You'd be surprised the number of guys secretly hopping on the higher compression bandwagon,,,, tractor and truck guys alike,,,,,, 
What compression ratio?
- Thread starter Evan@InglewoodTrans
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In theory higher compression (within reason) is better.
Just like a N/A gas motor, more compression equals more power (given same cubic inch), BUT tuning plays a MUCH LARGER role in the longevity of the engine especially when you start adding forced induction.
Detonation in a gasser can have a huge impact on piston/rod/bearing life and the higher the compression the more attention has to be paid to timing advance, fueling, etc.
The same holds true for diesels, with lower compression you have a larger window of "error", a little too much fueling here, too much timing there and there most likely will not be any issues. Once you bump the compression you exponentially decrease your margin of error (from a tuning perspective).
I believe increased CR is a good idea (fasting spooling, more complete burn, etc), but parts selection and tuning become CRITICAL. Even the best parts won't last long with poor tuning......
If you want a larger window of error and can give up a few ponies, a slightly lower CR would be better. I think the OP might have stated they are going with Fingers pistons, it would be interesting to see them in a high CR build that gets beat (with proper tuning).
Just like a N/A gas motor, more compression equals more power (given same cubic inch), BUT tuning plays a MUCH LARGER role in the longevity of the engine especially when you start adding forced induction.
Detonation in a gasser can have a huge impact on piston/rod/bearing life and the higher the compression the more attention has to be paid to timing advance, fueling, etc.
The same holds true for diesels, with lower compression you have a larger window of "error", a little too much fueling here, too much timing there and there most likely will not be any issues. Once you bump the compression you exponentially decrease your margin of error (from a tuning perspective).
I believe increased CR is a good idea (fasting spooling, more complete burn, etc), but parts selection and tuning become CRITICAL. Even the best parts won't last long with poor tuning......
If you want a larger window of error and can give up a few ponies, a slightly lower CR would be better. I think the OP might have stated they are going with Fingers pistons, it would be interesting to see them in a high CR build that gets beat (with proper tuning).
Giving up HP with lower compression is false information. Sorry, theres alittle machine shop down south that has dynoed engines back to back and actually gained horse power on the same engine with NO changes except pistons. They lowered the compression and gained power
If the higher CR engine was improperly tuned or turbo/fueling was not optimal for the setup, I will agree 100%.
I can build a high CR engine that has minimal power output
I can build a low CR enigine with substantial power output
There is more to power than just CR
High CR in and of itself does not equal more power
Low static CR with a ton of forced induction CAN produce as much as a High static CR with no or minimal forced induction (assuming similar platform and cubic inch).
There is also a point where too high is not feasable due to fuel restrictions (detonation in a gasser), parts availability, and logistics (getting fuel in quick and accurate enough).
The ideal CR depends on all factors, if it weren't for piston issues (and assuming proper tuning), I bet CR's would be higher than they are as many decided to lower CR thinking it would help piston life, and it's what mechanical engines have done due to lack of fuel control running big air and big fuel (not that they could run high CR's, but I believe given better fuel control, they could have been running higher, but they were stuck with mechanical injection timing).
Gas engines need lower CR as boost increases much more than a diesel
Both engines become much more finicky the higher CR is. Again, your window for errors becomes exponentially smaller as CR goes up.
I would also be willing to bet if you took an optimally tuned engine and ONLY decreased compression, you will lose power as it's simple physics.
On that same token, take an optimally tuned engine and increase compression, you most likely will not make more power (without tuning changes) and could actually lose power (if it passes the optimal threshold).
You still haven't taken in the information that is on this forum, and in this thread, about compression ratio though...it's not all about compression ratio, there are lots of other variables that can be played with to best use higher compression pistons and also lower compression pistons. In the example of my buddy's cummins, the one I mentioned earlier, he is running 15:1 arias pistons with a custom bowl design and he sunk his injectors into his head a few thousandths (Scheid injectors) and the thing fires up just as quick as my truck, spools faster than he ever imagined, so from our standpoint, it worked well. Take that same piston and keep the injector in the stock location and I'll bet he makes way less power and would barely be able to come up on the turbo.
Now you say a builder has dynoed this engine with no changes but pistons; what did he use for pistons? LBZ vs LB7? or was this not even a dmax?
Interesting facts to consider:
The displacement is not affected by CR
A 16:1 CR chamber leaves about 10% more dead air in the chamber than a 17.5 CR chamber after exhausting. (~4.5 cc)
The displacement is not affected by CR
A 16:1 CR chamber leaves about 10% more dead air in the chamber than a 17.5 CR chamber after exhausting. (~4.5 cc)
I know it all looks good on paper, I guess I like to see my proof on the track... Maybe one day they will turn around the duramax as we know it, but right now they are only breaking parts and smoking less on start up.
You'd be surprised the number of guys secretly hopping on the higher compression bandwagon,,,, tractor and truck guys alike,,,,,,
And are they accomplishing anything with it?
I know and talk with a lot of the owners of the strongest dmaxes in the country, and Cummins alike. None of the top runners are running high cr.
People have been playing with it for a couple years, and they don't seem to be geyting any closer to the top of the pack.
some of the big cummins engine pullers are running it, some of the top tractor guys are running it.
Would it work well in a street setup? I want to eventually do a motor, seeing how it will mostly be a driver and maybe a few weekends a year at the track. I just want it to behave as close to the way it does now, most it will get is maybe something twinned to my 66.
Our high cr cummins engine worked great on the street with the 76mm single on it with a manually shifted 47re and higher stall converter.
Fingers, have you found any correlations between CR and chamber pressures?
I know it's tough as it not easily feasible to rip apart a motor and just change CR with no other changes, but just wondering if the the pressure spikes seem to be affected by higher CR? I think it's safe to ASSume a higher CR would produce a higher chamber pressure, but are the rates of increasing and/or decreasing pressure different. If my thinking is correct, pressures within the cylinder would have to increase/decrease at a faster rate given a specific RPM when CR is higher.
Different cams would have an effect on chamber pressure (overlap and possible bleed off....at least on gasser engines, I have zero experience with turbo diesel cams) as well as the amount of force fed air and fuel, but didn't know if any "generalizations" can be made by looking at graphs from different motors.
Simple relation between CR and pressures. Higher CR, Higher pressure. No getting around it. But the analogy with gas engines is valid. Higher CR for power, lower to save parts.
Higher CR scavenges the chamber better and is more efficient, but at a cost.
I'm not saying you can't make good power with a lower compression engine. I'm saying you can make more with a higher one.
I'll get into some interesting things I have been working on with piston strength and stresses when I get off the road next week.
High compression in a 1000hp+ engine with good boost will give us more broken parts. Rods are pretty pricey to be raising cr and not gaining much power....
