Compression ratio question.
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From: Conroe TEXAS!
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Compression ratio question.
So how bad would things be if I dropped to 9.5:1? Whats the disadvantage? Can't be that much horsepower given up? Right?:o
Im still gathering parts for my motor, all I really have left is heads to purchase. Im going with the AFR 195's. If I have them milled to 58cc I'll get right around 10.1:1. Or, I throw the extra $300 in the motor somewhere else, and leave them at 65cc and end up around 9.5:1.
What do you think?
or 
?
Im still gathering parts for my motor, all I really have left is heads to purchase. Im going with the AFR 195's. If I have them milled to 58cc I'll get right around 10.1:1. Or, I throw the extra $300 in the motor somewhere else, and leave them at 65cc and end up around 9.5:1.
What do you think?
or ?
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From: Overland Park Kansas
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Are you reusing your stock block? What gasket?
Consider 55-56cc if you mill. Everything I read said to pack that sucker as tight as possible. (At least until 11:1 CR is reached).
If you go from 65cc to 56cc, I'm estimating the gain would be 20hp - (5hp from loss of flow*) = 15hp.
15hp for $250ish isn't too bad. If you want the best off-idle response and low-end torque, I'd go for it. That's a lot of what you'll gain.
gp
*according to AFR, ~5hp is the loss from milling 65cc to 56cc
Consider 55-56cc if you mill. Everything I read said to pack that sucker as tight as possible. (At least until 11:1 CR is reached).
If you go from 65cc to 56cc, I'm estimating the gain would be 20hp - (5hp from loss of flow*) = 15hp.
15hp for $250ish isn't too bad. If you want the best off-idle response and low-end torque, I'd go for it. That's a lot of what you'll gain.
gp
*according to AFR, ~5hp is the loss from milling 65cc to 56cc
Safety Car
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From: St Marys GA
Here's what you should do:
Leave it at 9.5:1, and you'll actually be able to use 87 octane (which will help with high gas prices)
Then, when you save up some more money for more car projects, supercharge it! You'll already have a low CR ideal for forced induction. At least this is my plan when I build a 427 for my Vette. Sometime...
Leave it at 9.5:1, and you'll actually be able to use 87 octane (which will help with high gas prices)
Then, when you save up some more money for more car projects, supercharge it! You'll already have a low CR ideal for forced induction. At least this is my plan when I build a 427 for my Vette. Sometime...
Racer
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I'm not sure you can flat mill them that much. You would probably have to angle mill them to get them down to 58cc. That will pose other problems. I take it you have already bought the pistons? How about having the block decked?
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From: Conroe TEXAS!
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It's a gmpp ZZ4 shortblock that im using. I think GM rates the possible ratios on the ZZ using a .051 gasket. I would be using the gasket AFR reccomends, .041.
Last edited by Firevette; Jan 14, 2009 at 10:21 PM.
Burning Brakes
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From: Pittsburgh PA
Heck if they used a .051 gasket with ZZ4 58cc heads to get 9.6 to 1 or so, a .041 gasket will bring that up .2 right there.
I doubt they used that gasket tho, because .051" gasket on a block that i dont believe is zero decked means the pistons are in the hole another .020-.025 depending, leaving a very large quench height of .070"+. NOT GOOD.
Find out if your block is 9.00 deck or 9.025 and if they pistons lay flat on the top of the block at TDC.
In any case, 9.5 to 1 with aluminum is abit low but still can make some banging power with the right cam. i wouldnt mill the heads
If your pistons are in the hole, run the thinnest gasket you can get away with and still seal ok. .028" should be ok, some have used .015" but thats cutting it close.
I'd still run 93 oct and run aggressive timing. Will help make torque that you lose with less compression
I doubt they used that gasket tho, because .051" gasket on a block that i dont believe is zero decked means the pistons are in the hole another .020-.025 depending, leaving a very large quench height of .070"+. NOT GOOD.
Find out if your block is 9.00 deck or 9.025 and if they pistons lay flat on the top of the block at TDC.
In any case, 9.5 to 1 with aluminum is abit low but still can make some banging power with the right cam. i wouldnt mill the heads
If your pistons are in the hole, run the thinnest gasket you can get away with and still seal ok. .028" should be ok, some have used .015" but thats cutting it close.
I'd still run 93 oct and run aggressive timing. Will help make torque that you lose with less compression
Thread Starter
Safety Car
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From: Conroe TEXAS!
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Heck if they used a .051 gasket with ZZ4 58cc heads to get 9.6 to 1 or so, a .041 gasket will bring that up .2 right there.
I doubt they used that gasket tho, because .051" gasket on a block that i dont believe is zero decked means the pistons are in the hole another .020-.025 depending, leaving a very large quench height of .070"+. NOT GOOD.
Find out if your block is 9.00 deck or 9.025 and if they pistons lay flat on the top of the block at TDC.
In any case, 9.5 to 1 with aluminum is abit low but still can make some banging power with the right cam. i wouldnt mill the heads
If your pistons are in the hole, run the thinnest gasket you can get away with and still seal ok. .028" should be ok, some have used .015" but thats cutting it close.
I'd still run 93 oct and run aggressive timing. Will help make torque that you lose with less compression
I doubt they used that gasket tho, because .051" gasket on a block that i dont believe is zero decked means the pistons are in the hole another .020-.025 depending, leaving a very large quench height of .070"+. NOT GOOD.
Find out if your block is 9.00 deck or 9.025 and if they pistons lay flat on the top of the block at TDC.
