Paul Mc

Have recently had a big end failure on my V4 STI Type R following a trackday. After speaking to various Subaru specialists I bought a low mileage complete V5 STI engine, had my heads skimmed and rebuilt and put them on the V5 block.

When the car was run up we found the compression far too low. We knew the V5 engine had a lower compression but didn't think it would have made that much difference, especially after having the heads skimmed.

My options now appear to be:

1. Rebuild my old V4 engine using new cosworth pistons.
2. Continue with the V5 engine (with my V4 heads) using higher compression pistons and running with my V4 turbo.

Would be grateful to hear any other opinions on this before deciding which route to follow. I'm not looking for a quick fix option before selling the car as I intend to keep the car for a while longer.

Paul Mc

Don't know what compression the new engine is running now it's fitted to my car, just know it's a late phase 2 engine (could even be a V6). When fitted with my (V4) heads the compression is way down and apparantly drove like a 200bhp car rather than 300bhp.

Bob Rawle

Ok so how do you know, sorry to keep asking, have you had the comprssion checked and what were the figures, there are a number of things that would cause a lack of performance.

Bob

Paul Mc

Yes the compression was checked, I wasn't there when it was done and the car is still with the mechanic so I don't have the figures myself. He did say it was down quite a bit, initially he thought it might have been a valve stuck open, this is why the compression was checked.

I have complete faith in my mechanic, who comes highly recommended, but wonder whether anyone else has come across this sort of thing before and trying to come to a decision as to which option would give me the best result in terms of performance and reliability.

R19KET

Paul,

Trying to be polite as possible, I suspect whoever rebuilt the engine hasn't a clue what he's doing.

Both the V4, and V5 have an 8:1 compression ratio, lke Bob says, BUT, the V4 uses a 1.4mm head gasket (IIRC) and the VF5 uses a 0.7mm head gasket. So I'd guess they've used V4 gaskets.

This will make the compression more like 7.4:1 (guessing, without working it out, and knowing how much was skimmed).

On top of this, you're still using the same ecu/map for 8:1 compression, so yes, it will run like a dog.

Choices: get a remappable ecu, and get someone like to remap it. This will greatly improve things, but's it's not ideal, running stock boost. Or, get the engine builder to replace the gaskets with the 0.7mm V5's, and then get a remappable ecu, and get someone like Bob to remap it, taking into account the higher compression.

First it would be worth confirming just how much they skimmed the heads, so that the true compression can be "estimated". Theoretically, it shouldn't have gone up much, maybe .1/.2:1, and remapped, it will work well.

Mark.

ustolemyname??stevieturbo

iTrader: (1)

Another thing that would be noticed when fitting such different head gaskets, is that it will be difficult to get the inlet manifold bolts to line up. Just a simple check when building would tell you it is wrong. After all, the difference in thickness is doubled, and one set of bolt holes will be out by nearly 1.5mm. That is a lot. Did whoever built the engine notice this happening?

R19KET

Steve,

It would only move the hole centres by less than 1mm, and the holes in the manifold tend to be a little larger anyway.

There are some "engine builders", who use the V4 gaskets on V5 engines as a matter of course. I've spoken to one company, who did it to a guys engine, after his V5 engine let go. This was done to lower the compression, and prevent it detting.

Firstly I pointed out that the car had a mappable ecu, so det wasn't an issue, and when I asked them what the new compression was, they didn't have a clue !!! When I suggested what it would be, and the effects, they just said they always did it on V5's !!! GREAT !!!

And these people are a well known company.

Mark.

ustolemyname??stevieturbo

iTrader: (1)

Well, I speak from experience.. Dont ask why I did it, but i did. I fitted the thin metal shim gaskets to an early engine, which would usually have the thick approx 1.5mm gaskets. ( difference here 2mm. )
I measured the thin metal ones at 0.5mm. The manifold bolts did not line up properly. And of course I built her up anyway ( am I stupid or what ) and not surprisingle the car went like sod all. And that tapping noise???? never did figure out what it was. The engine only started to rev freely above 6000 rpm. Not surprisingly the engine was quickly removed and refitted with the correct gaskets, No noises, and goin fine again

Paul Mc

Thanks for the replies so far.

