Opinions on Compression Ratio for Boosted applications

**PerformanceSS** · 07-14-2016, 01:12 PM

I'm about to freshen up my 1995 Z28 Lt1 Turbo motor. Turbo is a Precision PT76GTS and it's been up to 17psi and probably capable of 22-23psi. I'm currently running 8.5 to 1 compression, 388cu in, with 210 AFR heads,Th400 trans and 3.00 gears. I also have a 1995 TransAm that now has AFR 210's and I have run 9.7 to 1 compression on it for years 10-12 psi boost levels. I'm currently running a Ross piston on the motor being rebuilt and may simply go back with the 8.5 to 1 pistons, but if too many pistons are damaged, I'm looking for opinions on compression ratio and particular pistons that others have had success with. Naturally this is a reasonably economic build, so super high end stuff will probably not be affordable to me at this time. A few more details about the Turbo car: still running stock ecm, 2.5bar speed density setup, 120# Holley injectors, dual nozzle Methanol kit (Not trusting the kit), dual 255lph Walbro in tank pumps (one on Hobbs switch), stock fuel lines / rails ( car was originally setup at a street racer, most mods are somewhat hidden to the untrained eye). Also I haven't fooled with my cars until the last couple years and back then the only Head gaskets were the Felpro 1074's, I have ordered the MLS Cometics. Any advice on head gaskets? Any other notable upgrades to someone that's been out of the car scene for several years? I know the fuel lines probably need upgrading, what about fuel rails? What's the max HP the stock fuel rails can handle? What's available for aftermarket fuel rails if they are a limiting factor?

Thanks in advance for input!

Steve

***Fastbird*** · 07-14-2016, 03:19 PM

For starters, ditch one of the meth nozzles, you don't need it. Too many people are shooting WAY too much meth into their cars.

Your factory lines and rails will support in excess of 800 at the wheels and my money says a fair bit more than that. I had mine at 767/712 at one time and had no problems running an 84 lb/hr injector. No pressure drops, etc.

TBH 8.5 is too low. My car was 8.8:1 and I didn't like it. Felt it was too sluggish off boost. My Corvette (different architecture obviously) is running 9:25:1 and even it's a bit on the low end by todays standards. For the LT I'd probably target about 9:5 as a solid middle ground without starting to push it too much.

Both FI builds I've done (LT Trans Am and LS C5 Vette) have utilized Diamond pistons, custom made. Can't beat them for $600 for a set.

Something to consider that I didn't see you mention. What length rod are you running? If you're running a 6" rod, ditch them in favor for a 5.7" rod. The 6" rod pushes the wrist pin up too far and really reduces the compression height, sacrificing strength in the process. The 5.7" leaves a lot more compression height.

**PerformanceSS** · 07-14-2016, 03:49 PM

Originally Posted by Fastbird

For starters, ditch one of the meth nozzles, you don't need it. Too many people are shooting WAY too much meth into their cars.
Your factory lines and rails will support in excess of 800 at the wheels and my money says a fair bit more than that. I had mine at 767/712 at one time and had no problems running an 84 lb/hr injector. No pressure drops, etc.
TBH 8.5 is too low. My car was 8.8:1 and I didn't like it. Felt it was too sluggish off boost. My Corvette (different architecture obviously) is running 9:25:1 and even it's a bit on the low end by todays standards. For the LT I'd probably target about 9:5 as a solid middle ground without starting to push it too much.
Both FI builds I've done (LT Trans Am and LS C5 Vette) have utilized Diamond pistons, custom made. Can't beat them for $600 for a set.
Something to consider that I didn't see you mention. What length rod are you running? If you're running a 6" rod, ditch them in favor for a 5.7" rod. The 6" rod pushes the wrist pin up too far and really reduces the compression height, sacrificing strength in the process. The 5.7" leaves a lot more compression height.

Yea, I'm having fuel transition issues with the Hobbs switch bringing in the second pump and then the Methanol coming in also.
Yes, the last time this car was Dyno'd was in 2006 and it was 695/644.
My turbo car is a dog off the line 1.6-1.7 60ft times, but has run a best of 9.92 @146mph. I'm ok with it being sluggish off the line, easy on rearend and trans. My real concern was which compression will make good HP, but also have longevity. I'm sure boost levels would also make a difference. My ProCharged car with the D-1 and 9.7 to 1 compression is more responsive, but I run less boost on it.
Yes, I've seen many people go with Diamond pistons. $600 a set is about as good as it gets!
I'm running the 5.7's

**firebird_1995** · 07-14-2016, 06:32 PM

I'm kinda in the camp that if it ain't broke, don't fix it. I agree that 8.5 is pretty mild, however, if you decide to turn up the wick you will have a little more wiggle room for error. I would be comfortable with 9:1 on that setup in the low to mid 20# range. You said stock pcm, still running the opti?
Mahle makes dished pistons (-24, -31cc) in 2618 alloy now. They can be had for under 700 with rings. I've got a set now and I like the top land thickness on them vs others I've seen. I have not run these yet though, so no first hand experience.
Again, if you are looking to keep the budget down, I would have to really assess the condition of the piston to determine if I wanted to go with something else.

