jimlab

Me again, resident LT1 project car boy.

I thought I had the gearing question solved. Jordon recommended I keep the 4.10s in my car (assume a 2.97:1 first gear T56) to help with low rpm cam surge by getting the engine into higher rpms quicker at low vehicle speeds. Good plan, but...

After some long hard thought, I've talked to Mark a little and we'll probably have a new cam ground and dial my engine back a bit to make it even more streetable. The power numbers were excellent, but power is useless if you can't put it to the ground. It's also useless if you hate to drive what you've built. We made more than enough power to "sacrifice" some of it, and still end up with too much, when it comes right down to it.

So assuming a more mild solid roller cam, and maybe dropping back to 600-620 horsepower, I think that the 4.10s are probably more of a liability than a benefit. The big horsepower Corvettes with have traction problems even with 3.42s. I know that many F-body owners run 3.73s, but F-body owners also tend to be drag racers in many cases. In building a true street car, I don't think I need the gearing that I would if I intended to run slicks at the drag strip. I need something that will help me get power to the ground with street tires, or at the most, drag radials. I'm not worried about being as close as possible to peak power as I cross the quarter mile, I'm worried about blazing my tires if I even look at the gas pedal too hard.

I've pondered Car Test 2000 numbers for a few months now, and have come to the conclusion that Car Test is useless. Not only have I found many glaring inconsistencies (calculated top speeds higher than the possible gear-limited top speed, which indicates no factoring of wind resistance, not to mention other problems) but the numbers reported for even widely differing combinations (2.90s vs. 4.10s, for example) aren't different enough to believe that the results are being calculated correctly. I know for a fact that it's not calculating tire spin properly, generating tire spin only on launch and at the shift, but holding well in excess of 1.0 g of acceleration, even on street tires. No way.

I know from real world results (videos that I've seen of big cube LT1 Camaros, Z07 and built Z06 Corvettes, etc.) that the Corvette, at ~300+ lbs. heavier, has no trouble at all accelerating quickly both from a dead stop and at highway speeds with a 3.42 differential. Add a lot of horsepower, and they have severe traction problems in 1st and 2nd even with a 3.42 diff and P335s in back. Same goes for Camaros, I'm sure... put in a built 396 LT1, for example, and the car wants to blaze its tires all the time in lower gears, especially for those with 3.73s (or higher).

All I'm asking for is confirmation that going in the opposite direction (3.42, 3.23, and maybe even a little lower) is the right way to go for a street car that may see very limited drag strip use, but certainly on nothing more serious than drag radials.

And finally, the Saleen S7 seems to be confirmation of my theory concerning gearing. 550 horsepower @ 6,400 rpm, 520 lb-ft. of torque at 4,000 rpm, and a 7,000 rpm redline from 7.0 liters, and they gave the car a 3.22 differential with a 2.46:1 first gear. If I have a 2.97:1 first gear and more power, in a car that weighs almost the same, wouldn't it stand to reason that I'd pull even harder (or spin more) using a 3.23 differential also? Obviously the 3.23 diff isn't holding the S7 back any in the acceleration department.

I've got a friend assuring me that my car won't pull as hard on the high end if I lose the 4.10s... maybe he's right, but there's not much on the road that will pull as hard as my car would with 3.23s or 3.42s either, and having a car that doesn't hook until 60+ mph and having to pull high speeds to really use your power isn't where I want to be.

Any help would be greatly appreciated, as always.

atljar

Im in the same boat as you are now in gear selection, yet my car is not making anywhere near mega HP or TQ. IM in between 410s and 373s.

I dont think it is absoultly wrong to step down in gearing for high horsepower cars. Most of the M6 guys running blowers and nitrous are using the 373 gear instead of the 410 in mostly drag cars.

The problem associated with lower gearing in drag cars is the launch. You need enough power/clutch/gear to be able to launch the car without burning your clutch, and without bogging.

The lower (numerically) gear you select, the more wear and tear acts upon the clutch which is something to consider also in high HP aplications. A 500rwhp car and a 3.08 gear would = a very shortened clutch life.

If I were you, for a street only car, as mentioned, i stick with the 342 ratio. If you wouldnt mind running DRs on the street, i would go with the 373. Anything lower than the 342 IMO would be useless (top speed would be 300mph? if it wasnt for air), which is so under geared.

Hope this helps a little
jared

jimlab

Can you elaborate on where you heard/learned this? The Viper GTS-R has a 3.07 differential, and many other high powered sports cars (which I'll list in a moment) also have remarkably "tall" differential gearing. I've never heard that gearing had an impact on clutch longevity... only driver abuse and/or exceeding the power handling ability of the unit.

