Quick compression question..yer right!

Post Sat Nov 25, 2006 7:05 pm

OK,
I have been down this road B4 but here goes...
I had my cam replaced with a piper fast road ( 274 deg )
When 2 months later my car would not start I got it into a BMW specialist garage for a check up.
They could find nothing wrong except it needed a crank sensor, furry ecu connection and get this:
It is down 25% compession on all 6 cylinders.
It idles and runs smoothish for a camed car but does run rich!!
I do have one naging doubt about the cam job:
I kept the standard valve springs.
Surly this would not have such a dramatic affect on compression?
THERE ARE NO AIR LEAKS ANYWHERE AROUND THE HEAD.

Do you think the valve springs are to blame?

Problem is I have not driven another e30 325i touring so have nothing to guage it to.

Well, I said it was a quick question, lets hope there is a quick answer.
I am doing the valve clearence tomorrow and took the cover off today, had a little look and at the mo they are set to 0.30

Cheers Guys.

Post Sat Nov 25, 2006 7:09 pm

They should be set to 0.25mm cold.

Might be that they timing is out slighty, belt may have jumped a tooth.......

Post Sat Nov 25, 2006 7:20 pm

How would that affect the compression.
The belt is in its proper place, they went over everything ( so they say )
My car is in for spring change on Monday with a mech I can trust.
What should I ask him to check for the compression problem?

COULD IT BE THE SPRINGS???

Post Sat Nov 25, 2006 7:22 pm

Whoops,
Hope no confussion
my car is in for Suspension spring change on Monday.

Post Sat Nov 25, 2006 7:27 pm

A loss of compression of 25% across all six cylinders could be valves not closing at the right point of the stroke (timing slightly out), if the vavles are not closed before the compression stroke begins, then less compression will be the result.

Post Sat Nov 25, 2006 7:42 pm

Then I will ask my Ace Mech to take a look,

Cheers.

Post Sat Nov 25, 2006 9:23 pm

Some of your C/R loss is due to your cam, thats why traditionally firms like Alpina up the C/R when they install a sporty cam to compensate and to improve performance.

Post Sat Nov 25, 2006 10:09 pm

Should I skim the head or just put up with it?
It flys when it hits 4000!
Nxt move is e30zone wild chip and after maybe exhaust manifold, how do you think it will feel then?
BTW clarkson hates bikes, hes such a Wanker!!

Thanks Andy,

toughguych.

Post Sun Nov 26, 2006 11:05 am

as long as its not burning lots of oil, and the compression is even across all cylinders and valve clearances are ok i would put up with it.

changing the cams wil change the dynamic compression ratio by sacraficing low down performance for better breathing at the top end, with lots of overlap the starter probably can't turn the engine fast enough to get a true running compression ratio reading.

Post Sun Nov 26, 2006 11:28 am

Andy335Touring wrote:Some of your C/R loss is due to your cam, thats why traditionally firms like Alpina up the C/R when they install a sporty cam to compensate and to improve performance.

Andy - I'm not questioning your knowledge here, as I don't know any different, but could you tell me why this would be the case?
I'm interested as I checked the compression on my rebuilt M30 (with Schrick camshaft) recently and it also seemed slightly down on all 6 cylinders!
Left me slightly puzzled!

Many thanks,

Martin.

Post Sun Nov 26, 2006 11:39 am

because of the ovelap built into the cam, a higher lift high performance cam will open the valves further and for longer to take into account the momentum of the gas flow to make the engine more efficient at higher revs. when you do a compresion check youre only using the starter to turn the engine, so you dont get the high speed airflow you do when the engine is running, and some of the air can escape past the valves which close later with a performance cam.

which is why a heavily cammed car often runs like crap low down, go silly and you'll have to up the idle sped a long way so it will run without stalling

all the above is why manufacturers developed variable valve stuff, so you get a nice smooth engine low down with no 'loss' of compression, and yet it will alter timing and lift at the top end to account for high speed gas flow.

Post Sun Nov 26, 2006 11:44 am

Yeah, that makes good sense.
I suppose larger ports in the head, as mine has got, will slow the gasses down a little more also at cranking speed compounding the 'loss' of compression.
I'm not so concerned about my engine's health now!

Thanks dude!

Post Sun Nov 26, 2006 7:22 pm

DRIFTBOY wrote:
Andy335Touring wrote:Some of your C/R loss is due to your cam, thats why traditionally firms like Alpina up the C/R when they install a sporty cam to compensate and to improve performance.
Andy - I'm not questioning your knowledge here, as I don't know any different, but could you tell me why this would be the case?
I'm interested as I checked the compression on my rebuilt M30 (with Schrick camshaft) recently and it also seemed slightly down on all 6 cylinders!
Left me slightly puzzled!

Many thanks,

Martin.

(O/T but your question has been answered)

Congrats on getting your car in PPC !

Post Sun Nov 26, 2006 7:30 pm

Andy335Touring wrote:

(O/T but your question has been answered)

Congrats on getting your car in PPC !

Thanks!

My 5 minutes of fame at last!

And the pics are small enough that you can't see the rust!

Post Sun Nov 26, 2006 7:54 pm

I have a 273 degree cam on my 2.7 and I'm using standard (albeit new) valves springs with no problems.

Everything that's been said regarding lumpy cams and lower compression readings is spot on.

Cheers,

Iain T

Post Sun Nov 26, 2006 10:12 pm

Thank You To All!
I can relax with all the compression doubts now!