LSx Compression with Boost?

[RSX302]

I haven't purchased the heads yet for my new LQ9, but before I do, I wanted to get some different opinions/input around the appropriate static compression that I should run.

The Texas Speed 370ci LQ9 is set-up with Wiseco -25cc pistons with ceramic tops. The cam I selected is what I think to be middle of the road and should work pretty good. CompCams 224/230 .581/.588 114LSA 112ICL. This is very close to what I ran in the Ford 302.

91 oct is what I will be using with no more than 12psi intercooled boost.

With the above info, here are the heads/options I was looking at:
AFR215 - Only come in 65cc chambers which would give me ~8.6:1
AFR230 - Comes in 62, 65 & 72cc chambers. 62cc would give me ~8.9:1

I'm hearing that the LS based engines can run higher compression due to the reverse flow set-up feeding the heads first. Would 8.6 be too low for decent drivability? Has anyone driven around with this lower CR? I'm sure it will be ok, but how big of difference would it be from driving around in a 10-11:1 LS motor?

Also, what kind of timing do the LS engines like? The Ford liked around 24deg at 12psi. I also noticed that the Ford ran high EGT's above 9:1 @ WOT (1400-1500) is this the same with LS?

Thx

[RSX302]

Nothin?

After reading this article Spec Sheet 700hp LS1 (http://www.superchevy.com/technical/engines_drivetrain/completebuilds_testing/sucp_0805_ls_engine_power_packages/nitrous_ls2_turbo_ls1.html) I'm leaning toward the AFR215's at 65cc (8.6:1) and getting new 57mm turbos..(next size up from what I have 54mm) 800-900hp could be about right. ;)

[Flash68]

Ron -- I think you'll have much more luck and feedback from LS1tech forum if you haven't already posted there. I've been immersing myself there lately and learning tons in prep for an LSx swap sometime by end of 2011.

I also think your cam is on the small side compared to what everyone seems to be running on the LS motors over there. I think yours is the first that isn't at least 230 duration and .600+ lift. Should still be more than enuf power I am sure.

I'm looking at doing an LQ9 stroker with some L92 heads. Such an awesome bang for buck.

Good luck.

[Mike Norris]

Here is a basic and simple guideline I use at my shop. For an 11-1 compression setup such as an LS7 & LS3, I prefer to run a max of 6-7 PSI for stock pistons and 8-9 PSI for forged. The 10-10.5 compression LS1, LS2 & LS6 I feel comfortable with 11-13 PSI on forged pistons and 9-10 PSI with stock pistons.

I would say that keeping it to at least a static compression of 9.5 to one would be best unless you are really going to shove a lot of boost through there. The 9.5-1 should be fine on a properly built and tuned (key words) engine with 15-16 PSI.

The lower you go with the static compression, the less low speed driving power there will be. If you go with an 8.5-8.7 to 1 compression and run any amount of cam it can make the dynamic compression pretty low, so cam choice becomes quite important for sure.

Like I said, these are not hard rules, just basic guidelines I go with for my street builds.

Hope this helps.

Mike Norris

[68Formula]

You have to compare more than the SCR for these two heads when considering driveability effects.

The 215cc smaller port volume would be better with your displacement when not under boost (assuming AFR has optimized both respective heads for port velocity). Here's an interesting article. It's mainly referencing the effects at WOT, but it plays a large factor in part throttle driveability as well.
http://www.superchevy.com/technical/engines_drivetrain/cams_heads_valvetrain/sucp_0902_chevy_engine_port_variations_measuring/index.html

The better would be to have the 230s chamber size with the 215s port volume. One option would be to mill the 215s heads. Another might be to look at your gasket size. You didn't mention what volume gasket you assumed (or if you used it in your calculation at all). Gaskets that have a large diameter compared to the head and bore not only decrease compression ratio, but also increase crevice volume (which can actually promote pre-detonation). The gasket thickness also plays a role in the compression ratio, but it also affects the quench distance (which again can promote pre-detonation). In some cases, people trying to reduce compression ratio using the gasket to improve pre-detonation actually made it more susceptible. Depending on what your assumptions are, you might be able to find a boost compatible gasket with a small enough volume to get you closer to 9:1. And in doing so lessen the chance of detonation, and further improving drivability and power through the more efficient quench. Just make sure you don’t go below 0.035” piston to head clearance on any one, single cylinder or you risk piston/head contact. (Note: To be consertively safe at that distance, assumes you’re using forged pistons, and strong rods/bolts which you’d automatically do for a boosted engine anyway).

