Ordering New Transmission, but What Gears ?

[Aubrey N Bruneau]

I sold my 2.83 1st gear Jerico DR4 a few months ago ( thanks to Class Racer ! ).
Time to order the replacement.
I have the new whizzy ( for me anyway ) Advanced Clutch system now.
Obviously, my previous selection of a 2.83 1st gear was a miss-match. Most guys have told me that I need like around a 3.20. When I spoke with the guy at Jerico, he recommended no more than 3.08. His reasoning being, that the engine doesn't rev high enough. Sounded logical. Engine makes peak torque at 4500/4600 RPM

Car scales at 3635
Shifted at 7000
Thru traps at 7200 / 7300
Best previous 60 ft was 1.59

[Sean Marconette]

What was your SLR with the other first gear? 3.08 seems like not enough. The higher numerical gear the more you can work the clutch and accelerate the car

Sean

[mopacltd]

My thought would be to go to a 3.25 first

[Bill Belden]

I would go with 3.19 first

[Aubrey N Bruneau]

SLR ?
HHMMM...
"single lens reflex" camera ? ? ! !
You have to remember who you're dealing with here... the guy who shows up to increase car count, and give the new guys somebody to beat in the first round !
really don't know what SLR stands for

Thanks, guys. That's what I keep hearing from everybody but Jerico.
torque is pretty good on this engine... 490 ft lb, so I'm thinking recovery might not be such a monumental obstacle.

[Rory McNeil]

SLR equals Starting Line Ratio. 1rst gear x rear axle ratio.
example, 2.88 x 4.88 is 14.06 SLR.
My Mustang with 3.19x5.13 is 16.36 SLR. I would like even more SLR, but with the steeper 1rst gear, and only a 4 speed, the ratio drops thru the gears get pretty wide. Not as much of a problem for a big engine with good torque, but then you are also pulling a lot more weight.
Also, don`t forget that an engine that doesn`t like a lot of RPM will have less rear end ratio, so to get the SLR more favorable, the only other choice is more low gear. Of course more SLR also increases the chance of wheelspin......

[henrys57wagon]

Aubrey, SLR= starting line ratio. Here is a calculator.
When we raced I was one of the cars to increase count, did get into 2nd round the odd time at division meets.

http://www.wallaceracing.com/calc-slr.php

[terry1]

I don't want to steal the thread but; I have always done(and read) finish line RPM,shift RPM and launch RPM calculations off of engine max HP not max torque.
May be I've been looking at this wrong!?

[Adger Smith]

Terry,
You are right, but he has no real RPM HP he has a torque engine. A 409 will not RPM like a SBC. You have to "work" the engine differently. If you have ever seen a 409 piston and pin you don't want to RPM one of those. Just the sheer weight will take things apart. Right Aubrey? I feel 7,000 is really pushing it. BTY you aren't stealing the thread, you are adding to the discussion.. :~)
If I was gearing a good RPM engine I would go for 8-10% RPM over peak Hp as a shift point and then figure the % gear pull back based on that to keep the engine right around peak HP. I would also work the ratios to get it in high gear somewhere around the 330' on the track.
Sean added a good point about stick cars not having the TQ multiplier of a TQ converter. The old rule of thumb that we used was add at least 20% to the low gear ratio of a stick over an automatic.
My 330 ft mark may be a little early for a stick, but a good automatic will respond to that. It would be interesting to watch a stick Comp car with about the same power to weight ratio as he has and see where it goes to high. On second thought that may not be the real deal because most all Comp cars are 5 speeds. Hummm, might be interesting just to watch!! :~)

[Aubrey N Bruneau]

Thanks Adger. No, Terry, you're not stealing the thread.
Just looked at my old dyno chart. Same cam, using stock stamped rockers.
Here's what I have: ( stupid printout wasn't legible in places... or maybe I'm over 50 ? )

4000 450 350
4100 465 361
4200 467 373
4300 473 384
4400 475 398
4500 481 410
4600 479 420
4700 482 429
4800 479 438
4900 479 444
5000 478 456
5100 477 461
5200 477 472
5300 477 478
5400 476 481
5500 465 485
5600 461 491
5700 459 495
5800 446 493
5900 443 496
6000 437 500
6100 439 508
6200 429 507
6300 423 505
6400 419 511
6500 ??? 512.6
6600 407 510
6700 ??? 509
6800 384 501
6900 ??? 497
7000 372 497
7100 ??? 490
7200 357 486

You're right, need to be careful about RPM, but Adger, the failures have never been pistons !... it's the damn "super duper" race car parts on the uncontrollable intake lobe... shattering retainers !
Have a toned down intake lobe now.

I've used the "average horsepower" system.