Decompression plates
Turbo Mini Forums

Just found some at mike farriday engineering,also thick solid copper gaskets.

plates are used on vmax supercharged mini to lower cr

anygood??

Metro turbo weekend driver,Mini turbo in the making again!

as a last resort...

just mod your head and 8:1 is easy.

alex

AlexF

I've been thinking about this also. A lot of people have been grinding on about the importance of "Squish". I'm not that convinced how important it is on a regular blown engine, running comfortably. Certainly, it allows you to get an edge but must entail some serious setting up.

I figure it this way, and this is merely how I see it.

The aim of a tuner is to extract maximum torque from a given amount of fuel. To get the maximum combustion efficiency, it is necessary to employ the highest tolerable compression ratio, getting the most out of it by advancing the ignition to just before the point of detonation. What it leaves out is bumping clearances and how this influences the minimum gap.........

The point is, at high RPM, there are massive forces involved on the conrods, with a long stroke engine (1275, etc) this is made worse. At high revs where there is considerable flexing of the conrod. This can be in the order of about 1/2mm or perhaps a bit more. Since we are not generally using that much, it will be possible to run a thinner head gasket anyway. Too thin and too many revs and the clearance may cause the piston to strike the head. The point being that ideally the piston rises up the bore, compressing the gas, at a critical moment of maximum compression, the spark will fire it. Too much of a rise in mixture compression, too early will result in detonation too early and not the potential amount of power developed. So this has to be balanced against bumping clearance which is an unknown quantity, and can be assumed negligable at our RPM's.

So, if you imagine the graph of combustion space against cylinder volume, you can imagine that if the gap between the piston band and the head band (squish) is significant, a rise in compression will not occur so readily. So, what I think is that the gap becomes unimportant after the gap is opened up "a bit", I don't know how much because I don't have experience or a dyno. That is what the theory says.

So, following on from that, you may as well have a big shim to drop the compression ratio, or short compression height pistons. Achieving the same thing.

I'm not sure what sort of gaps we are talking here in real terms, it is also proportional to the density of the gas. (As boost pressure goes up, you can get away with a bigger clearance as the gas has increased inertia).{Again, to what level of significance, who knows}.

Bearing in mind, the sorts of gains got by minimising bumping clearances, maximising squish effect can be achieved by octane increaced advance liberating more power.

If Vmax are building big engines with steel shims, you can expect that he is maximising torque from "Big Potatoes" as the Bini blower is not happy at high RPM. Of course, if you can build a smaller, high boost engine, you can use the squish got from minimum bumping clearance, maximum piston/head "bands" and the most concentrated "dishes" and chambers you can afford.

From the Bini blower point of view, I actually think that a big capacity low boost engine is likely to be more efficient than a smaller engine with significant squish. Since the pistons are not available to do any tricks with, I guess shims give the best result.

But, the logical reply to my bollocks is "Shut up spouting shit and get out there and make one".....

With my build, I am trying to keep the squish high. Although I could probably quantify the whole significance of the thing, given dish/chamber sizes, but I can't be bothered. I'm too busy building one.

Bugger off, I'm getting there.

Rocker geometery is already lousy using aftermarket cams and roller rockers. Adding one of these plates makes it even worse...
Fine if you can source some pushrods around 1/4" longer than the A-series ones...

You won't be able to run near the point of det with large regions of "end gasses". They will just pop and destroy your ring lands.

Then theres the issue of running two head gaskets, not great for reliability. I suppose you could get round this using a very thick copper head gasket, but I still think the above reason is going to give you more problems.

Then as Dave has said the rocker geometry is also going to be seriously upset.

All in all why bother!!! You can easily take the head out to 28ccs and beyond with skill and care. So why use a very poor method of de-sompressing the engine that introduces all manner of problems.

Alex

AlexF

Depends, I heard you could skim the rocker pillars down a bit. Not sure how much though.

Looking from the point of view of rootes blowers, big capacities are a good idea, as are low compression ratios. If you look at the old stuff (inefficient blowers) they ran relaly low CR's. There isn't enough space in a "cheap" head or sometimes an expensive one.

I assume you have to use 2 gaskets, for the fire rings and "O" rings. So assuming the threat comes from having too much space over the rings (lack of squish). bearing in mind that the bottom gasket would give your bumping clearance, the top one being pretty much irrelevant, the piece of "guage steel" you would have to have laser cut (or bloody similar) could quite easily be cut bores undersize, thus getting the squish back. I'm not sure whether the gaskets are actually needed, but I assume the "Steel Gasket" sold by minispares is in fact a decompression shim. Using 2 head gaskets sounds a bit "African Land Rover" to me. Don't like the sound of it one bit.

