Ken Dowd writes:
I’ve got high compression readings of over 190 lbs on all cyls on my TR4. The head has been reworked twice that I know of, about .050 has been shaved. I got these readings after putting the engine back together with all new sleeves, pistons, rings and a head/valve job. I CCed. the head at 55cc and crunched the numbers on several online compression ratio calculators and figured my compression to be about 9.5:1 Compression ratio on the stock engine is 9:1. Would you expect to see such high compression readings with such a small increase in compression ratio?
I’ve searched the world over and cannot find a spec for compression on a healthy 2138cc TR4 engine. Do you have any experience with after market solid copper oversized (thicker) head gaskets to bring down compression? That is the only thing I can come up with. Your thoughts would be appreciated.
Some people would kill to have this problem. Because this isn’t a problem: TTAC’s award winning 24 Hours of LeMons team made it a point to bump up compression with hotter cylinder head on our ’72 Datsun 240Z. As long as the cooling system is up to snuff, this is a good thing.
An extra half-point of compression is easy to compensate for, netting more power in the process. Over 190 psi (assuming its still under 200 psi) of compression is fine for most motors, unless there’s a problem with this motor that isn’t common knowledge. But let’s assume everything’s kosher.
Assuming the TR4-spec timing curve is retained, you might need more CFM (Cubic Feet per Minute) from the carburetor to make your Hot Rodded TR4 play nice. Minor adjustments (or a high performance part) for your vintage carburetor will make it happen. I am sure a speed-shop can custom tune your carb with an A/F (Air/Fuel) gauge to ensure maximum power with a safe ratio of air and fuel. But I’d go to the drag strip on “Test ‘N Tune” days, bring your carb/distributor adjusting tools, and tweak to avoid detonation while searching for what tune gets you the fastest time slip.
What I’m trying to say is, you can have a lot of fun with this “problem” you uncovered.
But let’s revisit your concern, thicker head gaskets are a great way to lower compression. Everyone from backyard grease monkeys to the whiz kids at RUF do this so an engine is more accepting of forced induction. It’s definitely your Plan B if carburetor/ignition tuning doesn’t work out.
(Send your queries to email@example.com)