Ideal Compression Ratio - 350 to 500bhp

[Rusty]

As per the title, is OE fine at 9:1 or should I be looking to go lower with a thicker HG?

8.5:1?

[Error404]

Not really my expertise, but from what I have ready you want to drop it a smidge.
CR ratio with the MLS gasket is not 9.3:1 its actually just under 8.9:1!

I've attached my calculator I did recently for my Wossners that might save you some measuring

[Jamo]

If your going in that range 350-500 leave it at OEM CR, if you were going 500 above consider dropping it, you only really want to drop it for timing reasons

In regards to Error yes 9.3 is for the paper gasket it's around 9.0 with the metal gasked.

[Rusty]

As I said, 9:1

Cheers for that, saves me some effort.

[Mr30psi]

Has your head ever been resurfaced? I run my 3071R at around the low 9 range

[prj]

These engines love low cr.
If you are going over 400hp and are running pump fuel go to 8.0:1.
The result ia a much more responsive car any time you are hard on the throttle. On or off boost.
If running wmi then you can stay at 8.5:1.

The upsides also include much lower EGT.

The only downside is somewhat higher fuel consumption when cruising but this is probably irrelevant.

[A80Avant]

With a proper combustion chamber modification and cleaning of ruff edges you can get the best of both worlds. My cr is close to 9:1 and I'm still able to run fairly good timing.

Also you should take into account the turbo you are using. If the efficiency islands don't allow much power production above a certain boost pressure and therefore wont run then you don't need too low compression ratio as you lose the off boost reaction. If you are aiming for high power and combine that with cams and other relative equipment then a lower compression ratio is beneficial.

I'm running close to 500, I could have more boost pressure with a larger hotside on the turbo but I like the off boost response that the high compression offers.+ my combustion chamber has been smoothed out and therefore that helps with the elimination of hotspots that can predispose to knock and detonation

[driftin jo]

Lower compression makes bad throttle response is a myth, the only reason for
higher compression in a turbo engine is for fuel economy as prj pointed out above.
When cruising your effective compression is much lower than WOT and this combined
with low compression of say 8:1 will lower you mpg.
There is virtually no loss of power running 8:1 at WOT, what you loose from compression you can gain in timing.
Don't get confused with NA engines, they need a higher compression ratio to make power, especially up the rev range, turbo engines get all the compression they need,(and some they don't) once on boost.
The first turbo car I ever messed with was the first Audi turbo, it had 7:1 compression
ratio and no ECU at all. Timing was done mechanically by the distributor, (vac advance, boost retard). It did have lag but this was because it had a k26, I changed the k26 for a k24 off a later 200 turbo and the lag was gone, throttle response was as good as an S2!
Remember the old F1 turbo cars ? They ran 6:1 compression ratio !!
With today's crap petrol I would go for 8:1 if you want 500 HP as it will be so much easier for someone to tune it and much less stress on the engine.
As it's an S2, fuel economy is not really an issue as it is **** anyway, if it is an issue, sell it and buy a diesel!

[A80Avant]

When a turbo engined car is running off boost isn't it considered a NA one?...

[driftin jo]

When a turbo engined car is running off boost isn't it considered a NA one?...

Like I said above “ especially up the rev range”.

High compression at lower revs is actually counter productive, as timing has to be retarded, this is why high compression NA engines make little power low down and lots up top.

A turbo engine that runs 9.1 compression is actually suffering for it, the pay off is in fueleconomy when cruising. Most turbos even when low down in the lag zone still make 2-3 psi when you go WOT, my RS2 turbo does and at low revs, this raises the effective compression ratio
well over stock, but the engine is at low revs so the ECU has to retard the timing to prevent knock, this then make the engine LESS responsive then if it had 8:1 compression ratio.
This is even more pronounced with quick spooling turbos that make 7-8 psi low down before they get going as this is again raising the compression ratio before the revs get up and you end up making less power.
Try dropping your compression to 8:1, you will be pleasantly surprised at the results, you will make more power lower down and higher up the rev range, unless you are running 110 octane race fuel that is. The only thing you will loose is MPG when cruising.

[Rusty]

So how is best to drop it then?

[Error404]

Skim the pistons, ideally out the dish so you don't affect the squish / quench area, but I don't think that is so important on an FI motor, plus you need to be careful about the crown thicknes

[A80Avant]

Like I said above “ especially up the rev range”.

High compression at lower revs is actually counter productive, as timing has to be retarded, this is why high compression NA engines make little power low down and lots up top.

A turbo engine that runs 9.1 compression is actually suffering for it, the pay off is in fueleconomy when cruising. Most turbos even when low down in the lag zone still make 2-3 psi when you go WOT, my RS2 turbo does and at low revs, this raises the effective compression ratio
well over stock, but the engine is at low revs so the ECU has to retard the timing to prevent knock, this then make the engine LESS responsive then if it had 8:1 compression ratio.
This is even more pronounced with quick spooling turbos that make 7-8 psi low down before they get going as this is again raising the compression ratio before the revs get up and you end up making less power.
Try dropping your compression to 8:1, you will be pleasantly surprised at the results, you will make more power lower down and higher up the rev range, unless you are running 110 octane race fuel that is. The only thing you will loose is MPG when cruising.

On any given psi when the engine is not knocking which will make higher torque? The 8 or 9 to 1 cr?

I run up to 41deg timing off boost and 20 on wot and 1.8bar which is the effective island of my FP at high revs and doesn't knock. I have plenty of timing in the midrange except on WOT. Would I gain more on having it 8:1 ? Also, do you run the car on WOT or nothing?

What I am saying is that you cannot say to someone: do one or the other CR unless you know the rest of the parameters of the engine. Hence if for my setup I had less CR I would have lost top end and gained a bit of torque in the midrange. Lose top end due to the fact that although it may not knock because of the CR, the fact that the turbo is off the chart it would not make much more power anyway.

For what is worth that's my setup and that is how its been and worked fine. In my view a car is not made to go from idle to WOT while the in between map is useless.

[driftin jo]

On any given psi when the engine is not knocking which will make higher torque? The 8 or 9 to 1 cr?

.

Pulling the timing cuts power very quick, even a small reduction in timing looses you loads of power, so answer to your question, the amount of torque you make is dependant on timing thus 8:1 can make more power than 9:1 at a given rpm/boost, due to timing being better.

[A80Avant]

Pulling the timing cuts power very quick, even a small reduction in timing looses you loads of power, so answer to your question, the amount of torque you make is dependant on timing thus 8:1 can make more power than 9:1 at a given rpm/boost, due to timing being better.

I am not talking about when timing is being pulled. I'm talking about when Not being pulled. Theoretically you are saying that the 8:1 will be able to run more timing everywhere on the map.

I have stated my top and middle range timing. Perhaps Dmitri can add to this if he thinks more power will be made with a different compression adding more timing to that.