Sale ends todayGet 30% off any course (excluding packages)

Ends in --- --- ---

Selecting the right compression ratio

General Engine Building Discussion

Forum Posts



Tech Articles

Talk about engine building here. New products, tricky questions or showcase your work - If it's engine building related it's welcome here.

= Resolved threads


Good day all,

I am currently trying to decide which compression ratio I should use.

Vehicle is BMW 330e with b48 4-cylinder which is similar to the 6-cylinder b58 engine which is used on the supra.

The stock compression ratio is 11:1 and I am seeing aftermarket available at 10.25:1 (custom should be available on special order).

The main goal would be to have a reliable engine for daily use with the flexibility to take it to track and get the most out of the engine.

1. What compression ratio do you recommend and should I look at lowering the compression ratio?

2. Will lowering the compression ratio complicate things with tune?

3. Curious if I will be able to run stock map after lowering compression ratio?

Thank you.



Hi Shane are you non turbo or turbo ?

Hi stock the car comes turbo.

I will also be upgrading turbo in future.

if you are running on the track and intend to still be running pump gas then yes i would be dropping to 10.5 especially if looking at upgrading the turbo and no it will not make it harder to tune in the future

Regards Ross

It will run like a pig on the stock map after lowering the CR Shane. I've known some of the lads running as low as 9.2:1 on the E36 M3 but you really have to watch cylinder pressures adding more boost at those levels.

Lowering the compression ratio will allow more boost/power but its always a trade off. The lower the CR the more poor it is off boost and takes a little longer for the "umph" Higher the compression the more torque you'll get but then its a case of as always....knock.

On a 4cyl like the coupe we've always been alright on stock CR messing about drifting with a bar/bar & half all day long without issues.

I'd say run the stock CR and you can always add a spacer in with the head gasket and play about. If it doesn't work out just remove it.

Martin these are a direct injection engine so lowering it to 10.5 will not affect the tuneability in the future especially if he is going to be upgrading the turbos these engines are all ready knock limited hard out with pump gas, i have had a play with one or two in japan

Ah well in that case, abort!! petrol and a match!!! :D

Didn't know they were DI and thought the 30s were port. My experience is limited to 36s and coupes that we've used a missiles.

I'm not a fan with the DI due to the lack of support with them and the inerrant issues. This is why I gave up on DI in my VW and backwards engineered the head to port.

Sorry Martin I may stand to be corrected they may also be port injection in the uk

Hi Ross,

My goals are to have a safe tune for daily use on pump gas but on track will be on race gas where I plan to get the most out of engine.

Hence a bit difficult to decide.

CP has both a 10.25:1 and a 11:1 set of pistons so I am trying to decide.

I can custom order a different ratio which may take slightly longer lead time.



There seems to be quite a flourishing tuning industry for these engines, I'd suggest checking out what they are doing before making a call.

11:1 is rather high for a forced induction engine and there may not be a lot of headroom for increasing boost - hence the lower compression option as opp sed to the OEM spec' pistons.

There is one VERY important factor you didn't mention - what sort of fuel is available to you? You may already be on the limit of what you can use - maybe even a little over and the engine is detuning itself (boost or timing) to control potential problems.

You will also need to give some serious thought on where, and what, you are prepared to compromise - for example, for a road car you will normally find mid-range torque/power and good response as the aim, but for the track you may want to focus on high rpm torque/power.

Hi Gord,

My car is also a hybrid so the electric motor should help with some lag.

My pump gas is 91 octane but I am ok with a very conservative tune for daily use.

For track, I will be looking at X98 and MS-109.

The newer BMW B48 engine comes with lower compression, 10.2:1 and another one 9.5:1 (this one has different crankshaft).

Hence I am thinking to lower it to a 10:1 or even 10.25:1 (as CP has this one in stock).

I am also seeing numerous people breaking the 11:1 pistons and it had a shop saying the 11:1 compression ratio is too high for high boost applications and poor fuel which is possibly why BMW probably shifted to a lower compression in newer engines.

Majority of people with b58 supra are using the stock 11:1 ratio but this car has two extra cylinders from my b48.



We usually reply within 12hrs (often sooner)

Need Help?

Need help choosing a course?

Experiencing website difficulties?

Or need to contact us for any other reason?