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TR5/TR250 TR 250 Compression Ratio - is 10.4:1 too much?

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RobT

RobT

Jedi Warrior
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Hi Ya'all
Been a while since I posted here, but things have been pretty quiet for me on the Triumph front.

Over the winter I did get around to pulling the head on the TR250. It started off as just popping of the exhaust manifold for a tidy up, but noticed some old build-up from what looked like coolant seeping from the side of the head gasket. So off with his head!

All looks pretty clean and wear-free inside, so did some de-coking and lapped in the valves for reassembly. With the head off I checked the compression ratio using Richard Good's excellent guide: https://www.goodparts.com/tech_docs/TR6_Compression_Ratio.html

I measured both the overall thickness of the head, and the volume of the combustion chamber and get a compression ratio of 10:4:1 with both methods.

My question is: Is this too high? And if so, how should I drop it? Use two head gaskets?

Before the rebuild, I was getting occasional pinging, and using an octane booster generally helped, but wondering if, with regular 93 octane petrol, 10.4 is just too high a compression ratio for these engines. Has anyone else had experience running this high a ratio?

TIA,
Rob.
 
I took mine to 9.7. I believe when you get above 10.0, you should start considering adding racing fuel to the gas to bump up the octane.
But as mentioned above, add the gasket height. You may be okay.
 
What I have read generally agrees with what the others have already written, I had my head skimmed to get to about 9.5 to 1, no problems, don't have any actual experience with higher ratios.
 
BRSLimited said:
did you take into consideration the thickness of the head gasket? That makes a huge difference in the calculation.
Click to expand...

Yeppers -- added 4cc for the head gasket per Mr. Good's table (and checked -- the new one is .035").

So anyone running over 10:1 compression? Would adding a copper head gasket be O.K.?

Thanks guys

Rob.
 
I have (only) a copper gasket from Gasketworks on my TR4A. I used it to increase the compression a bit over stock (it is thinner than the compressed stock gasket).

You can order a variety of thickness:

Standard MATERIAL THICKNESS .021,.032, .043,.054,.063,.070,.086, .093, and .125"

I've had no problems with mine.

https://www.headgasket.com/index.html
 
If 10.4:1 is the CR when you include the gasket then this is too high for standard pump gas if everything else is stock. Your engine will ping. I had this problem in my Tr6. Mordy Dunst who runs Gasketworks can help out with a solid copper gasket. This is what I used to drop my CR down to about 9.7, which gives a good powerful engine that'll run on standard gas. The other alternative is a wacky camshaft which will drop the dynamic CR but this brings other problems associated with a race engine - narrow power band in a rev range too high for street use. Go with a copper gasket. They're expensive but less expensive than a new head.
David
 
Thanks David & others.

I think that is the route I will go. I did de-coke things and lap in the valves, so may try reassembling everything with a stock head gasket first, and see if I get any pinking, but if so, then switch to a copper gasket.

Thanks all for confirming my intuition.

Rob.
 
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