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I have a kit car with a modified 302 in it. It's a pretty light car and will do 0-60 in about 6.5 seconds. It has a 4 speed manual transmission and a 3.00 rear in it. Is there a rule of thumb or simple calculation that will approximate 0-60 times if the only thing that changes is the rear ratio?
I'm wanting to go to 3.50, 3.70, 3.90, 4.10? in the rear but would like to get a general idea of acceleration improvement with the different rear ratios (without changing anything else). But I also want it to stay fairly enjoyable to drive, ie., don't want to have to stop at every gas station to fill up.
Thanks!
it would be tough to make such an estimate... since it depends on so many things...but a gear change will help 0-60
but, gearing, quality of driver, weight, how high you stall/rev your motor before hand, any incline you might see, quality of road, tire size/stickiness, suspension configuration, engine's power band on both the hp and torque sides of things, and probably dozens of other factors.
Lots of variables. What’s your plans with the kit car? Street cruising, drag racing, time attack etc? 4.10 gears in my opinion would not be ideal for cruising especially paired to a 4speed.
Thanks torq'ta! I understand all the variables, but it seems to me there should be a rule of thumb if EVERYTHING is kept the same - only change being the diff gear.
In other words, if I change nothing but the diff gear then go to the same road, same conditions, same driver, same shift points, etc, etc. there should be a way to approximate improved acceleration. I bet the drag guys would know.
Drag guys worry about, 0-60 foot times, not red light to red light speed.
They say, 0.1-0.3 sec, for each gear change, and they say it takes, 0.2-0.3 sec, to shift, so lets just say 1 shift for 0-60, 3.55 = 6sec, 3.73 = 5.8sec, 3.90 = 5.6sec and 4.10 = 5.5sec, but you could have a extra shift, with 3.90/4.10, and now your back to 3.73 range, still just a S.W.A.G.
those are based on 1 to 1 final drive, do you know what your trans 4 gears are, I would assume 4th gear is 1.00, if you decided on 3.90, 4.10, and didnt like the cruise speed, you could go up in tire size like the charts show, to help bring down rpm's, a fox body Mustang tire is 26" tall at 65 mph, 3.07 gears=2500rpm, and 3.55 gears=3000rpm, 4.10 gears=3500rpm, so now depending on mufflers, and where they exit, drone inside vehicle could be unruly with 4.10gears, kinda why everyone goes to 5 speed trans, and 5th gear is , 0.63-0.67 final drive and brings rpm's way down to livable sound.
Hey torq'ta,
This is really helpful. Thanks. I think I can use that chart and pro-rate to my 6.5 secs at 0-60 and (realizing it is a S.W.A.G.) maybe make a more informed purchase.
4th gear is 1.00; I measured my tire sitting in the garage and got 25" dia. Already thinking along the lines of a different tranny with a lower final gear.
Thanks baddad. I'm leaning that way or maybe a 3.7 with an OD 4th gear or maybe a 5 spd.
I'm running 3.50's in my 77 Comet with 26.5 tires (295/50-15) and it's turning 2600 rpms at 60 mph with a Toploader 4 speed. It's happy running down the freeway at 75 at around 3 grand on the tach. This is with it's 400+HP 331. It's got Canfield heads, Ford Racing Z303 hyd roller (.585 lift & 228*@.050 duration with 1.7 rockers) Intake is a repop 3x2 setup with 3 Holley 250 cfm carbs. Cruising at these speeds all it takes is a blip of the throttle to zip up to 100 mph when the secondaries open up.This motor got 16 mpg in my 89 Ranger (supercab) with 3.73 rear and 28" tires on the highway. The Ranger weighed 3500 lbs and had the aerodynamics of a brick. I'm guessing you have a Cobra kit car ? Your final drive ratio needs to also take into account where your engine's powerband is. You don't want to be cruising at 1500 rpms on the freeway when the sweet spot in the rpms is 1000-2000 rpms higher. Fuel mileage will suffer and it's not good to have the engine lugging at cruise. Not good on the mains and rods either when this is the case
Thanks baddad,
You're close. I've got a Classic Roadsters Sebring (Austin Healey look alike). Engine is a 302 bored 30 over with 351M (truck) heads. I was trying for Cleveland 351s and got snookered by the junkyard guys. Anyway, it might be fun if I ever try to go in the nitrous direction. Don't remember cam numbers but yours look vaguely familiar. I know when I was researching that I decided on a cam that would have a bit of a rough idle that I tamed back down with rapid bleed down lifters. Has Zero-gap rings and a single Carter 625 4bbl. Wish I knew where the power range was - never had it dynoed. It's pretty torquey. Doing 0-60 times I just start in 2nd gear and go to 60mph - ~6.5 secs. With the original Mustang2 tranny with a 1.00 4th gear, I used to be about 2,000 rpm at 50 mph.
Thanks baddad,
You're close. I've got a Classic Roadsters Sebring (Austin Healey look alike). Engine is a 302 bored 30 over with 351M (truck) heads. I was trying for Cleveland 351s and got snookered by the junkyard guys. Anyway, it might be fun if I ever try to go in the nitrous direction. Don't remember cam numbers but yours look vaguely familiar. I know when I was researching that I decided on a cam that would have a bit of a rough idle that I tamed back down with rapid bleed down lifters. Has Zero-gap rings and a single Carter 625 4bbl. Wish I knew where the power range was - never had it dynoed. It's pretty torquey. Doing 0-60 times I just start in 2nd gear and go to 60mph - ~6.5 secs. With the original Mustang2 tranny with a 1.00 4th gear, I used to be about 2,000 rpm at 50 mph.
Mustang II 4 speed ? I'd look into replacing that ASAP before doing anything else to it. It's a grenade waiting to go off. Anyway, really no difference in those heads vs Clevelands, they had Thermactor passages where the Clevelands didn't. What pistons did you use ? And intake ? Thought about doing a bogus Boss 302 lots of times just never went through with it.
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