By on April 28, 2010

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.

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36 Comments on “Piston Slap: TR4 Compression Depression…Or Not?...”

  • avatar
    Da Coyote

    Darn, reminds me of my TR4 IRS. Fun to drive, although typically British maintenance required,

    It’s a shame the Brits let this all go.

  • avatar
    Paul Niedermeyer

    I’m hardly an expert on these engines, but I tend to not see a problem here. Just make sure you’re not getting pre-detonation (ping, etc.). Use premium gas, and check your ignition timing carefully. And enjoy. I’m not totally sure I agree with Sajeev’s comment about needing more CFMs from the carbs. That usually comes into play with a hotter cam and the resulting higher rev band, but I could be wrong. Enjoy!

    • 0 avatar

      Agree 100%. The carb tuning info was only for the just-in-case scenario.

    • 0 avatar
      Uncle Mellow

      “Use premium gas”
      Can you still buy premium gas ? When I started driving in 60’s London , super premium was 101 (which is what a TR would be run on ), premium was 99 and 95 octane was rubbish for old crocks. These days 95 is all that is available where I live , but 97 is”called” premium in places where you can buy it.
      I agree the S U carbs should not need adjustment. This model has a wonderful bulge in the bonnet to clear those S U ‘s.

  • avatar

    Look, an old saab engine!

    Aren’t there some after-market knock sensors you can install (with a led warning?)

    • 0 avatar

      No, the TR2 thru TR6 used pushrod engines. The early SAAB 99s used an OHC four produced by Triumph but it was more like the TR7 engine, or one half of a Stag engine.

  • avatar

    190 is a little high for 9.5:1 ….I would think 160-170 is more like it. However, so long as it runs fine without knocking…go with it. Use good fuel!

    I think my 14:1 A series is around 235.

    • 0 avatar

      The 1976 F-150 with a 360FE I used to own had 158-162 PSI in all 8 cylinders, and the stock compression ratio was somewhere around 8.6:1 if memory serves. And this was in an engine with 170,000 miles on it at the time! Why did I sell it again?

  • avatar

    the bump in compression isn’t a big deal. but a proper air fuel ratio and ignition timing will keep you from burning up your new valves and pistons.

  • avatar

    My 1963 TR-4 had some valve work done, after which it pinged a bit, even on 105 octane from a blend-it-yourself Sunoco pump, with factory-spec timing. I suspect that the shop had possibly shaved the head a bit to stop a coolant leak they (might have) caused. They didn’t say what all had been done or why, just that a couple of exhaust valves were replaced. I still have one of the old ones as a souvenir. My car had SU carbs with metal screen air cleaners that kept out birds, bugs, and small children, and little else.

  • avatar

    190 psi is wonderful. Enjoy the beautiful car!

  • avatar

    Can’t help you directly, but here’s some info that may point you toward a better guideline or answer in future.

    In the Bentley manual, Ch.3 p.11, the PSI range for a Brand New 1.8L 8v 10:1CR ’85 VW GLI motor, code HT, is 145-189psi.

    Best of Luck!

  • avatar

    Awesome looking car, I wish mine looked like that!

    If you are just concerned with the 190 number, quit worrying about nothing. The exact number depends on the gauge and the special weak springed schrader valve in it. That’s why most manufacturers say to go by the relative readings of each cylinder, not the absolute value.

    If you have no pinging/knocking/pinking with the standard 4 degrees BTDC static timing, no problem. Even if you do, I would consider retarding the timing a little before something as drastic as a thicker head gasket. Your Lucas distributor has a convenient “micrometer” scale; each hash mark is 4 degrees and there is an “A” and an “R” to indicate advance and retard.

    • 0 avatar

      As for the pinging, I had an experience with an old Mopar slant 6 that might translate to another tractor engine. In warm weather, I would add as much extra gasket under the carb as the manifold bolts would allow (about 3/8 to 1/2 inch on my car). The result was that although the timing spec was 0 btdc, I could advance the timing to 12 degrees btdc and get no pinging at all, along with better accelleration and higher fuel mileage. Also, hot starts improved dramatically. The only downside was a longer carb warmup cycle (I would revert to stock in the winter).
      So if that great compression gives just a bit of a ping, I wonder if some extra thickness in the carb gaskets will solve your problem. It would certainly be cheap and easy to find out.

  • avatar

    I dont think you have a problem either. Pinging could be corrected with timing adjustments and fuel mixture adjustments, possibly a cooler thermostat.

  • avatar

    Use the guage on another car and if you get a high reading there then it’s the gauge. Frankly as long as the reading is consistent accross cylinders I would not worry about and go for a nice drive.

