The engine in my car now is the V6 3.7L from the ml350 04 (with 55k SC), n/a 55 pistons with new oem rings, C32 cams and crank, static compression ratio is 8.2, but i just checked the cranking compression ratio which is low 125-130 psi! is it normal with this low static ratio? advantage disadvantage?
thanks.

 

How do you run a 55k sc with a v6?

 

Are they consistent? That's a little low, but you do have low compression. How'd you figure your SCR? Also, are you sure the chains are in right? One tooth off on the crank can send your compression into the toilet.

 

^ Everything he said.
In addition, slow cranking speed will lower your reading. Verify that the gauge is accurate.

 

I'm not surprised, C32's see about 150psi I believe and they are 9.0:1 so you seem fine in that department. I, however, find that 8.2 static is WAY low unless you are going to run BIG boost and the C32 cam profile may not be helping much either, you're dynamic compression may be lower than it has to be.

 

Thanks for all reply

by cutting the rear runners.

yes they consistent between 125 and 135 psi, the chain are right i installed them myself with the mb pictures in hand when i rebuilt the engine, for the SCR calculations you can take a look in my project thread in my signiture.

battery is new and gauge accurate, it read 160/165 on a stock E55K and 150/155 on 55 n/a w210.

i gonna see today what C32 will gauge if it is at low 150psi so with my 8.2Cr and the bigger bore pistons my cranking compression is right.
the 55 SC made high boost on my V6 because of that i went lower SCR, with stock pulleys i am seeing 1.5-1.6bar(23psi), so with a 180 pulley i 'll be in 25-27 psi boost, let it be on the safe side and i can run high timing also.

 

Check and see what the 55k motor's CR actually is and get back to me if you think 8.2:1 is low OP-Like they stated above slow cranking will hurt your numbers. Did you make sure the timing is correct this time?

 

I forgot about that, and now that you mention it I bet the C32 is the same. I can't remember what the number came out to but I do remember it was in the 8's.

 

It's between 7.97:1 and 8.11:1 depending on what the deck height is.

 

That is ridiculously low, even more of a reason for these engine to handle more power than what people are getting out of them. It just needs a 4.0L whipple.

 

This time the timing is correct and it is verified by consisting nbs on all pistons left and right, and yes i read before your posts about the lower compression that you find, but what can i do? the only amg pistons that can be installed are the 55n/a and the rods of the v8 cannot be installed on the v6 crank, with the c32 rods and 55na pistons i am 2mm below the block, the only way to up my compression is by aftermarket pistons.
how much it will cost me 6 customs pistons with rings?
btw my car is the only one in the world with the 55 SC, it made 555 crank hp with stock pulleys and 8.5 SCR(MB nbs).

 

The set of pistons with rings will cost you about 800$ from Wiseco, I'm not sure about other manufacturers. More if they need coating for an aluminum bore. What do you mean by "consisting nbs on all pistons left and right"?

 

only 800$!!! i paid 340$ for only the oem rings without shipping and customs.
i mean by consisting nb of cranking compression ration on each side that if the timing chain was installed wrong each side will get different readings.

 

Na 55 should have higher compression than 55k

 

It's possible to have the cams in synch with each other but the crankshaft off by a tooth. If the cams are both retarted in relation to the crank, your numbers will be low across the board, which is what I think your problem is.

Do the cam alignment tools that fit an M113 engine fit yours as well?

 

Did you verify that all 4 black chain links were lined up on the crank, idler and both cams? I think there are 21 links between each black link, as I recall and then it takes up to 14 full cycles to get them all aligned. The cam tools are ok for general setting but they all tend to be sloppy, bend easy and can allow for an easy one tooth off event.

If you used a 04 Ml 3.7 engine and N/A 55 pistons that have a listed 10.5- ish comp range, then how do you come up with 8.2? It takes a lot of measuing to get accurate comp ratio, when parts are changed. Even wrist pin height, ring height, rod lenght, deck, etc.

 

it defer with the cam profile the most when relating to cranking CR.

i see your point but no way that i did it wrong, i used the MB timing chain pictures.

 

yes the 4 black chain were lined up.
guys you need to read my thread about the SCR calculation it is along story, the w210 55 pistons along with their rods make 10.5 but i used them with the c32 rods cause they will not fit on the c32 crankshaft, the c32 pistons are higher then the 55 ones because of that they sit 2mm below the block and they lower my SCR, the w209 55 pistons which made 11 SCR on the w209 result by 8.5 SRC in my engine. plus the stroke is 84mm. all affect my SCR to be more low, if i use the 55K pistons they will lower my SCR to mid 7. the volume of the heads of the 3.7L and the 55 n/a are the same plus the gaskets are also the same. you can use the formula of the SCR to find the volume of the heads then use it plus add volume of 2mm to find final SCR.

