New engine is smoky at idle, did I screw up my specifications?

[Begle1]

Originally Posted by j.fondler

I guess i misunderstood and thought you had the big bowl marine style 155* pistons?


Are they “non-intercooled” pistons or the big bowl marine style?

I bought ".020-over 286210 Mahle pistons".

I don't know what that means or how I ended up selecting that particular piston when I did so over 5 years ago, but it seemed the consensus common recommendation amongst the people I talked to and it's not like I really knew what I wanted in the first place.

I think they're actually the same pistons non-intercooled first gens had stock, right? I'm unclear if they're also referred to as "marine pistons" or anything else?


And do over-sized pistons really have a different compression ratio? They don't modify the bowl a little to compensate?

[JWhite]

I can't offer advice on what Mahle does (if anything) for the compression, but if the bore size increases and nothing else changes, compression ratio will increase

[BigPapa]

Originally Posted by JWhite

if the bore size increases and nothing else changes, compression ratio will increase

MMmmm......I'm not sure about that one.

NOT definitively saying you're wrong, just don't sound right to me. From what I understand, you have to change stroke, piston compression height, dome/bowl shape, or the length of the cylinder to change compression.

[j.fondler]

Originally Posted by BigPapa

MMmmm......I'm not sure about that one.

NOT definitively saying you're wrong, just don't sound right to me. From what I understand, you have to change stroke, piston compression height, dome/bowl shape, or the length of the cylinder to change compression.

He is correct, bore increase alone raises compression, all else equal.

[1972RedNeck]

Originally Posted by BigPapa

MMmmm......I'm not sure about that one.

NOT definitively saying you're wrong, just don't sound right to me. From what I understand, you have to change stroke, piston compression height, dome/bowl shape, or the length of the cylinder to change compression.

If you do the math, it does increase ever so slightly. Doesn't really effect the combustion volume at the top of the stroke, but adds a little at the bottom of the stroke.


My brother runs a non I/C engine with about a half million miles in his Fummins. Engine is completely stock, he just put rings and bearings in it and ground the valves and had the pump rebuilt.

Had a the local diesel shop (really good) put together a set of marine 370's (not opened up at all) and skipped the timing a tooth with the pump at the stock mark. It runs great, knocks out the best mileage numbers for a 12V that I have seen, and yet it will sure clear the parking lot at idle. It will put a souped up Big Cam Cummins on a cold winter day to shame.

If the wind is just right at idle, it will choke you out of the cab in 15 seconds flat.

Pulls hard, runs great, never uses a drip of oil.

I think it's the nature of the beast with these injectors but as said, I would get some miles on it and play with the timing before you spend any money.

[BigPapa]

Well, thanks for the education. I learn new things from you fellows all the time.

[biggy238]

Volume increase of the cylinder.

Sent from my motorola one vision using Tapatalk

[BigPapa]

I understand you’re increasing cylinder volume and that compression will increase because of it, but I just don’t see a 0.020” increase in a 4” bore making more than a negligible increase in compression. Maybe somebody smarter than me can do the math and tell us how much the CR would increase for the over-bore.

[j.fondler]

Originally Posted by BigPapa

I understand you’re increasing cylinder volume and that compression will increase because of it, but I just don’t see a 0.020” increase in a 4” bore making more than a negligible increase in compression. Maybe somebody smarter than me can do the math and tell us how much the CR would increase for the over-bore.

An example from one Mahle cummins piston is 16.5/16.6/16.8 for stock/.020/.040

[ofelas]

Cylinder volume divided by combustion chamber volume (& accounting for the head gasket thickness + piston dish/dome) = compression ratio

[Smokem]

Originally Posted by BigPapa

Maybe somebody smarter than me can do the math and tell us how much the CR would increase for the over-bore.

0.14:1