What do you think?

Bugger off, I'm getting there.

You need a gasket either side of the shim...

2 gaskets on top of each other is very poor and gives you little CR drop.

Squish is any area covered by the piston that does not have an opposing area in the combustion chamber. The closer you can get the piston to the head the better your squish is.

Alex

AlexF

So if you made the hole in the plate the same size as the dish in the piston it would be sorted

Metro turbo weekend driver,Mini turbo in the making again!

I believe so.

The thing is, how do you seal these decompression plates? With head gaskets? Can you run them without. On ebay, someone was selling a turbo kit for something which had a steel gasket (decompression plate) that's what got me thinking about it. I can't see why it wouldn't work, I can't see it sealing properly. I suppose you could get the surfaces like a mirror and seal it with copper foil.

I'll ask my mate, he's the expert.

Bugger off, I'm getting there.

All this talk about squish is all well and good, but I've no squish in my head at all - not exactly a slouch either...
If you throw enough boost in there, it'll make power - maybe you lose 5hp, maybe a little more (who knows) - but the turbo will always compensate it - that's why we all love turbo's - us amateurs can build an engine for less than a grand that'll last longer, and make more power than a 10K miglia engine!!!

If you're aiming for chalengine miniwilliams, then sure, concentrate on the squish - but only after you've done everything else like perfecting a camshaft, sorting a good intercooler, and using a turbo capable of supplying efficient airflow at the boost and rpm levels needed for that kinda power...
For 99% of the guys on this site, little or no squish is not something to lose sleep over... A lousy valvetrain, no that is just wasting a LOT of power...

Stu, maching rockers does not help, when the prob is there on a stock roller rocker on an aftermarket cam - longer pushrods are needed! I've not found any UK pushrods that will do - but there must be soemthing close somewhere in the world!

Edited by turbodave16v on 19th Dec, 2004.

There's always the lathe and some bits of (quality) steel.

I'm using forged rockers.

I've temporarily run out of money at the moment and so the project has come to a grinding halt. Got loads to be getting on with though. The answer to a blow through Bini Supercharger is "Don't bother" I'm going to do it though but it is a BIG hassle. Don't have the space for a big intercooler. Rover TD should hopefully fit, but the dizzy has gone as has the oil filter!

Bugger off, I'm getting there.

I thought MED sold adjustable pushrods ?

Chris

No, it's going there come hell or high water.

The problem is that silly inlet port. Why the hell they had to cast it is beyond me. If I remember rightly from my trial fit, If I mount it with the port facing the engine, the piping for the air filter would foul everything or be hugely restrictive, that goes for mounting it the other way. So, the way to go is to mount the engine on shorter steadies, mount the blower sunny side up (inlet) and build myself a collector which points downwards and pipe that into an intercooler underneath. This allows me to keep the oil filter (which the other configuration fouls) also it allows more grille space where my intercooler/oil cooler/aux rad goes. It is very close (touches) to the slam panel so I might be able to lose a couple of mm off the casing (!) if that doesn't work (which it will) I will lose the bonnet catch and strap the thing shut.

My mate reckoned that we should just bracket the thing above the engine and bolt up the intercooler and then have bonnet bulges :) Fuck that.

I'm still wasting time with preparation and haven't really gone with a final design for the guts, but I'm thinking of going big and that will require a decompression plate. I can't see a problem with one in theory.

Bini blowers are REALLY poor performance wise. The only thing they have going for them is that they are cheap. 12PSI looks about the maximum it will sensibly blow, with a cooler.

Anyway, I'm getting there gradually.

Happy Christmas

Bugger off, I'm getting there.

jsut a quickie, incase you havnt considered it. and i could be wrong as its beena good while since i last handled a bini blower, but what about mounting it into the inner wing area? it might be a bit long, but might not!

Il try find it, but got an interesting pic of a bini blower atttached to a saxo. it runs on a driveshaft which is belt driven in teh usual manor, the shaft alows the blower to sit at teh other side of the engine. in the minis case this would be over the clutch housing and intruding slightly inot the inner wing on teh drivers side.

turbo 16v k-series 11.9@118.9 :)

Denis O'Brien.

They are gigantic.

I know a guy who was a BL ST development chappie, they had an 850 with a Cabin Blower off an aeroplane mounted in the passenger footwell. It got hot and it was loud but it worked.

I'm working on other things at the moment.

Bugger off, I'm getting there.

Metro turbo weekend driver,Mini turbo in the making again!

It would definately reduce piston to bore friction, and that's a good thing.