An idea I had.........
Turbo Mini Forums

I was having a chat to my engineer mate about dropping compression ratios. I'm actually a big fan of the decompression plate idea, I think if you match the bores of the plate to the piston bowls, you retain squish. That's a good thing.......

Anyway, we discussed about the snags of using shorter conrods/height pistons, or shortening the centres and had a moan about how the top ring takes a load of abuse..... not good. Then I had an idea.

It sounds daft and would take a fair bit of machining, but isn't impossible. The catch is installing the pistons, but I've thought of that as well. Get your blocked bored right out to the max, let's go 73.5mm, what you need to do then is get your boring man to take it out a tad more, to the top of your pistons. You then need a bloody good lathe with a fair old chuck on it and machine up some spigots to fit in the holes. These spigots would be a pain in the arse to fit the pistons through, so you can fit them with a strip of shim steel around the edge and the usual compressor. So, once your pistons are fitted, and well down the bores, you need to have your spigots in place. The beauty of this is that instead of being rectangular in section, you would fit them inside the chuck so that you could machine a U in the side, so if cut in half, they would look like this |( )| which would give you a bigger chamber still. Here is the catch. You would then need to get the whole thing decked flat, you could do this yourself with great care and difficulty, probably taking a load of time as well. I reckon it's a good idea.

I don't propose to do it myself as I have done basically bugger all to my mini in ages. I can't seem to be arsed Got the bits, just can't find the motivation to put it together again.

Pros.

It would allow you to run cheap short compression height 73.5mm pistons (for a long stroke job) with shallow bowls. This would need no bowl machining and retain the full strength of the piston. It would also allow you to get "that overskimmed full race head" which is worth nothing and fit that sucker, instead of mucking around with sculpting out chambers and buggering the shape up, piercing the water jacket etc. It would allow you some head gasket reliability. It would allow you to run a decent compression ratio. The rings should not break up if made out of good shizzle

Cons.

A machining difficulty. You would have to make everything within zillionths of an inch and then leave put your spigots in the freezer and your block outside on the coldest night of the year and install them quickly!!!!! Piston installation and getting the deck flat... think this involves some grinding paste and a big piece of glass and then grease in the bores to stop the grinding paste going in the bores.

People do a lot of buggering about with the mini and I reckon that this is a good idea in the world of forced induction. I'm not sure what the actual figures you would use but it seems like a good idea even for people who are using small capacity heads (eg BMW) with big engines to minimise the CR to get rid of the need to have a spangly crank and get the power by heading for 12000rpm.

What do you think.

Good idea or shut up you Tw@?

Bugger off, I'm getting there.

A better picture might help, because I havent a clue what you are describing lol.

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

I'll have some of what you are on! Hegemonkey.
Regards
dave

decomp plates are shite IME

AlexF

I agree with alex, 1 of 2 gaskets to blow.

Back once again like a renegade master

in the first bit where you refer to using the decompression plate to basically build out the chamber shape rather than extra boree height, i think this would be disasterous as there would be un sandwiched material would would probablt melt and shag your engine

"Turbo's make torque, and torque makes fun"

"did you know you can toast potato waffles?"

I fail to see the point. Within reason any compression ratio you may want to use is achieveable with parts/methods already available.

I have to agree with Tom. With modern engine management just around the corner for the A series and good ignition control already here people should be looking at higher CRs.

There is absolutely no need for an CR less than 8:1 now, no matter how much boost your running.

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

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

It all sound like a lot of fing around to end up with an engine with a lot of non standard parts to cause issues later on....

I have been looking into getting a low comp ratio as cheap as possible on the 7 port as it only has 24cc chambers, and eventualy i'd like to take the capacity out to getting on for 1500cc,

the one solution I keep coming back to considering the colossal piston speeds involved is a decent set of custom pistons with a short crown pin tocrown and a BIG dish from Ross in the US,

It does the job properly, by keeping the squish, give the piston better geometry, is a hell of a lot less arsing about and won't cost the earth in time or money.

Edited by Joe C on 6th Oct, 2007.

As folks say above....with the cost of machining, decent pistons with required dish and a good head would be far less. Reliablility with the proven knowledge around would be far better.
Its still good to have ideas though m8....and some make it to the engine.