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Toyota Supra Turbo 7M-GTE with R154 5-speed
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Discussion Starter · #1 ·
After shaving a head and having the block surfaced, we customarily use a thicker metal head gasket to keep from increasing the compression. I have only recently come to understand that boring a cylinder does not also decrease compression, but rather, increases it.

I Googled after having a long conversation on the subject and I still don't get it. Will someone please explain it to me?

Thank you,
David
 

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Same old calculation. Compression = total cylinder volume @ BDC : total cylinder volume @ TDC.

Boring increases the cylinder volume @ BDC, but not @ TDC (ok, ok, minus whatever the piston is under the deck)
 

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it sure does... more area when you bore and size the piston acordingly.
More area means more air ingestion.
More air ingestion and the same combustion chamber size means higher compression. Not by much but still higher.

So if you ever see any set of aftermarket pistons, it will have something like this

Dish area = -16cc

If that stays the same but the bore size is bigger... well you get an increase in compression.
 
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I've always wondered how a ~9.5:1 7M with a well ported head would react to 15-20lbs of boost.
probably pretty good. See 2jz and the 2jz head design, specifically the combustion chamber and lack of squish area.

Even the group A heads did away with the squish area.
 

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Debrucer,

When confronted with seeming conundrums like this, I like to use wild extremes of the situation in my head to help myself grasp it. So for this one, imagine a bore in the block you've enlarged out to the size of a 55 gallon drum, but remember you're keeping the head that same little 3" circle. So now, when the piston goes down, it draws in several cubic feet of air. Then when it goes up, it must compress all that air into the tiny little squish zone of the head. So it must reduce all that air down to perhaps a hundredth of its volume, or a 100:1 compression ratio.

As mentioned above, there would also be some additional space in the shape of a thin disc the diameter of the drum sized bore to consider, but this technique should fix it in your head as to why the compression ratio goes up when you enlarge bore.

Doug
 

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Discussion Starter · #8 ·
Debrucer,

When confronted with seeming conundrums like this, I like to use wild extremes of the situation in my head to help myself grasp it. So for this one, imagine a bore in the block you've enlarged out to the size of a 55 gallon drum, but remember you're keeping the head that same little 3" circle. So now, when the piston goes down, it draws in several cubic feet of air. Then when it goes up, it must compress all that air into the tiny little squish zone of the head. So it must reduce all that air down to perhaps a hundredth of its volume, or a 100:1 compression ratio.

As mentioned above, there would also be some additional space in the shape of a thin disc the diameter of the drum sized bore to consider, but this technique should fix it in your head as to why the compression ratio goes up when you enlarge bore.

Doug
Yes. If it wasn't already fixed, it is now. Thank you.

I think my logic was that the "combustion chamber" was not just the area in the head on top of the piston at TDC, but rather, as the piston went down that the combustion took place in the cylinder as well. Adding space above the head gasket was no different than space added with increased bore.

I will probably most surely be one of those guys who blows his 7m, but at least I will blame it on the builder :)
 

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Toyota allows 0.010 inch total removal from the head in the block from stock. What I did in my case was to use a 0.020 metal shim gasket sprayed with Permatex copper on the bottom that contacts the block and a standard Toyota head gasket on top of that. It got me back to stock dimensions and also valve timing which can vary depending on how much you take off the surfaces. I haven't had any issues in 7 years and 35K miles. I have ARP studs with nuts torqued to 90 foot-pounds.
 

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Discussion Starter · #11 ·
Toyota allows 0.010 inch total removal from the head in the block from stock. What I did in my case was to use a 0.020 metal shim gasket sprayed with Permatex copper on the bottom that contacts the block and a standard Toyota head gasket on top of that. It got me back to stock dimensions and also valve timing which can vary depending on how much you take off the surfaces. I haven't had any issues in 7 years and 35K miles. I have ARP studs with nuts torqued to 90 foot-pounds.
I totally blew it on calculating by not precisely measuring and in the process understanding the combustion chamber is only the dome above the head when piston is TDC on the compression stroke. I presumed that the chamber continued to be above the piston as the piston went down, and that therefore, boring was lower compression, not raising it as is true.

That's why I'm saying I'll be one of those guys.... I used a 2.2 MHG, but block and head each surfaced twice while I had it... and who knows truly before. I forget how we measured height, but we did, I just compensated wrong on the bore.
 
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