Have spoken to garage today and apparantly my car is running 125 psi ( is this the correct term ?).

After having gone down the route of replacing the engine to the V5 and spending a lot of effort sourcing a good complete engine (not to mention the expense) he wants to now rebuild my V4 engine, which has knackered 3 pistons and 2 rods. I don't particulary want to go down this route, after having already run up a rather large bill fitting the new engine, if it will be possible to solve the problem currently being experienced with the V5 engine and my V4 heads and gaskets.

I have already spoken to an engine builder I know well (who specialies in building Subaru rally cars)who has adivsed me to fit a set of V4 cosworth pistons with my V4 heads to the V5 block but I'm not sure what gasket to fit (V4 or V5).

I'm not mechanically minded myself but I can see the sense in this option - although I take on board your option of an ECU remap but imagine this would be costly (any ideas?). Having already spent over 4K on engine and labour/other parts not really wanting to start from scratch again.

amelnikov

there may be a hidden benefit to lower compression. A lot of people with turbo cars lower their c.r. to run higher boost. This way the car may be a little slower off the line but once the boost hits, just hold on!!! I say just leave it as is and crank that boost up!

R19KET

Paul,

Forget rebuilding the V4. The cost of rods, pistons, etc', will be by far the most expensive route.

The V5 engine can be sorted by a good engine builder, for relatively speaking, reasonable money.

What you need to know, is just how much the heads were skimmed, so the new compression ratio can be worked out. Then you can either have the V5 piston crowns machined to lower the compression, and use stock V5 gaskets, or use the V4 gaskets, and have the heads skimmed to match the compression, back to 8:1.

Depending on what you asked of your engine builder, will depend on who's responsibility the cost of the work will be down to.

Unless you are intending to run much higher boost, I wouldn't bother spending money on Cossie pistons, because the stock rods would be a bigger concern anyway. It would be worth asking your engine builder if the V5 did have forged STi pistons fitted, but I'd doubt he'd know Either way, you could confirm this when the heads come off.

I'm sorry to say, that if the engine builder new what he was doing, he'd just be sorting out the V5, and wouldn't contemplate trying to rebuild the V4. The chances are, that if you need 2 rods (?) and 3 pistons, the block will be iffy anyway.

Mark.

Paul Mc

Mark

I believe there was an 1/8 skimmed off the heads. As far as boost goes the car was holding 1.4 bar in 5th gear on track. It was normally only holding around 1.2 or 1.3 on the road but not very often would it get driven as hard in 5th gear. (ie: I need my license!!).

Cosie Convert

1/8th Sorry, just had to say Fookin ell an 1/8th !!! Why ???

Are you sure it wasn't 0.8 mm maybe ?? even so, Why ??

[Edited by Cosie Convert - 5/13/2002 10:35:20 PM]

ssubaru

turn the boost up and add more fuel ,might be laggy but will go like ****

R19KET

Paul,

I'm not certain how you're raising the boost, but whilst I could be wrong, IMO, 3 pistons, and 2 rods doesn't sound like big end failure to me. I suspect you have one or two other issues that might need addressing.

Mark.

Paul Mc

Mark

You could be right about it not being big end failure as I haven't seen the crank but it certainly sounded like big end (I've been here before in my last car!!)

I don't know why the boost is so high either, the car was standard when I bought it and has had nothing more than the usual bolt on mods since.

As for skimming the heads, I think I meant 1/8 thou not an inch. Do you have any idea what an ECU remap would cost?

Paul

R19KET

Paul,

Maybe 8 thou ? 1/8 thou would be a "polish" not a skim

The cost of remapping would depend on the ecu you chose.

Ecu/mapping range from circa £600~£****, but £1000 would get you a Link ecu, inc' mapping. Mail Bob Rawle at BRD.

Mark.

Bob Rawle

What compression do you think you have, stock is circa 8:1 (as it is on Sti4 or 5), I've seen cars down to 7.9:1 with no ill effect, in fact a benefit (aftermarket ecu etc)

Bob

[Edited by Bob Rawle - 5/12/2002 7:48:35 PM]