Yes, another compression ratio thread

[bshull]

I am about to pull the trigger on a new engine for my burb. It is a 95 K1500 GMC. I have just rebuilt the front end, replaced the IP, new tires, new shocks, new breaks, high idle mod, glow plug override, and a racor fuel filter/heater.

My plan all along was to use the original engine as long as I could but upgrade/refresh all the other components so I would be ready for a new engine. I have settled on an optimizer or p-400. I have been shopping around and I am about ready to purchase.

My question is what compression ratio?

My driving habits are 90% highway (usually interstate and running 5 MPH over the limit) and the rest is in town. I rarely tow and even then it is a smaller u-haul type trailer. I do carry about 1,000 lb of tools in the back. I live Kansas and my work travel takes me across N. Dakota, S. Dakota, Minnesota, Wisconsin, Michigan all the way down to Arkansas. I log about 60 to 70 thousand miles per year.

18 to 1 compression ratio sounds nice for the long drives running 75 mph down the road for hours on end but I am concerned about cold starts. The lowest temps I have found have been -20 deg F. I do not always have a power outlet to plug into at night so I need this thing to start with out assistance other than extended glow plug time.

Other plans with the engine install include a new cold air intake, tune, and turbo, I am thinking about an ATT for better MPG. My turbo choice will affect where I get the tune from.

I am under the impression that with a lower compression ratio you can run more boost for more power (since this is a highway traveler I only really need power to pass). Will the ATT provide enough boost with 18 to 1?

Which compression ratio would you select and why?

Thanks
Brian

 

[DZZ71]

Lower myself, I built my first one to the mid to high 16s ratio. The truck started in -25 with out being plugged in I couldent believe it. Just half to have a good set of glow plugs and the bypass mod to get the extended heat. Other then that I like it. The engine I'm doing now won't be as low since last time was an accident. I have no regrets

 

[great white]

I'm gonna say stay with the 20.2:1 that the Optimzer comes with for your uses.

I've got lots of reasons, but it would take too long to "hunt and peck" it out.....

 

[6.2 turbo]

I built a low compression motor,and hated it,it was around 16-1.

 

[DZZ71]

You must have been lower then that. I shaved 0.070 and you shaved 0.090 Another member figured mine to be around the mid 16s. I would think another 0.020 would knock yours down more then half a point. I have no white smoke at idle like you did eather. Maybe I'm wrong I donno.....

 

[Mike K3500]

I did some research before installing the +.010" Fel-Pro gaskets in my in my engine and what I was able to come up with was +.010" head gasket dropped compression approximately .2 pts.
Ultimately bringing my Optimizer from 20.2:1 to 20:1.

I personally wouldn't run 18:1 or lower for a daily driver that doesn't see any towing... Get your self a set of +.010" Fel-Pro's and a set of ARP head studs, you wil be just fine.

 

[Woody35]

I did some research before installing the +.010" Fel-Pro gaskets in my in my engine and what I was able to come up with was +.010" head gasket dropped compression approximately .2 pts.
Ultimately bringing my Optimizer from 20.2:1 to 20:1.

I personally wouldn't run 18:1 or lower for a daily driver that doesn't see any towing... Get your self a set of +.010" Fel-Pro's and a set of ARP head studs, you wil be just fine.

x2

 

[6.2 turbo]

My 6.5 is decked .010 and it went from 21.5 - 22.6. Now if you had 10-1 compression.010 would be closer to a .2 increase,but up around 20-1.the slightest change makes a big difference in ratio.

 

[chevyCowboy]

i put the .010 gaskets in mine and from what i was told/understand that should have brought me down to around 20-1 but that is on a 599 that was 21.5-1

 

[Ratman]

I'm certainly no expert on 6.5's, but based on the information you supplied, I would keep it as close to stock CR as possible considering you aren't going to flog it with big boost and fuel, or tow. This is just my personal opinion but, in your situation it would be kind of pointless to lower CR other than for lower internal stresses and increased reliability.