Also consider that I'm planning to use the 2.97:1 gear set T56 as opposed to the 2.66:1 gear set in the '94-up F-body cars. With a 3.23 differential, the final drive is actually a little higher than 3.42s in a '94-up F-body.

1st gear - 2.97 x 3.23 = 9.59:1
2nd gear - 2.07 x 3.23 = 6.69:1
3rd gear - 1.43 x 3.23 = 4.62:1
4th gear - 1.00 x 3.23 = 3.23:1
5th gear - 0.80 x 3.23 = 2.58:1
6th gear - 0.62 x 3.23 = 2.00:1

1st gear - 2.66 x 3.42 = 9.10:1
2nd gear - 1.78 x 3.42 = 6.09:1
3rd gear - 1.30 x 3.42 = 4.45:1
4th gear - 1.00 x 3.42 = 3.42:1
5th gear - 0.74 x 3.42 = 2.53:1
6th gear - 0.50 x 3.42 = 1.71:1

Here are a few other cars with incredibly tall differential gearing...

Viper GTS-R
3.07:1 differential
2.66:1 first gear
8.16:1 final drive

Ferrari F40
2.90:1 differential
2.77:1 first gear
8.03:1 final drive

Saleen S7
3.22:1 differential
2.46:1 first gear
7.92:1 final drive

McLaren F1
2.37:1 differential (?!?)
3.23:1 first gear
7.66:1 final drive

Lamborghini Murcielago
2.53:1 differential
2.94:1 first gear
7.44:1 final drive (and that's a 3,600 lb. car!!!)

All are high powered sports cars, some more so than others. All have plenty of torque. Most are fairly lightweight, with the McLaren and S7 coming in at right around where I'll be (~2,700-2,750 lbs.) and the F40 at about 150 lbs. heavier. The Viper weighs in the 3,300 lb. range, and the Murcielago is 3,600, as mentioned above. Yet all the manufacturers of these cars (built for street and open road course use) geared them this way, so they must have had some reason, and the pattern is too obvious to ignore. By comparison, even with the 2.66:1 first gear T56, with the 4.10 differential, my final drive is at 10.91:1 in first gear. No wonder the guys with 3.73s and 4.10s can't get any traction on the street.

Does anyone know how car manufacturers select their gear ratios? My Z06 has the same 2.97:1 first gear and 3.42s, and weighs 3,117 lbs. Someone at GM must have felt that the 2.97 gear set gave an advantage over the standard 2.66:1 gear set with the 3.42s, and they're right. To a point... when you slap that MTI Z07 422 in the car, suddenly it's got a little too much gear in 1st and 2nd. At that point, if GM were issuing the car with, say, 580-600 horsepower, I wonder what they would have selected for gearing for street use.

Anyway, I need to make a decision, since if I'm going to have a custom ring and pinion cut, I'll need some lead time so it doesn't add further delays to finishing my car. I've decided to stick with the stock housing, so my only option lower than a 3.90 is to have the gears made to order, unfortunately. Still, that's far better than ending up with a car that's completely useless below 60-70+ mph because it has way too much gear for the street.

Also, I've only found one 18" drag radial, and it's a P305/45-18, which is a big tire. ~28.8" tall compared to the stock P225/50-16 (~24.86" tall) or the fairly common aftermarket options of P275/40-17 and P285/35-18 (~25.66" and ~25.85"). I doubt I can fit the P305 under the back anyway, so I think drag radials are out unless I want to stick with 17" rims. I was planning on moving up to 18s.

Thanks!

OldSStroker

OEM gear ratios are selected for all around performance and driveability. As you pointed out, it's difficult to drive a car that has gobs of torque AND lots of gear and can fry the tires through 1st and maybe 2nd gear. It takes a very skillful driver to control that. Even Winston Cup guys have that trouble on the shorter tracks, and most of them have very educated right feet! That may be why there have been so many rumors of traction control in WC.

ZO6 is a good example. The steeper 1st on the Z06 went along with the larger, sticker rear tire to better launch the car. I also think they were looking to get sub 4 second 0-60 times on absolutely stock Z06. If it went to 500+ lb-ft, bigger tires still would be needed, and ASR would probably be the only way to get better launches, especially for magazine test guys, who almost always turn it off. It's a balanced system. Throw gobs more torque at it and the scale tips.

When you jack up the low-mid range torque significantly, you need to increase the traction to go along with it or the car not only is more difficult and less satisfying to drive, but it can be slower if you are always going up in smoke.

Big cams often put a big hole in the low-mid range torque curve which makes the car even more difficult to launch. If the engine then "comes up on the cam" with a huge torque spike at say 4000-5000, poof goes the traction again. Smoke isn't fast.

