The V8 Owners Forum -

Hi,

a new brainstorming

In our stock RV8 we have an quench distance of 0.065" (0.040" piston deck heigh clearance + 0.025" head gasket thickness).

Our RV8 has been designed to run whith dished piston and hemispherical head combustion chamber.

In theory if I use flat top piston I increase quench arera which itself increase pression rise which itself modifies burning rate.

But if the rate of pression rise is too high it will be done with the detriment of power.

So any recommandation with flat top piston ? Which is the maximum piston deck heigh clearance ?

Thanks.

In order to any of this to work correctly you will need to be careful or you could end up with less power!

The problem with a quench or squish band that is too large (i.e to big a gap) is that the fresh charge that is trapped in this area does not burn until well after TDC by which time it is too late for it to help in the process of creating useful pressure above the piston. The too large a gap basically does not move the fresh charge into the combustion chamber and stops the flame front from getting at the trapped charge. In this instance the engine would be better off with no squish band at all so the dished pistons actually help the standard RV8 because the charge gets exposed to the flame front. The lack of any decent sized squish band does mean that the CR is limited to a lower figure in order to prevent the 'end gases' detonating.

From my experience of messing about with bike engines and from what I've read the piston needs to be at most 1mm from the head but this depends on the size of the pistons, conrod strength, bearing clearance and the RPM that the engine must operate at. (The clearance could end up being less when the engine is running) Get the gap too small and you will blow the motor! How much smaller this gap gets on an RV8...who knows?

My guess would be to go for a squish band of around 1.0-1.2mm (Which will reduce at high RPM) and then hollow out each chamber to give a CR of around 10.5:1. I guess the timing could be pulled back a bit because the rate of combustion should be a bit higher.

Valve clearance would need looking at too!

All just my humble!

(Good question BTW)

Dimitri,

Hemi head? have you grafted on some interesting heads or is it top secret?

Also do you mean a pure Hemi or a modern 4 valve head configuration?

Regards,

Kevin.

Kevin,

no secret here, it just by comparaison to olds 215 which used a true wedge combustion chamber instead semi-hemispherical combustion chamber of buick 215/RV8. The combustion chamber in Buick 215/Rover is halfway between a wedge shaped head and a hemispherical head.
The combustion chamber is matched to the dished piston leaving an area around the rim for good squish effects and the spark plug is placed in the middle to give short flame travel.

Olds because they used flat top piston chosen wedge combustion chamber with more centerline valves.

With my combo I have at this time 0.040" piston deck heigh clearance and I would like to rate my pressure rise to +/-30lb/PSI/degree of crank rotation.

In practise if my block need to deck heigh surfaced according porting job and head combustion chamber shape.

quench and compression, Im running rather a lot of compression with flat tops, a tin gasket will give me 14.1:1 and a composite will give about 13.6:1,
thats with 36cc heads, this is very similar to my old 3.9 engine, and produced about 290hp on normal fuel
in my case, the way this works is a Grp A cam which is a stock lift cam, but runs a lot of duration and overlap, this bleeds off the compression and allows a more acceptable of around 11:1

Dimitri,

Out of interest do you know the cc. of the Olds chamber?

Kevin.