Z28 Boy

i am gettig new gears next week, and was planning on getting 3.73's. i have a 700r4 with 245/50r16 tires, which are 25.7".

basically, i lose about 10 mph of top speed in third gear with the 3.73s.

is there a huge difference with 3.73s vs. 3.42s? although i will probly not now, i dont want to "run out of gear" in the 1/4 with the 3.73s. if i ever get to a 110ph trap speed, i will be redlining it at the line.

but..with 3.42s i will only be spinning like 5 grand.

what do you guys think? is it worth the extra acceleration from moving up to a 3.73 from a 3.42? what kinds of gains can be expected from a moving up to a 3.73? thanks a lot.

-brian

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1991 Camaro RS - ultra blue metallic, t-tops
GMPP 350 HO crate engine - TBI, MSD ignition, SLP exhaust
700R4 - 2,400 stall converter, vette servo
10 bolt - coming soon - SLP zexel torsen differential, GM 3.73s
z28boy.fbody.com
z28boy@twcny.rr.com
AIM - "Z28 Boy"

theformula

Well I had 3:42s stock. It DID make a difference when I switched to 3:73s. However, I have NO first gear and it gets kind of annoying. But I did notice a difference in acceleration. Most people with autos notice better 60fts, but lose a little top end in the 1/4. Your gas mileage WILL suffer also.

The cost is kind of pricey to get the gears setup. I would only do it if you have the money to spend ($500 parts and labor at LEAST). Oh yea, don't forget your speedomter will be off and require you get new speedo gears.

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91 Formula
305 TPI 5speed
1LE/G92/WS6

Paxton SN92 polished kit, SLP airfoil, ported/polished plenum, March pulleys, Crane AFPR(43psi),Crane Gold 1.6rrs,MSD coil,MSD6AL (5200rpm limit),Holley 9mm wires,Ac delco R43ts plugs,stock chip,Ford SVO 24# injectors,Bosch O2sensor,SLP 1 5/8" headers,SLP catback,shortened shifter,3:73s w/Auburn posi,170* t-stat,JET 195* fan switch, Macewen white face gauges, Autometer gauges, Zoom hi-performance clutch.

[This message has been edited by theformula (edited March 14, 2001).]

Willie

iTrader: (1)

Depending upon the horsepower level of your engine, you can run out of gear before crossing the finish line. I calculated top speed as you did and based on what I saw, decided to swap my 3.42's for 3.73's. I did this last year. Unfortunately, I did not take into account torque converter slippage. You definitely will not have a top speed of 110 mph in third gear because of this. With my horsepower level, I was running out of top gear over two seconds before the finish. This was confirmed on my Auto Xray.

There are two "solutions". First is to stick with 3.42's (by far the simplest and cheapest). The other is to go with a mult-disc Vigilante lockup torque converter, the ONLY converter on the market today that's designed to be manually locked up. This will in effect give you an "extra" gear when you run out on the top end.

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Willie

Supercharged 1987 305 IROC-Z, Daily-Driver, Emissions-Legal.
Paxton (6-psig): 12.57 @ 111 mph.
Paxton (6-psig) & 50-hp nitrous: 12.04 @ 114 mph.
ATI D1SC (10-psig): ?

http://willie.camaro-firebird.org/

1987 "20th Anniversary Commemorative Edition" Z28 Convertible -- Super Chevy Show Class Winner, 1998.

[This message has been edited by Willie (edited March 14, 2001).]

AlkyIROC

When I was running 3.27 gears in a 9 bolt I was running out of track before I red lined. 3.90 would have been a better gear for me.

Once you know the power range of the engine, stick the tallest tire you can in the fenders THEN select the best gearing to cross the finish line at red line in high gear. 25.7" tall tires makes a very small footprint front to back. A 28" tall tire the same width, will have more rubber on the ground. Now you adjust the gearing to compensate for the taller tire. The lower gearing will give better torque multplication to get you off the line and the taller tire can grip the pavement better.

As an example with my car. I had 26" tall tires and 3.27 gears. Optimal gearing resulting in the same redline at the finish line would be.

26" tires and 3.90 gears (not available)
28" tires and 4.10 gears (currently in the car)
30" tires and 4.56 gears (rough guestimate)

The performance difference is all the same. I would still cross the finish line at my 6400 shift point. 4.56 gears would have a greater torque multiplication to get me off the line and the 30" tires would grip better so my ET would probably improve because of that choice.

It's really going to depend on your shift points. A car that shifts at 5500 doesn't need as much gear as a car that shifts at 7000 but then as above, tire size also is a factor.



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Follow my racing progress on Stephen's racing page
and check out the race car

87 IROC-Z Pro ET Bracket Race Car
383 stroker (carbed) with double hump cast iron heads and pump gas
461 Big Block installed and ready for the 2001 racing season

Best results before the 383 blew up
Best ET on a time slip: 11.857 altitude corrected to 11.163
Best MPH on a time slip: 117.87 altitude corrected to 126.10
Altitude corrected rear wheel HP based on power to weight ratio: 476.5
Best 60 foot: 1.662

Racing at 3500 feet elevation but most race days it's over 5000 feet density altitude!
Member of the Calgary Drag Racing Association

87 IROC bracket car, 91 454SS daily driver, 95 Homebuilt Harley