17.1 vs. 15.1???

[LTS Max]

I'm still learning in the diesel world vs gas motor and I'm curious why if your motor is built strong (rods/studs/etc) would you see a benefit from lowering the compression ratio down to say 15.1? I've read that lowering it takes some strain off the motor but don't you lose some potential power and drive-ability with that? If people are throwing nitrous on top of 60+lbs of boost with 15.1 ratio, can't you run 17.1 and 40-50 or more psi boost and stay safe with a built motor? Just interested in the pro's and con's of decreasing / increasing the ratios. Any comments??

Thanks

 

[Trippin]

Basically people are lowering the static compression ratio in order to increase the dynamic compression ratio with more boost/Nitrous.

The effect is to soften the torque in the lower rpm range with the hope that pistons, cranks, blocks etc will live.

Too much cylinder pressure too low in the rpm range is hard on parts.

As the rpm goes up, the reciprocating/rotating assembly is exposed to higher cylinder pressures for shorter but more frequent intervals.

 

[LTS Max]

Thanks for your reply. I'm curious though, with EFI Live aren't you able to effectively move the fueling and boost tables up in the RPM range to avoid the extreme torque at the low end? (ie-the post Pat mentioned the high horsepower/low torque tune) With gasoline motors they use to state that 8.1 was recommended for blown applications yet today you see some turbocharged motors running as high as 10.1. Even with todays lower octane fuels. Isn't there some gains to be seen in diesels with leaving the compression high yet tune the fueling to avoid the damaging low RPM torque? What compression ratio's are the Audi-Peugeot race engines running?

 

[Noreaster]

With gasoline motors they use to state that 8.1 was recommended for blown applications yet today you see some turbocharged motors running as high as 10.1.

Some of the high hp Cummins motors are down to 10:1 can you say ass loads of ether.

 

[Accelerator]

There was some good info here, any more comments?

 

[McRat]

We are limited by the temp required to ignite diesel fuel at idle.

A "Hot EGR" system might actually improve starting and idling with very low compression.

But without some kind of assistance, compression lower than 14:1 is going to make it impossible to start or idle even in hot weather.

 

[JoshH]

I had about 15:1 compression ratio on my first build. This time it's going to be closer to 16.5:1.

 

[Turbo Buick 6]

Thanks for your reply. I'm curious though, with EFI Live aren't you able to effectively move the fueling and boost tables up in the RPM range to avoid the extreme torque at the low end? (ie-the post Pat mentioned the high horsepower/low torque tune) With gasoline motors they use to state that 8.1 was recommended for blown applications yet today you see some turbocharged motors running as high as 10.1. Even with todays lower octane fuels. Isn't there some gains to be seen in diesels with leaving the compression high yet tune the fueling to avoid the damaging low RPM torque? What compression ratio's are the Audi-Peugeot race engines running?

I can tell you with gassers it has been proven many times that boost make more power than compression. Yes there are some turbo gassers running 10 to 1 but they would make more power running say 8.5 to 1 and more boost. I am going to ASSUME it is the same case with diesels.

 

[Turbo Performance]

I can tell you with gassers it has been proven many times that boost make more power than compression. Yes there are some turbo gassers running 10 to 1 but they would make more power running say 8.5 to 1 and more boost. I am going to ASSUME it is the same case with diesels.

I agree. Compression points don't gain you as much HP as boost does.

 

[JOHNBOY]

I am at 15:1 and I have a very large cam with some lobe overlap. Even with Freaking huge injectors the truck will start right up. Even if it is in the 30s outside. It will be a grumpy, smokey MOFO but it will start and idle. 05_LLY and UPSDRVR have both witnessed my cold start up.

My setup is not what I would call street freindly. It is very grumpy till the coolant temps get above 165*. I do not think think it would be nearly as bad if I still had the stock cam. But my cam is setup for RPM power. For that it works very well.

My compression is 15:1 for several reasons.
1 I delipped my pistons.
2 I need .100 cuts to clear the cam.
3 To lower static cylinder pressure and make power with boost instead.

What have I learned? I dont like 15:1 with my cam!:rofl: More Static compression with this cam would be a good thing. But more is not possible as long as I am starting with a stock piston. This winter I hope to replace the 15:1 pistons with 16.5:1 forged pistons.:coolspot:

But I am very happy with the power I make now.

 

[Stingpuller]

Johnboy

Your not happy with the power! None of us are happy with are power. We always need more.:rofl: Jeff

 

[duratothemax]

I had about 15:1 compression ratio on my first build. This time it's going to be closer to 16.5:1.

was that at the time you put it together or the time you took it back apart???

ben

 

[ChevyDieselLLY]

how would 16:1 be for daily driver and twins? (not sure if having twins or not matters when it comes to compression)

 

[duratothemax]

how would 16:1 be for daily driver and twins? (not sure if having twins or not matters when it comes to compression)

thats what mine was and I never had any problems down to 20* or so. It would run rough at that temp for a few minutes not being plugged in, but other than that I had no issues.

You sacrifice a slight bit of low end and around town "spunk", if you will, with the lowered compression...or at least thats what my butt dyno said when I went from 16:1 back up to ~16.7:1. I was running twins with the last motor, and still running them now.

ben

 

[mainer]

i can say my stock compression lb7 pistons/ stock engine started last winter at -37 F without being plugged in, ran rough then smoothed right out.....

cold starting is highly important to me as where i go to school it gets as cold as -30 and there is no where to plug in.

 

[[email protected]]

I limit my mm3 to 80-90 range where pulsewidth is 2200-2600, then I start ramping it up to 3200 once I get to about 2600rpms on my all stock LLY.

I don't think it actually gets used very much becuase it is hard to make get full power below 2600 becuase the turbo is so slow to spool up.

Can't you still make a lot of torque above 2600 even with the stock turbo, engine?