motor

RC45 · 02-19-2008, 11:48 PM

One of these days I will to try my hand at open road racing, standing mile events etc, and am busy working on a new transmission.

Without an unlimited budget, I am going to have to settle for a single set of transmission ratios to attend HPDE's, Standing Miles, Open Road Race Top End runs etc.

Here are some transmission scenaros I have worked up. I am inviting discussion and comment based on heresay, facts, personal experience, slide rule calculations, pure speculation and bar stool bench racing

a) Which setup will be the quickest.
b) Which setup will feel the quickest (but may not be)
c) Which setup appears to be almost ideal?

Peak torque for the current setup is at 5500rpm, rev limiter is set to 7,000 with shifting set for 6,900rpm. The MM6 is the 6 speed out of the C5 Coupe and C6 Z06, the M12 is the setup from the C5 Z06.

Let the games begin.
MM6 / 3.42 Combos

MM6 / 3.15 Combos

M12 / 3.42 Combos

M12 / 3.42 Custom 5th gear Combo

M12 / 3.15 Combos

M12 / 3.73 Combo

[edit] Looks like GM was already thinking along the same lines... this is what the C6 Z51 T56 looks like
Z51 T56 / 3.42 Combo

[edit..edit]Looks like GM really did their homework on the new ZR1 transmisison ratios - on the 20" rear wheel the ZR1 5th runs deep into 200mph territory, but with a 345/30/18 I would still need to shift at close to 200mph... not the ideal situation, but the transmission sure looks well spaced.
ZR1 / 3.42 Combo

gigdy · 02-20-2008, 12:08 AM

Thats a lot of pretty lines. Note following come out of my ass: The last one with the lowest Mph seems to make the most sense. It might seem that you would want to set up the first five to run courses, and 6th to have redline be at as fast as you want/ think you can go (probably not 341mph).

RC45 · 02-20-2008, 12:19 AM

Originally Posted by gigdy

Thats a lot of pretty lines. Note following come out of my ass: The last one with the lowest Mph seems to make the most sense. It might seem that you would want to set up the first five to run courses, and 6th to have redline be at as fast as you want/ think you can go (probably not 341mph).

Interesting take - I was also interested in peoples comments about the ideal rpm drop. I have heard suggestions of shifting at max HP, highest practical RPM, then being geared so that the RPMs drop to just at or above peak torque.

Others I have spoken with suggest its better if the RPMs drop to just below peak torque as thi sgives you more time in the gear.

Thoughts on those ideas??

5vz-fe · 02-20-2008, 05:17 PM

I would choose the earlier case for it gives you ability to shift up a gear at the end of the straight before braking and entering a corner while carrying most speed..

The later case does gives you a little longer time in the powerband, and a chance to hold a gear in a corner. But how many corners have you driven that it will require a shift in the first scenario and holding the same gear in the second? Plus, it's not like you are shifting like a granny.

My crappy logic aside, now the real reason. It just feels good to have the next gear picking up at peak LOL

gigdy · 02-21-2008, 12:28 AM

Originally Posted by RC45

Interesting take - I was also interested in peoples comments about the ideal rpm drop. I have heard suggestions of shifting at max HP, highest practical RPM, then being geared so that the RPMs drop to just at or above peak torque.

Others I have spoken with suggest its better if the RPMs drop to just below peak torque as thi sgives you more time in the gear.

Thoughts on those ideas??

Does it matter what the RPM drop is if you can break loose in every gear? Whats the use of dropping to max torque when you cant put it all down anyway?

gobs3z · 02-21-2008, 01:09 AM

I'm not going to chose which one i think is best but with your motor, the first four gears should be tall where you use the torque to get through them and shift at 6k-6300 and the top two gears should take advantage of the horsepower and be spaced very close together for peak speed. The torque will accelerate you quickly to a very high speed (165+) while the hp will let you get those extra mph where the acceleration won't be so great and hence the close ratios up top. It looked like none of your gearings had that. The Vette tranny's have overdrives that are basically useless (other than great mpg obviously) and waste a good portion of top end acceleration. I'm obviously no expert, but maybe i helped.

RC45 · 02-21-2008, 01:22 AM

Originally Posted by gigdy

Does it matter what the RPM drop is if you can break loose in every gear? Whats the use of dropping to max torque when you cant put it all down anyway?

