Performance Q&A Tech Section

Our Resident tech guru, Big Mac, tells all.

Kevin McClelland Apr 1, 2009 0 Comment(s)

As for your question about stall speed, I think you're right on target with the 3,500 rpm range. We've been very pleased with our B&M Nitrous Holeshot 3,600 10-inch converter that we run in our wagon in bracket mode. The Nitrous Holeshot is fully furnace-brazed and fully balanced, and it has heavy-duty needle bearing thrust packages and new turbine hubs, which are key features along with anti-balloon plates. It's not too loose driving the car around the pits, but it flashes very well and stalls easily on the brakes to 2,600 rpm and would probably go higher. The only problem is that my 9-inch stocker slicks won't handle much more. It's back to that gear thing! Check with B&M and look into the Nitrous Holeshot (PN 20482).

Source:bmracing.com

Killer Cruiser
Q
My '70 Chevelle is nearly show-quality and is driven on the weekends and in good weather only. My recent upgrades include an LS2 (510 hp from Turnkey Engine Supply), electric fans and fuel pump, tubular control arms, all-new suspension hardware, a TH700-R4 trans (which I'm not totally impressed with), and four-wheel disc brakes (13-inch front and 12-inch rear). By the way, all of my upgrades were from articles and ads I saw in CHP magazine.

I have a 12-bolt non-posi with 3.31:1 rear gears (I believe, based on some calculation). What would be a good rearend gear setup to deliver better 0-60 performance without sacrificing highway driving by pushing the rpm too high? I do understand that everything is a trade-off. My driving style is mild. I prefer cruising, and I'll never drag race the car; however, I'd like a little more off-the-line performance. Any suggestions? Thanks.
Sam Dommer
Via email

A Sounds like you have a very nice blend of late-model performance and early-model style. I ran into a very similar situation when I installed an LT4 and a TH700-R4 in my '65 El Camino. The engine bay had a ZZ4 in it previously and I had 3.31:1 rear gears. The car ran great, but we wanted a lower dragstrip e.t. so I swapped out the rear gear to a 4.10:1. The car did pick up 0.1 second in 60-foot, but that was about it. It also made the car less of a weekend cruiser. The major difference I see between our cars is that you must have around a 26-inch-tall tire, as you're running 12-inch rear brakes. My car had 14x7-inch Rally wheels and a 24-inch-tall tire. This makes a huge difference when working out gear ratios.

First, a very easy formula to determine engine speed at any mph is final gear ratio multiplied by the constant 336. Then multiply the result by the desired mph. Then divide the sum by the tire diameter. This will give you the engine rpm at that mph. To get your final gear ratio, you must multiply the rearend gear by the Fourth gear ratio of your TH700-R4, which is 0.70. You need to go for a drive and determine what you consider a proper cruising rpm at freeway speeds.

Everyone has his own opinion what this should be, and I'll give you my recommendation. Currently, with your 3.31:1 rear gear, you have a final gear ratio of 2.31. This ratio, with a 26-inch tire at 2,200 rpm, results in 73.71 mph. A 3.54:1 gear has a final of 2.48, which results in 68.70 mph at 2,200 rpm. With 3.73:1 gears, you have a final of 2.61, which works out to 65.20 mph at 2,200 rpm. Finally, the 4.10s give you a final of 2.87, resulting in 59.32 mph at 2,200 rpm. So now go out and drive your car and decide how much engine noise and feel you want at cruising speed. If you were running a big-block, you could tolerate a higher gear, but the torque curve of your LS2 could use a little slow-speed help. I wouldn't go any lower than 3.73s in the rearend with your engine. Again, this is my opinion.

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