  • avatar

    To address your problem, most of the advice you have been given is sound.

    As Sajeev mentioned, if you really want to dial things in you need to see a tuner that has a WBO2 unit and some basic datalogging capabilities.

    Make sure they use the Innovate WBO2, it’s as accurate as the old OEM standard Horiba unit. The other consumer/shop level units are just not as good as the Innovate.

    They can readily map your fuel and spark, and identify if there are adjustment issues or if there is a mechanical issue.

    As for thick copper head gaskets, they are a rigjob, to put it generously. There are times when they are necessary because all else has failed, and you need to make it run now, or you just can’t source a fresh head cost effectively. But they are non-optimal even in a non-force-inducted scenario. But for 10:1 comp, you’ll likely get away with it just fine.

    Though I haven’t any connects at RUF, I seriously doubt they are currently using thick copper gaskets to lower compression. No serious tuner does that I’ve ever met. Perhaps they did 20 years ago, but that was known to be a work around, even back in the day. With state-of-the-shelf in engine controls and tuning tools, nobody with any racing cred does that anymore. It screws with the squish area, and is simply not very strong. Especially in a force-inducted scenario.

    • 0 avatar

      RUF uses thicker gaskets to make a 911 Carrera into a supercharged 911 Carrera. Unfortunately I might be wrong on the copper part, but I *specifically* asked how they lower compression and head gaskets was the answer.

    • 0 avatar

      i would have thought a shorter piston and connecting rod.

    • 0 avatar


      Who the F told you that? Not that I don’t believe you, just that I want to run it around the community so we can laugh our collective asses off at him/her. Pa-F’n-thetic.

      From Bosch to Corky to Bob lowering comp with a head gasket is the mark of an amateur who is 20 years behind the curve.

      I can get custom Mahle slugs run for well under $200/per. What kinda joker is gonna half-ass an engine by running a thicker gasket? (Which is more likely to blow, and f’s up the squish?)

  • avatar

    After the TR6 CC, I went googling to find out more about TRs in general. You should check out this site:

    Click on the link for discussion forums,

    because the correct thickness head gasket allows the liners to poke out of the block by .004 inches, which is needed.

    Plus there are all sorts of links to myriad TR clubs and people apparently willing to help.

    I was fascinated by TeriAnn Wakeman’s web pages on her TR3. Every Triumph service bulletin is available in .pdf format by year. She also became an expert on fitting Weber carbs to a the TR engine, and I mean EXPERT. Kas Kastner, an old Triumph racing hand, has complimented her on her cylinder head page. This is serious stuff, folks.

    Anyway, you may be familiar with all this stuff, but in case you aren’t, a few pleasant hours of surfing awaits you.

    I’d read some of this info which is germane to your engine before following any general advice given here. The cylinder head is a weak point on TRs if skimmed, apparently, and may end up thermally cracking.

  • avatar

    Relative compression is the right answer. There are so many factors that go into compression readings (the gauge… wet/dry/cold/hot…)… at the same compression ratio, different engines will have different compression readings due to bumpsticks. I lost 30 psi of compression due to my new cams, thanks to what feels like three hundred sixty degrees of overlap.

    190 psi sounds like a nice, round number… not really problematic at all… for some modern small fours at around 9.5:1, rebuilds are suggested only after compression drops to below 130-160.

    If you’re concerned, I’d go with the advice given regarding the carb and timing.

  • avatar


    Man you post a lot of stupid crap on this forum. When I upgraded the turbos on the S4 i had I basically gave the very reputable tuner a blank check and told them to do whatever was necessary to get the best and most reliable performance including rebuilding the engine for lower compression. Instead of selling me the expensive engine rebuild they sold/installed a head gasket set to lower compression. Even if i went with even larger turbos they told me the only uprade i would need is rods and they would still use headgaskets and stock pistons to lower compression. This tuner campaigns several successful racecars in a national series so I htink they know what they are talking about.

    • 0 avatar

      That’s a rather personal attack on a fellow poster. A turbocharged injected S4 is a much different animal than a carbureted TR4.

      The question here isn’t really how to lower compression, it’s whether the compression should be lowered. If 190 psi is the actual number, that’s great. Now all he has to do is prevent pinging via carb and timing adjustments.

      However, “wmba”s comments on thermal issues are worth investigating.

    • 0 avatar

      If you’d like me introduce you around at the next PRI (Performance Racing Industry) show just let me know. You can meet and speak with dozens of engineers from F1, IRL, NASCAR, and SAE. As well as Bosch and host of top engine builders. If Pankl is there, I can introduce you to the president of the company that cast your block. Also some Porsche engineers (who will heap scorn on the 911, after enough beer to make them not fear retribution…).

