
D ÷ B = RPM

To run 48 mph with the 454, using 2nd gear, the math looks like this:
48 x 4.00 x 1.64 = 314.88 ÷ 0.0777 = 4052 rpm
At 4052 rpm, the engine makes 640# torque x 6.56 torque multiplier = 4198#

To run 48 mph with the 408, using 2nd gear, the math looks like this:
48 x 4.57 x 1.64 = 359.75 ÷ 0.0777 = 4630 rpm
At 4630 rpm, the engine makes 570# torque x 7.495 torque multiplier = 4272# (1.7% more torque)

So they are pretty close coming off the corner, with the 408” having a very slight edge, but as these two engines accelerate up through their respective rpm bands … and their torque curves get closer to each other … the 408” with the lower gear ratio will have a significant advantage & accelerate quicker.

Of course we can't accurately compare two scenarios without knowing the power curves of those two engines relative to rpm.

:cheers:

[Rod]

uummmm^^^^ what he said

[CampbellshotrodsAZ]

^ I was going to input my 2 cents, but yeah, I can't compete with that. Haha.

But if you aren't on the highway all the time, and racing, gearing your car to run higher in the powerband will gain you hp just by being in the proper RPM range. I've experienced this last weekend, in the opposite way. I'm returning my 98 T/A to stock so I can move on, so I pulled the 9 inch with 4.11's, and installed a stock 3.42 rearend. It feels a lot more gutless now, and takes longer to get anywhere till I get into the right RPM range. Even on an otherwise stock engine, the 4.11's made it much more fun to drive.

[TheJDMan]

If you are running a manual transmission, switching to a lighter flywheel will also make a dramatic improvement in engine performance.

[Motobrewmaster]

Great reply Ron. You do such a good job explaining everything and having numbers to work with makes it even more clear. I have been doing this for matching motorcycle gear boxes and final gear strokes for road racing for years. It can really make a difference with small engines that have very different power curves (i.e.- V-twins vs IL4's)

I have a question though. One thing that we found in motorcycle racing was that shifting more sometimes upsets the bike a bit more thus making it a bit more difficult for a smooth drive. Thus many times the V-twin engines become a bit more "ridable" as they don't have to shift in corners as much. It seems that sometimes it was worth giving up a bit of HP/Tq for an uninterrupted power being sent to the ground. Do you find this in the cars also? What are your thoughts on this?

I have attached a graph that I generate to help make gear choices and shift points. I use dyno, tyre and gear info to generate the curves. It isa good visual tool for my little brain. Do you use something like this for your car setups? It definitely helps make some decisions on the bikes. Oh and the bike in the graph is a 2000 Ducati SS750 setup years ago for Ultra Light Superbike racing and GP2 racing.

Looking forward to your thoughts and insight. Cheers, Brew

[Ron Sutton]

Great reply Ron. You do such a good job explaining everything and having numbers to work with makes it even more clear. I have been doing this for matching motorcycle gear boxes and final gear strokes for road racing for years. It can really make a difference with small engines that have very different power curves (i.e.- V-twins vs IL4's)

I have a question though. One thing that we found in motorcycle racing was that shifting more sometimes upsets the bike a bit more thus making it a bit more difficult for a smooth drive. Thus many times the V-twin engines become a bit more "ridable" as they don't have to shift in corners as much. It seems that sometimes it was worth giving up a bit of HP/Tq for an uninterrupted power being sent to the ground. Do you find this in the cars also? What are your thoughts on this?

I have attached a graph that I generate to help make gear choices and shift points. I use dyno, tyre and gear info to generate the curves. It isa good visual tool for my little brain. Do you use something like this for your car setups? It definitely helps make some decisions on the bikes. Oh and the bike in the graph is a 2000 Ducati SS750 setup years ago for Ultra Light Superbike racing and GP2 racing.

Looking forward to your thoughts and insight. Cheers, Brew

Hi Brew,

I have friends (engineers) that do the graph method. I typically do a spread sheet. But regardless, we all look at power to the wheels off each corner at specific rpms. We want to insure we have the best combination for a particular course.

The example I gave did not require more shifts. But if we find a combo that requires more shifts, we weigh that hard against a combo that would have less shifts. Today, teams have simulators to compare the two combos. Back in the day, we had to look at the numbers and make the best decision we could from experience of that track, how that driver races & the power curve of the potential engines.

Take care !

:cheers:

[M.Rad.]

Hi everyone, thanks for the great replies. Ron, I'm still trying to properly understand your graph and explanation since I am far from being an engineer and don't have the extensive practical experience as you do.

So, from what I can gather:

two identical cars, with engines that have the identical power BANDs (except that one is say 100hp/tq less throughout), and the lower power car having say 4.10:1 gears while the higher power car has say 3.00:1.

In a side by side run would result in faster acceleration for the lower powered car until it reached redline in top gear, minus any time used for shifting? Is that close to correct?

Thanks again to everyone for their input on this.

Regards,
Momir.

[Ron Sutton]

Hi everyone, thanks for the great replies. Ron, I'm still trying to properly understand your graph and explanation since I am far from being an engineer and don't have the extensive practical experience as you do.

So, from what I can gather:

two identical cars, with engines that have the identical power BANDs (except that one is say 100hp/tq less throughout), and the lower power car having say 4.10:1 gears while the higher power car has say 3.00:1.

In a side by side run would result in faster acceleration for the lower powered car until it reached redline in top gear, minus any time used for shifting? Is that close to correct?

Thanks again to everyone for their input on this.

Regards,
Momir.

Hi Momir

I can't say that. 100 hp is a lot to overcome. We'd have to run the calcs and see how much gear reduction would be required to achieve that.

Take care

Ron Sutton

[M.Rad.]

Hi Ron, yeah I wasn't trying to pin down a particular power difference vs gearing yet. Just the general concept.

It seems that if I get enough gearing to overcome 100hp/tq deficit, I would have a car that would be faster to it's top speed-which may only be 75mph in top gear! Har, har, har!

[hotrodalex]

It's all a tradeoff. Are you looking to make a street car faster?

I have 3.36s in my daily driver and decided that I would rather increase HP and keep the highway cruising rpm. If I just drove to car shows and the track, I'd get a higher rear end gear.

[M.Rad.]

Hi hotrodalex! What I am trying to accomplish is to make mt 70 Torino (~4200lbs with a 351Cleveland 4V projected HP ~380/375 and 3.70 detroit locker)feel like my 06 Magnum SRT8 (~4300lbs with a 370ci modern Hemi ~425/425 and a 3.06 diff.

I'm assigning this power /tq number to the 351C in order to preserve acceptable street manners. So my deficit would likely be closer to 45-50hp and similar tq.

That's the dream anyway...


Regards to all,
Momir