Better understanding gear ratios relation to horsepower

[Voltwings]

Ok, so this being my first RWD performance car im still a little new to understanding gear ratios effect on performance. I understand the numbers and the relationship, a 4.10 will allow the rpms to climb faster than a 3.31 ... but i want to basically talk through what im thinking and have someone clarify for me.

Take a honda for example, small motor with not much torque and a high rpm band, it uses shorter gears to get up in that high rpm horsepower sooner ( similar to wanting a 3.73 on this car). Now take an ls3 corvette for example which comes with a 2.56 gear stock. My thinking here is that the corvette has a very broad, flat torque curve, so theres no sense in ripping through it as fast as you can, you want to ride that horsepower out (make sense?)

Basically the meat of my question is, is there an "optimal" gear ratio for a given horsepower level? i understand that may be a bit vague, and there are a ton of variables, but obviously you dont want a 4.10 on a 800 whp turbo car you know, theres got to be a breaking point somewhere.

I suppose it more depends on your powerband than your overall horsepower however (corvette vs civic again), but if anyone would like to add some insight or clarity to my question i would greatly appreciate it. Thank you.

 

[vortecd]

There is much more then just HP. RPM, torque and weight can have factors on it also

 

[Shiken_Feddas]

There are far too many variables to just say at this hp level, you need to start using this gear. Using your own example, yes an 800+ hp car can thrive on a 4.10 gear, If its setup for 1/8 with the right trans. There are optimal gear ratios for N/A, turbo, and N2O cars. The type and gearing of your trans has a lot to do with it as well, the mt-82 vs. a t-56 for example.

But a simple answer is that most people who dd their car use a 3.55 or 3.73.

 

[JohnRichard]

To simplify... If the engine can carry it. as close to 1 to 1 transmission gear ratio, set your final gear ratio to match desired speed at HP peak.
Yes you can over do it... No the HP of a stock GT will not carry the car to 200 mph.

 

[Voltwings]

ok, i see where everyone is coming from so let me try to refine my question a bit. I understand there are still a lot of variables, but lets take the corvette example i made earlier.

My initial thinking was you would want the longest gear your horsepower could support, that way you could just ride it out, and since shifting takes time.

I think part of my confusion is i am used to working on and tuning stock turbo 4 cylinders, where the hp kind of flat lines. a solid 330 whp from 4800 to 6200 rpms, i wouldnt see the sense in a steep gear just quickly ripping through that. However, seeing as how the coyote makes more power as rpms rise, i suppose a steeper gear to get into the horsepower faster makes sense... Why do corvettes use such long gears then? seriously, the ls3 vette traps like 115 stock with a 2.56, surely theres got to be some justification to what im thinking? I realize the ls3 is a complately different motor as well, and one with a LOT more torque, which may be part of the reason for the longer gear...

 

[13DIB5.0]

Subbed for this.

 

[CharlieR]

You seem kinda stuck on 1 car(only the auto has a 2.56 gear, standard trans w/ 2.66 or 2.97 1st gears get at least a 3.42 rear ratio). You have to take torque, weight and transmission gear ratios into account also. The 6A vette weighs 3100#'s has a 4.02 1st gear and has the torque of a 6.2 V8. 6a mustangs have 4.17 1st gear, 3.15 rear, weigh in the 36-3800 range with the torque of a 5.0 V8. MT-82's have a 3.66 1st gear and 3.31(3.55&3.73 optional) rears V-6 cars have a 4.23 1st in the standard trans. You need all the variables to determine the "right" gear for a car, a 600 hp n/a standard car most likely will like a different gear than a 600 hp turbo auto car.

 

[Black Fire]

Basically the meat of my question is, is there an "optimal" gear ratio for a given horsepower level?

The answer is...NO, It depends on the car and what you are doing with it. (Drag, Autocross, Road Race, High speed runs etc...) Auto manufacturers make compromises when building and designing cars and final drive is one of those.
The aftermarket allows people to tailor the car to their driving style. But consider everything (Horsepower/gearing/suspension/etc...) as small parts of a total package designed to meet a certain goal.
Decide what the goal is and choose accordingly. But keep in mind you may have to make compromises of your own in the efforts to maintain safety and streetability

 

[nonliberal]

When comparing gear ratios from different cars you have to take into consideration the transmission gears as well. Final drive gears are only half the story. The Vette likely uses the 2.56 final for high speed cruising such as on autobahn, but makes up for the first few gears with lower tranny gears.

 

[Voltwings]

Ok, well i wanted this to be a "help me understand" thread, not a "help me pick" thread, but i suppose i kind of need to say what my goals are to really understand which gear to run.

