Low or High Compression?

Turbobark

Turbobark

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Feb 22, 2014
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Hey all,
New to this forum so I hope this is the right section, figured I should post this here instead of Cummins Forum. I have my engine apart and was wondering what you guys thought as to whether I should lower or raise my compression? It will have a single turbo, cam, intercooler, ported head and a big scheid VP and injectors. This is a part time street truck but I mostly drag race with it. This time around I will have it tuned with a blue chip redline and an Edge Drag Comp. The fastest pass I made is in my sig with a fairly low boosted launch and no tuning except for tire size, I was thinking maybe run some common rail pistons but before I pull the trigger I thought Id ask.
Thanks in advance.
 
The H.O. Piston is a point or two higher, designed for the same spray angle. Not sure it would really be worth it or not.
 
I'd say with a vp, nothing less than stock compression.
 
BRE said:
I'd say with a vp, nothing less than stock compression.
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I agree.

I think Karl with redline diesel and and johnny from stainless diesel both run higher compression on their VP trucks. I want to say they are 18:1-20:1 area. Don't quote me on that and I could be totally wrong, just thinking I heard that.
 
99GreenCummins said:
I agree.



I think Karl with redline diesel and and johnny from stainless diesel both run higher compression on their VP trucks. I want to say they are 18:1-20:1 area. Don't quote me on that and I could be totally wrong, just thinking I heard that.
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The way I see it, with the lack of rpm, lowering compression isn't gonna help anything at all.
 
DieselheaD said:
The H.O. Piston is a point or two higher, designed for the same spray angle. Not sure it would really be worth it or not.
Click to expand...

Just my $.02 but i would throw in a set of H?O.'s with a top & side coating. The vp trucks have the lowest compression ratio in stock form in the 5.9 family, so a bump is going to put you in the range of a CR.

FYI, idk if you can run a redline & comp at the same time....
 
If you can swing 14mm head studs, I don't see why you wouldn't go higher compression.

You're never going to make enough power to break the block, probably not even 12V rods with a VP (no offense, none intended), and so your biggest concern with higher compression is destroying head gaskets.
14mm studs, fire ring, and you should be good.

Can you run a CR MLS head gasket in a VP or will it mix oil/coolant like it will on a 12V? That might be a good route too if you can.
 
CorneliusRox said:
You're never going to make enough power to break the block, probably not even 12V rods with a VP (no offense, none intended)
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I disagree with that. vp's make stupid torqure down low.....bye bye block.

I personally would leave the compression alone and spend the money on a good head and turbo.




then a freakin p-pump swap!
 
I would think 12mm head studs(625's probably needed) and o-rings with stock gasket would hold anything a VP can make, even with higher compression, might need to run some pretty aggressive o-ring protrusion like .012-14".
 
Thanks for the replies guys! The head is fire ringed already and i havent ordered the turbo yet. If I run the HO piston how much would that affect turbo spool up? Any suggestion on turbo size? Was planning on running a s467.7/87/.90, maybe with that extra point in compression I could go bigger on the compressor wheel? Thanks
 
I would PM Smokem(Weston) on here, let home know what you fueling is like and go with whatever he recommends. He'll be able to help you out with pistons as well.
 
BRE said:
The way I see it, with the lack of rpm, lowering compression isn't gonna help anything at all.
Click to expand...

You may want to change your way of seeing it, compression is based more on cylinder pressure than RPM. Don't underestimate the amount of money and time that has been spent dynoing parts, if high compression really was an advantage, wouldn't you think it would be common?
 
Smokem said:
You may want to change your way of seeing it, compression is based more on cylinder pressure than RPM. Don't underestimate the amount of money and time that has been spent dynoing parts, if high compression really was an advantage, wouldn't you think it would be common?
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I had always figured a lack of compression is used to make them live. However, I realize some guys lower compression to stuff more air into the cylinder. The only thing I may not quite get is when we have sprint car engines, the good ones are running 18:1, with engine builders saying higher is better, however you sacrifice lifespan. I attributed the same concept to diesels, although be it a different fuel, and forced induction.
 
Smokem said:
You may want to change your way of seeing it, compression is based more on cylinder pressure than RPM. Don't underestimate the amount of money and time that has been spent dynoing parts, if high compression really was an advantage, wouldn't you think it would be common?
Click to expand...


I wasn't necessarily saying higher is better but in my opinion (a lot less knowledge than most here) lower compression wasn't gonna benefit him in his case with vp and such.
 
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higher static compression is a mechanical advantage. but compression is just one of many variables used in any given setup, to get the best performance for a intended use.
 
If I may, and please Weston and those in the know, tell me if I am off ........

You need to understand static and dynamic compression ratios. Static being whats listed, lets say what? 17.0:1 on an HO?
Now dynamic....... you take your expected boost. Your boost divided by 2 is your dynamic, plus your static.

So,
35psi of boost 17+17.5=34.5:1
With twins making 70psi 17+35= 52:1

My point/questions would be this -
How is the engine used? Where would there be a binefit/drawback to this?
What turbo configuration? The turbo has a huge effect on the uliltimate output of the engine.....

So, would an increase in static compression work well on a mild built daily driven/tow engine with moderate rpms? My thoughts are yes.
Why are huge power makers lowering compression? My thoughts are these engines are typically making huge boost numbers at higher rpms. Dynamically, the block/head gasket will support only so much.

So my thought/question to those that know and lay awake at night thinking about this stuff is where is the line drawn? If a guy were to build a 400hp tow rig not expecting to spin 4k in hills that want good spool and mileage is there a legitimate advantage to increasing static compression?
Yes there are timing and injector and cam choices to be considered. I am asking purely from an efficiency point of view. Can/could an increase in static compression yield a reasonable gain?
On the high rpm high boost side I see why boost is reduced. Its in the math. The engine will only hold so much cylinder pressure, and using boost to make up for static compression seems to be the deal, unless I totally missed something. And even there, seriously high cylinder pressure. For arguments sake 14.0 pistons + 200psi = 114:1 dynamic compression. Just math, not specifics.

So, I'm just curious if in the lower hp range (400hp or less) would there be a benefit in HP, fuel economy, and turbo spool if a guy were to increase static compression.......

Thoughts?

BTW not specifically VP directed........
 
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Diesel Nut said:
On the high rpm high boost side I see why boost is reduced.
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Sorry, need to change that to "On high RPM high boost side i see why compression is reduced"

was typing faster than I was reading and by the time I realized it more than 15 min had passed and I couldn't edit it..........:doh:
 
I was once told that you will never learn anything worth knowing by reading the Internet...
 
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