I read a tech article in a World's products catalog years back, detailing how properly selected rear gear ratio (along with spark plug heat range/ign. timing and carb. selection/air fuel ratio) will load an engine and increase cyl. pressure/heat(energy) to increase(or at least achieve) performance. It also explained how too much gear would cause not enough load, and thus no heat in the cyl. but just RPM's. Anyone care to discuss this concept in greater detail??? I have not yet come to grasp this idea enough to put it into good use, and need to do so as I ponder the correct rear gear to run with my mild combo. I want enough gear to light the tires on command, but don't want to wing the motor past it's usable torque peak in first gear. Moving the car foward as much as possible before the 1-2 shift occurs, efficiently using the torque/combustion chamber heat of the engine combo under the proper load. Does this make any sense??? Or have I finally dropped below the limit of dead brain cells one can still think with??
For 1320 drag racing, gear to turn usually 10% past the hp peak through the lights. Short shifting into 2nd usually helps ET. There was a recent thread on Speedtalk.com about this. For the street use, consider rpm at freeway speeds.
Yes, I know about the RPM @ 60mph-ish method, but using an overdrive auto. trans(4L60 with around 2,200-2,500 stall & lockup conv.)....this is not so critical. In this case...I am more concerned with the most efficient use of first gear to get the car moving quickly from a dead stop. Matching the axle ratio to the first gear, and the motors torque curve.
Good example. pontiacs a a bit more long legged than a smallblock chevy. a pontiac will run at it's optimum with a higher rearend gear than a smallblock will. same car tires tranny everything.. a 3:73 in a pontiac is like a 4:11 in a chevy. has alot to do with rotating mass and stroke length= piston speed counts too. efficiencywise... gas burns at a fairly constant rate. There's a certain point at which the piston speed is going to overrun the combustion. lower piston speed allows for more time for the combustion to do work. less time then trequires more combustion to do the same work. So yes piston loading is a factor but there is a point of diminishing returns. the best you can do is to optimize the physics of your engine and try not to hurt it by missmatching parts. Like ultra low gears mated with stock heavy rotating masses that cant catch up to the gears
Let me rephrase the question. What rear gear do you guys think I should run in my 55 210 post car? I am going to be using a 9.0:1-9.5:1 iron head 350 Chev. with the old 327/350 hyd., a performer dual plane w/ 800 Q-jet, 14" K&N, recurved HEI, 25% underdrive crank pulley, 1 5/8" equal lengths into 2 1/4" duals w/ turbo mufflers, a 4L60 overdrive w/ 2,200-2,500 stall lockup conv., and 235/70-15 rubber in back. Not gonna be a race car, but don't want a slug either.
There's no way of knowing what rpm the converter will actually stall at until the engine power is established...true but just kidding. 4l60 gear ratios <table class="wikitable"><tbody><tr><td>1st-3.06</td> <td> 2nd-1.63</td> <td> 3rd-1.00</td> <td> 4th-0.70</td> <td> </td></tr></tbody></table> I'd bet a 3.42 or 3.73 gear would get you around town quite snappy and still get good mileage running the interstate. lets do the math comparing it to a th350 trans <table width="75%" align="center" border="0" cellpadding="0" cellspacing="0"><tbody><tr align="center"><td width="25%" bgcolor="#33cc33">2.52:1</td> <td width="25%" bgcolor="#3399cc">1.52:1</td> <td width="25%" bgcolor="#cc6699">1:1</td></tr></tbody></table>1-(2.52/3.06)= .18 there is an 18% difference between 1st in the 4l60 compared to the th350. the 4l60 with a 3.37 gear will start out like a th350 with a 4.11 gear. .70 is 30% overdrive or 70% of the rearend gear ratio. 4.11* .70= 2.877 In 4th gear a 4l60 will have 30% less engine speed. dropping your 4l60 into a car formerly with a th350 combo with a 4.11 will be like changing the 4.11 to a 2.88. lets pick out the 3.73 gear... mathmatically it's going to launch like 4.40 gears and cruise the pike like 2.61 gears. figguring for converter slippage maybe 4.56 and 2.73 action out of a 3.73 gearset??? Oh yeah, don't forget converter lockup so the top gear cruizing is prolly going to be more like the old 2.42 mileage gears they used prior to the overdrive. Does that help?
