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I'm sure alot is based on the mph the vehicle is capable of (100 mph vehicle vs a 140 mph one) traction available and weather factors. But an old rule of thumb formula I had shows mph and HP ratio that might help. This formula seems to work for my car which has a 103 mph trap speed, 3650 weight and an engine dyno figure of 395 Hp.

Based on your information and supposing a 100 mph trap speed, the HP necessary to increses the speed by 2.5 mph is 23 to 25 HP at the rear wheels or approx 30 engine HP. ET would change with trap speed but may not be a straight ratio due to the factors I mentioned above.

The formula is (MPH x .00426)3[cubed] x vehicle weight This gives HP needed to move the vehicle that fast. Remember that there is up to 25% HP loss from engine HP to rear wheel HP due to the drivetrain.

I'm also sure there are other formulas out there and people with a better grasp of the subject. Just my 2 cents, but hope it helps.
 

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I did the math and came up with a 35 H.P. increase. This is of course, if there were no weather differences between the days you ran these times and speeds. What changes did you do to get the extra MPH? My calculations show going from 365 to 390 H.P. at the flywheel. In addition to the old Mopar formula you have to first correct the trap speed to the old "full" quarter mile speed first, as the trap speed is now an average speed at 1287' instead of 1320'. The correction, taken from a data logger, is appx. 1.006. So your real trap speed would be "108.6 current speed" vs. "104.1 prior speed" and using the formula as quoted in FTG53's post. I find that with an automatic there is a 19-20% loss to the rear wheels. With a standard it runs more like a 13-15% loss.
 

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Discussion Starter · #8 ·
Harold,I did a head swap.Stock closed chamber ovals heads to ported 2.19/1.88 closed chamber ovals

So your real trap speed would be "108.6 current speed" vs. "104.1 prior speed" and using the formula as quoted in FTG53's post. I find that with an automatic there is a 19-20% loss to the rear wheels. With a standard it runs more like a 13-15% loss.
previous speed was 105.5 MPH
 

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In cars at 11.00 or slower I would guess every .1 faster or 1mph faster equals approximately 10 hp.
When I did my head swap from stock home ported heads with 1.94/1.6 valve to aftermarket 200cc heads with 2.05/1.6 valves (and bumped compression from 9.5:1 to 10.17:1) I went .9 ET and 7mph faster which I guessed at 75-100 gross gain.
 
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