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Discussion Starter · #1 · (Edited)
I've been doing some calculations on 7M-GTE Compression Ratios, and thought I'd share the info with the masses. It seems to me that when trying to figure out compression ratio, I've noticed that a lot of people don't take gasket bore size into account, which can have a great effect of the compression ratio. I've calculated CR for many popular aftermarket Head gaskets on the 7M-GTE.

Gasket | Bore | Thickness | CR
Stock     83         1.5         8.39:1 (Toyota advertised 8.4:1)

Figuring Compression ratios can be tricky, because one unknown is how much you will mill the head. I mill the head every time I do a job to ensure a good RA. If a straightness check is good, I'd take as little as 0.05mm off, but many machine shops aren’t so conservative. Since a lot of people have also done a HG job more than once on an engine, I'm going to give all of the following numbers with a milling depth of 0.254mm or 0.010" So if anything, the numbers might be a little bit high, but within 0.02 CR.

Gasket | Bore | Thickness | CR
Greddy     83         1.0         8.87:1
Greddy     83         1.5         8.55:1
Greddy     83         2.0         8.25:1
Greddy     85         1.5         8.5:1
Greddy     85         2.0         8.19:1
HKS         86         1.0         8.82:1
HKS         86         1.2         8.68:1
HKS         86         2.0         8.16:1
HKS         86         3.0         7.48:1
Fel-Pro     83         1.5         8.55:1 (Being a composite gasket, compressed height may vary slightly)


In determining these numbers I used the more accurate information possible. I cc'ed multiple heads and pistons from 7M-GTE engines to ensure accurate measurements as well as compared these numbers again other's finding and some published numbers I found. I took into account piston to wall clearances and top ring height also to ensure the most accurate possible calculations.

When choosing a gasket, compression ratios are not the only important thing. Gasket bore size has a few effects on the combustion process. When the bore size is larger than the cylinder bore size, gasses will become trapped within the area between the head and the block. This is further exaggerated on a 7M engine because they run a negative deck height (piston pops up out of the block). The gasses in this area are not reached by the flame front, and even if they were, they are too cool to combust because of the increased surface area of the trapped area. These unburnt gasses will decrease efficiency (fuel mileage) and increase emissions. This may be of concern as more and more areas are subjected to snifffer emissions tests. Properly operating catalytic converters should take care of the slight increase in hydrocarbons though. On the positive side for those who are pushing their engine hard, these unburnt gasses will have a cooling effect on the piston and exhaust valve. Though slight, it could help to combat detonation and preignition, much like high cam overlap can on race engines.

When choosing a target compression ratio, generally speaking, the higher it is, the more efficient the engine will be (fuel permitting). In my opinion, for a street car, a high compression turbo engine can't be beat. Throttle response in greatly improved as well as low-end power. Though extremely high boost levels can't be achieved, that may be insignificant on a street car. You've probably heard about "area under the curve", and that's what a high compression turbo car is all about. With efficient turbo and intercooling it can be done very reliably. Many European manufacturers have utilized this theory. Most notably is SAAB who now has implemented their LPT (Low Pressure Turbo) setup on all of their cars. Audi too has traditionally run high compression on turbo cars. The sought after 5-cylinder 20v turbo has a compression of 9.3:1 and runs 9psi from the factory, 15psi on a chip. Why all this mumbo jumbo about other manufacturers? The point is, don't be afraid to try it. If you have an upgraded intercooler and stay within the maximum efficiency range of your turbo and use premium high-octane fuel then it's well worth it on the street. You'll never win a dyno contest, but I've seen well-built cars over 100hp shy some dyno queens, and still kill them in the 1/4 mile. More power doesn't always mean faster. Think outside the box. Fun factor of a well-built car and the added fuel mileage is well worth it.

Oh well, just thought I'd share, I'm done with my ranting now. :p

Anyone who's done a turbo swap onto an NA engine or run higher compression on your turbo engine, what's you opinion?
 

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not sure I'm following your calculations correctly. Stock Hg is 1.2mm when crushed - giving 8.4:1 CR. with .010" milled, a 1.5mm MHG ought to be damn close to stock CR
 

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Are there any "known heights" on the head that we could measure up against to determine how much had been milled off the bottom?
 

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Discussion Starter · #4 · (Edited)
ma71supraturbo said:
not sure I'm following your calculations correctly. Stock Hg is 1.2mm when crushed - giving 8.4:1 CR. with .010" milled, a 1.5mm MHG ought to be damn close to stock CR
That's a popular farce. Stock HG is 1.5mm not 1.2 as many seem to think. This was confirmed by checking factory Toyota Replacements, talking with a Fel-Pro engineer, information provided though a machinist’s engine measurement database and measuring the engine itself. The stock 7M-GTE head gasket is defiantly 1.5mm in thickness. Actually, my calculations come up with 1.47mm, but I said 1.5 just because it's really damn close.
 

