Toyota MR2 Message Board - View Single Post

te51levin · 05-17-2009, 01:11 PM

Those listings are kind of misleading. They don't tell you what CR you are starting with to get the numbers they quote, and there is a lot more involved in compression ratio than just the head gasket. You really have to do some math to know where you stand.

Your engine displaces 1587cc, which is 396.75cc per cylinder. The stock CR is supposedly 8.9:1, so each cylinder has 8.9 times more volume with the piston all the way down (bottom dead center, or BDC) as it does when the piston is all the way up (top dead center, or TDC). If we do some algebra, that shows that the effective chamber volume (combined volume of the combustion chamber, head gasket, and piston dome) is 50.2cc.

The stock head gasket is 1.2mm thick, and the bore of each cylinder hole in the gasket is 82mm, we know that the volume of the head gasket is 6.3cc per cylinder. Now, if you replace that with a different head gasket, you'll need to know the volume of that. Let's say you want to try the 1.6mm HKS gasket. Since each hole in the gasket displaces 8.4cc, you'd be adding 2.1cc per cylinder to the effective combustion chamber volume, taking it from 50.2cc to 52.3cc. That would actually lower your CR from 8.9:1 to 8.6:1, and this is on the gasket that they're advertising as "9:1".

Now, having got the example out of the way, I'd actually suggest sticking with a stock Toyota gasket unless you are really planning on turning up the boost. If you are, you will still probably want to avoid using head gaskets that are any thicker than stock, because that tends to destroy quench that occurs when the piston and cylinder head come very close to each other at TDC. In other words, it's possible to increase detonation even though you are reducing compression ratio. If you have access to decent fuel and/or decent management and/or decent intercooling, I'd prefer to leave that CR alone. If I changed it either direction via the head gasket, I'd go with a 1.0mm or 0.8mm unit.

I'd love to offer you a 0.8mm TRD head gasket, and they're only like $45 through me, but Toyota doesn't have any in the country right now, and I don't know when they will.

05-17-2009, 01:11 PM	#2 (permalink)
te51levin LithiaToyotaParts.com Join Date: Oct 2005 Location: Copperhead Road Posts: 3,685 Thanks: 50 Thanked 324 Times in 268 Posts My Google Map iTrader Rating: (193/100% )	Those listings are kind of misleading. They don't tell you what CR you are starting with to get the numbers they quote, and there is a lot more involved in compression ratio than just the head gasket. You really have to do some math to know where you stand. Your engine displaces 1587cc, which is 396.75cc per cylinder. The stock CR is supposedly 8.9:1, so each cylinder has 8.9 times more volume with the piston all the way down (bottom dead center, or BDC) as it does when the piston is all the way up (top dead center, or TDC). If we do some algebra, that shows that the effective chamber volume (combined volume of the combustion chamber, head gasket, and piston dome) is 50.2cc. The stock head gasket is 1.2mm thick, and the bore of each cylinder hole in the gasket is 82mm, we know that the volume of the head gasket is 6.3cc per cylinder. Now, if you replace that with a different head gasket, you'll need to know the volume of that. Let's say you want to try the 1.6mm HKS gasket. Since each hole in the gasket displaces 8.4cc, you'd be adding 2.1cc per cylinder to the effective combustion chamber volume, taking it from 50.2cc to 52.3cc. That would actually lower your CR from 8.9:1 to 8.6:1, and this is on the gasket that they're advertising as "9:1". Now, having got the example out of the way, I'd actually suggest sticking with a stock Toyota gasket unless you are really planning on turning up the boost. If you are, you will still probably want to avoid using head gaskets that are any thicker than stock, because that tends to destroy quench that occurs when the piston and cylinder head come very close to each other at TDC. In other words, it's possible to increase detonation even though you are reducing compression ratio. If you have access to decent fuel and/or decent management and/or decent intercooling, I'd prefer to leave that CR alone. If I changed it either direction via the head gasket, I'd go with a 1.0mm or 0.8mm unit. I'd love to offer you a 0.8mm TRD head gasket, and they're only like $45 through me, but Toyota doesn't have any in the country right now, and I don't know when they will.