Decompression plate idea. What do you think.
Turbo Mini Forums

I gave a lot of thought to these things. I cut my own head (about 33cc) and it was a pain in the arse. So, looking for an idea on the cheap, I got excited about decompression plates. Everyone got excited about losing squish, so I reckoned it would be a good idea to match the plate bores with the bowls/chambers. I still didn't like using 2 head gaskets, that does strike me as a weak point and a bodge.

So, I had the idea, since getting these plates is relatively cheap, it would be easy to copper plate them in your shed. You should need virtually no copper as all you are doing is creating a good seal. This could be done in the kitchen sink using a bit of copper sulphate, battery acid and a bit of copper pipe. The surface would be 100% pure and of homogenous thickness.

I can't see sealing being a snag. But my mate was talking about ringing it properly which would make things a bit more tricky.

What do you reckon? Load of bollocks or a good idea? Reasons?

I have time on my hands again!

Bugger off, I'm getting there.

still a bodge IMO its the thickness of the plate whats the problem inorder to de compress

although the copper idea is good

Several points:

Would the copper layer be thick enough to act as a head gasket?

Also I'm guessing that you'd have to re-plate the plate every time you remove the head.

Finally there is still the age old issue of the length of the push rods that needs to be addressed.

ferriday engineering is all i have to say!

Do the job properly, or dont bother, imho

thats a good link j ,i like his style!
my experience with this is ,a) running twin head gaskets on my mini as described in the gt17 thread ,,not a success..b) running a stainless steel plate between two normal head gaskets on a mini this was totally reliable up to 15 psi .
c)running a 1.8 mm mild steel plate between two shim steel gaskets ,basically taking apart the multi layer steel gasket ,making a thicker centre plate ,and putting it back together with hylomar , totally reliable in my 500 twin bike engine ,for 10k miles ,up to 22 psi boost ,and around 135 bhp at its best

,the audi rs2 uses a spacer plate as stock .

regards robert

Edited by robert on 16th Nov, 2006.

Medusa + injection = too much torque for the dyno ..https://youtu.be/qg5o0_tJxYM

The Audi did indeed run such a plate but I think their hands were tied as originally they didn't have enough around the chamber to remove material to lower CR and the S2 was never thought of as a volume seller so they'd not have had a huge amount of development money for a new instalation.

It did work well enough but they are know as having head gasket problems, my local VW/Audi center is never without an S2 with it's bonnet up!

Having said that I have run them on a VW 16V motor when piston options have been limited and again no material to remove in the head area to lower CR adn that was ok for quite a while.

If you can do it with material removal that is best if not then try a de-comp plate but keep an eye on it!

Every day is a school day ...........

How fast and how expensive ...... the same question...

The only time I would personally consider a decomp plate would be when I was constrained by not being able to get enough volume by using pistons available or digging out the chambers.

In our A series turbo application the market is swarming with large dished pistons and suppliers that will supply 28-30-35cc cylinder heads, so I see no reason to use what is a bodge at best when the proper components are out there to be utilised.

the ferriday engineering gaskets have a sealing ring, a bit like a wills ring but it dosent need the head machining. I havent used one but they seem pretty good.

J

You mentioned that you cut your head to 33cc, what sort of CR are you looking for?

Mine is only 28cc with a CR of 8.3!!

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www.fusionfabs.co.uk



1/4mile in 13.2sec @ 111 terminal on 15psi

Tom is spot on here, I hate to admit

you've gone and done it now v

Yep Tom's right,

by the time you've pissed around sorting the decompression plate you may have well got the chambers hogged out in whoich case you'll have better squish.

I wanted to do a 1430 7 port turbo but with only 24cc in the chamber its not going to happen without either running the pistons down the bore or the equivalent of 3 head gaskets even with 18cc pistons... i'll not be going there....

Ok, What you are essentially doing is trying to make a head with a massive chamber which the piston sees a squish to. Instead of mining out a huge chunk and busting through water jackets, etc, a lazer cut plate is relatively cheap. Made out of gauge steel, accurate stuff IMO. The idea is that the block and head are flat but they aren't really, you need something to clamp up to which facilitates a good seal, which I reckon a nanometer of copper would do the job.

If I look at my head I scooped out, the squish was compromised a bit by "going for it", if you match the pistons withe the plate, there is no problem. Instead of seeing it as a sandwich, see it as a stick on part on the bottom of the head. Perhaps, if there are areas to consider, you could use a HG and also a copper side. All it has got to do is stick there. I suppose if you were worried and this IS a hassle, you could carefully weld the plate to the head (which might warp) to effect a good seal. I think epoxy is stable up to those temperatures. How about sticking it on? Scooping out chambers is a bodge if you ask me, it buggers up the ideal chamber shape. A decompression spacer, merely chucks it down a bit, you can keep your beak, shape around the exhausts, etc. I was thinking this as I have a 36-31 head on my mini with high compression (na) chambers. Rather than go to the hassle of getting a new head, porting it and scooping the chambers out, I'd rather pay £100 for a plate and bung it in, make it a bit thicker and leave my cast pistons the same. They're cool for 6500rpm. As for pushrods, I went on a mission to find these and I think Kent ones are about 1/2 longer. Failing that cutting some down isn't really a problem.

The 1430 idea is a good one. Perhaps even 1480, I'm convinced that low boost, high efficiency, large capacity is the way to go on the cheap.

My beautiful head got toasted in my shed fire, it took ages to do and the thought of picking up another grinder and doing it again is sick. I'd rather channel my energies in a different direction.

Bearing in mind I'm supply teaching at the mo, it's gone dead and I'm out of throwaround funds. Mini needs new tyres, pads, etc, etc, etc, before I think about putting the other engine back together. :(

Bugger off, I'm getting there.

Would have to be some serious glue ..... araldite rapid certainly would cut it with combustion temperatures!! LOL

The VW 16v I ran a decomp plate on had it "glued" to the top surface of the block, can't remember what adisive was supplied but I will try to find out for you.

IIRC it was very important to get both head and block decks ground first and to build and torque the engine whilst the adhesive was still curing. Care had to be taken to make sure that you didn't get adhesive into the bores as well.

Any joint / interface you have is a potential failure point and that is where the reliability concerns come in with decomp plates.

Every day is a school day ...........

How fast and how expensive ...... the same question...

how about just a sheet of copper? Sure it wont be as good as copper plating the head (that IS what you meant wasnt it? OR have I got the wrong end of the stick here?) But it would be cheap to make, it would be possible to re-use it if it was made properly, and would then in theory, still leave you with one proper head gasket, which would still be your safety barrier


thoughts on this one?

1) not properly doing the job of enlarged chambers

2) cheaper than grinding

3) leaves more meat in the head, possibly keeping it stiffer than a massively ground-out chamber

4) ground heads will inherently have less metal between chamber and water jacket, posibly leading to better cooling, this would keep that jacket quite thick



several pros and cons, but it all boils down to taste, and etiquette

"Computer games don't affect kids, I mean if Pac Man affected us as kids,
we'd all run around in a darkened room munching pills and listening to repetitive music."


Kristian Wilson, Nintendo Inc, 1989.