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compression and bore increase hp gain?

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wideopen231:
I have been looking at increasing bore and compression for boost in power over the winter,

Yes I know I will not know how much it is worth w/o some solid runs this year. Few more chances to get that info left,

Going from 4.250 bore to 4.310 or maybe 4.375 with compression increase of 12:1 to 15:1. According to desk top dyno before pc died it is worth 60 to 70 hp. Rather I go with a .250 bigger crank is up in air since requires new rods and the ones in motor are about new. At 150o a set i rather not do both. I could reinstall old rods with 128 runs but that was my change-out point for those in blower motors.
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So question is agree or disagree with the power gain from bore and increase in compression

gordon:
What is bore and stroke currently? I got lost following the numbers in your post. Also is this thing Blown Alky currently or just N/A? Other thing to consider is what is your realistic comfort zone on Max RPM on a run? Bore and stroke changes do make a difference on shape of power curve as does cam timing and head flow.

JEFF/21C:
I think there are typos! need more info but from I understand maybe this is a na motor on alky,bigger bore normally helps head flow and higher compression if it a result of a bigger dome sometimes impedes flow and combustion, which might require dual plug heads depending on heads  if after market heads call tech and ask questions.
Gordon is right need more info on your set-up

wideopen231:

--- Quote from: gordon on November 08, 2023, 03:32:50 AM ---What is bore and stroke currently? I got lost following the numbers in your post. Also is this thing Blown Alky currently or just N/A? Other thing to consider is what is your realistic comfort zone on Max RPM on a run? Bore and stroke changes do make a difference on shape of power curve as does cam timing and head flow.

--- End quote ---

4.250 going to 4.310 or 4.375 bore/ Rpm not a concern motor use to be spun at 11.000 when had screw blower on it and these rods are better than one I had then. Not going 11k probably 8500 max but 9500 would not worry me.
 Headflow is around 540 at .800 if memory serves. N?a but with nitro at times. Cam is 289/292 at .050 with .825 lift.
 Stroke is 4.250 and may drop the 4.50 in,just not wanting to buy new rods at 1500 a set and have new set. Rods are .340 over stock and could drop back to .220 over if goin g to 4.50 crank and use same pistons. With big influx, I could just build both. Anybody have more than you need?LOL at 511 and 540 ci just need money.
Compression is 12:1 going to 15:1 with .040 gasket and dwon to around 13.5: with gasket changes.

gordon:
I would go to the .060 overbore of 4.310 , use your 4.250 crank with the longest rods you can hang on it and have a fairly short pin height . Compression is good for alcohol  but you do reach a point where the dome to get it starts plying xxxx with air flow and flame front. 13.5:1 sounds like a good place to start. Also if I remember right the 4.310 bore with a 4.250 stroke is 496 cu in. not 511.  Put it together and have fun. Later on if you want more put the 4.5" crank in for alot more grunt but don't buzz it past 7500-7800 max

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