PeachParts Mercedes-Benz Forum - View Single Post

400Eric · #26 01-18-2010, 07:28 AM

Nice pictures Jay. Thanks for posting.
Jay, how come the dish on the next piston looks different? Not much of that piston shows in the picture but what does show is clearly different.

Those chambers look unmodified to me which tells me that AMG kept the C.R. in check with that piston dish alone.

There probably isn't anybody here who beats the long rod ratio drum louder than I do. (You can ask GSXR!) Same goes for bore to stroke ratios. But the available room (deck height) in any given block is put to better use by using it for more displacement rather than using it for a better rod ratio. That's why AMG built a C36 instead of a C28. In other words, the ideal bore to stroke ratio and rod length to stroke ratio that the 2.8 has still doesn't enable it to spank a 3.6 which has a fairly poor bore to stroke ratio and rod length to stroke ratio. Even when you throw in the fact that the bigger engine isn't as well "fed", (i.e. the bigger engine doesn't have increased breathing equal to the increased demand created by it's increased displacement), The bigger engine still prevails. Whether we are talking about a 302 Chevy vs. the truly awful 400 Chevy, or a 427 vs. 454 Chevy, or 400 vs. 455 Pontiac, or 360 vs. 401 AMC, or etc., etc. The list is endless, the bigger engine will win the race despite making less power per cubic inch, despite having poorer bore to stroke ratios and poorer rod length to stroke ratios and despite not being as well fed as the smaller engines. Food for thought. (Pun intended.) Also, if you chose to build the smaller engine, to get the same performance out of that engine you have to build it to make it's power in a higher RPM band which leaves you with a "peaky" engine with no low end torque or throttle response which is not a good formula for good fuel economy or good emissions or long engine life since you are always winding the pee out of it to do the same job as the bigger engine with the rotten rod and bore to stroke ratios.

I still say the approach to the valves is much better on the later head. Look at those cross sectional views in that engineering paper. Look at Jay's pictures. Those early heads were compromised by M.B.'s strange desire to mount the CIS fuel distributor above the manifold which caused the manifold to wind up mounted low and the intake ports in the head as well. They wouldn't have spent a ton of money to redesign and retool the head if those retooled heads didn't accomplish a pretty decent gain. They certainly didn't need to raise those ports to improve emissions! They probably did other compromises to meet more stringent emissions and fuel economy requirements and then got most of the power back with the improved head.

Don't be so quick to condemn those windows in the main webs. This is to manage air pumped by the pistons as they fly up and down their bores. This "bay to bay" breathing reduces those pulses by spreading them throughout the crankcase and is beneficial to ring seal and power production. (Remember, there is a windage tray at the bottom of those bores.) Ribs are ribs and are beneficial whether they are outside and/or inside the block.

I don't know my BMWs very well. What years and models are that awful 3.2 in? That's a truly abysmal rod length to stroke ratio. I'm surprised that even BMW would release an engine with that awful of a ratio.
Regards, Eric