what is the comp ratio on these engines? What happens when you change the ratio? Why do guys change it? What is the pros and cons of this?

 

I think they are 16.4 to 1 as SO and HO is 17.4 to 1.
Raising it may create too huch cylinder pressure.
High HP diesels are run with a lower CR and more boost!!!

 

You definately dont want to raise the compression, if anything you want to lower it to get cylinder pressure down

 

That sounds correct to me. One thing to note is that is static compression ratio and when the engine is boosted it is drastically higher!

 

so what do those guys gain by lowering the comp ratio? Is it to reduce the cyclinder pressure for higher boost and higher rpm? If it's good to lower it then why don't they come lower from factory? Being higher(stock) would combust the fuel better, right? Do you gain power or torque or is it to just be more reliable at higher hp?

 

Sorry but the compression ratio does not change with boost. The peak cylinder pressure changes but the ratio is fixed by the difference in cylinder volume at the end of each stroke. I am surprised you got that one wrong.

 

Anything lower can effect cold starting and emissions.
It did surprise me to find the HO motors are higher with about the same boost.

 

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Flatbed24V:

I have a 1996 Dodge CTD 12 valve with a Scheid Diesel built engine. Scheid Diesel lowered my compression for all of the reasons you stated above. Right now, I am a little over 800 H.P. on #2, and my compression is around 14.0-14.5. As you pointed out, I am turning 4,500-4,800 RPM and the wastegates are set at about 75-80lbs., but I can hit 100lbs without them. Some of the "disadvantages" of the lowered compression have already been pointed out by the Members here including: hard cold weather starting, excess blue smoke at idle. Very low compression (10-12) as is used on alot of the Dodge CTD Sled Pulling Trucks or Drag Race Trucks, requires the use of ether to get the trucks started in alot of cases. As you said, my lower compression gives me lower cylinder pressures, more reliability and help with EGT issues to name a few.

I would say that the truck builders (GM, Ford, Dodge) don't lower the compression due to emissions issues.

Hope this helps you some.

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John_P

 

thanks guys that helps me understand, so to change it you would, change the piston, or head, or head gasket, or a combo? If you added a .020 thicker head gasket, is that gunna make a big difference or little to no difference?

 

My truck is right at 15.1 and anything below 60 degrees you have to plug it in to get it 2 start!

 

One extra item is that higher compression allows for better efficiency. The high CR is a key element that allows the diesel cycle to be more efficient than the Otto cycle (gasoline engines).

 

Nope, you are exactly right!!!! My mind was thinking about cylinder pressure and my fingers were typing about compression ratio.

What I was trying to point out that the reason you lower the compression ratio is so that you can run higher boost pressure with lower cylinder pressure and that more boost = higher cylinder pressure.


Its what I get for being online when I should be in bed!!!!

 

intersting mine started this morning at 14 degrees and was not plugged in. and my compression ratio is 14:1

 

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No problem on any of that Quad!

IMO,....your second paragraph and explanation was worded very well!

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John_P

 

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Boy, I wish my 96' would do that Robert! Any temperatures below around 40 degrees and if my 96' is NOT plugged in, I have a hard time starting it unless I use ether. But,.....I am also running Scheid's Forged Arias Pistons and that may add to it.

However, in December I was up in Illinois and the overnight temperature got down to 3 degrees above zero! I had the 96' plugged in at the hotel and the following morning the truck started on the first try. So,....I try to keep that baby plugged in when it gets cold!

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John_P