***popo8*** · 07-14-2016, 06:46 PM

On my procharged 385... im at 9.5 and see 10 lbs of boost. I think at 9.5 the car is completely fun to drive even outside of boost, but then again its a 6spd with 3.90s.

LTXtech.com IS my drug...
OWNER/ADMIN

**PerformanceSS** · 07-14-2016, 07:08 PM

Originally Posted by firebird_1995

I'm kinda in the camp that if it ain't broke, don't fix it. I agree that 8.5 is pretty mild, however, if you decide to turn up the wick you will have a little more wiggle room for error. I would be comfortable with 9:1 on that setup in the low to mid 20# range. You said stock pcm, still running the opti?
Mahle makes dished pistons (-24, -31cc) in 2618 alloy now. They can be had for under 700 with rings. I've got a set now and I like the top land thickness on them vs others I've seen. I have not run these yet though, so no first hand experience.
Again, if you are looking to keep the budget down, I would have to really assess the condition of the piston to determine if I wanted to go with something else.

Yes, I'm still running the stock ECM and the Opti Spark! I'll admittedly say I've changed several Opti Sparks over the years, but have had some luck with both the MSD billet and some of the cheap eBay stuff. The turbo car with 8.5 to 1 has leather interior and AC, it's a pleasure to drive around with good street manners. I'm just considering options and people experience/opinions, if it's becomes cost effective to buy another set of pistons. At that point I can change compression if I choose. Otherwise, I'll go back with the current Ross pistons.

**PerformanceSS** · 07-14-2016, 07:14 PM

Originally Posted by popo8

On my procharged 385... im at 9.5 and see 10 lbs of boost. I think at 9.5 the car is completely fun to drive even outside of boost, but then again its a 6spd with 3.90s.

LTXtech.com IS my drug...
OWNER/ADMIN

Yes, that's close to the compression I run on my other car (1995 ProCharged TransAm). I normally run 12 -15 psi on it and it's 9.7 to 1 compression. Just curious if anyone has any input about at what point (boost level) should you lower compression. Or does that even matter? I see many people on the Turbo forum running high compression and somewhat high boost, not sure if that's the way to go, but I think better controls over fuel and timing probably affords us the ability to push the limits a bit. My 9.7 to 1 is much more responsive on the bottom end, but not sure if it's all compression or more so the difference in one being a turbo and the other centrifugal supercharged.

**PerformanceSS** · 07-14-2016, 07:18 PM

Timing tables for high boost?
Maybe this should be another topic, but what's the consensus on timing on boosted applications. What does most feel is a safe timing curve to start with. I'm sure tuning and diagnostic will eventually determine max timing based on knock, but where has most of you that tunes your car ended up with a safe tune? I guess I'm asking for suggestions for timing at say 4 6 8 10 12 14 16 18 20 20 psi of boost. I know that there's unlimited factors, but it appears we have several fast boosted cars and it may be beneficial to share : Example my current Supercharged tune for 10psi is approx. 4-33 deg 6-28 deg 8-25deg 10-21deg I had an initial tune that was closer to 15 deg @ 10 psi and I was surprised to see how much difference it made to slowly move it up to 21 deg. I gained .3th in the 1/4 mile from only the timing move. Anyone know of any down side to running real low timing? Other than the obvious performance loss.

**MoeHorsePower** · 07-18-2016, 11:53 AM

boost is not boost as some say, Supercharger vs Turbo are different tunes, you have boost vs load and load vs boost. can you run a super charger tune on a turbo? or vice versa, of course but you will not be maximizing or be efficient. I run 9.5.1 compression, and at 12lbs make 710rwhp, will be upping it to 15-17 lbs, Longevity is all in the parts and especially in the tune..

***Fastbird*** · 07-18-2016, 02:35 PM

As for the timing question.........Ask 5 different people you'll get 6 different answers. The car wants what it wants. My old LTx/Turbo setup wanted a LOT of timing. I think at 20# we were at 22-23° of timing on the big end still. Tuner couldn't believe it and the motor liked it. Got a buddy making 1K RWHP with his setup and at 24-25# I believe he's still in the 18-19° range. Watch your knock sensors, read your plugs, back it off once it starts telling you it's unhappy.