Squish height - what is this ?

[DamirGTI]

Hi Guy !

Please , can you explain me what is "squish" ? or squish height - what does this means in terms of engine tuning ?

Thanks !
Damir

[Guy Croft]

Damir, hi

the squish height is the small clearance between the opposing (flat) regions of the piston crown and flat region of the head; designed to give added compression and thus vortex-generated turbulence to the charge to mix it better and increase the burn rate.

'Swirl turbulence is the end result, sometimes called, in this case 'squish-induced swirl turbulence' to distinguish from port-induced swirl.

There is of course also huge turbulence during the compression stroke anyway, especially at high rpm, generated by the piston shearing the charge off the cylinder walls. This creates very high speed vortices that improve the charge mixing.

This may be worth reading:

http://guy-croft.com/viewtopic.php?t=97

If you use the search facility there is quite a lot written about those two phenomena as well. And before you ask 'do I need more?' I'll just say that as a rule the original manufacturer's design is probably as good as it gets (for the standard engine) and highly tuned competition engines don't need more than standard ones, they can manage with less. The more squish (or port swirl too) the higher the efficiency losses.

GC

[DamirGTI]

Hello Guy !

Ok. thanks ! , but is all this area on the pictures of the head and piston crown (painted with blue colour) squish area or not ? Is it necessary that these two squish areas on the piston crown and head must be of the same shape ?
What i mean is what will happen if i fit some other type of pistons with different crowns like reverse dome pistons instead of dished pistons .

As i understand this squish area also serves to push the air-fuel mixture from the cylinder walls towards the spark plug .

Thanks !
Damir

[Guy Croft]

Damir, hi

I thank you for posting those nice photos but I don't want to go into squish in any more depth than that which I have already posted. Sorry!

In order to analyse squish and its influence on burn and emissions - you need 'industry standard' methods, we don't have those kinds of things at hand, and as I already said, race engines don't need that much anyway, and too much is power loss. Copy the OE setup where possible and reduce it maybe, never increase it..

GC

[DamirGTI]

Hello !

Ok. no problem Guy !

Regards
Damir