Increasing Compression Ratio

[DefeXion]

Has anyone thought about increasing the compression ratio on an STR engine before? I want to try, but the engine doesn't really make things easy like a nissan does.

I was wondering if there is any room for bigger domed pistons in the cylinder or is it possible to rais the heads with spacers of some sort. And also, if there are any after market pistons that will already work.

[Nick Mann]

Raising the heads will reduce the compression ratio.

There can't be much space as the engine is an interference type, but how much there is I don't know. I don't remember anyone talking about this before, so you could be on new ground!

[Turbo_Steve]

I've not heard of this either!
For all the effort, and expense, wouldn't it be easier & comparable price to supercharge it?
OR even buy a VR-4? (sorry!)

Much as I admire the sentiment of improving what's there, unless you're after massive gains, it's rarely fiscally prudent to open the engine....bolting bits on is generally cheaper.

[Louis]

Apart from skimming the heads, ( which may be a prob due to the tight clearances there anyway)
I would say the other option like you mentioned would be a raised area on the piston crown, but again you would need to be careful due to clearances.
I used to have a merc, and I had the head skimmed when I got it and it was fine, got it done again about 60,000 miles later, and got a tapping noise which I thought was tappet settings, but it turned out it was the vale tapping the top of the piston. Lasted about 10.000m then the big and on that piston went. (I'm guessing due to the constant tapping.
Remember, there is always a way, good luck

[bradc]

It would be easier to just buy a VR-4 :) Probably cheaper too

[DefeXion]

cost... insurance ? ahaha.

I just want to learn as much as I can with this car, and there's plenty to learn about NA engines aswell as turbo engines. For now, I'm just trying little things here and there and feeling their effects (with some guidance of course).

I was talking to a friend about his nissan pulsar which he has increased to 12.5:1 compression and it goes like the clappers. In his the piston heads are flat, so he got domed piston heads with a cutaway for the valves.

If I'm in new territory then it's going to be too costly anyway (confound the anti bolt onness of the mitsi). I still like finding out about things like this which is why I ask the questions :D.

[Kenneth]

Another thing you can do (or get someone professional to do) is change the shape of the combustion chamber.
This reduces (by welding in extra material) the area available in the compression chamber and hence the compression ratio increases.

[DefeXion]

hey, now that doesn't sound like a shabby idea. What do your reckon that would cost?

might need to buy a second hand engine if I want to try this. I'm considering revisiting the dohc na's at kempys. They are only 1000 dollars after all for a pretty low km's engine. Coilpacks FTW.

[Kenneth]

I have no idea sorry, never had reason to get it done... look up cylinder head reconditoners in the yellow pages and ring around.

[bradc]

seriously, I would suggest getting one of the dohc engines from Kempy's, they will give you lots of parts to experiment with, including a new intake manifold :)

We also have no idea what the compression ratio of those engines is, or what the power rating is.

Another idea would be to get a 6A13 DOHC Mivec engine with 12.5:1 compression. Dave at RPW in Perth sells them

[Turbo_Steve]

Sounds expensive?

[Gowf]

seriously, I would suggest getting one of the dohc engines from Kempy's, they will give you lots of parts to experiment with, including a new intake manifold :)

We also have no idea what the compression ratio of those engines is, or what the power rating is.

Another idea would be to get a 6A13 DOHC Mivec engine with 12.5:1 compression. Dave at RPW in Perth sells them


I must admit i do like the Mivec's....you always have the option of twin turboing it with the vr4 manifolds and tub's. I know of 1 that has just been done, but just waiting to hear the results.
But i do seem to agree with the general consensus that if you want a quick galant, buy a vr4.

[Turbo_Steve]

I think he's just toying with the idea of a quickER Galant, though :D

The biggest bang-for-buck you're going to get is going to be a supercharger (no intercooler needed if you're only looking for a mild increase...25-50bhp) and / or a set of cams.

Cams are going to be your best bet, really, as they CAN be had ultra cheap, and will give a modest improvement in power, and probably better pickup in the midrange (depending on what you choose).

Guys, anyone know what cams would fit? (not seen any 6A13s in bits yet)
Just wondering if FTO cams would fit? And be any good? GTO cams, maybe?
Thinking in terms of cost vs benefit as the challenge seems to be keeping the price less than buying a VR-4 :D

[Eurospec]

6A12 (FTO)cams will fit in it from a GR (not a mivec- they wont) but they wont give you anything.

You can get cams cut though but not cheap.

As for the compression ratio thing, yes raising the cr on a non turbo car will make more power. It would also on a turbo car too, but you have to be careful of knock. You will run into it pretty fast on a turbo car.

The gains you would get would not be massive, and about the only ways i can think of to do it would be to either skim the heads quite harshly. Asside from the physical clearance issue, you get into problems with physical deck strenght (ie the head will start to twist) and the fact that the cambelt will now be too long, meaning you wont get the cam timing on, though you might with vernier pulleys. The other way would be to go with higer deck pistons (now you are taking the top and bottom end to bits) which would possibly give better results.

i really wouldnt do a domed piston. I'd pretty much guarantee that with the basically pent roof chambers a domed piston will push the mixture away from the plug and into what should be the squish areas and a loss of power would result.

Hope that helps,

Cheers,

Ben.

[peter thomson]

A long time ago when I had a sohc 2.3 engine modified it was the block that was skimmed to raise the compression up to 12.5:1 .Not sure what was done to the head but at the time it was described as a stage 5. Polished ported and bigger valves.