In any case, 9.5 to 1 with aluminum is abit low but still can make some banging power with the right cam. i wouldnt mill the heads
If your pistons are in the hole, run the thinnest gasket you can get away with and still seal ok. .028" should be ok, some have used .015" but thats cutting it close.
I'd still run 93 oct and run aggressive timing. Will help make torque that you lose with less compression
It is a .051 gasket. It allows 10.1:1, with a 58cc head
...ok i need to go find a C/R calculator.
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Thread Starter
Safety Car
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From: Conroe TEXAS!
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Safety Car
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From: Houston TX
Firevette,
I would have the heads milled by AFR to the 58cc vs the "off the shelf" 65cc that AFR also sells. Spend the money now to get the SCR up or you will be kicking yourself that you didn't later. Plus, in order to get down to the 58cc level, requires angle milling, which requires resurfacing of the intake flange, which will be costly later (if you find someone that can do it right, which can be a problem).
Aaron
I would have the heads milled by AFR to the 58cc vs the "off the shelf" 65cc that AFR also sells. Spend the money now to get the SCR up or you will be kicking yourself that you didn't later. Plus, in order to get down to the 58cc level, requires angle milling, which requires resurfacing of the intake flange, which will be costly later (if you find someone that can do it right, which can be a problem).
Aaron
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If its a question as to which I'd rather have, I'd say the higher compression, but still low enough to run on pump gas.
Higher CR helps efficiency and thus MPG, but higher CR means more expensive gas. I havent done the calculations but I figure that with gas (likely) going back up some, the higher CR will outweigh the detriment in that area.
Higher CR helps efficiency and thus MPG, but higher CR means more expensive gas. I havent done the calculations but I figure that with gas (likely) going back up some, the higher CR will outweigh the detriment in that area.
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From: Overland Park Kansas
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If its a question as to which I'd rather have, I'd say the higher compression, but still low enough to run on pump gas.
Higher CR helps efficiency and thus MPG, but higher CR means more expensive gas. I havent done the calculations but I figure that with gas (likely) going back up some, the higher CR will outweigh the detriment in that area.
Higher CR helps efficiency and thus MPG, but higher CR means more expensive gas. I havent done the calculations but I figure that with gas (likely) going back up some, the higher CR will outweigh the detriment in that area.
When I looked it up, I found guidelines that showed lower octance gas requires compression to be lower than the 9.5:1 - 10.5:1 range discussed here. IIRC, 8.5:1 lets you run on regular. So, you should run premium gas whether you mill or not (speaking to the OP's config). But, I've also seen people say they're running regular gas and not getting many knock counts.
One of my friends believes his actual stock compression was only 8.8:1 (after measuring cylinder volume), so maybe that's why some cars can run on lower octane. I guess there really is some variation in motor parts!
Last edited by GREGGPENN; Jan 15, 2009 at 07:17 PM.
Thread Starter
Safety Car
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From: Conroe TEXAS!
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Firevette,
I would have the heads milled by AFR to the 58cc vs the "off the shelf" 65cc that AFR also sells. Spend the money now to get the SCR up or you will be kicking yourself that you didn't later. Plus, in order to get down to the 58cc level, requires angle milling, which requires resurfacing of the intake flange, which will be costly later (if you find someone that can do it right, which can be a problem).
Aaron
I would have the heads milled by AFR to the 58cc vs the "off the shelf" 65cc that AFR also sells. Spend the money now to get the SCR up or you will be kicking yourself that you didn't later. Plus, in order to get down to the 58cc level, requires angle milling, which requires resurfacing of the intake flange, which will be costly later (if you find someone that can do it right, which can be a problem).
Aaron
My question is, when will I start running into accesory drive and valvetrain geometry problems? Id like to avoid that, seems that a lot of material needs to go to get down to 58cc.
That's definitely something to look at. When you consider 1 point only gets 10hp, the small amount of increase doesn't seem worth it. OTOH, the break-points for compression vs octane seem fuzzy. Plus, any increase in efficiency or burn quality can help with emissions and mpg, right? So, the benefits of higher compression can be worthwhile.
When I looked it up, I found guidelines that showed lower octance gas requires compression to be lower than the 9.5:1 - 10.5:1 range discussed here. IOW, you should run premium gas whether you mill or not (speaking to the OP's config). But, I've also seen people say they're running regular gas and not getting many knock counts.
When I looked it up, I found guidelines that showed lower octance gas requires compression to be lower than the 9.5:1 - 10.5:1 range discussed here. IOW, you should run premium gas whether you mill or not (speaking to the OP's config). But, I've also seen people say they're running regular gas and not getting many knock counts.
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From: O'Fallon Missouri
I have a 9:1 compression C5 and my C4 is ~11:1. The throttle response is so much nicer on the C4, and nothing like the sound of a high compression engine firing up. Just hit the key and BOOM!!
Last edited by STL94LT1; Jan 15, 2009 at 07:02 PM.
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Same deal with the 10hp. But, I am also wanting best possible burn -- having side pipes behind my doors. If you get better burn, better mileage, and better low-end response....
Keep in mind 10hp is a pretty good percentage of the power being made at idle. (see post immediately above this one -- for typical feedback I got on the subject).
Last edited by GREGGPENN; Jan 15, 2009 at 07:14 PM.
Safety Car
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No problems with accesory holes lining up with the heads angle milled from AFR. They are taking minimal material from the head in the overall height of the cylinder head.