How tight can the squish go...?

OK chaps, in the past I’ve kinda put a self imposed minimum limit on my squish band clearance of .030”. Although I’ve been as tight as .028” before, with alloy rods, revving to 8,000 and not had contact.

I’m now planning to run steel rods with lightweight pistons (less stretch), and alloy barrel (should expand and increase the gap when hot), and have a Maney crank in Maney cases (should flex less?).

It’s a 920 with an 89mm stroke, so I’m unlikely to rev it beyond 7,000rpm (I assume).

With all that in mind, how tight can I go with the squish?

How tight have people running a similar spec gone?

Try this.

https://www.rbracing-rsr.com/squishcalc1.html

Mine makes max power just over 7000 and I know Martin used to take his to 8000 when needed.

Not sure I’d trust that Ralph!

Do you know what squish you’re running?

Just been in the garage to read my notes but didn't write that down (bit slack) I seem to remember quoting it somewhere and you mentioned I could have gone tighter but I cannot find that either.

Jim did ask me how I was going to start my bike when I ordered the gapless rings so a compromise might be wise re the squish because of the resultant compression.

The recommendation I got years ago from C.R. Axtell for standard stroke 750 race engines with stock crank and rods in iron cylinders was .040". I've stuck with that for over 40 years now. I've run them as tight as .035" with no problems. But rebuilt an 872 cc race engine for a friend, that had broken at the crankcase. It was running a Nourish one-piece crankshaft, titanium rods, and an iron cylinder. It had been built at the insistence of the owner (Martin Adams of Commonwealth Racing) with .020" squish, against the recommendation of the builder (Johnson Motors, I think). He thought that with stiffer crank and Ti rods it wouldn't flex and stretch as much as stock, so the tighter squish would be good. He was wrong. The pistons had very clearly been hitting the head at high rpm, and had essentially ripped the top of the crankcase off. I rebuilt it for the owner, who again insisted on .020" squish. But I ignored that, and set it at .040". The engine ran great, and after it ran successfully at Daytona, I told Martin what I had done, and he was so happy that it finally ran well that he didn't mind.

So, bottom line for my experience with Commandos is that .040" works and .020" doesn't. According to several race engine builders, once you get the squish below something like .050", making it tighter doesn't gain any real improvement in turbulence, so the only reason to go below .040" would be for higher compression ratio.

I have noticed signs in the cylinder head that even at .040" the piston is very close to the head, but only in bigger bore engines (920 cc) with very loose piston-to-cylinder clearances (.009" - .010"). In those cases, I think the piston was just rocking so much that squish clearance was marginal.

Other folks may have had other experiences, and for sure other opinions. This is just mine.

Ken

Thanks Ken, you may just have already answered the questions I was gonna ask next...!

The first measures I’ve taken are giving me a squish of .026” which is tighter than I’d like, especially after your post!

With that squish, it is looking like 10.4:1, slightly lower than I was shooting for (I think the stage 3 big valves must alter the combustion chamber volume somewhat) but so long as it’s above 10:1 I’m happy.

The next thickest head gasket is another .020” and that’s too much to add.

I can use a .010” thinner head gasket and add a .021” copper base gasket which will add .011” to the overall height, giving me .037” squish without too much lowering of the CR.

Or I could use the .020” thicker head gasket and skim some off of the head. You can only cut the metal off once though ...!

Steel rods and short pistons in the 1360 @ .040" squish and no sign of contact in there. No carbon in the squish area so it seems to be working ok.
Rev limited to 6500.
Different engine than the Commando, I realize. It has a much narrower, stiffer crank than a Commando.
Terry Prince said aim for .045" to 050" but do not go below .040".

Same as Ken he claimed no power advantage below .050" squish.
Others claim there is, but I wonder if it's just that normal male inclination to always go too far with a good thing....until ...oops

Glen

Fast Eddie said:

OK chaps, in the past I’ve kinda put a self imposed minimum limit on my squish band clearance of .030”. Although I’ve been as tight as .028” before, with alloy rods, revving to 8,000 and not had contact.

I’m now planning to run steel rods with lightweight pistons (less stretch), and alloy barrel (should expand and increase the gap when hot), and have a Maney crank in Maney cases (should flex less?).

It’s a 920 with an 89mm stroke, so I’m unlikely to rev it beyond 7,000rpm (I assume).

With all that in mind, how tight can I go with the squish?

How tight have people running a similar spec gone?

Click to expand...

Tighter squish is better but there's a diminishing return after you have achieved the adequate quench to suppress detonation.

After meeting adequate quench to suppress detonation, what is at stake is additional turbulence generation and displacing as much air fuel mixture closer to the flame kernel at the spark plug(s). Depending upon the squish area, the volume displaced becomes somewhat trivial and there are additional pumping losses as the piston reaches TDC.

If you really want to get things tight snotzo suggested putting a few machinist pop marks on the crowns of the pistons in the squish area, measure their heights and then run the motor, pull down and remeasure to see if they were kissing the head and by how much. Taking the "how much is left" measurement gives you a value you work with to get the clearance down to a gnats ass.

With the custom pistons I (NYCNorton) had made for the Nourish, clearance is down to around 20 something thousandths of an inch with steel rods. The motor is so happy!

As for the Steve Maney crank flexing less, flexure is a function of stress and the modulus of the material. There's practically no difference in modulus between the factory crank material and the material Steve used. Steel is steel as far as modulus goes. So as for stress, thus flex, I don't think the cross section of the stressed members of a Steve Maney crank are much different so I would expect crank flex to remain about the same as stock.

Thanks chaps, I think you confirmed what I already thought deep down: not to go too tight on the squish, and the .026” I have currently is too tight.

I’ll shuffle with the gaskets, I should be able to get to .037-ish and keep the CR above 10.1 me thinks.

Reduce clearance until it goes bang, then open it out.

Fast Eddie said:

Thanks chaps, I think you confirmed what I already thought deep down: not to go too tight on the squish, and the .026” I have currently is too tight.

Click to expand...

Maybe.

If you are willing to muck about with it a bit to dial it in then do so but if you want to just button it up and run it then I would have a look at setting clearance to what is generally accepted practices.

Once you suppress detonation, what is left is squeezing residual A/F mixture into a useful part of the combustion chamber. Take a look at your quench area and multiply by the clearance to get volume and compare to the nominal combustion chamber volume. Express it as a percentage point and you will see.

Is there an ideal percentage to aim for?

Glen

worntorn said:

Is there an ideal percentage to aim for?

Glen

Click to expand...

Ideal squish area as a percentage of swept area, yes. Ideal percentage volume, I don't know. The expression as a percent of volume is to illustrate the diminishing returns.