IMO, if you want a quick street car with the kind of torque/weight ratio you have, you need an engine that has a fairly flat torque curve, and pulls from the low 2000's at part to full throttle. It's hard to do that and make 500+ hp (NA), but many of the cars you mentioned do just that. 250 degree (@.050) cams in a Chevy won't make it happen. I don't know about your engine, but I've seen a 6 L SBC pull over 520 lb-ft and over 550 hp on the dyno with cam durations below 230 degrees on 92 octane. It was a very aggressive roller with lots of lift and had great heads. It also idled below 900 with no lope. 85% of max torque was available from 3000 to power peak at 6300.

With that kind of a torque curve, and the car weight and tires you have, 3.23-3.42's should be easier to drive, be very fast, and not buzz the engine during cruise. You'll still have trouble getting it all down w/o slicks, but you should be able to control it with your right foot and not your left (slipping clutch).

The key, IMO, is to build a driveable engine to fit the chassis, not build a killer motor (from 5000 to 7000+) with a huge hole in the low end torque curve, and try to get it to launch on street tires or DR's with less than 50% of the weight on the 2 driving wheels.

IMO, with this killer type of engine, and low gears (3.07's etc.) you have to slip the clutch during launch to keep the engine above 2500-3000 because it dies or even stalls under full throttle in those ranges. That's not much fun in my book, and it's SLOW!

I think you are on the right track with a milder cam, and mid 3's gearing.

Sorry to drone on so long. My $.02.

rskrause

To compose an intelligent response, we need to know how your motor is set up. IOW, what rev range do you plan to use, and what is your redline? I use 3.42's because I have a low rev setup (6,000rpm redline, peak hp at 5,500). Works great at the track, where I just touch the rev limiter through the traps, and on the street where I can cruise at 75mph at less than 2,000rpm. That's the beauty of the M6! I am contemplating a TH400, but really think the M6 is the better choice beacuse of the wide gear range except for extreme duty track use.

Rich Krause

graham

Interesting gear ratio combo. Looks like something more useful in a street motor with a wide torque band than a 600-620hp street-race motor. Thats alot of power to try to drive it around too much. The only reason to get a higher gear, to me, would be because of the trans ratio. I would go 3.73:1. Yeah 3.42's are gonna give a little more 'running room' in 1st but then again, wouldnt 3.73's make the car hit the tires quicker?

atljar

Maybe i shouldnt have used the phrase "very shortened clutch life", but with a numerically lower gear more time will be spent slipping the clutch getting the car started from stops, which equals more wear and tear on the clutch. Seems to me as that would shorten the lifespan of the clutch?

Correct me if anyone knows other wise, but i always thought a lower numerical gear is easier on the rear, harder on the clutch. A higher numerical gear is easier on the clutch, harder on the rear.

jimlab

My redline is an arbitrary 7,500 rpm. In its current iteration, the engine is still making nearly 640 horsepower at 7,400 rpm, but it is starting to fall off, and peak power is around 6,900-7,000 rpm. If we re-cam the engine for better drivability, I am sure that my peak will fall a bit, and that I'll be shifting a little sooner, but still over 7,000 rpm.

Horsepower looks like this... (my engine dyno numbers compared to converted RWHP figures for a stock '93-'95 RX-7 TT and a fully modified "360 RWHP" RX-7 with stock twins)
http://home.earthlink.net/~jimlab/pi...Horsepower.jpg

And torque looks like this...
http://home.earthlink.net/~jimlab/pics/rx7/Torque.jpg

396 LT1, 215cc raised runner heads, 2.10/1.65 valves, Hogan intake, 1,300 cfm monoblade, Crower ultralight crankshaft, Crower maxi-light rods, custom JE pistons @ ~12.5:1 static compression, Cam Motion solid roller in the 26x/26x range with over 0.650" lift and a 112 lsa, I believe.

The car will weigh in the 2,700-2,750 lb. range and was 50/50 balanced and should be pretty close even with the V8. The widest tire I can probably put under the back would be a P285 or P295.

jimlab

Quick question... is there any correlation between the number of teeth on the ring and/or pinion gear and strength? I'm assuming that the lower the number of teeth, the better off you are?

The reason I ask is because I can achieve a 3.23:1 ratio with a 42-tooth ring gear and a 13-tooth pinion gear (3.231:1), but I could make a nearly identical 3.27:1 ratio with a ring gear with only 36 teeth, and a pinion gear with only 11 (3.273:1), which I assume would be quite a bit stronger. 6 less teeth on the ring gear and 2 less on the pinion have to count for something, right?

My stock gears are a 41-tooth ring gear and a 10-tooth pinion gear.

OldSStroker

While it's true the teeth are larger on the 36:11. you are putting the same amount of torque through fewer teeth, so each one has to take more of the load. The finer pitch 42:13 with more teeth in contact spreads the load out more and might be stronger. The quality of the steel, machining and heat treatment are probably as important however.

