gear ratio for 327 sbc

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gear ratio for 327 sbc

Post by dth1160 » Mon Oct 11, 2010 12:28 am

'78 chevy monza, 2480lbs without a driver. A 327 sbc bored .030 over. 31" tires, 4.86 Ford 9" rear end, PG tranny with transbrake with 1.98 straight cut 1st gear, Erson roller cam, 675/645 with 4600 to 7800 rpm range. It dynoed 530 hp at 6400 rpm, then peaks again at 7100 rpm to 520 hp. Shift point seems best at 7600 right now. Best run was 7.90 in 1/8th mile and 10.91 in the 1/4 mile. Launch was 4800 rpm and tires hooked up pretty good. Shifting at 7600 the engine rpm dropped to 5800 after shifting then crossed the 1/4 mile mark at 7000 rpm and 120.47 mph. The car actually squeeled the tires slightly when it shifted which was at about the 300 ft mark. The 60 foot mark time was 1.51 seconds. I know I need more gear. Also need new tires and will probably go to a 30"x 12.2x15 due to the 31"x13x15's rubbing the side walls slightly inside and out. What gear would be best for 1/4th mile and which gear for 1/8th mile crossing the finish line in the 1/4th mile at about 7800 to 8000 rpm's. I figure crossing the 1/8th mile line would be at a lower rpm range like at about 7600 maybe(I am not sure). I will appreciate all input. Will probably opt for the best 1/8th mile gear or a range in between since most tracks in my area does not allow a transbrake running the 1/4th mile.



Re: gear ratio for 327 sbc

Post by CHT » Thu Oct 14, 2010 6:55 am

If you crossed @ 7k the tyre size/gear ratio suggests way above 650rpm slippage which is a fair amount.
Your terminal speed suggets there is a problem 120mph & 10.9et.
Lets assume you saw 7 but was actually 6.8 thats still getting on for 500 slippage, what was the speed @ the 1/8th mile?
With say a 5.69 gear 30" tyres at 120mph should be around 7630rpm (no tyre growth factored in) which with the correct convertor and "good" top gear clutches you should see 78-7900 on the tach at the 1/4mile stripe

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