6 replies to this topic

[Royalasaforce]

So basically as the title suggests do you need the decompression plate when going forced induction.
I've got a 998 ATM (soon too be a 1275 in the future) pocketed block and 12g940 head with the chambers enlargened to 23.2cc each and by doing so my compression is 110psi across the board +- a psi here or there. ( I can't remember the number before the head change but I remember they wasn't that consistent).
I know that the psi isn't as high as it should be but I'm thinking that could be down to the head work lowering the compression so could not fitting the plate raise the compression back to acceptable levels? If I pop a piston ring it won't be the end of the world as it'll motivate me to rebuild the 1275 😂. Also no vacuum advance if that means anything

[DeadSquare]

60 years ago, we just fitted the supercharger and upgraded to premium fuel.

 

For the last 30 years, I have always fitted a decompression plate since melting a piston.

[Lplus]

So basically as the title suggests do you need the decompression plate when going forced induction.
I've got a 998 ATM (soon too be a 1275 in the future) pocketed block and 12g940 head with the chambers enlargened to 23.2cc each and by doing so my compression is 110psi across the board +- a psi here or there. ( I can't remember the number before the head change but I remember they wasn't that consistent).
I know that the psi isn't as high as it should be but I'm thinking that could be down to the head work lowering the compression so could not fitting the plate raise the compression back to acceptable levels? If I pop a piston ring it won't be the end of the world as it'll motivate me to rebuild the 1275 . Also no vacuum advance if that means anything

The standard 998 head has a chamber volume of 24.6 cc,  so the chamber volume is still less than standard.  The pockets will obviously have a volume too.  It also depends on what the C/R of the engine was as standard.

 

A decompression plate will lower the compression plate further instead of bringing it back to acceptable levels.  Static compression tests won't really tell you the compression ratio anyway, you need to calculate the static compression ratio taking all the various volumes into account.  What the static compression ratio should be for a supercharged engine might be the sort of thing the developer would know.

[Shooter63]

The problem with decompression plates is that you loose squish, which can then lead to early onset of detonation, if possible try and open up the chamber volume or dish the pistons to bring the compression down to where you want it.

Shooter

[Spider]

As shooter has mentioned, if you need to de-compress it, find another way. Fitting plates is a last resort.

[Icey]

It’s hard enough to keep one gasket together with boost, two is, well, doubling the problem!

[Earwax]

My thoughts for what they worth ( adjusting for current market conditions - of course.- a joke)

 

Your current guesstimate needs to be tightened up in regards to CCing the head -  but 110psi across all 4 cylinders would lead me to try and do leak down tests. My reasoning is 110psi is notionally already below say 7.5  or 8.0 CR. ( my guesstimates now - so they also could do with checking....)  On current petrol/gas with Static CR of about 8 to 10 0r so I have seen figures for cylinder pressures about 140 -190psi for road cars. in reasonable nick. below that normally points to pressure losses ( hence the leak down test) but also usually are only in 1 or two cylinders

 

Regarding a compression plate - again the width and extra ccs reducing overall CR need to be factored in with some degree of accuracy.  ( I would say the same for pulley/ boost )

 

Personally , i would start with better numbers than those you are showing - the grunt you are planning to add will absolutely find its way to the weakest bits ( so a motor refresh at the very least and then throw some pistons with adequate cc to reduce CR as required RATHER than the compression plate    OR  just start the 1275 build with this is mind. or just build a NA 1275 with a medium short diff ratio    Good luck whatever you decide 

 

For my curiosity mainly - to Deadsquare and others who have run SC in the past /old school--  did you run Dizzy with Boost /timing changes  and what about Carby set up .  Nowdays - it seems the Direct injection is almost always hand in hand.