Squish Gap
Squish Gap
(OP)
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 $$ than I've got.
Anybody got an opinion or any rules of thumb as a good starting point?
Thanks!
The stock combustion chamber with a flat top piston will give a pretty big squish area - maybe 70% of it opposite the spark plug and 30 "behind" the spark plug.
Finally to my question - what would be a reasonable "squish gap" - ie clearance between piston top and head face? And should it be uniform or tighter opposite the spark plug?
My previous race engine building experience was with unlimited CR engines with very short stroke and more open wedge chambers where we were fighting for CR and got everything as close as we dared. But that's not the case here - there should be no problem getting the max allowed CR.
I can "visualize" - ie gut feel - that possibly there is such a thing as too much squish from a turbulence point of view. Also I would think that a very tight squish would produce some pumping losses.
Obviously the optimum would be discovered empirically on the dyno. And more time and $$ than I've got.
Anybody got an opinion or any rules of thumb as a good starting point?
Thanks!





RE: Squish Gap
RE: Squish Gap
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
RE: Squish Gap
RE: Squish Gap
The technical content of your post is not offensive at all but it is difficult to read.
RE: Squish Gap
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
RE: Squish Gap
Regards,
Dan
RE: Squish Gap
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!
RE: Squish Gap
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