With your tire limitations, I doubt rear end strength will be a problem, unless it's the original RX7.

OldSStroker

You were able to pull 400 lb-ft @2000 rpm at full load on an engine dyno with the 26x/26x (@.050) cam? That's a torque curve to die for! 400 lb ft @ 2000 and still at 8000! That's one dyno run I'd like to observe.

With that kind of torque curve, why are you worried about gearing and driveability? Just let the clutch out about 2 grand and control the wheelspin with your right foot.

I'm also very impressed that 215 cc heads will pull that kind of hp at 8000 on a 396. What kind of flow numbers?

rskrause

Jim: I remember you posting on a related matter a while back. There was a lot of discussion then re: gearing for traction. This doesn't make much sense to me in your case. I assume that this isn't going to be a drag car, right? If you aren't going to have a DR suspension and a huge set of slicks on it, there is no way you will be able to launch it hard no matter what your rear gear is. You have have to use the traction control device known as your right foot! Most likely you won't be able to use full throttle on a 295 street tire in first or second gears, and maybe third as well no matter what gear you select!

When you come right down to it, you don't need a T56 six speed at all with your TQ curve and a curb weight of less than 3,000lbs. If you go with 3.2x:1 like you are talking about, sixth gear will be useful from maybe 120mph and up! IOW, the spread of ratios is more than you need. I would go for a gear that allows me to use 1-4 as normal driving gears, 5 as an OD, and never figure on using 6th. This would be maybe a 3.73, I'd have to do a little calculating to be sure. Alternatively, you could gear it to use 2-6 and never use 1st except in a parking lot or traffic jam. That would entail a 4.11 or more. The advantage here is that you would never use 1st gear under high throttle, which might protect the transmission. Of course, I think you will have no problem with transmission or rear end longevity no matter what - you will just spin the rear wheels long before anything breaks.

Rich Krause

jimlab

Good point, and thanks for bringing that up. I had forgotten that more splines are usually better on the input shaft and output shaft of a transmission (as well as increased diameter) to spread the load further, so the same must also apply to ring and pinion gear teeth. Thanks!

jimlab

I expect that a 2,000 rpm launch would result in horrendous tire spin, regardless of what gearing I have, as Rich pointed out. I'm not looking to launch all that hard, but I am looking for some sort of advantage in being able to use "more" of my throttle pedal without fear of breaking the rear tires loose.

I'm already horribly well aware that even with a 3.42:1 differential, that with the 2.97:1 T56 or even the 2.66:1 T56 that I'll have a hell of a time in 1st and 2nd if I get happy with my right foot. I know that the MTI Z07 converted Corvettes have incredible traction problems in 1st and 2nd, even with P335s in the back, if you're not careful with the throttle. I expect to have to "tip in" the throttle, but it'll be a lot easier, I suspect, with a 3.xx gear ratio as opposed to some drag racing 4.10s in the back.

The only reason the car had 4.10s in the first place is because the rotary (obviously) makes almost no low end power.

The current plan is to step the cam down some and try to make the engine a little more docile. In Mark's words "it's streetable, but it's definitely a popcorn popper".

~34x cfm on the intake @ 0.700", 2.10" valve.

The best pull on the base maps had a big dip in the curve through peak power, but made a very promising 640.8 @ 6,400 before the dip, and 636.7 @ 7,400 after the dip. We're still waiting on the modified front cover (cam sensor) for the eDist, so I don't even know what kind of numbers it would have made tuned if I left the cam as-is, but they saw 647 on one pull, on the base maps, and peak power should have been in the 6,900-7,000 rpm range. It's down to ~622 at 7,600 and falling off, so I was planning on using 7,500 as the "redline".

jimlab

Yep. But it would be a lot worse with 4.10s, probably.

I've sort of figured that, although it galls me to haul around an "extra" gear I might rarely use.

It's been suggested by a couple people to keep the 4.10s and start in 2nd gear... but that has to be hard on the transmission and clutch. The alternative is to start in 1st and *really* short shift 1st gear and travel around a gear higher than I normally would. I'd rather have a more "normal" arrangement, if possible.

I was leaning towards the G92 T56 (2.97:1) because they're less expensive (brand new) than the '94-up T56 (2.66:1). Mark said the 400 lb-ft. rating wasn't anything to be concerned about. The 5th and 6th gear ratios are better, although 1st through 4th would be equivalent to my Z06 (3.42 diff) and I know for a fact that it's already hard to launch "aggressively" without spinning, even without the low end power of a stroker engine.

An ideal gear set would be 1st through 4th from a '94-up T56 with 5th and 6th from the G92, (0.80, 0.62) I think. The 0.62:1 6th gear puts me at ~1,930 rpm @ 70 mph, which is just about right.