Compression test results

[Mixzzz]

Just did a comp test on my M20B25 in e34.

12 turns
1 - 12.0
2 - 11.0
3 - 12.0
4 - 11.5
5 - 11.75
6 - 10.9

6 turns
1 - 8.5
2 - 9
3 - 10
4 - 9.5
5 - 10.5
6 - 9.25

It was done with engine hot

It took quite a few turns for the gauge to get to those numbers - good or bad? I have the never ending rough idle trouble that I cant manage to solve - no its not vacuum leak(although the vacuum level wasnt the highest)

[2mAn]

What do those numbers represent? I just did mine, and usually the numbers are reported in PSI, so is that BAR?

Also, 12 cranks I think is much too high, so Im going to use the 6 crank ones. Assuming those are BAR numbers, it translates to:

1-125psi
2-132psi
3-147psi
4-140psi
5-155psi
6-135psi

Your high is 155psi and the low is 125psi and the rule of thumb is that you want them ~10% of each each other and these are 20%. I still dont think its bad, and overall the numbers are good IMO.

Idle issues for me suggest a vacuum leak but there could be other issues. How are the valves adjusted?

[Kershaw]

Is that actually good? My motor was 160-170psi. 125psi seems pretty low to me.

[e30davie]

If your "12turns" numbers are accurate, i would put your engine in the "Tired with a possible valve seating issue on a few pistons" category.

So your rough idle might be a dodgy valve on a cylinder or two.

I had a burnt out valve on an m20 engine that was so bad that when the engine was cold the cylinder wouldn't fire, but when it was hot it came good.

Leak down test next to figure out more:)

if it does turn out to be valves, SOMETIMES that seafoam/cylidner head cleaner stuff can help clean them up and extend life. But otherwise, head off and valve grind time.

[zaq123]

do a leak down test...it will tell you much more about your engine vs the comp. test. 10-15% loss that is even across all cylinders is acceptable. If there is a bigger loss, with the leak down test you will know exactly where the problem is.

[Mixzzz]

So these are the wet test results with 6 turns
1 - 10
2 - 10.5
3 - 11
4 - 10.5
5 - 11.5
6 - 12

Forgot to mention - yeah those are bar. The head was taken off in march due to overheating the engine and blowing the headgasket around 6 cylinder. Changed the head after all due to worn valve seats, had the valve guides pressed, few valves changed and all lapped at machine shop. At home took the finest rockers from 2 sets and grinded them to be nice and smooth(just a little bit). Put in M20B20 camshaft because its impossible to find used 2.5 camshaft in decent condition and new one wasnt in the budget at that time. The only difference is that its 6 degrees advanced.

The engine is missing out a cylinder at idle here and there - cant pinpoint a specific one and its not constantly one cylinder, it varies.

• AFM is good(at least passed the test with multimeter)
• spark plugs are only few months old
• Distributor and rotor seem reasonable, tried with another used set and nothing changed
• spark plug wires are 2 years old but gonna change them soon
• All vacuum lines and gaskets have been replaced
• injectors from m50b25 - all with nice 4 point spray pattern

Not really sure what to check now

[ForcedFirebird]

If you have m50 injectors and don't have a matching chip, you are running rich and probably slightly fouling plugs at cold starts.

As far as cranking compression, I never count the revolutions, just crank it until the gauge stops and taking note if any took significantly longer than the rest to come up. As others said, you really need a leak down test, they are much more accurate than compression testing and will even pinpoint the leak.

[Mook613]

If you have m50 injectors and don't have a matching chip, you are running rich and probably slightly fouling plugs at cold starts.

You base this on what exactly?
Lots of misinformation about M50 injectors being higher flowing because of the 17lb vs 14.5lb numbers. But you need to look at the actual flows at the same pressures. E36 pressure is at 3.5bar and E30 is at 3.0bar. At the same pressure they have very close flow rates and well within spec of what a stock ECU can handle for trimming.
The benefit of the M50 injector isn't the flow rate but rather the increase in pintel holes from 1 to 4 which creates a significantly better dispersion and better ability to create a more complete and stable burn when ignited.

