The mother of all "compression ratio" threads...

[cc-rider]

Not sure if this thread will take off or not. But I'd love to get a big thread going on forced induction compression ratio.

I've been doing a ton of searches and reading a lot of threads. Answers on this subject are all over the board. And for good reason. So many variables and trade offs related to compression...octane vs boost vs timing (probably the 3 most dominant factors), IAT, bower type, turbo, throttle response, gas mileage, etc, etc, etc. I've been reading a lot of strong opinions that the old school 9.0:1 CR for forced induction is not the way to go with these LS motors. 10:1 is king? But then there are many tried and true "old school" (low CR / high boost) putting up some impressive numbers as well.

I'm wondering if we can get some detailed write ups from those with experience explaining the logic behind their position? Mention all the variables and give some details. I've got a feeling that there isn't a perfect right or wrong answer here. Maybe turbo is better with high CR while blowers better with low CR????

Let's hear it!

[ttshovel]

Originally Posted by cc-rider

Not sure if this thread will take off or not. But I'd love to get a big thread going on forced induction compression ratio.

I've been doing a ton of searches and reading a lot of threads. Answers on this subject are all over the board. And for good reason. So many variables and trade offs related to compression...octane vs boost vs timing (probably the 3 most dominant factors), IAT, bower type, turbo, throttle response, gas mileage, etc, etc, etc. I've been reading a lot of strong opinions that the old school 9.0:1 CR for forced induction is not the way to go with these LS motors. 10:1 is king? But then there are many tried and true "old school" (low CR / high boost) putting up some impressive numbers as well.

I'm wondering if we can get some detailed write ups from those with experience explaining the logic behind their position? Mention all the variables and give some details. I've got a feeling that there isn't a perfect right or wrong answer here. Maybe turbo is better with high CR while blowers better with low CR????

Let's hear it!

I believe it depends on the setup... with the 1.9l blower I have 10.9:1 compession. And I will add compression when this motor goes... compensate for the lack of blower!

As far as the turbo goes. The stock 5.3 guys like to use 317 heads, which net 8.4:1, while other guys like the 243 or 799 heads which net 9:1

These guys run big boost 25+ range and have set the sbe records.

If I had a bigger blower such as a whipple or kb like yourself, I would run the lower compression and let the blower do the work

Again, I believe it just depends.

[cc-rider]

Originally Posted by ttshovel

I believe it depends on the setup... with the 1.9l blower I have 10.9:1 compession. And I will add compression when this motor goes... compensate for the lack of blower!

As far as the turbo goes. The stock 5.3 guys like to use 317 heads, which net 8.4:1, while other guys like the 243 or 799 heads which net 9:1

These guys run big boost 25+ range and have set the sbe records.

If I had a bigger blower such as a whipple or kb like yourself, I would run the lower compression and let the blower do the work

Again, I believe it just depends.

Thanks for the thoughts. I've got a few thoughts running around in my brain but I'm kind of with you. For my KB, lower the CR and let the big dog eat.

But the thing that concerns me with top mount PD blowers on these LS motors is that there is such a tiny space in the valley to fit an intercooler brick (at least as compared to a DOHC engine). I think the rule of thumb is that your going to get heat to the tune of 10 degrees per pound of boost and then it's up to your intercooler to get that back down. The bigger the boost, the bigger the temp rise....yet that thin intercooler brick gets no bigger and therefore removes less heat. But then again with the lower CR, the octane goes a lot further too.

So damn many variables. But most detailed discussions with builders who have experience with KB's say the same thing...lower the CR and let the big dog eat and control the detonation with timing. But I'm open to more opinion here. Part of me wonders if a middle of the road build with 9.5:1 and boost in the 10-15 psi range might be better all around.

And to be clear...this thread should be about all FI options with the LS engines.

[blake-b]

When I was in the planning stages, I asked my cam guy (Martin@Tick) what compression ratio he would run based on my proposed combo (twin turbo) and he said he'd run 9:75:1 so that is what I had pistons made for.

I also plan on running 20-25 lbs of boost.

[BullF-16]

Im at pretty stout boost, 25psi on my 427lsx. I too tossed and turned at night about what CR would be best. Lots said 10 or higher lots said 9 or there abouts. I figured id go middle of the road since i planned on pretty high boost. Im running -23cc dish wisecos which i calculated at about 9.4 CR

Worked pretty good for me on the twins

[Todd in Vancouver]

Very interesting thread. I would like to go twin turnos on my project cat that is at 10.8:1 compression. I'm thinking lower boost and higher static compression for quick spool up.

Definitely following along on this one.

