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Triumph TR6 - Squish? - exactly what is it

Hello all

I am interested in changing the compression ratio and the expression 'squish' comes up in conversation from time to time.

It involves the distance between the top of the piston at TDC and the top of the block and the subsequent effect on compression ratio.

It seems to be very important to engineers who prepare serious race engines and less important to those of us who tinker around uprating bits and pieces. Unfortunately, I haven't been able to fully comprehend the concept or effects of too much or too little squish(!) (something to do with the sun on my ever-receding hairline?) and therefore can't decide how important it is to me in the engine modification.

If some-one could help with a technical explanation in 'non-technical terms', it would be a great help.

Thanks.
Roger H

Roger--"Squish" is actually the "squish area" formed between the piston top and head bottom surface at TDC. The design of this area is important in gradually increasing combustion pressure and burning velocity, thus improving anti-knock quality and combustion efficiency. Squish area designs are evaluated by comparing the fuel-air flow (g/s) at various intake valve lifts and also by the rate of heat released as a function of crank angle. Some (most?) modern engines (e.g., Toyota) use a "taper squish" area to maximize flow velocity and combustion efficiency. In this design, the top circumference of the piston is angled to match a corresponding relief cut in the head.

Rick O.
Rick Orthen

Forgot to add that "squish" would be important to racers since the best squish area design would minimize detonation (knocking) at higher CR's. In other words, a better squish allows higher CR's.

Rick O.
Rick Orthen

Thanks Rick for the reply.

I guess that this means that in the '6', it is the head design that determines the squish area(?) and its efficiency. If so, it would obviously be a major exercise to modify this in a TR6 head and I'm not thinking that big!

This whole question has come about after a visit to an engineer who races Lotus and specialises in Cosworth engines. He is also on the visit list for some Group A and WRC teams when they are over here in the west. I am definitely not that serious!!! - I just wanted to skim a few thou off the head to increase the compression.

He suggested that the 'amount of squish' was more important than purely a reduction of the volume of the combustion chamber in the head (ie by skimming the head). He also said that at the very least, the pistons must be at least level with the top of the block at TDC. (Apparently this can vary from engine to engine, piston to piston, and every thou below the top of the block is a reduction in power!)
He then went on to say that, dependent on head gasket design/type and valve clearances, he would suggest decking the block to a point where the pistons protrude above the block at TDC!! - a frightening concept for me with an ever decreasing confidence in my basic knowledge of engines.

I had hoped that getting the head skimmed may have been enough to give me an increase in compression and I didn't want to take the whole engine out but now I'm beginning to wonder.

Have you (or anyone else) had any experience with this concept? - I know the TR is a bit of an old banger compared to the high tech buzz-boxes around now but I would still like to get her somewhere near her potential.

Regards
Roger

Roger H

Roger--There are many folks running with skimmed heads (I'm not one of them though). Perhaps one will speak up about their experiences. What you didn't mention were the mods to your valve train/rocker gear that would also be required after a head shave. Depending on the amount of material removed, you're looking at shortened pushrods and/or shimmed rocker pedestals.

Yes, properly squished cylinders have the piston traveling above the cylinder deck into the head. But you're not talking flat top pistons here; most likely saddle-shaped.

Rick O.
Rick Orthen

Hi Rick

Thanks for your interest. The necessary top end work is not too much of a problem. At the moment the head has quite a bit of work done to it and I'm looking at a higher lift cam so valve to piston clearances do have to be looked at.

My understanding of the basic engine differences between cars produced for the US (carb) market and the UK (injection) models is the cam and the compression ratio (ie the head). I believe all heads for the 2.5 litre Triumph 'sixes' are fundamentally the same (porting and valve sizes aside) so the increase in compression for the PI models must be due to less combustion volume in the head.

I have heard of 10.5:1 or 11:1 being able to be run (given the right fuel) - I use 96 RON so I'm aiming between 9:1 and 9.5:1.

I will be getting the work done soon - if anything interesting arises I'll re-post.

Thanks for your explanation of 'squish' - it has helped!!

Regards
Roger

PS I was told that piston to valve clearances should not be too small as at high temperatures, the con-rods and pushrods lengthen and bring them even closer together - something I would prefer not to experience!!
Roger H

When I redid my engine the head was shaved making the push rods too long. Turned out with the amount of work the machine shop had to do. I thought it best that they reassemble the engine for warranty purposes. At any rate instead of using shorter pushrods, they used pushrod ball adjusters from a MGB. From what they told me this is a common practice. I've had no problems.

Doug
Doug Campbell

That's a good thought Doug - I don't know much about MGB's but I'll check it out - Thanks
Roger H

Skimmed .090 thou from my head to bring compression back into the realms of reality (8.5 to 1 - The fuel here is so sh*tty you cant go much higher) and did nothing but adjust the valves with whatever was already there. Everything works fine. Peter
Peter Gooch

Peter
Perhaps .090 thou is right on the edge of where one would need to make some changes. My head was shaved .120 thou and there was no way the stock setup would work.

Doug
Doug Campbell

This thread was discussed between 17/07/2003 and 29/07/2003

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