That's exactly what I decided on! And just in time to win a nice used late model GM truck 3.42 8.6" ring & pinion on Ebay for $30 last night !! Thanks for all your time & help. Ken
Take a hint from GM production vehicles. In the last few years they have been gearing their cars with a 3.42 rear with an overdrive. This puts the overdrive RPM at around 2000 at 70 MPH (in overdrive). So.........if your cam, carburation (injection), rear tire diameter, even closely mimics their formula you can't go wrong with similar gearing. Frank
How to gear a car for maximum cylinder pressure: 1.000 rear axle. start in 4th gear. Are you beginning to see what I meant by "not relevant"?
If you have a performance map for your engine and the mass and drag characteristics if your car, you could probably run a simulation for best gearing for acceleration. (You could also use the same information to pick the final drive ratio for best fuel economy, but that's not quite as exciting.) We did that in Engine Lab (at U of MN), bt that was quite a while ago, so I'm a bit rusty.
Torchmann and Fab 32 have posted some excellent answers. Backed up by facts, figures and the requisite math. A couple of 'rules' I've heard and read over the years: Gear the car so cruise RPM is equal to the engine's torque peak. Works especially well for pickups and the like. A 12:1 overall gear ratio - diff ratio multiplied by 1st gear ratio - used to be considered optimum for the launch, but it seems that a lower overall ratio is used nowadays and I attribute that to improved tires and better chassis science. IMO, high stall converters ain't so swift for the street. The converter in my 32 roadster is a 2400 RPM rated B&M and it locked up well at highway speeds when I ran a 3.70 diff. Flashed at 2800 RPM and that probably due to the 462" BB Buick engine. 30" tall tires and 2400# 32 roadster fwiw. T-400 trans. All of which means, the trans ran cool running on our desert highways with the usual 75 per speed limit. (Not all highways in Arizona are 75, so pay attention.) 3000 RPM @ 70 MPH. Since I've gone to the 3.00 diff, same tires etc. (2400 RPM @ 70 MPH) I note the trans runs hotter. Not too bad on the long level runs, but climb a local grade that's 6% and 12 miles long will bring the trans to 220-225*. The tach indicates the converter hasn't locked up when on the grade and as you know, slippage = heat. When the 3.70's were in the car, the same grade on a slightly hotter day would have the trans at 210-215* A lot of guys will tell you, "no problem" with trans heat on their combo, but most of them don't run a trans temp gauge so my question there is, "how do you know?" The main reason I went for the 2400 RPM stall converter was when the car was under construction I was aiming for mostly street use and an occasional run at the nostalgia drags. I didn't make it before the rules were improved safety-wise. (I have a 2 1/2" OD .120 wall four point roll bar in the car and a four pointer won't pass tech.) Along with that, I figured the big cam and dual quads would have a high idle and the higher stall would help in keeping the light car with a somewhat high idle from creeping. I don't mind holding the brakes on, but maintaining high pedal pressure with your foot gets to be a drag. As it turned out, the engine idled at 600 rpm and 'creep' doesn't seem to be any worse than similar cars running stock converters. After running the 32 for a while I realized a high stall wasn't really needed and the tires will spin or chirp with even a mild hit on the throttle which brings the fast responding engine's revs up pretty quick. To that end, the project roadster has a 2200 RPM stall B&M converter, the engine will run the bigger cam and dual quads among other things. I expect it to be mild mannered and the torque peak in the right place for the 2200# car. 28" tall tires and a 3.25 locker in this one. As you can see, a lot of choices to be made, but with the advantage's of running an OD trans, other than a too-high stall converter, you can make some moderate errors in gearing choices.
One more effect can be heard when listening to one ton or big trucks with turbo diesels. You won't hear the turbo spool up much when the vehicle is in a lower gear because it doesn't take as much torque to make the vehicle move or keep it moving while using a deeper gear reduction. When you get into the higher gears, more torque to accellerate or to maintain speed, listen to the turbo sing.
Yeah, I figure late model powertrain combos are a good starting place for gearing. That's why I ditched the plans to build a steel 327 small journal cranked '65 283 with 3 deuces, a torque hyd. cam and a late saginaw 6 cyl 3 speed on the column with a 3.31 auto. impala open ham in my 55. It def would of had the period look I love, but the dependability and driveability I have come to demand from my recent builds would be marginal. I think the 3.42/3.06 1st gear multiplication, with the slightly loose convertor, locking up in 4th gear to minimize slippage and trans. heat will be a pretty good street setup with my mild 350. I think it will compensate for the slightly mismatched .050" duration 327 cam specs and relatively low 9.5:1 C.R. of my engine. The resulting lower than usual cyl. pressure may enable me to get agressive with my ign. curve and initial timing and go a little hot on the plugs and lean on the jetting without detonation concerns. I played around with this theory on another 350 years back, with pretty good results, but this motor has TRW forged flat tops and I will get a little more zealous this time.
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