Raising CR on M30B35

[Arny535]

Hello folks!

My 3.5 engine is disassembled for overhaul now, and I'm planning to make a few enhancements in the process.

Among other things I'm interested in raising the compression ratio of the engine. It is designed for 91 RON gasoline, and the lowest octane gas I can obtain now is 95 RON, therefore it is a shame for the engine to have the low CR 9 compared to the older (European) models which had CR of 10. Since more compression means more efficient engine, I would gain at least minor improvements in power and gas mileage and make better use of the fuel available.

Now, for the method, there are several options and I've already found out about their pros and cons, so I hope someone here could give some good advice...

First method that is quite simple would be to swap pistons from an older engine with CR 10. The downside is they would be older than the engine, and also I've heard (don't know if it's true) the newer pistons are of a better design, and the older piston type wouldn't be recommended.

Other methods are based on lowering the compression chamber height by other means. I've made a rough calculation that I'd need to lower it about 0.45 mm for a 0.5 CR raise, so if I want more than 9.5 i'd need to lower it at least 0.5 mm. The question that arises here is, by how much could it be lowered with original pistons before they would collide with the valves?

The next method that first comes to mind is head skimming, but the markers on the head are at 0.3 mm, after which the head becomes too thin. Obviously that is not enough if I don't intend to risk the head.

A safer method proposed is milling the block, which can be skimmed as much as required. Unfortunately I haven't been able to find a machine shop which could do it satisfactorily (yet).

Another method would be a thinner head gasket. The normal gasket is 1.72 mm thick and I could order a custom one, say 0.5 mm thinner. Now, for that I'm not sure if it's a good idea. What are potential drawbacks for that, and should it be done at all?

So, here are my thoughts. I'd like to hear some advice on that... Old M30B35 tuners please help with your experience

Thanks in advance.

[Ross]

I would investigate what parts BMW used in the Euro motors v. our version. Pistons?
Probably pricey for pistons and certainly if the head is different.
Any self respecting machine shop ought to be able to deck the block for you. I'd avoid thinner gaskets as this is a long motor and only uses four bolts per cyl.

[Arny535]

I would investigate what parts BMW used in the Euro motors v. our version. Pistons?
Probably pricey for pistons and certainly if the head is different.

E32/E34 both Euro and US have CR 9, and older engines (E23/E24/E28) Euro models were CR 10, while US models were lower. The only difference between the three versions is in the pistons (Actually newer engines also have a bit larger intake valves, but that doesn't affect this). Since I'm in Europe, I could find a piston set from an older car, but I don't like the idea of having pistons older then the rest of the engine. Having a new set of those would certainly be expensive... And I'm still not sure about that considering the rumors I mentioned earlier about the piston quality.

Any self respecting machine shop ought to be able to deck the block for you. I'd avoid thinner gaskets as this is a long motor and only uses four bolts per cyl.

I'm not quite sure how good a machine shop here can I find
Thanks for the advice about the gasket!

[632 Regal]

Raising the compression alone will not give the gains you are thinking. You would need to do other things in combination to help the compression such as exhaust, camshaft, intake etc.

[Arny535]

Raising the compression alone will not give the gains you are thinking. You would need to do other things in combination to help the compression such as exhaust, camshaft, intake etc.

Of course, I'm planning to make other mods as well while the engine is being overhauled.

I don't actually expect much from raising the compression... if I gain one MPG and a few HP from it, I'd be satisfied Anything more than that would be quite welcome, but I'm not really holding my breath...

[Jeff N.]

Before you get too far in spending $$, you need to have a plan on how you will tune the engine to accomidate your compression / head / cam changes. Figure this out first, then commit to spending the $$.

The stock motronic maps will not allow you to unlock the changes you're making.

I'm currently using a Perfect Power SMT-6 to retune. You may want to consider something like this.

My mods include:

- 3mm overbore
- compresssion bump
- head port
- cam change
- exhaust change
- MAF

[Arny535]

My mods include:

- 3mm overbore
- compresssion bump
- head port
- cam change
- exhaust change
- MAF

Hello Jeff,
how did you do the compression bump?

Other than that I don't plan much mods, I was planning cam change and minor intake and exhaust adjustments. I wonder if that won't be too much changes for default motronic maps or a generic chip like MarkD's? Or for that matter, Miller's GENIII chip from their MAF kit, which is very likely I'd also have.

[Jeff N.]

Compression change was via a larger bore with the same head size. I didn't calculate the compression (ie: cc the head, etc) but the reported compression based on the pistons should be about 10:1.

If you change your cam and your compression, you need to tweek the timing and generally customize to the setup. No one has an off the shelf chip for that. Not Conforti, not Mark, no one. It will OK on a tuners' chip but you just won't get the $ return from your expensive mods. This is why I went to the piggy back.

I'd look to run a factory configuration and get the matching ECU or make a plan to get control of your fuel and timing before you move too much further ahead.

[Jeff N.]

sorry...one more thing. I don't think a 1/2 point compression change would cause any issue with the factory chip. It's when you start laying in cam, intake and exhaust changes that this becomes imperative.

[Arny535]

sorry...one more thing. I don't think a 1/2 point compression change would cause any issue with the factory chip. It's when you start laying in cam, intake and exhaust changes that this becomes imperative.

I was hoping to get away with that. Even the cam I'm considering is quite a mild one so I don't think it would get default maps out of whack. Intake and exhaust mods would be really minor, just to get a bit more flow to match other mods, nothing that would require any remapping anyway.

If I decide for more drastic changes, I'm planning to use a MegaSquirt. Btw. a friend of mine is working on a more exciting project, M30 with a S38 head and turbo. I'm working on a MegaSquirt setup for his engine (he does really need that).