My daily driver/flogger is a 19:1, -and realistically, I think it is the perfect engine for the way I drive. It tows adequately, and is a blast to drive around town empty in daily-driver role. It still gets upwards of 16 mpg on the highway if I'm easy on the go pedal and keep it below 60 mph (with 4.11's she runs around 2000 RPM at 60).

The 19:1 has so little rattle, that several people have told me the only way they can tell it's a diesel is from the smoke and the turbo whistle (most people just think it's a 454).

My vote is to keep your CR above 19:1 anyway, -preferably closer to 20:1 for your application.

 

[chevyCowboy]

i like my 20-1 it runns smooth and loves the 14-15lb of boost i dont drive with a light foot to often and i routenly get 14-16mpg with most of my driving around 55 to 60mph with a fair bit of stop and go city mixed in.

 

[bshull]

Thanks for the replies! Since I do not tow I think I will go with 20 to 1 compression. It is a little lower than factory which should relieve some lower end stress while maintaining a good cold start.

With an ATT turbo I am guessing that I would not see high boost numbers since I will not be working the engine too hard but if I ever try to tow something the drop in compression ratio should be able to sustain 15 psi boost with ease.

I hope I have figured it out right, once again thanks for the replies

Brian

 

[buddy]

Here is a file I built to determine your CR changes with different methods of altering it.

I would stick with 20:1, and with an Optimizer if it starts at 20.3:1 I would just stay with it there, unless you are taking the heads off anyway, then you could add the +.01 gasket and it will take you under 20:1 but still fine.

at stock levels above 20:1 CR, a .01" change is almost a full point change

 

[Mike K3500]

Eventually when I get my Optimizer finished.. I plan on running 20 lbs of extra atmospheric pressure to it. With the studs it should be fine, I would think.

 

[Ratman]

Here is a file I built to determine your CR changes with different methods of altering it.

I would stick with 20:1, and with an Optimizer if it starts at 20.3:1 I would just stay with it there, unless you are taking the heads off anyway, then you could add the +.01 gasket and it will take you under 20:1 but still fine.

at stock levels above 20:1 CR, a .01" change is almost a full point change

Buddy, according to your spreadsheet mine looks to be closer to 18.5:1?

The 18:1 marine pistons have .032" reduced compression height, and the block is decked .010" with stock thickness gaskets. What would that yield?

Oh yeah, -with diamond cups...

Thanks, Rich.

 

[chevyCowboy]

i forgot this in my post my block was decked .004 so by adding the thicker gasgets i only ended up with .006 extra. now here is an odd ball my block was bord .030 over and oversized pistons installed dose that have any affect on the compression ratio. i have always figured it wouldnt.

 

[Mike K3500]

Yes it will, but how much... that is for somebody smarter then me to give the answer. It does because with over sized pistons you are creating more cylinder volume, therfore you are compressing more cylinder volume in the same size combustion chamber as you were with std. bore pistons.

 

[buddy]

Buddy, according to your spreadsheet mine looks to be closer to 18.5:1?

The 18:1 marine pistons have .032" reduced compression height, and the block is decked .010" with stock thickness gaskets. What would that yield?

Oh yeah, -with diamond cups...

Thanks, Rich.

The lower compression height, is that acheived by shaving the pistons or are the swirl bowls still full depth? youre probably looking at about 18.5:1. The diamond precups are what brings the CR down from like 21.3:1 to 20.2:1. That was GM's method.

 

[buddy]

i forgot this in my post my block was decked .004 so by adding the thicker gasgets i only ended up with .006 extra. now here is an odd ball my block was bord .030 over and oversized pistons installed dose that have any affect on the compression ratio. i have always figured it wouldnt.

The oversized pistons are pretty negligible, like a 0.02 change in ratio, because you are also increaseing the combustion chamber. The combustion chamber is not only the precombustion chamber, but also the small volume between the piston and the surface of the head.

 

[chevyCowboy]

so if i understand that right by adding the .01 gasgets minus the .004 that it was decked and adding the .02 for the lager pistons i should be at like .026????????? im confused i really want to understand how all this affect each other but i always end up giving up cause i get a head ach!