10:1 vs. 11:1 Compression: Power Difference?

90mustangGT · Jul 17, 2005

What would be a ballpark difference between two engines, otherwise the same, but one having a point more compression than the other.

I have not decided on what motor I am going to do, but weather it's a 306 or 347 the compression ratio is an issue. 11:1 may make more power, but staying around 10:1 compression is still mild boost freindly, maby for the future.

Grn92LX · Jul 17, 2005

I'd guess 5-10hp. If your building the engine with the plans for a supercharger I know your smarter than to use a stock block. Theres also diofferent ring gaps needed for boost i believe.

BlackFox5.0 · Jul 17, 2005

10:1 is still high for boost. I wouldn't go over 9:1 for boost.

90mustangGT · Jul 17, 2005

Only 5-10hp? That's it? I thought there would be a more than that. Building a motor for a S/C, not exactly but I kind of want to leave that option open. That way if I decide to get a blower, I can at least use it while building another motor.

I am just currious, I really trying not to rack my mind on the motor right now because I am trying to build a workable chassis first to put that motor into.

Grn92LX · Jul 17, 2005

5-10hp was just a guess. Remember, it only costs $80 or so to mill heads so for the few bucks it costs, it helps. I have zero interest in a blower so I went with higher compression.

90mustangGT · Jul 17, 2005

Grn92LX said:
5-10hp was just a guess. Remember, it only costs $80 or so to mill heads so for the few bucks it costs, it helps. I have zero interest in a blower so I went with higher compression.

I hear you on that one, you got that S trim then decided against it.

I might just go ahead with the current 306 but add Probe pistons that are for the TFS reliefs. They would put me closer to 10.8:1 compression. I'd like to do a 347, but that will cost more, and I already have the 306 shortblock.

Modular2v · Jul 18, 2005

there is usually around a 20- 25hp gain per point in compression

Grn92LX · Jul 18, 2005

90mustangGT said:
I hear you on that one, you got that S trim then decided against it.

I might just go ahead with the current 306 but add Probe pistons that are for the TFS reliefs. They would put me closer to 10.8:1 compression. I'd like to do a 347, but that will cost more, and I already have the 306 shortblock.

Is your block zero deck? Are your tfs heads milled? If not, then you'll end up with around 10:1 or so. My block is zero deck and my heads are 55.6cc (stock TW are 61cc) I also run a .039" gasket (fel pro 1011-2)

Yeah for me, the blower was a mistake. But at least I learned how to install one

Your back and forth on the 306 vs 347 lol. If your keeping it n/a, build the cubes!

sodaking663rd · Jul 18, 2005

Grn92LX said:
5-10hp was just a guess. Remember, it only costs $80 or so to mill heads so for the few bucks it costs, it helps. I have zero interest in a blower so I went with higher compression.

i'm curious why you don't want a blower...can you explain?

89MustangGX · Jul 18, 2005

I believe the general rule of thumb is 4% per point of compression. This obviously means if you run a blower and/or make more power -- more compression will be worth even more.

I think there was a dyno test in a mag a little while back. It was on a race motor, but it dynoed something like 400hp then they upped the compression four points and it went to like 475hp. They put it under boost under low compression and made 800hp and then upper the compression and made 975hp or so. Numbers were similar to that IIRC.

Blk91stang · Jul 19, 2005

11:1 compression is the MAX I would go on a street car. Anymore might give you reliability problems (see 2nd paragraph). A point more of compression not only add's more hp, it's the best way to make a lot of torque!

Just something to throw in.. I have about 12.7:1 compression on my 85' and I've been through 3 stock starters before dropping $350 on a Mean Green starter to stop them from breaking when cranking. The car also hardly turns over when it's hot... Major pita. Just something to think about..

90mustangGT · Jul 19, 2005

Grn92LX said:
Is your block zero deck? Are your tfs heads milled? If not, then you'll end up with around 10:1 or so. My block is zero deck and my heads are 55.6cc (stock TW are 61cc) I also run a .039" gasket (fel pro 1011-2)

Yeah for me, the blower was a mistake. But at least I learned how to install one

Your back and forth on the 306 vs 347 lol. If your keeping it n/a, build the cubes!

The pistons come up slightly higher than the block deck, not much but I set a cylinder head on top and turned over the motor and the pistons were bumping the head. (no gasket)

The heads have been milled because they are used, but have been machined and the deck surface ground.

306 is cheaper than a 347, but I am tempted. I have a 306 shortblock, but I need different pistons, which will be about $500, then I need to balance it, and have the block re-align honed and such and buy new fasteners. For about $500 more I'll have a rod and crank combo to go along with the pistons with more throw. Since the car probally won't be ready until next spring, I have decided to do the motor last, right now I'm doing the chassis build, this winter I will do all I can to get something mean in that chassis.

emagdnim · Jul 19, 2005

What kind of heads are you running 90mustangGT ?

Dark Knight · Jul 19, 2005

try this
http://www.wallaceracing.com/calc-crchg.php

Jason@AFR · Jul 19, 2005

89MustangGX said:
I believe the general rule of thumb is 4% per point of compression.

Agreed

90mustangGT · Jul 19, 2005

emagdnim said:
What kind of heads are you running 90mustangGT ?

TFS Twisted Wedge with Track Heat springs.

Grn92LX · Jul 19, 2005

90mustangGT said:
The pistons come up slightly higher than the block deck, not much but I set a cylinder head on top and turned over the motor and the pistons were bumping the head. (no gasket)

The heads have been milled because they are used, but have been machined and the deck surface ground.

306 is cheaper than a 347, but I am tempted. I have a 306 shortblock, but I need different pistons, which will be about $500, then I need to balance it, and have the block re-align honed and such and buy new fasteners. For about $500 more I'll have a rod and crank combo to go along with the pistons with more throw. Since the car probally won't be ready until next spring, I have decided to do the motor last, right now I'm doing the chassis build, this winter I will do all I can to get something mean in that chassis.

Even with a gasket you'll still hit. So either way you need new pistons, rings and balancing. I would do all new bearings as well while its apart.

How sure are you about your compression? Piston sticking out of the hole + milled TW heads + an aftermarket flat top piston will = over 10:1 I'd say. My block is zero deck. I would stay n/a and build the stroker. It'll be fun and less hassle. If it gets boring, throw an n2o hit on it.

10:1 vs. 11:1 Compression: Power Difference?

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