[sr71bb]

Well I am running more compression than most at about 11.3 to one with about 22 pounds of boost. More compression makes the engine more efficient and helps make a broader power curve. In roll racing which is my favorite type of competitive event, we typically jump out to a 2-3 car lengths advantage at the punch even over cars that weigh 800-900 pounds less and that have maybe 2-300 more PEAK HP. Because of the wider power band, at that point in time it is very difficult to run this car down even if you have a weight and peak power advantage. The bottom line is the AVERAGE HP/Torque is higher than on most cars and is certainly higher compared to if I was running typical boosted compression levels.

The disadvantage here is that you really have to have a GREAT tuner because the combustion temperatures and pressure is HIGHER with this type of set up. I knew from the beginning that we were going to run E85 and METH injection and that I would run head studs to accomodate this set up. With this set up your ability to run a lot of nitrous as well is limited An LSX block or ERL block with six bolt heads probably would be required here on this set up if you wanted to run say more than a 150 shot in addition to the 22 PSI boost.

[old motorhead]

Camshaft selection will have influence along with timing, boost level and static comp ratio. Where the intake valve closes (cam timing) on the compression stroke can move the dynamic compression ratio around. Close the intake valve later lowers the DCR. Close it earlier and it goes up. N/A spec cams generally result in higher DCR's. Blower cams, somewhat less. I've seen turbo cams all over the board.

Wider LSA's and less advance on the cam lowers DCR. Bigger intake duration, with everything else being equal, will also lower the DCR. Interesting puzzle isn't it?

[sr71bb]

internal Combustion Engine = An air pump powered by controlled internal (you hope) explosions.

On top fuel/funny cars the combustion pressures are so great that it ALMOST turns the liquid nitro mix into a SOLID. No wonder you see those engines occasionally grenade into a thousand pieces!!!

GREAT point about cam selection. Boosted cams with more exhaust duration help relieve some of the pressure.

[10aSSee D]

ADM put mine at 9.5:1 since I would be running 93 pump gas and nothing else. More forgiveness on detonation in case of a bad tank of gas.

[cc-rider]

Originally Posted by 10aSSee D

ADM put mine at 9.5:1 since I would be running 93 pump gas and nothing else. More forgiveness on detonation in case of a bad tank of gas.

I like that middle of the road 9.5:1 number for pump gas. You have a very similar build to what I hope to end up with. PD supercharger and all. I like your car. One of my favorite cars on this site.

[SSE 4 2SS]

My old TT motor was 9.4/1 compression ratio and we ran as much as 24 pounds of boost. This was after installing the LSA heads with ARP over sized aged studs, and we still had problems blowing out head gaskets and or pushing water.

The new motor is 10.5/1 compression and we will be north of 30 pounds of boost with 76 mm turbos. We will push the boost to whatever the fuel system will handle. My cam is around a 118 LSA and I don't have the rest of the specs available. It is a solid lift cam in the .800 lift range.

The difference here is a proprietary head gasket, that has a "fire ring" type of sealing surface. It's a copper gasket for the most part rather than a cometic gasket. I am not permitted to go into any further detail. but we have been told the motor can take upwards of 50 lbs of boost though we wont be anywhere near that high...

The higher compression ratio will allow for substantially more off boost power than the previous motor. we will also be spinning this motor way higher than before... we are looking for 8,100 -8,200 rpm shift points and can run to 9,000 rpm if needed for a given track distance.... Final tuning will determine the limits...

[JantzenOKC]

Originally Posted by SSE 4 2SS

My old TT motor was 9.4/1 compression ratio and we ran as much as 24 pounds of boost. This was after installing the LSA heads with ARP over sized aged studs, and we still had problems blowing out head gaskets and or pushing water.

The new motor is 10.5/1 compression and we will be north of 30 pounds of boost with 76 mm turbos. We will push the boost to whatever the fuel system will handle. My cam is around a 118 LSA and I don't have the rest of the specs available. It is a solid lift cam in the .800 lift range.

The difference here is a proprietary head gasket, that has a "fire ring" type of sealing surface. It's a copper gasket for the most part rather than a cometic gasket. I am not permitted to go into any further detail. but we have been told the motor can take upwards of 50 lbs of boost though we wont be anywhere near that high...

The higher compression ratio will allow for substantially more off boost power than the previous motor. we will also be spinning this motor way higher than before... we are looking for 8,100 -8,200 rpm shift points and can run to 9,000 rpm if needed for a given track distance.... Final tuning will determine the limits...

Man that is one crazy build you got going on.. Your boost numbers remind me of this picture.

[granatl]

Mine is 9.25:1 and (so far) 16.5 PSI of boost. Turbonetics 7875 turbo that feels more like a supercharger as fast as it puts down the torque.
I suspect since i run e85, it would help fuel mileage (yeah, who cars) if it was a higher C/R. I do seem to get around 17-19 MPG on the highway on e85.