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De Compression Plates
- Thread starter Ridgeback
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minichapperz
Active Member
never seen or used them, ive had to use a shim on an engine when the head had to have a fair amount skimmed off but tht was a tiny amount
If they're made of a good metal and to a good tolerance then they would work at least in the sense of provide a safer way than just doubling your gaskets. But you'll still need two Head gaskets, one either side of it. Which also changes the effect of the bolt rundown strategy.
Where it gets messy is the things like head bolts. You'd reduce the thread engagement if you used the same bolts, which obviously how much will depend on the thickness of the plate. Finding the right bolts to the same spec but a couple mm longer would be impossible, meaning you'd need them specially made. God knows how much that'd cost if anyone would even do it for you. You could washer up under the heads (possibly) but doubtful in the tight package of a cylinder head, as well as adding to the bodge job factor.
I can bet you will go through gaskets more often the thicker the plate that is used as well. Plus real combustion will be poorer, worse emissions which is possible to throw out your MOT result.
Serious ways of reducing compression ratio are:-
1) New pistons with different crowns
2) Shorter con rods
3) Cylinder head with different fireface and more combustion volume.
If I could personally choose for my own job for instance, i'd use one of those 3.
Anything else, the jury's always out.
Where it gets messy is the things like head bolts. You'd reduce the thread engagement if you used the same bolts, which obviously how much will depend on the thickness of the plate. Finding the right bolts to the same spec but a couple mm longer would be impossible, meaning you'd need them specially made. God knows how much that'd cost if anyone would even do it for you. You could washer up under the heads (possibly) but doubtful in the tight package of a cylinder head, as well as adding to the bodge job factor.
I can bet you will go through gaskets more often the thicker the plate that is used as well. Plus real combustion will be poorer, worse emissions which is possible to throw out your MOT result.
Serious ways of reducing compression ratio are:-
1) New pistons with different crowns
2) Shorter con rods
3) Cylinder head with different fireface and more combustion volume.
If I could personally choose for my own job for instance, i'd use one of those 3.
Anything else, the jury's always out.
Last edited by a moderator:
Ridgeback
Member
The head bolts wouldn't be a problem or material for the plate, as my cousin Phil,is one of the top gearbox R&D guys at mclaren,so I'm sure he'd knock something up for me 
As for the head gaskets, I'd read that you use an ordinary gasket at the block\spacer joint and a non setting sealent at the spacer\head.
I know what you mean about the best way being the low compression piston route,but I thought I'd see if anyone had heard any real horror stories.
Reading up on the drag forums,some actually prefer the plates,as they will go before any real damage is done to expensive internals,yet others steer well clear :|
As for the head gaskets, I'd read that you use an ordinary gasket at the block\spacer joint and a non setting sealent at the spacer\head.
I know what you mean about the best way being the low compression piston route,but I thought I'd see if anyone had heard any real horror stories.
Reading up on the drag forums,some actually prefer the plates,as they will go before any real damage is done to expensive internals,yet others steer well clear :|
If you've got a man on the inside that always helps. But those head bolts will have to be very specific, so if you don't get it right you will run yourself into more trouble than the job was worth.
The sealant type job i'm not so sure. All the seals have to hold combustion pressure - which on a good turbo will be 140bar easily. And it's at a constant running surface temp of around 250-300°C.
As for the plates going before the engine internals, I'm not sure exactly what type of failures they'd be talking about, I'd have to see the evidence and some data.
If you don't get closer to which side you're sure about, I'd say you should err on the side of caution and those that say no for a reason, and steer clear. I wouldn't do it personally - but this is your motor if you think the bases are covered safely feel free to try it out. If it's going to be a daily runner type deal are you really going to be comfortable with that setup for long term motoring??
The sealant type job i'm not so sure. All the seals have to hold combustion pressure - which on a good turbo will be 140bar easily. And it's at a constant running surface temp of around 250-300°C.
As for the plates going before the engine internals, I'm not sure exactly what type of failures they'd be talking about, I'd have to see the evidence and some data.
If you don't get closer to which side you're sure about, I'd say you should err on the side of caution and those that say no for a reason, and steer clear. I wouldn't do it personally - but this is your motor if you think the bases are covered safely feel free to try it out. If it's going to be a daily runner type deal are you really going to be comfortable with that setup for long term motoring??
Ridgeback
Member
The decomp plates are old tech which were used in the states,on big block supercharged engines,kicking out massive HP.
Sorry,that was the application I was looking at,not the Turbo route.
The thinking behind running them on v expensive engines,was that the seal going on the plate,would be a tell tale sign that something was amiss, before $$'s worth of damage was done to the internals.This would be a budget build on a budget engine,hence not wanting to part with huge amounts on forged pistons,rods etc.Not looking at running big amounts of boost,so will figure something out with the pulley sizes.
Sorry,that was the application I was looking at,not the Turbo route.
The thinking behind running them on v expensive engines,was that the seal going on the plate,would be a tell tale sign that something was amiss, before $$'s worth of damage was done to the internals.This would be a budget build on a budget engine,hence not wanting to part with huge amounts on forged pistons,rods etc.Not looking at running big amounts of boost,so will figure something out with the pulley sizes.
