M30b35 compression psi

Sat Jan 18, 2014 7:28 pm

Hi

As you all know i have a low performing newly built engine. What should the compression be in psi across the six cylinders? And the compression ratio?

Sat Jan 18, 2014 7:41 pm

175 min

Sat Jan 18, 2014 7:43 pm

I got 195 across all 6 but I have 0.5mm skimmed of the head and the valves had just been re-lapped

Zaust is spot on - anything less than 175 is a bit low

drop me a PM with your number and I could give you a call in the morning if you want to talk it through

Sat Jan 18, 2014 7:49 pm

190 across all six on mine, as far as I know a completely standard unopened engine

Sat Jan 18, 2014 7:53 pm

Well I think I've found my problem then. Listen to these current figures cylinder 1 is 90psi cylinder 2 is 174 cylinder 3 is 175 cylinder 4 is 160 cylinder 5 is 165 and cylinder 6 is 176. The bottom end has had cleaned up pistons new piston rings cross hatched bores and shells. These have just been taken from my mapper as the car is on mega squirt and has a massive over fueling problem which I think has washed the bores on cylinder 1 causing low psi.

This is not good is it?

Sat Jan 18, 2014 8:00 pm

it would be ok if you sort out the over fuelling problem

90 is quite low for bore wash though, when I had this I was getting 130psi ish

are you dead sure your valve clearances are right on cylinder 1

Sat Jan 18, 2014 8:04 pm

The head was built by a professional builder who owns a race team but I can check it. Really deflated now as you know Jim when I picked up the bottom end from you I've been here twice now. That why I rebuilt it thinking it was worth doing. With these figures does this mean the bottom end is fooked now ?

Sat Jan 18, 2014 8:13 pm

Blimey, I didn't realise it was you mate, nice to hear from you

Why would the bottom end be fooked ? Low compression can only be caused by issues with the cylinder boundary, rings, head, gasket or timing

I had the same problem with my M20 engine and it was ok in the end

I put too larger injectors in there and it was getting bore wash. I think also some of the rings were stuck. Anyway, I changed to normal injectors, gave it a good thrash and the compression came right up

Don't despair, you engine sounds fine.

If you have access to a garage get them to do a leak down test - if done properly this can tell you exactly what the problem is - do this before you take the head off

If it were me, I would get the engine running on normal fuelling first then convert to megasquirt, do everything one step at a time.

Also use a feeler guage and just turn the engine so the cam lobes are both pointing down in no.1 cylinder, then check the valve clearance. If you don't have a guage, you should just be able to rattle the rocker - also look at the hole where you adjust the adjustor thingies when doing the head - look across them all and see if they are all in the same place.

Double check your head bolts are tight.

If you have done all this and you have new piston rings it must be fuel

Does the engine run ? a good thrash would probably help if you have bore wash or a stuck ring.

Seriously, get a leak down test done on cylinder 1 as this will tell you exactly the problem.

I very very very much doubt your engine is knackered as you have just re-built the bottom end.

Sat Jan 18, 2014 8:20 pm

Have you tried a few squirts of oil down the spark plug holes and compression testing again to see if there is any difference in reading?

Sat Jan 18, 2014 8:22 pm

Yer, that is worth doing also. if it's just one cylinder it does sound like a stuck ring although I've been told that this is very rare on BMW engines.

Sat Jan 18, 2014 8:24 pm

Thanks Jim very helpful. I don't know if this might be a cause but I have fitted m5 throttle bodies on the car. So as they were m5 throttle bodies I fitted the m5 injectors aswel but I'm not sure if they are the same injector.

The engine is on mega squirt from day one as it was like this before I bought it. The wiring is on a kit car fuse board so wouldn't know where to start to change now.

the TPS went down so I've just changed that but now it developed a misfire. Let's hope after a thrash on the rollers it may sort itself out then.

Sat Jan 18, 2014 8:29 pm

Yer, it might just be fine after a good run

one other thing maybe worth doing is take your injector rail out and plug each injector in one at a time pointing it into a jam jar, then turn over the engine. A crude way of measuring that the injectors are giving our roughly the same amount of fuel (take fire precautions !!)

if it's not ok after a run, I would get a leakdown test done, the kits are on ebay, get a 2 dial tester and they are not expensive. You could probably sell it on and get your money back.

I'm no engine guru though, I've had a whole world of problems with the m30 and still have them.

Sat Jan 18, 2014 8:32 pm

Ha ah well at least we can learn of each other. My mapper seems to think that it will pull through but it's just concerning, however it did pull 207bhp and 211 ftlb of torque with only 5 decent ish cylinders lol ....

Sat Jan 18, 2014 8:35 pm

wow, that's amazing, should be getting at least 230bhp then with a wall of torque

where are you getting the car mapped if you don't mind me asking ?

Sat Jan 18, 2014 8:40 pm

Telford of all places. He is a mega squirt specialist. Really nice guy too! Bailey performance.

Sat Jan 18, 2014 8:42 pm

Jim do you know if the m5 injectors are larger than the standard m30b35 ones?

Sat Jan 18, 2014 8:52 pm

not 100% sure, the original injectors can take a lot of tweaking before adversely effecting the AF ratio. It's all in the mapping mate, you can put bigger injectors in no problem at all but they could be problematic and cause non-running if not mapped properly.

Give it a good run and take it from there

Don't worry at all about it - it all just needs setting up / balancing now to get running properly. Let the mappers earn their £££

Sat Jan 18, 2014 10:21 pm

what cam shaft are you using? is the cam timing correct? the cam timing has a massive effect on compression numbers

Sat Jan 18, 2014 10:37 pm

The cam is standard mate.

Sat Jan 18, 2014 10:42 pm

mikeyce30 wrote:Jim do you know if the m5 injectors are larger than the standard m30b35 ones?

Im sure M5 injectors are different, either way standard 3.5 M30 injectors are good for 260hp + so id use them.

Sat Jan 18, 2014 11:33 pm

mikeyce30 wrote:The cam is standard mate.

is the timing spot on? a few degrees can make alot of difference

also do a leakdown test

Sun Jan 19, 2014 9:22 am

Thank you guys. Appreciate all your feedback. I will let you all know what I find.

Sun Jan 19, 2014 6:22 pm

As per previous thread you have had things like the valve throats bored, I would find a good standard b34 head wack it on and the check your comp. Bottom end when rebuilt should not be giving problems even with a little bore wash. Plus if it is fresh you have no carbon build up to help matters.