Decompression and engine theory.

TheBigNasty

TheBigNasty

Project bish
Joined
May 9, 2007
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First off who started this fad? Is it really a good idea?
I've always been told when super/turbocharging gas motors that lowering the compression would net more power. Do diesels act the same way? Is the reason it works for gas motors because of the ability to detonate under higher static cr's?

From what I've been reading, diesel likes a higher cr to burn more complete. There's a point to where the additional power lost to compression overcomes the benefits of the more complete burn. So it should be at that cr that the engine is most efficient. I'm willing to bet it is somewhere around this point is where the cr for the stock cummins is set. So by lowering the cr we are loosing efficiency and also causing lag because the lower cr takes longer to light the turbo as well.

So I ask, what is the point? What am I missing?

I am about to take mine back apart and either put a stock 12v gasket on and mill the head .015-.020 or put a .020 and mill the head .035-.040. Either way the injectors should sit back in where they should but do I want to keep the slightly lowered cr or get back as close as possible to stock?
 
Well, we push more fuel into our engines, at a higher boost. Plus, we crank our timing up quite a bit, all in the effort to gain performance. Throw a little water/meth or nitrous in there, all this stuff adds up to higher cylinder pressures. We do things like o-ring the head, put in studs and the like in hopes of keeping the head attached to the top of the block. Dropping compression a bit lowers those cylinder pressures. You're right about the compression ratio on a stock engine being ideal for stock boost levels, stock fueling and stock timing. I'm sure Cummins does their level best to be sure it's the ideal ratio for the given power level. But go away from stock, is it still ideal?
 
my thought on this was that if you lower the compression ratio you are effectively making the combustion chamber bigger to allow for more fuel and air to be burnt.

just my thoughts
 
I think by lowering the CR you are making the cylinder bigger so thiers more room (like making the engin bigger) for boost to pack more air in to the cylinder...witch makes more power.

But i don't know a lot about this...just how i think it works!
 
Milling the head has virtually zero effect on CR since diesel heads are flat and have no combustion chamber (you can think of the combustion chamber as being in the piston crown).

Dropping diesel CR by a considerable margin has its drawbacks such as harder starting, more blue smoke, lower MPG, poorer off-idle response, but like PRattenbury mentioned it's basically a necessity when running a lot of fuel, which requires a lot of air (and boost) to keep EGT in check. Running too high of cylinder pressure results in blown head gaskets, excessive blowby, oil leaks, etc.
 
droping compression alows for alot of things such as big turbos 3.0 and bigger and more by droping compression you can ad the fuel to to get a turbo that large lit with out such high cylinder pressure
 
he is talking about milling the head in order to get the injectors back down to their original height in relation to the piston. the question is what should he do to the compression ratio.

I guess the question is, if you give the cylinder another .020" of height (.020" thicker gasket) will the added air that you can pack into that space make up for the amount of power you are going to lose from lowering the compression 1 full point (16.3:1 to 15.3:1 or 17.2:1 to 16.2:1)? And will it actually makeup for the difference and still make more power?
 
I know............ Been drinking............ St. Paddy's and all......... Not reading very carefully LOL
 
If you are not going to turn more rpms, decompressed is not desireable. You will loose bottom end torque, spoolup, idle haze yes but you dont have to go to 9 to 1 like the pullers do. Id say for a street truck, 15 to 1 is ideal. More room for air and fuel, little stress of the head studs, it will still start okay, and help your motor live longer for the way we treat them. Id say the average vp44 motor turning 3300-3600, just run the 90 thous marine gasket and go. Cranked ppump, start cutting pistons for 14 0r 15 to 1 compression for street, between 9-12 to 1 for race only .Straigt decompressing will loose hp so you have to make up for it somewhere. my 2cents. Tim
 
mod trucks. This i dont know first hand,but coming from some pullers and a guy that used to work for Schieds. Plus wasnt there a truck in the Diesel power mag that listed his at 9 to 1?
 
I guess i should have said "like i have heard"! Maybe one of our mod pullers will get in here and give us a little insight on this first hand.
 
i know a guy that has his compression around 11:1 on a street truck...he was to use ether everytime he cranks it when the motor is cold
 
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