Squish Gap

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I'm just starting to research building a racing engine. This is based on a 70s - 80s VW 8V watercooled (Rabbit, Scirocco, Golf etc.) engine. The class rules specify an 11:1 CR and an unmodified combustion chamber so the basic design is pretty fixed. A flat top piston with the right head milling will give approximately the correct CR.

The stock combustion chamber with a flat top piston will give a pretty big squish area - maybe 70% of it opposite the spark plug and 30 "behind" the spark plug.

Finally to my question - what would be a reasonable "squish gap" - ie clearance between piston top and head face? And should it be uniform or tighter opposite the spark plug?

My previous race engine building experience was with unlimited CR engines with very short stroke and more open wedge chambers where we were fighting for CR and got everything as close as we dared. But that's not the case here - there should be no problem getting the max allowed CR.

I can "visualize" - ie gut feel - that possibly there is such a thing as too much squish from a turbulence point of view. Also I would think that a very tight squish would produce some pumping losses.

Obviously the optimum would be discovered empirically on the dyno. And more time and $$ than I've got.

Anybody got an opinion or any rules of thumb as a good starting point?

Thanks!

 

[turbododge]

The standard rule that I have heard, for iron block V8 engines, steel rods, moderate rpm is .040 inches. More if you rev very high, have aluminum rods. Less if you don't rev very high, have tight piston to bore clearances. Above .045 you start to lose effectiveness, less than .035 it can get dangerous to the engine. I have never heard of anyone complaining about having too much squish (too little gap) as long as they didn't actually kiss the heads.

 

[patprimmer]

On a Golf moror, you should be able to run tighter than a typical American V8, but I don't how much. About 0.032" comes to mind, but that is an unsubstantiated vauge memory.

I have never heard of not enough squish either, and all my best engines showed some signs of soft contact between pistons and heads, like maybe just a zero clearance touch during an over rev.

Regards
pat

 

[stix]

This1 Is Slightly Confused & Way Out Of Field Here But Wonders, Wasn’t “Squish” Used 2 Promote Mixture Homogenosity (Is This A Real Word?) By Means Of Turbulence. The More Turbulence, The More Homogenous The Mixture, The Bttr The Flame Front, The Bttr The Overall Extraction Of Heat From The Mixture & Reduced Losses Caused By Detonation. That “B”ing Said, The Aim Would B 2 Increase “Squish” 2 The Point Of Absolute Mixture Homogenosity. Depending On The Flow Conditions Prior 2 Spark, This Could Mean Either An Expanded Or Contracted Area. As Far As Pumping Losses Go, Compression Ratio Has Far More Influence On Losses Than “Squish”. This1 Also Wonders If The Point Is Theoretical Or Practical, As The Question Was Put Forth “should it be uniform or tighter opposite the spark plug?”. The Answer Would Depend On Both Placement Of Sparkplug & Intake Port. Some Elementary Guesses Could B Made By Inspection Of Design, More From Some Fluid Modeling Of The Design & Finally As Put “On The Dyno”. This1 Also Wonders If There MayB Changes Made 2 Sparkplug Angle That Might Not Alter The Chamber & Wonders Further If There Might B Changes 2 The Valve Cant Allowed That Would Not Alter The Chamber. If In Fact Such Changes R Allowed, Then Would Have 2 Go w/ Both. Either Or, Valve Cant As Sparkplugs Can B Indexed 2 Compensate, The Point After All Is 2 Ensure Mixture Consistency. The Other Constabulation On My Part Was The Use Of The Term “Squish” 4 What Is Called Deck 2 Piston Clearancing, A Zero Deck Height w/ The Piston “B”ing The Ideal w/ The Thinnest Gasket Possible. Perhaps This1 Should Have Read That As Valve 2 Piston Clearance, In Which Case TurboDodge Is Very Close 2 The Mark. Not 2 Discount PatPrimmer, This1 Assumes The Rods R Allowed 2 B What The Designer Has In Mind. Further This1 Notes That Answer 2 The “Squish” Problem In Both Function & Form May B Addressed By The Piston Face If Other Than A Flat Top Is Involved. My Apologies If This Post Offends Any1

 

[strokersix]

stix,

The technical content of your post is not offensive at all but it is difficult to read.

 

[patprimmer]

Not at all offensive, but dam near impossible to read I thought.

The valves canot be canted as the specification is a standard combustion chamber.

Turbulence generally, but not always helps to improve the homogenious nature of the mix, and in some instances it can detract from it by causing seperation of the heavy fuel from the light air dur to centrifugal force sending the fuel to the outside.

Squish also helps control detonation and propogates flame travel by greatly increasing turbulence just after ignition.

Squish or quench or whatever you call it only realy works by the piston rapidly displaceing gasses towards the combustion chamber as the piston gets very close to the head, hence the need to go as close possible without causing actual damage from impact.

Regards
pat

 

[dynodan]

Regarding squish area usually the rule of thumb is 50 % of bore area as a good starting point. I've seen where more than this, especially with a tight flat squish zone can lead to very high squish velocity which in turn can cause top land damage. This can be reduced by using a small taper in the squish if the area can't be changed.

Regards,
Dan

 

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So it looks like the consensus view is:

1. Make the squish gap as tight as possible w/o mechanical damage from contact.

2. Make the squish area as large as possible except do not exceed ~50% of bore area.

The VW head has vertical valves. Basic chamber layout is much like a wedge chamber V8 except that the valves are vertical and it is not a crossflow.

So does everyone agree that I want the piston tight to the head "behind" the spark plug as well as opposite the spark plug?

Thanks!

 

[patprimmer]

Yes. Keep it as tight as possible.

You wont be able to get 50% squish area with a high performance, big valve head, especially if the chambers are opened up to unshroud the valves.

I would expect that land damage is more the result of distance from the bore to the open area of the chamber, as the damage will be a result of pressure and temperature build up as the gas is forced to flow through the small gap between the piston and the head. The longer and tighter the flow path, the more pressure it takes to move the gas in the time avaliable.

The Golf 2V motor is quite a small bore as I recall

Regards
pat