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Discussion Starter · #5 ·
Adam W said:
Are there any "known heights" on the head that we could measure up against to determine how much had been milled off the bottom?
Yup, I was gonna post that, but I don't have the info here with me now. I'll get it and post it soon.
 

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sammydafish said:
That's a popular farce. Stock HG is 1.5mm not 1.2 as many seem to think. This was confirmed by checking factory Toyota Replacements, talking with a Fel-Pro engineer, and information was provided though a machinist’s engine measurement database and measuring the engine itself. The stock 7M-GTE head gasket is defiantly 1.5mm in thickness. Actually, my calculations come up with 1.47mm, but I said 1.5 just because it's really damn close.
The thickness of the stock gasket in the box is worthless. It crushes to 1.2mm
 

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Discussion Starter · #7 ·
ma71supraturbo said:
The thickness of the stock gasket in the box is worthless. It crushes to 1.2mm
Thickness of the gasket in the box is nearly 1.6mm. Do you honestly think I'd write a post like this and not know something like that? I've been doing this a long time and spend half my day working with engineers and well respected engine builders. I shared some research I've been doing for the past week or two with the rest of the Supra community, please don't turn this into a flame war. Trust me, I know what I'm doing.
 

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you very well may be correct, but IMHO you carry the burden of proof as it will be very difficult to convince me that Reg Reimer is wrong. And a .3mm difference is substantial enough that it would have been noticed before.

Instead, I'm guessing you CC'd your heads and then calculated the HG thickness you would have needed to get to the stock 8.4:1 compression ratio. This is fine assuming you bought the car new and know for a fact the head had never been touched, but 90% of the heads out there have been machined at least once
 

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Simmer down Jeff... Thank you Sammy for this excellent information.

One thing I'd like to point out... A thicker head gasket will tend to decrease quench, for very much the same though opposite reason mentioned by Sammy that the wider gasket will increase quench. A thicker gasket will increase the distance from the piston to the head, allowing for more complete combustion of the fuel / air charge. This slightly increases effeciency, but greatly increases combustion chamber temperature (as it heatsoaks).

The best setup on street turbo cars is moderate compression (9ish) with a piston and headgasket setup for lots of quench. This will give you the thermodynamic efficiency of high compression, with the detonation resistance of high quench.

-B
 

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I didn't intend to come of harshly, but I do express my skepticism regarding a rather significant change in data regarding a part which has been looked into countless times in the past. The information that has been circulating within the Supras Owners Group for the last eight years could, in fact, be incorrect -- but I doubt it.
 

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Heh - if your bored and want to do another calculation...

7M-GE Compression w/ 10~ thou taken off and 1.5 mm GReddy HG: ?
7M-GE Compression w/ 17~ thou taken off and 2.0 mm GReddy HG: ?


Thanks
BK
 

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Brysta89 said:
Heh - if your bored and want to do another calculation...

7M-GE Compression w/ 10~ thou taken off and 1.5 mm GReddy HG: ?
7M-GE Compression w/ 17~ thou taken off and 2.0 mm GReddy HG: ?


Thanks
BK
First one is very slightly lower than stock (.3mm = .0118") 8.4:1 (7mgte)

Second one is lower than stock (.8mm = .0314" only took off .017") 8.2:1 (7mgte)

For the 7mge they are: 9.2:1 and 8.9:1


these calculations are assuming 1.2mm is the stock crushed height
 

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If we assume stock is 1.5mm, then your 7mge would have:

9.4:1 for the first one, and 9.1:1 for the second one


So this .3mm discrepency boils down to a .2:1 change in compression ratio. Not a huge deal for your average build, but significant if you are planning on pushing the limits
 

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I emailed Reg R and got an interesting response. .95mm crushed :scratch:


Actually its closer to .95mm.

Hope this help.

Reg



(this is the email I sent):
I have always been told the stock HG is 1.2mm when crushed. A member of
Supraforums recently did some calculating and came up with 1.47mm crushed. He
then talked to the Fel Pro engineers and they confirmed their gaskets are 1.5mm
(crushed). So I'm now curious where you got your information, and whether you
think a .3mm difference is possible (I would assume such a relatively large
difference would have been noticed by now).


Thanks,
Jeff Gore
 

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Excuse me, i have a question maybe a bit out of topic ..
Do you know how much over all milling is possible before the valves touch the pistons in case of a broken timing belt ?
Same head gasket thickness of couse ( 1.2 mm HKS ) !

Grizzly from Germany
 
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