[Mk2] [Turbo] Compression ratios and head gaskets

[MR2 Rich]

Thanks Bob, you have been most helpful.

[Adam86]

Just to add ive now got mine back having being rebuilt with 86mm 8.0-1 wossner pistons.

Off boost it feels identical to the 8.5-1 pistons and drives perfectly. No noticeable loss in off boost performance whatsoever.

I'm still running the engine in so can't comment about on boost but im happy knowing that once on boost it will be a safer setup. Although im going to have a fuel and ignition map anyway.

[MR2 Rich]

Trickster - it did happen and it happened lol.

1.2 bar, st205cc and a hybrid CT20 to be precise.

I should point out though that the car has done a fair few track miles and a few flat out runs where ive touch the rev limiter in 5th gear.

So a hard life lol.

A Rev 3 is safer than a Rev 1,2 at that boost because Toyota knew what they were doing and lowered the compression ratio down from 8.8:1 to 8.5:1 when they raised the boost.
Now why would they do that small change if compression ratio does not matter, it’s because it does, even that small amount if you want to get the best out of the boost being run.
If anyone understands how engines work they know the ignition timing needs to stay as designed and if the boost is being raised on the same fuel the compression ratio has to be lowered.

All the MR2 Turbo engine running in this country with stock ECU are all working under the control of the knock sensor. But that can only do so much and in the end the head gasket will blow or the ring land break under detonation.

Why does the ignition timing need to stay the same Bob. Genuine question btw.

[ashley]

Because if you retard it to reduce combustion pressures, you end up with the hottest part of the combustion cycle occurring once the piston is running down the bore and so super-heating the cylinder walls, and discharging burning charge into your exhaust manifold. Localised super-heating of the cylinder walls is the main issue in the 3S engines given the inconsistency with which Toyota managed to cast them

[MR2 Rich]

Because if you retard it to reduce combustion pressures, you end up with the hottest part of the combustion cycle occurring once the piston is running down the bore and so super-heating the cylinder walls, and discharging burning charge into your exhaust manifold. Localised super-heating of the cylinder walls is the main issue in the 3S engines given the inconsistency with which Toyota managed to cast them

Thanks for explaining that Ashley, can you go the other way with ignition? (The opposite of retarding?)

[Peter Gidden]

Because if you retard it to reduce combustion pressures, you end up with the hottest part of the combustion cycle occurring once the piston is running down the bore and so super-heating the cylinder walls, and discharging burning charge into your exhaust manifold. Localised super-heating of the cylinder walls is the main issue in the 3S engines given the inconsistency with which Toyota managed to cast them

Thanks for explaining that Ashley, can you go the other way with ignition? (The opposite of retarding?)

Too much advance and you'll get pre-ignition, or pinging. A sure way to destroy an engine.

[MR2 Rich]

If it is essential to have it set exactly as Toyota set it, why isn't it not non adjustable?

[Peter Gidden]

If it is essential to have it set exactly as Toyota set it, why isn't it not non adjustable?

Cheaper production and manufacturing tolerances.

Distributorless cars have non-adjustable ignition timing.

[MR2 Rich]

If it is essential to have it set exactly as Toyota set it, why isn't it not non adjustable?

Distributorless cars have non-adjustable ignition timing.

That's why I asked. Cheers.

[2mad]

Why not just run 50/50 water methanol injection to up octane levels . It worked great on my car, after an ecu reset power levels were very nice.

[greeny]

Adam if that was a stock engine with stock turbo there is no way on earth you detonated that heavily on 99 fuel at 1.2bar of boost! You ran lean or overboosted heavily and didn't realise, and trust me it doesn't matter what CR you run if you have atht same issue it will smash your built engine to bits just the same!

Odin well observed I am building another engine, I used 2 3s based blocks which i already owned for my previous 2 builds which bother did 20k of hard driving with 500+ hp, both cracked behind the water pump which is a common issue and not something that you can build around without a different block.

This build is 5s based to get around that issue which I had planned years ago if I cracked another block which I did at 700hp on the dyno after running that for over 20k mile at 500hp, I wouldn't build a 3s block ever again.

There is a reason I rarely post on this forum, and I won't be posting again. Plenty of people here to give advice, I'll keep mine to myself, sorry if i offended anyone.

Strip it down and cc' those chambers right now!

Have to remember that Toyota also built in a safety margin with these engines and they're not tuned on the ragged edge stock.

[greeny]

The lower the inlet temperature the more volume of air there will be in the inlet. So under compression there will be more air to compress, this will give a higher pressure and a higher temperature

I always thought hotter air has more volume than colder air

Yes you are right, the hot air has a bigger volume but less weight because it has expanded from the heat.

Colder air has less volume but more weight. The more air by weight that an engine can breath the more power it will give.

Teach them about mass bob!

[gavinda]

Yes mass and density, not volume and weight, I gave up teaching for free on this thread when I read that