 

in my built thread page 2 post 34:
I mentioned before that the 55K pistons will be used in this project, so after some work and research the 55K pistons will be swapped with 55 N/A pistons from W209 or C55, reason:

Compression ratio formula:
CR=(D + PV + DC + G + CC) / (PV + DC + G + CC)

D = Displacement

PV = Piston Volume
DC = Deck Clearance Volume (distance between the top of the piston at TDC and the deck of the block)
G = Gasket Volume
CC = Combustion Chamber Volume (engine heads)


If i started by calculating 55K engine:
let's take PV, DC, G and CC as one factor V and try to calculate V for 55K engine (DC is same for all 112/113 engine, G also the same, CC is also the same only 55k differ by a very little volume-assuming the same now we can get the difference between piston volume by calculating V)
formula now is: CR= (D+V)/V

D for all 55 engines=3.14*bore*bore*stroke/4= 3.14*97*97*92/4= 679864mm2
CR=9=(679864+V)/V so V 55k= 84983mm2


now for 37 engine, D = 3.14*97*97*84/4= 620746mm2
if we want to use the 55k pistons in the 37 engine the 55k piston will be lower by 1.3mm from deck block, so the volume made by 1.3mm is 3.14*97*97*1.3/4=9607mm2
CR for the 37 with 55k pistons will be= (D37+V55k+V1.3)/(V55k+V1.3)=
=(620746+84983+9607)/(84983+9607)= 7.56
also the heads of 55k(Combustion chamber) are a little bigger from 37/55NA so this nb will vary a little up to 7.7 or 7.8.


so this nb is very low for compression ratio, we have a choice to mill the block 1.3mm and we get CR at 8.3 to 8.5 and maybe the camshaft timing also will deffer by milling the block 1.3mm.
so we did not go with this route, we went with the w209 55 pistons,
W209 55/C55 has 11.0 as CR and the pistons are also lower by 2mm from deck of block.
CR w209 55 = 11= (679864+V 55)/V55, V 55= 67986
V of 2mm= 14780

CR of 37 with 55 w209 pistons = (620746+67986+14780)/(67986+14780)
= 8.5


Result if we use 55 w209/C55 pistons we get CR = 8.5 and we will not touch the block or camshaft timing. and this is what i want with lower CR for making room for bigger pulley and more power.
i swapped the 55K pistons with 55na ones without paying anything, so the problem is solved (cheap). Also 55 n/a pistons are less weight than the 55k:
55k= 582g

55n/a w209= 532g
C32= 512g

 

You can also try calling Diamond Piston for custom pistons. I've seen prices around $2,000 for strange pistons no one else will want to buy.

 

I went with diamond pistons myself and have always ha great results with them. As
For the CR all you need is the CC's of the chamber and the piston dish, bore,stroke, head gasket bore and thickness and the deck height. That's the accurate way to get your CR.

 

Assuming your calculations are correct, and you have a true 125-130 psi cranking compression. My next question would be if all of the timing gears and cams from the same engine? Perhaps with the mixing and matching parts, the cams are retarded even though you lined up the marks? Lining up the marks gets you in the ballpark, but in your case, you might need to break out a degree wheel and check the true timing. Next hurdle, are the cam timing specs listed by MB?

 

Even if the specs are not published, you can still put on the degree wheel and use some judgement. If you're reading an intake valve closing angle of 80'ABDC with stock cams you know you're off and are killing the DCR.

 

thread up, i am still power hp down, before i was getting 550 hp crank, now 480/490 hp, i am suspecting that before we installed the timing chain one tooth advanced, can a one tooth advanced made an increase of 60/70 hp and cranking CR from 125/135 psi to 160/170 psi ? how much degrees does one tooth affect?
now i am 100% sure that the timing chain is installed correctly and the cams and crank and the chain are from one engine(C32).

 

I rebuilt cls 55 motor and now cylinders 1234 have only 120 compression and cylinders 5678 have 155 compression. less than 10 percent leak on all cylinders. i bought timing tool and cams are apparently lined with tool at 40 degrees as seen on videos. car idles like crap but runs ok when under full power. i must have one cam off if passenger side cylinders only have 120 compression. Can anyone comment if they have any answer.