[ForcedFirebird]

You base this on what exactly?
Lots of misinformation about M50 injectors being higher flowing because of the 17lb vs 14.5lb numbers. But you need to look at the actual flows at the same pressures. E36 pressure is at 3.5bar and E30 is at 3.0bar. At the same pressure they have very close flow rates and well within spec of what a stock ECU can handle for trimming.
The benefit of the M50 injector isn't the flow rate but rather the increase in pintel holes from 1 to 4 which creates a significantly better dispersion and better ability to create a more complete and stable burn when ignited.

Because it works, doesn't mean it's proper. Even with he reduced fuel pressure, the injectors are still 10% larger than stock. Even with modern ECU's, LTFT of 10% will cause issues, so it certainly will on the Motronic.

Based on the fact that I have live tuned hundreds of Motronic ECU's using a wideband and/or dyno, and 90% of my shop work load for the last 7yrs has been e30's (and 90% of those m20 powered).

As far as the pintle, it's a toss up. The m20 is a single valve with a stream injector spraying directly on the back of the intake valve which instantaneously evaporates the fuel. Fogging is redundant, no harm, it's more about forcing the ECU to use huge trims - they will always run on the rich side, specially in open loop, and plugs foul prematurely.

One other thing to think about is the m50 injector is intended to run at 3.5bar. We have had injectors in the machine and played with pressures and/or cycle times and it's amazing to see what happens when the injector is taken out of it's intended range. Lowering the pressure makes the droplets larger. Some injectors can handle 90+% duty, others go crazy at far less.

[jeffnhiscars]

You base this on what exactly?d.

Of all the people to challenge FF is the last one you should second guess.

[digger]

There is a 7% difference between m50 and m20 injectors at the same pressure. I can't see this ever being an issue in itself, but I still don't get why people need to upgrade to something bigger when if you require a new set What is so bad about getting the correct flow ones? it's likely to be a whizzz bang 4 hole one which you would never know the difference despite perception of big power and fuel economy improvements from the Internet

[ForcedFirebird]

Of all the people to challenge FF is the last one you should second guess.

lol, no harm. Just had to warranty an engine block since my guy honed the mains to .0016" which is the small side of factory, so I let it slide (crank was balanced/polished, showed true). After it warmed up, the bearings got tight. Had to eat 24hr of labor all said and done, but everyone was satisfied with the dyno results yesterday. In short, not anyone special, just e30's are my day in/out at work, commute in them, go race them on the weekends, then come on r3v and squawk about them. :)

There is a 7% difference between m50 and m20 injectors at the same pressure.

Didn't do the math, but 7% is still a lot to ask the ECU to trim on a long term.

I can't see this ever being an issue in itself, but I still don't get why people need to upgrade to something bigger when if you require a new set What is so bad about getting the correct flow ones? it's likely to be a whizzz bang 4 hole one which you would never know the difference despite perception of big power and fuel economy improvements from the Internet

Exactly. Just like the 24v internet hype over 3.5" MAF's (or m30 AFM on m20 back a decade ago). No reason to monitor airflow through a pipe that's larger than the throttle plate :/

Either way, put larger injectors and fresh plugs in an m20, drive it for a week to work and compare them to stock doing the same. Don't need a wide band for evidence. They actually like to be lean compared to most other engines I've dealt with. They are happy in the upper 13's at peak TQ N/A, where many cars will have all kinds of KR.

[varg]

There is a 7% difference between m50 and m20 injectors at the same pressure. I can't see this ever being an issue in itself

I can. I've spent a fair amount of time tinkering with standalone EMS over the past decade, and 7% is a lot. And... it's not the only difference in the injectors. They'll have different dead times and flow vs pressure curves. In open loop that's costing you power and fuel economy, in closed loop you'll be lucky if an old EMS with a narrowband sensor like the E30's can make up for that.

[digger]

I can. I've spent a fair amount of time tinkering with standalone EMS over the past decade, and 7% is a lot. And... it's not the only difference in the injectors. They'll have different dead times and flow vs pressure curves. In open loop that's costing you power and fuel economy, in closed loop you'll be lucky if an old EMS with a narrowband sensor like the E30's can make up for that.

yeah sure those things are all real, but the consequence of 7% are usually small enough that its still not something that most are going to worry about. budget minded people have been swapping in different injectors on the M20 running motronic for decades. i ran some 18lb on a stock m20 for about a month to diagnose some issues before buying a new set with stock flow, the sky didnt fall down though it was temporary

[TobyB]

Heh- with carbs, if it'll stay within 7% of where you want it
over the entire rev/load range, you're doing pretty well.

But I digress.

t