      Sincerely, if what your tuner did for you works for you, great! If I ever have time to attend any 928 things, perhaps we can hang out.

      I’m just relating the knowledge that I have garnered from 25 years of my working with exotic cars and, more importantly, actively searching out those on the cutting edge of tech who are producing the best results.

      Perhaps read one of Corky Bell’s books, or call Bob Norwood (he’s got over 100 world records and tunes for Ferrari). They may provide for you a more credible source for the same wisdom that I share.

      When I tune, I use the same optimization that the top-of-the-hill uses. There’s plenty of Lingrenades out there, doesn’t mean they know their ass from a hole-in-the-ground. Just means they can/need to market and are pretty good at it. And/or they drive well.

      BTW – Thanks Gslippy. I do appreciate the support, but I have learned not to fear scorn. I’ve been dumpin’ on carbs and Rootes blowers since the 70s. Wanna know how many people used to think I was effen nuts?

      PPS- I’d at least MegaSquirt the TR6. I can make look very original except to the trained eye.

    • 0 avatar

      “I’ve been dumpin’ on carbs and Rootes blowers since the 70s. ”

      Well there’s a thing, whilst they made some lovely Sunbeam, Hillman, Singer, Talbot, Commer, Karrier and Humber branded cars and trucks in their time, I’m pretty sure ROOTES didn’t dabble in making superchargers. Some of their Commer trucks, most notably those powered by the ‘Knocker’ TS3 three cylinder – six pistoned diesel used a ROOTS type supercharger though.

      “If Pankl is there, I can introduce you to the president of the company that cast your block.” Pankl? Surely you mean Eisenwerke bruhl?

    • 0 avatar


      “Rootes” blower is merely a style of forced-induction – as is Lysohlm, or turbo, or centrifugal turbo, or axial flow. Whatever “Rootes”, the company, does not matter. It’s merely a generic type of forced-induction.

      As to ‘Eisenwerke’ do tell? Far as I know from Keith Allende and those inside Porsche, they cast the 928 blocks. If you have contrary info, I’m intersted…

      BTW- Right, wrong, or indifferent, “Rootes” is the de facto nomenclature here in The States. Just sayin’….

    • 0 avatar

      “gave a blank check”

      Well right.

      Keep posting Porschespeed. Aleays learn a lot from you! Keep up the good work.

    • 0 avatar


      Thanks! I don’t post links, or massive SAE docs, or my bonafides, for a reason – I want the reader to do their own homework and make up their own mind.

      I could care less if those who don’t bother to do even basic research hate me. But, if one person who actually cares does the work and finds out what is really out there, I’m a happy man.

  • avatar
    Eric Bryant

    25 replies and no mention of dynamic compression ratio (DCR)? Damn. Anyways, static compression ratio (what you calculate from swept volume, chamber volume, and so on) is only one factor in determining the dynamic compression ratio and the cranking pressure that you’ll observe during a compression test.

    Probably the most significant contributor to this is the intake valve closing event, so any changes in lobe duration, lobe center, or advance will change the dynamic compression ratio. There are also other factors, such as connecting rod length, which are somewhat less intuitive but affect the outcome (in this case, by changing the dwell time at TDC and BDC, and the velocity of the piston as it moves off those two points).

    I built the LT1 in my Impala SS with a static compression ratio of 11.7:1, but it uses a relatively long-duration camshaft and thus the DCR is about 9.5:1. This is still a bit higher than the 9:1 rule of thumb for premium pump fuel, but the LT1 has reverse-flow cooling which maintains a lower combustion chamber temperature (I also have aluminum heads, which further contribute to this effect). The LT1 also has relatively precise control of the ignition events (at least for a distributor-based system), and so one can run a bit closer to the edge without worrying about spark scatter.

    • 0 avatar

      Eric Bryant,

      It would appear that you are an engineer (or a true motorhead) that knows how to program his TIVO. Congrats!

      The reality of your compression ratio is determined by one’s cam(s). The old ‘muscle cars’ had stupid high DCRs. Especially the ‘ringers’ that the manufacturers used to send to the magazines.(With acid-dipped bodies and frames and built blueprinted engines the public would never receive…).

    • 0 avatar
      Eric Bryant


      I am indeed an engineer (albeit of the electrical type), I can indeed program my TiVo (although my wife’s shows get higher priority in the Season Pass list), and I’m not half-bad at picking the right camshaft.

    • 0 avatar

      God love ya Eric!

  • avatar

    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?

    • 0 avatar
      Eric Bryant

      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.

  • avatar

    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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