Lowering compression ratio m104
 

I've done a bunch of reading but never came up with any definitive answers. What's the best way to lower the compression ratio significantly on the m104.980?

While I'm posting, has anyone found any aftermarket internals that are stronger than stock? And will work/fit?

I'm working on a turbo system, will be running ms3 pro and e85.


Thanks for your knowledge, the information on this site is incredible.

How much boost? What HP target? With E85 I'd suspect you could get an acceptable ratio pulling material out of the chamber. Up to about 500 HP I'd be more worried about everything behind the flywheel than anything in front of it.

The goal for the car is going to be 800-1000 crank horsepower, on 30+psi.
I'm still very new to the performance mercedes side, and expect this to be a multi year project and the rest of the drivetrain will be another huge project.

Here's some pics of the car so far, mishimoto type r intercooler, turbo bandit manifold and 80mm turbo

https://imgur.com/a/SeD7QdW

Still mocking up some parts, the wastegate for the turbo bandit system wont fit the sl due to the steering setup, so that will have to be cut and rerouted.

Next will be pulling the head and having it checked, machined, etc. And same with the motor. Coolant passages had tons of sludge and wasn't circulating coolant well at all.

Please step over to the rabbit hole, Alice will be your guide.

Where do I meet this Alice you speak of?

@Carroll, Slick, & Co.

I'm not familiar with them Frank, any more details?

https://www.racetep.com/manufacturer/engine-parts-and-performance/pistons-and-rings/jepistons/bmw/m50-m52-m54-and-s54/mercedes-je-custom-6cyl.html


I did come across this, considering were both in SoCal, I may just have to give them a call

Somewhere on this site, someone got to just under 500 HP from a M104 but it was a huge struggle.

The inline 6 2JZ Toyota guys get big HP so I'm probably follow what they are doing. Or just use the proven, easy 600 HP 2JZ. ( 600 HP is considered low for these motors )

A 500SL Like You've Never Seen (Or Heard) Before - Speedhunters

https://www.carscoops.com/2015/12/naughty-mercedes-r129-sl500-hides-dirty

Also, unless you are looking for vintage MB part numbers, ignore Frank.

That is a great looking car, and although I want to do something far from the norm, I want to do it with the mercedes inline 6.

When I originally bought the car I was leaning towards big turbo junkyard LS motor swap, but the more I drove the car and read about the m104, the more I wanted to stick with it.

I think with the price of the rods and pistons, I will do some work with the head and leave the motor as is for now till I can get everything fit and running, especially since I have no experience with megasquirt lol

Just take your pistons and send them in to bet profiled. Then drop the compression. Should take 6-8 weeks and any good machine shop should be able to cc your combustion chamber and tell you how much needs to be removed to drop the compression ratio. Those are 10-1 engines. The crank and rods will most likely support those hp numbers. We have made over 700 w/tq on stock om606 crank and rods and pistons. Rods finally bent but the rest is rock solid. The m104 shares the same basic setup. Rods are a tiny bit smaller with smaller piston pins than the diesels. But if you wanted to just get it over with you could do rods at the same time.
Good luck. Sound like fun. I think the headgasket is going to be the hardest part. But I think it is doable. I certainly am going to turn mine up some day and see.

Thanks for the replies. I really appreciate all the work you guys have done on these motors and all the info you guys are putting out there for the rest of us.

Has anyone conquered the head gasket issue? Have read post about custom ones being made, sealing some spots, etc., but unfortunately like most post, the ending product isn't talked about.

The best way is to get custom pistons. (Or machine the stock pistons)
If you are using E85 you really don't have to lower the CR, 10:1 should be alright.

You can use forged Conrods for a Honda H22 they have the same big and small end dimensions as the M103 and M104
but at 143mm they are a little shorter so the compression will end up at around 8,5:1 on the 104.980.
Forged conrods also have the advantage of being lighter then stock, that means less wear on the bearings at higher rpms.

From my little experience the headgasket, has no issues sealing the combustion chamber, but the water and oil passages tend to leak.
Good thing on the M104 is that a really big M12 bolt is used for head, which has the advantage of being a standard 1,75mm thread pitch.
So you can get off the shelf 12.9 allen head screws, torque them to 140Nm(~100ftlbs), and there shouldn't be any issues.

I've the same problem.


I found this : Ferriday Engineering


Copper head gasket or decompression plate (aluminium).

the using of h22 rods are with honda bearings right?

becasue m104 rods are 51.6mm in bearing shell and honda rods 51.1, but with bearings fitted should be both near 48mm