After doing turbo builds in the past, i really want an NA car, just simple horsepower. The plan has 2-3 stages i plan on doing it in:
1. full bolt ons with CJ intake manifold - goal of 450 whp (on the dyno i always use)

2. Cams + cylinder heads for 500 or so (again, on the same dyno)

3. maybe a small shot of nitrous but it will stay under 600 whp.

That being said, it would seem like i would want a 3.73 gear to get me up into the meat of the rpms and horsepower. However, thats a lot of horsepower to hit so soon, so maybe sticking with the 3.31 or a 3.55 would make it a little easier to put the power down... i suppose there really are too many variables, but gear i feel is one of the more important ones. I also have little intention of taking it to the drag strip, I do race my friends occasionally so it will be more set up for roll racing ( we have back roads and don't weave in and out of people, keep your "take it to the track" comments to yourself.)

Im addressing all suspension components i can before i even add power to the car, already have upper and lower control arms on order, the car will receive an adjustable panhard bar and will be lowered, and will be off these god awful 235 pirelli's.

 

[JohnRichard]

all your plans 1 2 and 3 changes the torque peak location... I would suggest finishing the engine then move towards the transmission and gearing you need for maximum acceleration over time.

Build your transmission ratio to land after upshift slightly before torque peak.
Set your final gear ratio to match desired speed at HP peak at point of destination.

You have to be real here, do not set this goal as what you truly desire... yes I would like to go 300 mph but the car just wont do it but 120 at the end of 1320 in a NA Mustang GT sounds just as good to me...
28'' tall tire forth gear manual 1.32:1 HP peak 6800 RPMs 3.55 gears
28'' tall tire forth gear manual 1.14:1 HP peak 6800 RPMs 4.10 gears
The main reason forth in the manual, the long throw to fifth
The main reason in an automatic being in fourth, fifths gear ratio is .87, now your rear gearing is working as a torque deduction.

 

[Voltwings]

all your plans 1 2 and 3 changes the torque peak location... I would suggest finishing the engine then move towards the transmission and gearing you need for maximum acceleration over time.

Build your transmission ratio to land after upshift slightly before torque peak.
Set your final gear ratio to match desired speed at HP peak at point of destination.

You have to be real here, do not set this goal as what you truly desire... yes I would like to go 300 mph but the car just wont do it but 120 at the end of 1320 in a NA Mustang GT sounds just as good to me...
28'' tall tire forth gear manual 1.32:1 HP peak 6800 RPMs 3.55 gears
28'' tall tire forth gear manual 1.14:1 HP peak 6800 RPMs 4.10 gears
The main reason forth in the manual, the long throw to fifth
The main reason in an automatic being in fourth, fifths gear ratio is .87, now your rear gearing is working as a torque deduction.

The tires will be 275/40/19 on a 19 x 9.5 rim, which comes out to a 27.7" tire ( im assuming you rounded?), and i plan to shift at or below 7500 rpms for the health of the transmission ( i know its going to be hard to reach my hp goals that way, but it is what it is) so lets assume ill be working with the stock mt-82 gearing. That being said, i believe by your information above, that would make a 3.55 "optimal" for my setup?

 

[kcsvt94l]

This big thing your missing is "optimal" is a relative term in this situation. Optimal for what ? 1/4 mile ? economy ? DD? Autocross? roll racing to 140+ ? I can tell you if you plan to stay N/A and want something middle of the road 3.55's are your best bet sorta ok at everything but not the BEST at anything. If you're wanting to maximize your 1/4 mile times. 3.90's are best for the MT-82. If you want to roll race & or Autocross 3.31's are fantastic.(Also, probaly going to net you the best MPG on a highway trip.) Once people start doing blower, turbo, Nitrous they generally go down to a 3.31 because of the 800WHP+ 4.10 analogy you referred to.

I can tell you that the CJ setup w/o the cams/head work. On a 3.73/3.90 gear is going to be good for low 11's high 10's with suspension mods & minimal weight reduction on a 28" slick with skinnies up front. If you throw cams at the car it's going to be knocking on mid 10's again providing you have the supporting mods.

If you're completely dead set on getting the best possible 1/4 mile time 3.90's other wise I wouldn't hassle with it as the you're kinda splitting hairs to go from a 3.31 to a 3.55 or a 3.55 to a 3.73. I personally love my 3.31's though I realize it's not the best gear for the 1/4. It's awesome to roll race with because I can hold them so long.

So what's Optimal for you may not be optimal for someone else.

Hope that makes sense.

 

[Voltwings]

the goal is backroads cruiser / the occasional roll race, so I suppose for the money put in itd be best to just keep my 3.31's. I just have a very busy mind and love to understand how things work, but was having a hard time wording the question I was trying to ask on google. I see why now.