[CarlC]

Ron, I agree with Mike. My experiences with Magnuson are right in line with what he states. Keeping the static compression higher and overall boost pressure lower makes for a smoother powerband that is typically easier and more fun to drive.

For mine, which is much more sedate than yours, we had to be carful during the tune to make sure there was no detonation @ 9psi on a stock piston. If memory serves timing at WOT is 20*, but the tune is purposefully very safe since the car is on-track quite a bit and pump fuels are not all created equal. The cam is also far smaller and on a 119* LSA, so cylinder pressure builds pretty quick.

[RSX302]

Ron -- I think you'll have much more luck and feedback from LS1tech forum if you haven't already posted there. I've been immersing myself there lately and learning tons in prep for an LSx swap sometime by end of 2011.

I also think your cam is on the small side compared to what everyone seems to be running on the LS motors over there. I think yours is the first that isn't at least 230 duration and .600+ lift. Should still be more than enuf power I am sure.

I'm looking at doing an LQ9 stroker with some L92 heads. Such an awesome bang for buck.

Good luck.


Yea..I got a crash learning lesson with LS engines in the last 12mo. So many different configurations and parts! "I've gone cross-eyed"

I purposely stayed under .600" lift for a couple of different reasons. With a "street set-up" boosted motor, it doesn't buy you a whole lot anyway. Duration on the other hand...

[RSX302]

Here is a basic and simple guideline I use at my shop. For an 11-1 compression setup such as an LS7 & LS3, I prefer to run a max of 6-7 PSI for stock pistons and 8-9 PSI for forged. The 10-10.5 compression LS1, LS2 & LS6 I feel comfortable with 11-13 PSI on forged pistons and 9-10 PSI with stock pistons.

I would say that keeping it to at least a static compression of 9.5 to one would be best unless you are really going to shove a lot of boost through there. The 9.5-1 should be fine on a properly built and tuned (key words) engine with 15-16 PSI.

The lower you go with the static compression, the less low speed driving power there will be. If you go with an 8.5-8.7 to 1 compression and run any amount of cam it can make the dynamic compression pretty low, so cam choice becomes quite important for sure.

Like I said, these are not hard rules, just basic guidelines I go with for my street builds.

Hope this helps.

Mike Norris


You have to compare more than the SCR for these two heads when considering driveability effects.

The 215cc smaller port volume would be better with your displacement when not under boost (assuming AFR has optimized both respective heads for port velocity). Here's an interesting article. It's mainly referencing the effects at WOT, but it plays a large factor in part throttle driveability as well.
http://www.superchevy.com/technical/...ing/index.html (http://www.superchevy.com/technical/engines_drivetrain/cams_heads_valvetrain/sucp_0902_chevy_engine_port_variations_measuring/index.html)

The better would be to have the 230s chamber size with the 215s port volume. One option would be to mill the 215s heads. Another might be to look at your gasket size. You didn't mention what volume gasket you assumed (or if you used it in your calculation at all). Gaskets that have a large diameter compared to the head and bore not only decrease compression ratio, but also increase crevice volume (which can actually promote pre-detonation). The gasket thickness also plays a role in the compression ratio, but it also affects the quench distance (which again can promote pre-detonation). In some cases, people trying to reduce compression ratio using the gasket to improve pre-detonation actually made it more susceptible. Depending on what your assumptions are, you might be able to find a boost compatible gasket with a small enough volume to get you closer to 9:1. And in doing so lessen the chance of detonation, and further improving drivability and power through the more efficient quench. Just make sure you don’t go below 0.035” piston to head clearance on any one, single cylinder or you risk piston/head contact. (Note: To be consertively safe at that distance, assumes you’re using forged pistons, and strong rods/bolts which you’d automatically do for a boosted engine anyway).




Ron, I agree with Mike. My experiences with Magnuson are right in line with what he states. Keeping the static compression higher and overall boost pressure lower makes for a smoother powerband that is typically easier and more fun to drive.

For mine, which is much more sedate than yours, we had to be carful during the tune to make sure there was no detonation @ 9psi on a stock piston. If memory serves timing at WOT is 20*, but the tune is purposefully very safe since the car is on-track quite a bit and pump fuels are not all created equal. The cam is also far smaller and on a 119* LSA, so cylinder pressure builds pretty quick.
Thanks guys for the input..

