Difference in Pistons on 280SL engines between USA spec and EU spec cars

[Nicolas Aristodemou]

I have sourced a set of Mahle 1st oversize pistons for the 280SL (130 engine) which is for the european model; these have the following specs: 87.00mm dia, three rings 2.0/2.5/4.0mm a length of 80.6mm and a compression height of 48.6mm.

My car is a USA 1970 SL model and from what I can find out the corresponding Mahle 1st oversize pistons (87.00mm) are identical to the european model apart from the piston height -79.90mm USA vs 80.6mm EU- and compression height 47.90mm USA vs 48.60 EU.

Would the only difference be a slightly different compression ratio or are the two types different and hence the sourced pistons would be incompatible with my USA 130 engine ? I haven't stripped out the engine yet to have a look but I would like to have  most of the parts with me in advance.

Thanks for any info on this issue.

Nicolas

[MikeSimon]

I may be wrong, but I don't think there is a difference in pistons between US and Euro. Compression was the same, unless you had a "low compression" engine (which is somewhat elusive).
Possibly the pistons for another version of the 2.8L  M130 engine were different.

[Shvegel]

It could be an error in writing the specifications.  The larger diameter pistons will have a lower installed height to account for the increased size of the bore. 

[WRe]

Hi,
cars with US Emission Control System (model year 1970/71) had slight lower pistons with 4 rings.
...WRe

[Nicolas Aristodemou]

Hi,
cars with US Emission Control System (model year 1970/71) had slight lower pistons with 4 rings.
...WRe

Slightly shorter pistons could mean lower compression ratio then......

My point is that if I could use the slightly higher pistons in my 1970 emissions control USA 280SL if there would be any adverse effects....

[Benz Dr.]

I was under the impression that replacement pistons are made slightly shorter so that you can deck the top of the block. You actually want the top of the piston to protrude a little bit to keep the total compression higher.

In the case of stepped pistons, the compression will be lowered anyway.  I have a set of NOS flat top 87.00 mm pistons in case someone needs them.

[Shvegel]

Dan, 
If you think about it if the standard and oversize pistons had the same installed height the oversize piston would have a higher swept volume as the bore is bigger.  Basically a bigger squeeze into the same size combustion chamber.  Given the same stroke the bigger piston squeezes more air.   In order to keep the compression the same you have to create more volume in the combustion chamber.  They do that by lowering the piton crown a touch on the oversize piston.

[MikeSimon]

I think that is what the "stepped" pistons were supposed to achieve...

[Benz Dr.]

I think that is what the "stepped" pistons were supposed to achieve...

On average, you will get about 150 PSI compression with stepped pistons on most rebuilds. Normal compression should be 160 - 175 on our engines with 150 being rebuild time according to the BBB. Most older engines with normal wear will often be in the 145 - 155 range and still run well. Going to all of the trouble and expense to rebuild an engine and only getting worn out results, never filled me with a sense of great accomplishment.
The amount of a larger bore .040" in this case, can't add that much, but compression is HP, and trust me, there's a big difference in performance between 150 and 175 PSI.  You are allowed about .020" of protrusion out of the top of the block with pistons installed. I aim for about  .010" so that I can always deck the block again if needed. Having the piston top .020" below deck height and stepped pistons is akin to peeing into the wind.

[MikeSimon]

Dan: You have done that many times and have more experience, however, consider that the stepped pistons are used in conjunction with an overbore which in itself, increases the compression ratio.
In a 2.8L motor, with a Bore x Stroke of 86.5 x 78.8, the combustion chamber volume to achieve 9.5:1 compression is 54.5cc
An increase of 0.5mm in the bore to 87mm will raise the compression ratio to 9.6:1. (87.5 bore will give you 9.7:1) An increase in the combustion chamber volume of 1cc through a stepped piston will keep it at 9.5:1
Even if the loss of volume through the stepped piston is larger than that, I have a hard time believing you will lose 15-20 PSI in compression.