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Lower compression pistons

SmithvilleD, what I was meaning to say was I haven't seen any numbers, like say from a dyno, you know... that thing that every 6.5er seems to be afraid of :confused:

I can see how lowering the compression ratio reduces peak cylinder pressures, but when you pack in more air to make up for it, isn't that kind of counterproductive because you will be once again raising the peak cylinder pressures?

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The theory I've heard is that by lowering c/r and increasing boost you can get more air/oxygen into the combustion chamber. Which in turn should lead to more power assuming you have enough fuel available.
 
However, lowering the compression ratio reduces the heat which to burn that fuel which is why higher is better. And higher will push out more exhaust gases, so its like a bust for getting fresh air volume in.

If the engine is out and heads are off, then you might change out the piston rings, there are some better ones out, help make sure you have good compression at whatever ratio youre at.
 
I believe , at least when your boost is up, that combustion pressure should be the same which should produce as much or more heat, extra oxygen, than standard compression ratio. The trade off would be when boost pressures are down. So it should produce more max power but lack a little when boost is down. Everything is a trade off it's finding the combo you can live with.
 
The cylinders inability to "scavenge" exhaust gases is such that you only push out whatever you could with the piston and at high boost its worse with backpressure, so the exhaust gases left in the cylinder, more when the compression ratio is lower, will negate any cylinder volume opened up by lowering the ratio.

since air squishes, replacing metal piston volume with more air/exhaust gases will result in lower compression pressure and heat.
 
The theory is that structural limits (failure) are reached/exceeded at peak cylinder pressure.

For a given power level, the peak cylinder pressure spike is higher in proportion with higher compression ratio's. I don't claim to have an iron grasp on the thermodynamics of this, but it's a common belief among engine builders.

So the idea is to maintain as high, or higher mean/average cylinder pressure across the power stroke, while keeping the peak pressure within structural limits.

Practically speaking, you never know an engine's individual failure limit, 'til it fails. However it's pretty much always a deflating/expensive experience. Best you can do is look at as many other's empirical experiences as possible, then choose your path & build it.

I'm not trying to say lowering compression on 6.5's is a necessary thing. However I do believe there is some degree of inverse relationship btwn compression ratio & durability as performance is pushed to structural limits. My cautious approach relates to a very low tolerance for engine failures.
 
Diesel Depot obtains their various compression ratios by a combination of deck height, piston height, rod length and gasket thickness.

I opted to go with 20:1 in my rebuild. This is what was done to obtain it with mine.

I looked up the build sheet and it looks like the compression was gained by using .005 shorter rods and .010 shorter pistons. It looks like we didn't have to use the thicker head gaskets.

Chevyinlinesix, I have no fear of putting my truck on the dyno and fully plan to. The problem is I keep having bills come up for $4-500 that are more important than spending the money for the dyno.
 
I wish I could find a dyno in my area.

Lower compression ratio will be more reliable, anyone could agree with that, saves your piston rings. At high compression and high boost we might be putting them to the test, but thinking 20+ psi boost, assuming you can keep all the temps in check.
 
Diesel Depot obtains their various compression ratios by a combination of deck height, piston height, rod length and gasket thickness.

I opted to go with 20:1 in my rebuild. This is what was done to obtain it with mine.
I looked up the build sheet and it looks like the compression was gained by using .005 shorter rods and .010 shorter pistons. It looks like we didn't have to use the thicker head gaskets.


Chevyinlinesix, I have no fear of putting my truck on the dyno and fully plan to. The problem is I keep having bills come up for $4-500 that are more important than spending the money for the dyno.

I'm sure you are aware that changing connecting rod length has no affect on compression ratio.

As for dyno, I mean take a stock 22:1 compression ratio engine and dyno, then lower compression and dyno, then adjust boost and dyno again. To see if there is any power gain from it.

I just wonder why there seems to be so few dynos done of these engines, there are many things more important than engines, I'm just trying to get people to dyno their stuff too :)
 
While the piston will be .01" further down in the cylinder at TDC, it will also be .01" further down the cylinder at BDC. The connecting rods only change the distance from the crankshaft's connecting rod journal to the piston pin.
 
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Compression ratio is the ratio btwn the volume above the piston at bottom dead center & the volume above the piston at top dead center.

Volume above piston at TDC = small volume

Volume above piston at BDC = big volume

The 0.010" difference in volume is a far more significant portion of the chamber volume at TDC than at BDC
 
shorter rod does lessen the ratio.

The displacement is the same, bore area x stroke, but the volume left at TDC is greater.

Therefore the ratio of displacement to leftover volume has changed. With .01" more at TDC that adds 0.13cu in volume to 2.322cu in.

So now the compression ratio is 49.454cu in:2.452 cu in which is 20.17:1 vs 21.3:1 ratio.

4.06in bore diameter, 3.82in stroke.

pi x radius squared x stroke = cylinder displacement volume = 3.141593 x 4.121 x 3.82 = 49.454cu in x 8 cylinders = 395cu in

49.454 / 21.3 = 2.322 = volume left at TDC with original stock compression ratio.
 
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I think I read that right SmithvilleD :)

Basically, the only way to change compression ratio is to change;
Deck height
Compression height
Chamber volume (cylinder heads, piston dish/dome, gasket, precups)
 
FWIW - I'm trying to dig up my notes. It's been about a year since I did this work. We cc'd a prechamber w/ glowplug & injector installed. Did our best to cc the piston top flow pattern & then estimated the gap volume above the top ring. Seems like the std Felpro head gasket compressed was something along the lines of 0.041 or 0.045"
 
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I have my cc numbers for all that stuff right beside me in my stack of papers, but I am going to take a shower and going to bed now, haha. I was struggling to explain it myself, just took a few tries to get the words narrowed down :D
 
In essence, the amount the rod is shortened moves the piston further away from the head by that amount - both at TDC & BDC. So you're adding that 0.13 ci volume to the area above the piston, both at TDC & BDC.

It's easy to confuse displacement/swept volume with the total volume of the cylinder/chamber (which is always gonna be a bit bigger volume, by the volume still above the piston at TDC). Otherwise you'd be compressing the mixture to 0 volume at infinite pressure:eek:
 
shorter rod= less compression......a piston and cylinder are a piston and cylinder.....when i build chain saws .017 to .020 squish is worth about 15psi of compression between the 2....and you are doing nothing than raisin' or lowerin' the jug.....same as lengthin' or shortin' the rod....
 
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