Funny enough I thought about this, as with street tyres this bastard will spin the tyres at the 4th to 5th shift already hehe

Originally Posted by gobs3z

I'm not going to chose which one i think is best but with your motor, the first four gears should be tall where you use the torque to get through them and shift at 6k-6300 and the top two gears should take advantage of the horsepower and be spaced very close together for peak speed. The torque will accelerate you quickly to a very high speed (165+) while the hp will let you get those extra mph where the acceleration won't be so great and hence the close ratios up top. It looked like none of your gearings had that. The Vette tranny's have overdrives that are basically useless (other than great mpg obviously) and waste a good portion of top end acceleration. I'm obviously no expert, but maybe i helped.

Well, all those scenarios are based on a stock Tremec transmission as a starting point, playing with rear axle ratios or changing 1 or 2 ratios at the most.

A completely custom ratio'ed close ration transmission is the answer, but I am not ready to bury $5000 in a custom transmission yet.

79TA · 02-21-2008, 01:34 AM

I'm not sure what kind of revs you want to have while open road racing, but I've always really enjoyed being able to cruise at 70 mph at about 2000 rpm. That was a big part of how I ended up choosing 3.42 gears for my Trans Am's rearend (not yet installed).

RC45 · 02-21-2008, 01:49 AM

Originally Posted by gobs3z

I'm not going to chose which one i think is best but with your motor, the first four gears should be tall where you use the torque to get through them and shift at 6k-6300 and the top two gears should take advantage of the horsepower and be spaced very close together for peak speed. The torque will accelerate you quickly to a very high speed (165+) while the hp will let you get those extra mph where the acceleration won't be so great and hence the close ratios up top. It looked like none of your gearings had that. The Vette tranny's have overdrives that are basically useless (other than great mpg obviously) and waste a good portion of top end acceleration. I'm obviously no expert, but maybe i helped.

Here is a quick n dirty one Iworked up based on the stock transmission with a new 4 & 6 gears. 1.10:1 & 0.74:1 instead of 1:1 & 0.56:1.

Trying to use the upper range in the higher gears:
M12 / 3.42 /Custom 4th & 6th Ratios

Originally Posted by 79TA

I'm not sure what kind of revs you want to have while open road racing, but I've always really enjoyed being able to cruise at 70 mph at about 2000 rpm. That was a big part of how I ended up choosing 3.42 gears for my Trans Am's rearend (not yet installed).

My current setup lets me cruise at 1800rpm, the above transmission would spin 2300rpm at 70mph - which would suck gas at an astonishing rate of knots hehe.

Although I normally cruise at 60mph any way which would be about 1900rpm with the above custom setup.

gobs3z · 02-21-2008, 01:08 PM

That M12 / 3.42 /Custom 4th & 6th Ratios makes a lot more sense, the stock 6th gear is not getting wasted anymore. I'de go for something like that.

RC45 · 02-21-2008, 03:02 PM

Originally Posted by gobs3z

That M12 / 3.42 /Custom 4th & 6th Ratios makes a lot more sense, the stock 6th gear is not getting wasted anymore. I'de go for something like that.

That's what I thought - thanks for taking the time to look.

By the way, I am chatting to a guy in Cali that has a 550rwhp LS7 in his 2800lb RX7, and would like to have the 600+rwhp I have.

I can only imagine the insanity of such an RX7 hehe.

gobs3z · 02-21-2008, 04:48 PM

Yeah can't imagine anything beyond 500whp in the car, it runs 11s with 350 crank hp. It's funny how technology has gone to more power instead of less weight. Good for him if he gets there, but good job on your car, i'm all about natural aspiration.

79TA · 02-21-2008, 08:34 PM

it would seem that your vette has enough power that you could just leave it in 5th all the time

RC45 · 02-21-2008, 11:18 PM

Originally Posted by 79TA

it would seem that your vette has enough power that you could just leave it in 5th all the time

Hehe - I can actually stay in 6th gear... dont need any other, bu tthere's no fun in that hehe

RC45 · 02-23-2008, 12:45 AM

Well, I went in to order my custon geared transmission today....

soon tuened to

as I had not considered the mechanics of the Tremec 6 speed... there is essentially no 4th gear perse... its the direct drive, hence the 1:1 ratio.

I understand 1st, 2nd, 3rd, 5th and 6th can be customised (with complete gear sets) but 4th is the direct drive and unchangable.

So, its back to the drawing board - LOL