Wow..I knew the LS engine can take a little more compression but damn..

The compression that I calculated was with the .040" MLS Cometics. The AFR230 at 62cc, per Air Flow would drop a little low end torque. So it's a toss-up. It would be best to run the 215's with a 62cc chamber, but I was kinda hoping not to start playing around with milling. I can see if Air Flow could do that for me. That is a connection I have.

The Texas Speed LQ9 bottom end should be good for 1000-1200hp. It is running the factory 3.62 crank, but running forged pistons, rods and ARP fasteners. Per Texas Speed, they are running 1200hp on a factory crank with no problems....Yet

So I guess in the end this motor should be pretty bullet proof at 8.6-9.0:1 (850ish hp), but maybe a little lazy down low..(Comparatively) I'm sure it will be night and day compared to the cubically challenged 302 I was running. 20degs timing seems low, but I'm with you Carl..I would like to build in a good safety margin so next time I'm flogging at the track, everyone stays happy. With the lower compression, it looks like the 24 degs will work with the LS engine at WOT. One thing is for certain, 700rwhp and 700rwtq will be establish. That's 150rwtq more than I had thoughout the powerband from the 302. I'll need a stock pile of rear tires. lol

[68Formula]

So are you getting a short block assembly? If so, ask how far down in the hole the piston is, and then how much to zero-deck the block (rather than milling the heads). This would leave you with a .040" quench distance (because with a zero deck the quench is only the compressed gasket thickness) and it increases the comp ratio (the amount depends on how much they take off to zero deck). Not only is it good for increasing the SCR and better quench, but if their engines are based on seasoned blocks, it ensures a good sealing surface for the heads (important on a boosted application).

[RSX302]

So are you getting a short block assembly? If so, ask how far down in the hole the piston is, and then how much to zero-deck the block (rather than milling the heads). This would leave you with a .040" quench distance (because with a zero deck the quench is only the compressed gasket thickness) and it increases the comp ratio (the amount depends on how much they take off to zero deck). Not only is it good for increasing the SCR and better quench, but if their engines are based on seasoned blocks, it ensures a good sealing surface for the heads (important on a boosted application).

Already got the shortblock and the pistons are at zero deck. New LQ9 block. 4.030"

https://static1.pt-content.com/images/pt/2011/01/DSCF0010-1.jpg https://static1.pt-content.com/images/pt/2011/01/DSCF00041-1.jpg

[killer67]

One a forged bottom end with I did 9.5-9.6 compression. With only 12 lbs, you should stay out of the danger zone. 215cc would give real crisp power below 2000. Above 3000-3500, (especially big bore beads), you will get a nice hp gain. You can change gaskets for a slight increase in compression. IMO, 8.6 may seem sluggish in vacuum

[68Formula]

Already got the shortblock and the pistons are at zero deck. New LQ9 block. 4.030"

https://static1.pt-content.com/images/pt/2011/01/DSCF0010-1.jpg https://static1.pt-content.com/images/pt/2011/01/DSCF00041-1.jpg

Thanks for the pic, now I can't stop drooling.

So mill the heads and you'll be good to go. Keep in mind, although that timing even under boost seems low, if AFRs combustion chambers mimic the factory design it's not that bad. In naturally aspirated form, the factory heads chamber design is so efficient that you'd be hard pressed to find needing much total timing to reach peak power.

[joejaze]

I posted this article in another thread but thought it would be beneficial to this discussion as well.

http://chevyhardcore.com/tech-stories/engine/preparing-your-ls3-for-surviving-boost/

[RSX302]

Thanks for the pic, now I can't stop drooling.

So mill the heads and you'll be good to go. Keep in mind, although that timing even under boost seems low, if AFRs combustion chambers mimic the factory design it's not that bad. In naturally aspirated form, the factory heads chamber design is so efficient that you'd be hard pressed to find needing much total timing to reach peak power.

The chassis dyno will ultimatly tell what the timing will be. give motor what it wants..right.

[CarlC]

You can argue with them, but it's expensive.

There's more on the table with mine but I'm not concerned about that. In the end a few more horsepower won't make me or the car any faster on a road course.

[RSX302]

You can argue with them, but it's expensive.

There's more on the table with mine but I'm not concerned about that. In the end a few more horsepower won't make me or the car any faster on a road course.

I hear ya.. this is one reason I like the lower compression with the my new motor. I'm not going to melt it.. like you said..get's expensive.