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just a ski boat with bark
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Discussion Starter #1 (Edited)
So as some of you know I recently grenaded my old BBC combination. It was a true 12.5:1 compression 454 bored .060" over with some home ported 990 heads, tunnel ram and large flat tappet cam. It was able to push my old Hondo between 101-104 mph depending on prop and gear combination.

This go around I would like to get away from race gas and/or av-gas and just run our locally available 91 octane unleaded pump gas. With the Heads I ordered 4 weeks ago and shelf available pistons it looks like 10:1 is about the highest compression I can get without milling the new heads. So how much power will I lose dropping the compression 2.5 points anybody ever done something similar to this? The thermal effeciency chart I found says 5.6% so if I made an honest 600hp before the compression drop it would lose about 34 hp, does this sound about right?
 

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I cannot guarantee the accuracy but running the compression change on a 540BBC using a dyno simulation program dropped the Hp over 20%.
 

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Fastmarlin is correct, dropping 2.5 points you are going to loose 100HP. Compression is like icing, the more of it the sweeter the taste. If you want to make up for the loss then put some stroke in it.
 

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just a ski boat with bark
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Discussion Starter #4
Fastmarlin is correct, dropping 2.5 points you are going to loose 100HP. Compression is like icing, the more of it the sweeter the taste. If you want to make up for the loss then put some stroke in it.
Are you serious, 100 hp by dropping from 12.5 to 10:1 on a mild 600ish hp 467?!?!?!
 

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steelcomp was here
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So as some of you know I recently grenaded my old BBC combination. It was a true 12.5:1 compression 454 bored .060" over with some home ported 990 heads, tunnel ram and large flat tappet cam. It was able to push my old Hondo between 101-104 mph depending on prop and gear combination.

This go around I would like to get away from race gas and/or av-gas and just run our locally available 91 octane unleaded pump gas. With the Heads I ordered 4 weeks ago and shelf available pistons it looks like 10:1 is about the highest compression I can get without milling the new heads. So how much power will I lose dropping the compression 2.5 points anybody ever done something similar to this? The thermal effeciency chart I found says 5.6% so if I made an honest 600hp before the compression drop it would lose about 34 hp, does this sound about right?
X2 to the above. The only way to make it up would be displacement.

If you're building a new deal all together, you can get away with 11:1 on 91 if you pay attention to the details on your build. With a TR and your new heads, there's no reason you can't make an honest, reliable 700hp on 91 with a .25" stroke and not have to turn it 7500.
 

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just a ski boat with bark
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Discussion Starter #7
If everthing else stays the same yes.
Well I would have went with a different cam but 100 is way more than I would ever expect. I was thinking around 50 hp and hoping the thermal effciency chart was accurate with around 35.
 

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Resident Ford Nut
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Well there are a couple people posting here that know far more than I do, but after doing 3 Engine Masters Engines with Danny Crower my bet would be approx 4% loss per point of compression. So in your case if you are losing 2.5 points it would be approx 10% or 60hp, but it does depend on the cam.

If you're going new go big with at least a 496". Or what about a 502" short block?


In any case depending on the cam my bet is 10% loss.

S CP :D
 

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just a ski boat with bark
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Discussion Starter #9
X2 to the above. The only way to make it up would be displacement.

If you're building a new deal all together, you can get away with 11:1 on 91 if you pay attention to the details on your build. With a TR and your new heads, there's no reason you can't make an honest, reliable 700hp on 91 with a .25" stroke and not have to turn it 7500.
I was wanting compression between 10.5-11:1 but I can't find any off the shelf pistons to do that. I really did want to angle mill the new heads I ordered that I haven't even recieved yet. But a true 700hp would be fine with me I guesstimated the last one at 625hp from the numbers it ran.
 

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just a ski boat with bark
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Discussion Starter #10
Well there are a couple people posting here that know far more than I do, but after doing 3 Engine Masters Engines with Danny Crower my bet would be approx 4% loss per point of compression. So in your case if you are losing 2.5 points it would be approx 10% or 60hp, but it does depend on the cam.

If you're going new go big with at least a 496". Or what about a 502" short block?


In any case depending on the cam my bet is 10% loss.

S CP :D
Well this is only a hobby for me and I have 2 454 blocks left I can use and th eheads I ordered for the old combination might be too small to go very big. After talking my wife into heads a Dart or Merlin block will be way out of the realm of possiblilties. lol
 

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"BEER BUDGET RACING"
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Well this is only a hobby for me and I have 2 454 blocks left I can use and th eheads I ordered for the old combination might be too small to go very big. After talking my wife into heads a Dart or Merlin block will be way out of the realm of possiblilties. lol
what is that other crank you have Loren?
 

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steelcomp was here
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I was wanting compression between 10.5-11:1 but I can't find any off the shelf pistons to do that. I really did want to angle mill the new heads I ordered that I haven't even recieved yet. But a true 700hp would be fine with me I guesstimated the last one at 625hp from the numbers it ran.
With the stroker, any piston that has a 20cc dome should get you there, and almost 11:1 if you mill your heads down to ~115cc. You don't have to angle mill them, either.
 

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just a ski boat with bark
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Discussion Starter #14
With the stroker, any piston that has a 20cc dome should get you there, and almost 11:1 if you mill your heads down to ~115cc. You don't have to angle mill them, either.
The SRPs I have for that crank are 18cc domes.
 

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steelcomp was here
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The SRPs I have for that crank are 18cc domes.
KB/Icon makes a 20cc dome piston, IC789...about the only one I know of. Everyone else seems to be 18cc in that range. If you mill your heads to 114, zero deck, .042 gasket, you'll end up with 10.6 with the 118cc, and 10.8 with the 20cc dome. (That's for a 496...it's alittle less with the 489). I wouldn't lose sleep over .2 in compression. That 10.6 would be a nice ratio for the 91. You couls also go with a .039 gasket for a little better quench and another .07 in compression.
 

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just a ski boat with bark
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Discussion Starter #17
KB/Icon makes a 20cc dome piston, IC789...about the only one I know of. Everyone else seems to be 18cc in that range. If you mill your heads to 114, zero deck, .042 gasket, you'll end up with 10.6 with the 118cc, and 10.8 with the 20cc dome. (That's for a 496...it's alittle less with the 489). I wouldn't lose sleep over .2 in compression. That 10.6 would be a nice ratio for the 91. You couls also go with a .039 gasket for a little better quench and another .07 in compression.
Thanks for the info Scott but since I already own the SRPs I just can't justify buying new pistons for 2 more cc's. They didn't list a piston weight for that IC789 in their online catalog.
 

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just a ski boat with bark
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Discussion Starter #18
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