compression ratio - PeachParts Mercedes-Benz Forum

02isb · #1 09-08-2011, 06:06 PM

what is the stock compression ratio on the 603.961 300sdl. searched but didnt come up with much. just curious.

thanks

sixto · #2 09-08-2011, 06:08 PM

Static compression ratio is 22:1

Sixto
87 300D

vstech · #3 09-08-2011, 06:15 PM

heh, this kinda came up recently... the static should be 22:1, however, there is a more elaborate CR formula...
why are you asking? what's in the works?

02isb · #4 09-08-2011, 06:23 PM

thanks for the quick replies. nothing in the works. just curious why mine seems to relie on the glow plugs so much for quick starting even when warm.

thanks again

vstech · #5 09-08-2011, 06:27 PM

I think the later vehicles need GP's working well due to the aluminum head and it's ability to rapidly transfer heat. makes it hard to build up ignition temp without the gp's.

but CR is a useless number unless you are modifying or building the motor. the important number for starting a car is cylinder pressure. and a DIESEL compression tester is needed for that. cylinder pressure on cold engine should be 300 to 475 PSI on a good running engine to start, and warm numbers should be in the 400-500psi range.

vstech · #6 09-08-2011, 06:40 PM

additionally, the motor needs to crank at a high enough speed to develop heat quickly for the motor to start without the need for glow plugs... Funola has a few threads on the subject of Starter RPM and cranking speed videos of various engines...
best to make sure your oil is clean, synthetic is best for fast cranking. also have a good fully charged battery of sufficient capacity to power the starter for cranking this high compression vehicle... and the battery cables need to be clean on both ends, battery and body/starter. ALSO the engine to frame grounding cable needs to be in place, clean and tight! of course the starter needs to be in good shape too!

02isb · #7 09-08-2011, 06:42 PM

static ratio does affect starting. (i thought) compression makes heat. i know there are other variables but. had a 6.5 gm that used glow plugs and would start eaisly when outside temps where above 50 with out even waiting and gm raised the c/r to 21.5 to aid in cold starting. both being idi. my di isb5.9 cummins dosent use glow plugs of course but a grid heater and it starts really easy with out waitng even below 30f but thats direct injected and a 16.3 c/r.

just tought maybe mercedes had a rather low c/r for reliablity thats all. my engine is in good shape. just have to wait for the glow plugs or it will smoke and my wife hates it at first start up

. dont bother me

thanks again

jt20 · #8 09-08-2011, 06:48 PM

Considering how easy it is to cold start an early type of engine without the aluminum head, I am inclined to suggest that the electronic idle system is to blame.

It is temperature based, so when you start your car warm, there is less fuel supplied for idle. But on a cold start, more fuel is supplied.

Another thought is the delivery valve (pressure valve) o-rings. They are more prone to leak when hot. (leak: meaning letting air in)

vstech · #9 09-08-2011, 06:50 PM

oh, sure CR is critical for diesel startup, I just saw your question about problems starting without using the GP's and figured you were trying to fix it or test for a problem... gave you some numbers to look into on your particular motor.
22:1 and the tiny pistons MB uses give pretty reliable service compared to the flimsy huge pucs GM uses...
I didn't know GM upped the CR on their motors for starting aid... bumping to the gear reduction starter sure did help!

02isb · #10 09-09-2011, 12:45 PM

Quote:

Originally Posted by vstech

oh, sure CR is critical for diesel startup, I just saw your question about problems starting without using the GP's and figured you were trying to fix it or test for a problem... gave you some numbers to look into on your particular motor.
22:1 and the tiny pistons MB uses give pretty reliable service compared to the flimsy huge pucs GM uses...
I didn't know GM upped the CR on their motors for starting aid... bumping to the gear reduction starter sure did help!

nope no real problems other then its old

i appreciate your concern

funola · #11 09-09-2011, 02:15 PM

I have my glow plugs manually controlled with a toggle switch. If I shut the engine off when up to operating temp, glow plugs are not needed to start instanly for at least half hour. I'd think that should be no different with your engine with aluminum head. Maybe it's a fueling issue (air)? I know that before I got rid of a slight air leak, it would not start instantly, even with long glow times.

bigblockchev · #12 09-09-2011, 05:46 PM

My Land Cruiser has a comp ratio of 18.6 & starts instantly just like a gasser without waiting for glow plugs. It also has a published compression spec of 500 psi though I have seen readings in other cruisers of the 550-600 range. I also have a VW TDI with a comp ratio of 18ish which doesn't even kick in the glowplugs till it gets close to freezing & it starts instantly. Older VWs with the Bosch Duraterm fast glow plugs require a 7 second glow to get going. The wife's 603 engine gets real recalcitrant if it loses 2 plugs, 3 and you have to plug it in for an hour. My feeling is there is some element of design which assists the ignition process over and above the compression ratio. I would like to hear from some of the guys with CDI based engines to find out what their experience has been. Cheers Dan

vstech · #13 09-09-2011, 06:00 PM

550-650 psi from 18:1 CR goes to show how much higher the cylinder pressure is with modern ring paks, and combustion chambers, not to mention cam timing and cranking SPEED!