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TR4/4A Compression Ratio

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RJS

RJS

Jedi Warrior
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Hi

Mine is a '66 TR4A. Stock compression is 9.5:1. During a rebuild, the pistons were upsized from 86mm to 87mm. Also, I believe the cylinder head has been skimmed definitely once, maybe twice. Not sure how much though - forgot to take precise measurements when the head was off several years ago to replace a head gasket.

Any idea what my compression ratio may be? Higher, lower? Some have said increasing bore increases CR. And other say it reduces CR...?

Thanks

Bob
 
Per the manual, stock compression ratio is 9.0:1.

Technically, increasing the bore increases compression ratio, but the effect is minuscule. Shaving the head has a much stronger effect, but it's impossible to guess what your compression ratio is unless you know how much the head has been shaved.
 
Thanks all,

This is helpful and I will read these articles in more depth. But at least I now know my CR is somewhere slightly above stock 9.0:1 (and not lower than), and mostly determined by the head shaving.

Steve thanks for correcting me. I could have sworn I saw 9.5:1 somewhere.

Bob
 
If you used one of the currently available composition type head gaskets, your compression ratio is quite possibly lower than you think. They are quite a bit thicker than the original gaskets (they also seal up a lot better if the liner heights aren't perfect).

My experience is an 87mm engine that has had about .020 milled of the head over the years will yield about 9.1:1 with the currently available gaskets. Completely deshrouding the intake valve also increases chamber volume and will lower compression.
 
Herk said:
If you used one of the currently available composition type head gaskets, your compression ratio is quite possibly lower than you think. They are quite a bit thicker than the original gaskets (they also seal up a lot better if the liner heights aren't perfect).

My experience is an 87mm engine that has had about .020 milled of the head over the years will yield about 9.1:1 with the currently available gaskets. Completely deshrouding the intake valve also increases chamber volume and will lower compression.
Click to expand...
Interesting. I did go with a composite head gasket with silicone sealing strips. Worked a treat. Torqued down to 110 lbs-ft with ARP studs

Bob
 
It is not too hard to CC the chamber on your head , then you will be able to see the actual comp ratio, and act
accordingly. For daily use don't go too far past 10/1 should run fine on 89/92 octane.
Get a piece of clear plastic(plexiglass) 4x4in . Drill a small hole in the middle.A bit of grease to seal it to the head
and a hypodermic to measure in the fluid.
Mad dog
 
mctriumph said:
It is not too hard to CC the chamber on your head , then you will be able to see the actual comp ratio, and act
accordingly. For daily use don't go too far past 10/1 should run fine on 89/92 octane.
Get a piece of clear plastic(plexiglass) 4x4in . Drill a small hole in the middle.A bit of grease to seal it to the head
and a hypodermic to measure in the fluid.
Mad dog
Click to expand...
Good advice.
 
Although they're a little more hassle to install, the embossed steel shim (MOSS 694-505) will boost compression a bit and more importantly tighten up the quench height which helps with detonation resistance.
 
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