Lowering the compression ratio mechanically reduces cylinder pressure, which is easier on the rotating assembly. It allows the engine to easier handle higher boost and fuel volume, there is more area in the cylinder. The correct way to do it is machining out the piston's bowl, like this:

 

:whs: Another way to do it is like this.

 

Who does this? And how much does it cost? Do you just use Mahles for this? Thanks.

 

I just used the stock pistons as they were fine and had my uncle cut them for me. Most everyone uses Mahles (which is the oem manufacture).

 

Another vote for machining the bowls!

 

 

How much does this effect starting in colder climates?

 

That depends on how much compression you take out of it.

 

Mine is 15:1, and with glowplugs, it starts up fine outside, but will chug the white smoke, for a very long time, and will blow white/blue for a minute or two going down the road under light throttle. I try to keep it in the heated garage and it likes it..

But if you don't wait for the glowplugs..

 

PSJunkie~

that's in the tunes, IMO.

We've got a few 14:1 engines rolling around that don't smoke that bad.

Dave

 

We've got a few 14:1 engines rolling around that don't smoke that bad.

Dave

How about starting in -10 to 0 deg F? Or have you had any lowered CR engines as daily drivers in that temp range? Block heater required at that temp with lowered CR?

 

So, at what point in an engines growth would it be a good idea to do this? Past 600? Going to twins?

 

IMO, if you are going to be using the truck competitively...and will be pushing more than 50-55psi of boost (with plans to go higher) then dropping the static CR has its benefits.

If the truck is going to hang around the sub 500hp arena for it's existence...then stock CR is fine...and the costs associated with "just" dropping the CR a point or so to maybe...16:1 wouldn't be worth the expense, IMO

Dave

 

How about if you're going for 70-80lbs of boost on a street truck?
Also, seeing as how this reduces cylinder pressure, does it reduce horsepower?

 

Yes, and yes - apply more boost and make up the difference. There's a few posts on here from the past talking about the mechanical benefits of lowering the static compression ratio.

 

i plan to go 15:1 whenever i have to build a motor

 

What about starting issues on one with dropped CR?

Please tell me I dont need ether.....

 

The ones I've been around always started pretty good. As long as your glow plug system is in good shape, I think you'll be fine. Usually it's engines that have had big changes in timing that are hard to start.

Justin

 

i am sure i will anyway w/ my new dummy glow plugsointlaugh:

 

Thanks Dave. I'm about to have a whole 'nother animal with a lot of crap i'm still learning about. :doh:

 

So Dave, are you saying that you run the same pressure and the extra volume makes up for the horsepower without the excessive CP's?

Sorry, but it just makes sense to me that if you lower the CR, and add air, you're right back in the same bucket.

 

YES! you ARE in the "same bucket", with regard to cylinder pressures...BUT, you now are making that CP with POWER (more air with which to burn more fuel)...instead of nothing other than piston material hanging around in there...

This is one of these circular logic things that are easy to get mixed up on...

take any CP value, at some CR...say factory ~17.6:1

now cut the piston, to lower the CR...now ADD back in oxygen (in the form of higher boost)...and take it right back to the PREVIOUS CP...you just made more power, without increasing CP.

 

so i'm taking it like this correct me if i'm wrong...

on a pretty heavily modified engine if your over fueling big time and your cp are throught the roof(to an extent) you can lower the compression ratio and add more air to drop the cp and burn the excess fuel more efficiantly..

 

Dave, that tuning issue should be fixed soon

Garrett, no you shouldnt have to run ether, if you do have glow plugs. When i bought the red truck with the race engine in it, it had 13:1 pistons, with dummy glow plugs. Would not start cold, without ether. Thats a two-person affair and quite the pain in the ass. Now at 15:1 i have worries that it wont at-least "start" in 20-30* weather, not plugged in. My headers come too close to the block heater element to get it plugged in, i don't know what to do there.

 

Thanks guys. The ether comment was more or less a joke.....but I was wondering about start up ease.

 

Why put in dummy plugs? :shrug:

 

No risk of breaking off a glow plug tip in extreme temperature/pressures, and risk sending it through the engine.

 

yup. and its straight puller now and wanted to get rid of some junk on the top of the motor and reduce mantinance.

 

I live in Alaska and negative ten isnt extreme, but if you call it extreme that must make me hardcore...or stupid, ahhaa anyways, i was also considering going lower compression not a whole lot, but maybe enough to clean up my smoke, i may just need better tunes but if i go lower compression then i can go witha bigger turbo....does anybody know what kind of miles per gallon you can achieve by chaning the compression ratio? i probably wont ever get those other things

 

daily driving an engine that would benefit from 14:1 or lower CR...in Alaska...in sub-zero temps...would be extreme

don't confuse lower CR with increased engine efficiency. Quite the contrary. You would actually want to increase your CR to improve mpg's.

From an efficiency standpoint, the higher the CR the better. More 'squish' with which to combine & mix the air(boost) and fuel...the more complete (efficient) burn to extract the most power per unit of fuel.

When you start cramming in 2, 3, 4, 5, 6 times or MORE fuel per cylinder...PER injection event...you would NEED multiple times more oxygen within that cylinder...so that you could begin to extract a reasonable amount of power from that much fuel....

that's where the retardedly high boost comes in...along with creative ways to COOL that boost...so that the oxygen content is more useful.

...and that puts us right back into the trading "aluminum" for densely packed boost solution by lowering the static CR.

take care~

Dave

 

(all other things constant) lowering the compression ratio would (and does) lower CP.