Piston Slap: TR4 Compression Depression...Or Not?

Sajeev Mehta
by Sajeev Mehta
piston slap tr4 compression depression or not

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.

Sajeev answers:

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 mehta@ttac.com)

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3 of 36 comments
  • Matt Matt on Apr 29, 2010

    Maybe I'm taking the completely wrong approach here, but: 14.696 psi a * 9.5 = 139.6 psia Assuming no huge pressure difference from atmospheric (reasonable given the lack of forced induction), why are we all guessing what it should be? Shouldn't it be 140 psi, assuming his 9.5:1 was correct? Or am I way off base?

    • Eric Bryant Eric Bryant on Apr 30, 2010

      When calculating cranking pressure, you need to start with dynamic compression ratio (not the static ratio). Then, raise it to the power of 1.2-1.3 to account for the energy added to the compressed charge (varies somewhat depending on the cranking speed of the engine and a bunch of other thermodynamic factors), and then multiply by atmospheric pressure (14.7 psi nominal) to get the absolute cranking pressure. Subtract one atmosphere to get gauge cranking pressure. If you take the engine I mentioned above with a calculated DCR of 9.5 and use the above method, you'll get a calculated gauge cranking pressure of 204.3 PSI if you use an exponent of 1.2. Guess what I see when performing a compression test? About 200-205 PSI.

  • JimC JimC on Apr 29, 2010

    Matt, the air gets hot when you compress it very suddenly and this raises the psi even more (if you want to make your head explode then read up about thermodynamics and fluid mechanics, otherwise press your personal "I believe..." button and go with it). I just looked up the spec on my long-gone twin carb 1986cc Volvo engine and it was 156-185psi (9.3:1 stock). It isn't clear whether the letter writer is in the UK, the US, or elsewhere. If he is in the US, before I would let anyone touch the carburetors (they look like Zenith-Strombergs, not all that different from SUs) I would seek advice from the nearest British car club on finding a trusted mechanic. These carbs can work great if they are properly maintained, but their greatest flaw is that even the most worn-out, ill-maintained example will provide either barely enough or not quite too much fuel to allow the engine to start and (barely) run. For starters they are very sensitive to throttle shaft bushing leaks (then the usual suspects of worn needles and jets, dirty fuel, leaky oil dashpots). Bubba the American V8 hotrodder is the first guy I'd get to fix anything Holley, Carter, Edlebrock, etc. and the probably last guy I'd get to touch anything SU or Stromberg. Hope this helps!

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