Picking the right gears for 1/4-mile

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MopaR&D

MopaR&D

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I should be getting closer to swapping in my early B-body 8 3/4 rear into my '70 Duster here soon and I would like to start doing some amateur drag racing at Bandimere this coming summer if I have the cash. The rear end comes with 2.94s which I will keep for highway cruising but I'm wondering what would be best for 1/4-mile performance. I will be running 28" tall Firestone Firehawk street tires at first but depending on how well they hook up I might get a pair of shorter drag tires. My engine is a 360 with Magnum heads, 10.5:1 comp, and the small(er) Voodoo 60401 cam and currently the engine seems to run out of breath around 5500 RPM.
 
I was going to say 4.10s based on my (admittedly limited) experience with my Duster- with the built 340 it had in it (11:1, big cam, torker, 780 dual feed holley, 727 with 3500 stall) it would hit about 110 mph (and top out) right at the end of the quarter mile.


With the stock 1979 360 in it now it tops out at a lot lower speed lol.
 
Between 4.10 & 4.56 depending on the tire dia. and how much engine you really have. Only track time will tell. I'd probably do the 4.10's w/the 28's and if it wasn't enough, look into a 26" drag tire for bite and some more rpm.
 
Ok good advice... I also have a cheap torque converter that stalls around 2600 rpm and slips quite a bit but a lot of that is probably due to still having the stock rear end with 2.76 gears. My engine is pretty under-cammed, later I might swap in the 60403 voodoo or similar single-pattern cam.

So 4.10 gears won't make my engine wind out too quick? Again it seems to run out of breath around 5500 rpm and has monster torque in the lower and mid-range...
 
The 4.10's sound right. check your spring pressures for the size of cam you have and also make sure your lifters aren't pumping up solid . If you have adjustable rockers, check the lash. Maybe it needs more carb also. Good luck !!
 
65Vsignet said:
3.91 or maybe 4.10
Click to expand...
This is what I'd be thinking of, depending on your RWHP - as this would possibly change things. Try looking at this http://robrobinette.com/et.htm and do the math.

Even take your car down the qtr with the highway gears - the times will be down but the MPH will be close, from there you'll be able to figure out if you want 3.5:1 or ???
 
If it's out of breath at 5500 it's probably beyond the horsepower peak before that. If it was me I'd be looking at gearing so in high gear you're around 5300 at the stripe. If I estimate for a little over 100mph That's 4.10s.
 
As some others have said, a 28" tall tire on a street/strip usually works well with 4:10s, providing you can buzz the engine enough. There are some other variables, though. What torque converter are you using? What IS the recommended range for the cam. Are you sure your issue is a cam one & not an intake or fuel delivery problem?
Keep in mind too, that you can effectively "change" your gear ratio by changing tire size (28" to 29" or 30", for example) to help with your running the car. If it were me, I'd start out with 4:10s & find someone that's willing to lend you their other sized tires for a night of T&T to see what you come up with.
 
You have to know the trap rpms desired; that means you have to know the power band of your set-up. Then change accordinglyl; either internal combo, or tires, and/or ratio.
 
go to richmondgear.com gear and transmission calculator plug in your numbers and it will give you ratio
 
Well I do know it could use more carb, it only has a 625 cfm AFB right now. I took it to a test & tune night at bandimere and it ran a best of 93 mph trap speed at 5500 ft. elevation with crummy tires and stock gears. In fact 4.10s might be easy to get, my buddy's old dodge van supposedly has them.
 
Actually it's 5800', but thats no big deal. I've run 3.91 & 4.30 gears with 27" tires and the 3.91 was at 6200 at the stripe and the 4.30 was at 6400. I don't know the specs on the voodoo cam you have but if you are out of breath at 5500 you might consider 3.73's. I would give it more cam and gear it at 4.10. With 10.5 compression it should tolerate more cam but be mindful of our altitude and don't go too big. With a comp xe-274 solid cam my 318 went 100 mph at bandimere with the 4.30 gear but didn't have the power to get past 6000 rpm.
 
OldmanRick said:
Between 4.10 & 4.56 depending on the tire dia. and how much engine you really have. Only track time will tell. I'd probably do the 4.10's w/the 28's and if it wasn't enough, look into a 26" drag tire for bite and some more rpm.
Click to expand...

4.56 is too low if he runs out of motor at 5500 imo.
 
The cam is pretty small, it's the 256/262 version with .454/475 lift w. 1.5 rockers, I have 1.6s so it's a bit more. Lunati says it makes power from 1000-5500 rpm and describes it works for smaller milder engines... Like I said monster torque curve that starts low at 2000 rpm with this 360/magnum/10.5:1 setup. I think I'll try 3.91s at first but with this torque I can tell I'll need some traction aids. I'll also try a bigger carb and have the exhaust redone, the pipes off the shorty headers were not well made.

green1 my ET was 15.3 sec.
 
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