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im running 10:1 now

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G

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DaveH, I missed your reply, but I think 2 or 2.5mm per 1 unit of compression is close enough!

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I think you can get a new Toyota head for $1100

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does anyone know what mm the lexus is300 headgasket is?

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You have to take the head off to mill it. You may want to resurface the block deck at the same time.

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Thanks for the help and for my impala i put i thinner headgasket i had .60 i puta .50mm

so what would that .10 on a inch eqaute tocompression on my impala? would that raise it half a point or just .1? im still trying to get all this down. Thank for the help ~Brett

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Xdreamer, sorry I misunderstood your gaskets on the Chevy. Yes, obviously dropping it .1mm would raise the compression. How much? I don't know because the rule-of-thumb is exactly that...only an aid to get you started.

If you have an engine with a short stroke and a big bore (called over-squared) that is say 3 litres and you shave .1 inch off the head (or thin the gasket which has the same effect) you raise compression more than in a 3 litre engine with a long stroke and small bore. For the MKIII engine (7M) the bore is 83mm and stroke 91. The compression is 9.2:1. The MKIV engine (2JZ) is square, with both bore and stroke at 86, and compression at 10:1. The HP for the 7M is 200, and 220 for 2JZ. The torque went from 188lb-ft to 210. (I’m mixing metric and Imperial, but most people don’t think in kilowatts of power for engines, even in Europe they use a metric horsepower!)

To calculate what happens when we take 2 mm off the head, we need to do some simple volumetric calculation. I’ll use the MKIII here. Since the Volume of a cylinder is given by V=pi*r**2 * H (ignoring head shape), for the 7M you would have (83/2**2)*3.14* 91, or 492,365.907 cubic mm, or 492.37 cc for one cylinder; times six gives 2954.19 cc for the total engine, which is what Toyota claims. That means that at 9.2:1 the volume of one cylinder is compressed from 492.37 cc to 53.52 cc. Since the cross-sectional area of the cylinder is constant at (83/2)**2)*3.14 or 5.41 sq-cm, the height of the compressed cylinder is 53.52/5.41 or 9.89 mm. Now take 2 mm off the head, effectively lowering the compressed height to 7.89. Working backwards, we have a compressed volume of 5.41*7.89 or 42.69 cc. However we changed the stroke of the engine by the same 2mm, so the uncompressed volume of our milled cylinder went from 492.37 to 481.54 based on (83/2**2)*3.14*89. Consequently, our new compression ratio is 481.54/42.69 or 11.28:1. A significant compression increase!

Diesel engines with their 20:1 compression don't even vary the amount of air each stroke. Power is controlled strictly by the amount of fuel supplied. Interestingly enough the turbocharger was invented to use with diesel engines in 1905 exactly because of this lack of throttle air control. They were adapted to aeroplanes prior to WWI not for speed, but simply to keep the manifold pressure up to a sea-level equivalent as they became capable of flying higher. In fact, that is their primary purpose on aviation engines even today. But I digress....

Another consideration is the torque applied to the head gasket. If you follow Toyota's 58 lb-ft recommendation for a non-metal gasket, you have a lower compression than if you go to 72 lb-ft, because the higher torque will compress the gasket slightly more. The temperature range of the spark plug can also slightly change the compression because the tip reaches farther into the cylinder depending of the plug's heat range. These things have a greater affect as the compression ratio gets higher.

Could you turbo an 11:1 engine? Sure, but you will run into pre-combustion problems. I would say if you plan to turbocharge, you need to lower the compression so the turbo has room to work. There are lots of write ups on other threads here, on the SOGI form, supras.com, or MKIV.com. On the other hand, if you plan to stay NA or put on a Roots-type supercharger that has a fixed output, 10.5:1 is workable.

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