Compression problems m137.960

[limpan200]

Hi, im helping my friend whit his track day car. Magnus Jinstrand's V12 Shelby Cobra Kit Car - Hot Rod Magazine

He's been having problems with the engine, it's a m137.960 from a cl600 -01 to -03.
The biggest problem is that there is a significant compression difference between left and right cylinder bank. The driverside bank has lower copression. The shutoff valve system and vanos is not conected to the EMS (vems standalone).

Is there anyone that has the same engine that has done or can do a compression test so we know if this is the way the engine is supposed to be.

Right now he has fitted tuning camshafts which was supposed to eliminate the problem but the result was just minor diffrence from the original.

If there is anyone that knows a lot about these engines we would very much whant to get in touch with you as information is hard to come by.

Best regards Linus

[Duke2.6]

I can't help you with the compression problem, but I read the article, and I see a lot of beautiful fabrication, but little or no engineering content. I see this a lot in the hotrod and restorod world.

Long tube headers make no sense on a supercharged engine because at WOT inlet pressure is always greater than exhaust pressure throughout the exhaust cycle. Unless boost is only a few psi, wave dynamics from long tube headers won't create enough rarafaction anytime during the exhaust cycle to be of any benefit. The best exhaust system is a set of streamlined manifolds, and the OE exhaust manifolds would likely work better, especially if ported.

Similarly, long inlet runners are of no use and actually cost power because of increased friction. Long inlet runners can enhance VE on a naturally aspirated engine due to inertia pressure as the valve is closing, but does little good on a boosted engine because inlet pressure is already high due to the supercharger.

The best models for supercharged engines are the engines used by American Funny and Top Fuel cars. An adapter is used to connect the Roots blower to the cylinder heads, which yields the shortest possible inlet tract.

Likewise, on the exhaust side, individual exhaust pipes are just long enough to clear exhaust gas from the chassis.

Valve timing for a boosted engine also needs to be different than typical for a NA engine. Boosted engines should have little overlap, again, because inlet pressure is greater than exhaust pressure, so the fresh charge is not short circuited directly out the exhaust, and the split overlap point should be well after TDC. Hold the exhaust valve open longer than a NA engine to avoid cylinder pressure buildup late in the exhaust stroke, and it may need to be opened relatively early to achieve blowdown by BDC. Then open the inlet valve late and close it late.

VE is a function of high boost pressure and short, low friction inlet runners, and on the exhaust side, minimum plumbing is used to keep friction as low as possible commensurate with safely routing exhaust clear of the chassis and driver.

The best way to work out a custom engine configuration is with the various inexpensive engine simulation programs. The one I perfer for both racing and road engines is Engine Analyzer. I used EA to design a new camshaft for sixties/seventies base engine Corvettes. Along with massaged heads this camshaft yields OE idle behavior, but 20-30 percent more top end power and another 500-1000 useable revs without the loss of any low end torque, so it maintains the smooth idle and stump-pulling low-end torque of the base engine, while making about the same top end power and revs as the Special High Performance hydraulic and mechanical lifter camshafts that kill low end torque and cause a rough idle due to the high overlap.

Duke

[pawoSD]

Just throwing it out there, but that particular V12 isn't known for being one of MB's better engines. They had a lot of reliability/durability problems and MB dumped it for a new engine after a very short time. Was this engine tested as solid/good before all the mods done to it? A lot of them had issues with rounded cylinder walls and such causing compression loss/other issues.

They should have used an M120 motor, those are beasts!

[ILUVMILS]

Check the cam timing before you do anything else.

[limpan200]

Thank you for your imputs. The engine was never going to be a top end performance unit, it was chosen because it fit in the car and would make the build one of a kind. The easy and cheap way would have been a box ls unit or something similar.

The thing is that the engine was working really well, huge amount of torgh for this lightweight car. But 2 years ago it blew up, probably because of uneven combustion between the cylinder banks. As it was rebuild he tested the compression and noticed the problem.

[limpan200]

Cam timing is correct.

[limpan200]

The engine has been completely rebuild whit forged rods and pistons which lowers the copression to better suit the boosted engine. All work has been done by professional engine builder.

[limpan200]

Wasn't this engine the base for the first Zonda supercar? If so they would have fixed the issue whit different camcykles for left and right cylinder banks.

I'm worried that the chutoff cylinder system or the vanos system is somehow producing the symtoms, they are not conected to the EMS so they should not be able to adjust anything. But maybe we should lock them in place to be sure?

[97 SL320]

The Zonda used the earlier version ( M120 ) from the mid 90's and was non turbo.

There is a guy making a Zonda clone, www.srfoosecars.com he claims $ 40 K.

[per eriksson]

Today we put a degree wheel and dial indicator on it and it does infact have different cams left to right, max lift is the same but duration and separation differs.
Right cam has about 11 degrees more duration on intake and 8 on exhaust.

WTF?

The aftermarket cams that was ordered and supposed to be made to get rid of this issue has about the same difference so that company didn't do their homework for sure.

[97 SL320]

Something like that should be documented in the factory shop manual and parts listing.
I wonder if this was done to broaden the power band and keep it flatter.

Cranking compression might be lower but dynamic ( engine running ) compression could be higher due to better cylinder filling.

There are a lot of sharp engine builders over at speedtalk.com that could help out. ( Mentioning that the car was in Hot Rod would help too. ) Be sure to follow up on this site so the thread does not hang.

[tyl604]

Good thread.

[per eriksson]

Do anyone have a good descrpition of the variable valve timing system used on this engine and how it operates?

The install and initial setting is double and triple checked in respect to the markings and procedures given in WIS.

BUT.....
Looking at the numbers it seems as the left cam is roughly 20 degrees retarded, right cam is where it should be if we assume the lobes are symmetric in respect to TDC.

I'm keen to set the cams the old fashioned way instead.

[97 SL320]

Did you check the left cam in relation to that banks TDC?

I'm not sure how the variable cam timing works on this specific engine but, have a look at a M104 inline 6, it is probably similar. These have a helical spline system. There is a inside spline on the sprocket , a inside / outside spline piston and a outside spline on the cam side. When oil pressure enters ( or maybe leaves ) the area behind the piston, the ID / OD spline slides back and forth altering cam timing.

[per eriksson]

We've decided to try to advance the left cam one tooth, that gives us the same basci position for both cams.
Waiting for a part that should arrive monday or tuesday next week and then we'll test it.