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Triumph TR6 - Does anybody know... much I can shave off a '74 head (7.75/1 CR) to bring it to a reasonable CR - 8.5, at least ?? that will still run on today's fuel, and how much metal is there to shave on that year head. TIA Peter
Peter G

Hey Peter I believe that your engine is 7.5:1, so if anyone can provide the data for this engine that would be great.

Would turning up the compression to 9.5:1 give a whole host of new problems? is there enough clearence between the valves and pistons. Also the gas sucks here in Canada as the oil companies continue to add MMT and the octane range is 87 reg - 91 prem so I don't want tons of pinging.

If I put in a hotter cam (fast road) would it be better to keep the compression as is (7.5:1) or should the head be shaved as well.

1969-1971 8.5:1
1972-1973 7.75:1
1974-1976 7.5:1

TR6 PI 9.5:1

Peter-According to Kastner's comp manual, the early 8.5 cr heads are 3.460". This is measured from the valve cover surface to the combustion chamber face. Reducing the head to 3.400" will produce about 9.5, slightly more if the block has been surfaced. I think any of the hotter cams would need at least a 9.5 CR.

Why not just tell the machinist you want to achieve XYZ compression ratio and let the expert figure it out?

My 73 head measures about 3.373 cross section. The invoice from the machine shop reads "surface head .160"

My static pressure is around 190psi. She makes a pretty good bang and doesn't ping.


John Parfitt

So John, what compression ratio did you ask for at the machine shop seeing that you use the same crappy gas as I do and you get a good bang with lots of static pressure.

Thanks for your input. My head is currently 3.550 - 3.460 means I need to remove at least.090. Sounds as if there is plenty of material to cut w/o worrying about breaking into waterways.At least we have real gasoline here, even if the best is 92 - I grew up with 101 octane and 10.1 CR as normal (in England) 35 years ago.Dream on! Peter
Peter G

The standard UK-spec head was 9.5. Its common here to go to 10.5. As far as I can see the UK heads were made by skimming ythe fatter USA spec heads. But the squish on low compression heads is poor because the piston stops rising well below the squish area (its the flat platform opposite the vlaves covering about one third of the combustion chamber). Improved squish will help reduce detonation risk, but 91 octane seems rather low- in UK we still have 95. But I gather you use a mean between RON and MON octane measurements whereas we use RON, so a direct comparison becomes confused. If your 91 octane is closer to our 95 then 9.5 should easily be feasible, maybe more.
Of course you could stay with the 7.5 and fit a blower, then you'd really feel the imporovement.
Peter Cobbold

All, but Peter in particular,
After 27 years, how on Earth can you be sure that no one has skimmed a bit off the head before? Measuring the skirt of the head is not reliable - do you know what the thickness was originally? Your head may already have a CR higher than 7.5, so that choosing a standard height to skim off could easily give you a LOT more compression than you want. With the paraffin you guys have to use, that would be no fun.

I would ALWAYS start by measuring the volume of the combustion chamber. It's not difficult, just fiddly, and needs a bit of easily aquired kit. Lots of books describe the process (Eg How to build modify & tune cylinder heads. Burgess & Gollan, Pub.Veloce SpeedPro Series), or go to

Do it right, and Good Luck!

Thanks for the interest and info John D, I checked out the site you suggested and it gave me plenty to chew on - I do do want to do it right. Peter
Peter G

You need to know four things to calculate compression ratio.

1) combustion chamber volume
2) bore swept volume
3) deck height (any unswept bore volume)
4) installed head gasket thickness

Compression gets you power in a normally aspirated engine. Cams, port work, induction systems, headers healp it breath. With typical US pump gas, I would not recommend going above 9.5 to 1. Anything over that and you need to pay attention to engine sounds and consider octane boosters or racing fuel if your engine isn't happy.

I need a person to help complete a TR6 1971 project.
I am two thirds complete but need an experianced person to finish the project.Warm garage,lots of tea and coffee and terms to be arranged.Would suit retired/semi-retired.
Telephone David Mayes 01603 4086780 Norwich
David Mayes

This thread was discussed between 14/12/2002 and 20/01/2003

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