Expiremental OM617 build

atypicalguy

10-12-2016, 05:16 PM #30

(10-12-2016, 06:54 AM)R-3350 im running a HE221W right now if i go for compounds in the future ill probably go away from holset turbos at least for the HP turbo. their turbos tend to be optimized for a wide map combined with fast spool that pushes the compressor efficiency quite far down. with compounds you want turbos that are efficient in a smaller band due to the narrow operating band that each one actually sees.

as for the rpm increase vs BSFC the reduction is not an independent variable. it falls on its face for a few reasons the reduction in VE due to the horrible flow of the head. this combined with the restrictions in the pre-chambers preventing the expansion of the combustion into the chamber fast enough, and the timing mechanism not advancing far enough. i have worked on most of these things already and intend to keep experimenting as i go. the need for high boost to get enough air into the engine is solved with aggressive porting and a better cam. those are two of the things im working on, theres a lot more to be done too that may help get this engine to perform closer to its more modern brothers.

i have a rather large water to air set up im installing later this week the core is 11X6X4.5 so i don't see out growing that one for a while.

Yes it is definitely easier to choose turbos that have compressor maps more readily available. The efficiency thing is one reason I chose the hx30 over the 221 for my hp turbo. I chose aN he451 for the lp, which is a bit bigger than an hx40. It maxes out around 80lb/min.

For true compounds yes everything should function in a narrow band of pressure ratio, but in reality the hp is usually undersized for early spool, so it does nearly all of the work until the big turbo spools. So it should spool early and have a wide map. After the big turbo spools, it does most of the work and the little turbo only adds a little bit. So it also needs a nice wide map. The fact that the hp turbo is operating at lower efficiency at higher rpm doesn't really matter much, because it isn't doing much compression work. There is a guy on yellowbullet.com named kjewer1 who runs a drag car with compounds, and has provided a huge amount of detail on how his setup has evolved over time. I am basically trying to emulate that type of performance: early boost off the line, large turbo doing heavy lifting later in rpm range. My driving is in town and spool at 3k rpm is a bit late.

I am running a 606, but spent a bit of time thinking about a 617 first. To run 150cc and afr 17:1 on the 606, I would set the small gate at 65psi. It would have to be even higher for a 2 valve head to achieve similar airflow for 150cc fuel I think. Of course there are many people running 13:1 with smoke and high egt, so that is always an option, but I look at it as cheating :-)

Looks like you have thought things through and have a great build going. The om617 is so common that it it is a shame more people don't rework the heads etc. As you are doing. Post some photos of your intercooler if you can; that is my next step so I am looking for ideas.

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