BJC51984

iTrader: (60)

So I sold my procharger and am changing setups going back to a NA heads/cam setup...I have a forged ls6 with -8.6 CC dished diamond pistons and PRC stage 1 LS6 heads with stock graphite gaskets...TSP told me I should have around 9.6:1 CR when they built the shortblock...not looking to change pistons...right now I am looking at going with an MS4 cam or something close to that...my question is how much HP am I leaving on the table by only having 9.6:1?... I was considering having the heads milled/tighter gaskets but my concern is ptv clearance limiting my cam choices...am I better off leaving lower compression with a Max effort cam or bumping the CR and getting a cam with more ptv clearance...thanks

Schantin

iTrader: (6)

You'll be losing some low end torque with the lower compression, but not sure it would be enough to feel.

When I went from my STS turbo to na a few years back, I did it in 2 phases. The first was pull the turbo, etc. I ran for 2weeks with the turbo cam (228/228 .588/.588 on a 116LSA with Patriot LQ9 heads. Probably had a compression around 9.5 to 1 or so.

Switched a month later to milled and ported 243 heads and a .228/.232 .600/.601 111LSA cam. Got more top end power, but even with a now 11.0 to 1 compression, it wasn't that noticeable of a difference. I recorded a change of 3/10 s at the dragstrip. So in poor-mans math, i probably had a difference of 30hp or so between the low compression and high compression setup.

itsslow98

iTrader: (18)

With a big cam like the MS4 a higher compression ratio will definetly help with tuning and driveability. I would say its worth the money to have them milled.

mark21742

Going up a full point in compression (like 10:1-11:1) will give you a 3-4% increase in tq and HP all acrossed the rpm range of the motor. You want to raise your compression when you run a bigger cam because you do not start actually compressing air/fuel until your intake valve is closed, which is where you get dynamic compression.

For instance I'm running 12.5:1 static compression, and a 255/263 cam and my intake valve hangs opened until 85 degrees after bottom dead center which gives me a 8.22:1 dynamic compression....

So even when running 12.5:1 I can still run 92/93 octane pump gas with gull timing and 0 knock.

ausls1

been told the same when i asked similar questions before i started my build and was told that its barely noticeable and roughly only a 3-4% or rwkw difference. i ended up getting -8cc pistons from wiseco with the big valve cutouts standard gives plenty of ptv room, iv got thinner head gaskets and 63.5cc heads for n/a but can still drop my cr with thicker head gaskets and different heads if i want to go F/I down the track as i wasn't sure i'll go that way for a long time now.

batboy

iTrader: (6)

With the MS4 cam, PTV clearance is always a concern. If I were doing this, I'd probably have the heads milled .020" and get a lower lift cam. Even dropping back to the MS3 might be all you need to do.

DONAIMIAN

iTrader: (18)

You do have -8cc pistons, I doubt the MS4 would be as big of an issue with milled heads because of that, but its always best to check.

chrisfrost

iTrader: (8)

Think about this : compare overall volume between flat tops vs. dished at bottom dead center,,,,,,,,,,answer : dished pistons when at bottom dead center have a larger cylinder and combustion chamber volume than do flat tops do . Having said that care to guess at which pistons given everything else being the same make more power ? Yep , dished
Also go over to wallace racing and look at/use some of their engine and car perf calculators ,,,,,I think they have the most calculators than any I've seen . Going from say 9/1 comp to 12/1 comp will only gain You like 23hp with alot of added heat and stress on You're engine .

BrntWS6

iTrader: (7)

You'll might as well go with the 2.5 ls6 heads with ms4 cam. They come with .650" dual valve springs and the stage 1's come with stock ls6 springs. By the time you add springs to the stage 1's your not too far off, and the 2.5's come with larger intake and exhaust valves.

Just curious why you would take your Procharger off?

PREDATOR-Z

with 8cc valve reliefs? And what does lift at .050 have to do with PTV clearance?????? (at the durations we run)

LS1-450

iTrader: (33)

It's best to list cam specs. We don't all memorize vendor cam specs & it's impossible to answer without the specs; especially cam duration.






That's an awesome set up you got there; nice.

Hy82e

iTrader: (14)

I'm in the same position as the OP, where as I'm question what compression to target. I'm building up from a stock 2008 6.0 LY6 ( dished valve relief, L92 heads, vvt) with a 9.6 to 1 from the factory.

Anyhow I guess I'm little confused of the opinions I've heard. From my understanding we stand to lose 15-30hp on the table running low comp. Which at times even in this thread seems to be a small gain to be had. But then we spend $100's and $100's of dollars for gains of 30 hp at a time... ?

I apologize if this seems obvious to most I have my noob card ready if it needs to be hole punched.lol

mark21742

We all had to learn at some point

Main thing is it really depends on what you want to do with it....if you leave it with the dtock compression and want to keep the vvt you can have good gas mileage and insane power if you slap on a superchargrr or turbo.....if you want to stay N/A and build power you will want to mill some off the heads, maybe thinner gasket and ditch the vvt so you can run a higher lift cam.... The vvt only let's you run around s .525 loft cam.

BJC51984

iTrader: (60)

I already have stage 1 PRC LS6 heads bolted to the engine...I don't plan on tearing that all apart to upgrade to stage 2.5 since it just offers slightly bigger valves...and I already have PRC .650 dual springs...also the MS4 that I have is 239/242.649.609 111 lsa...I really don't want to tear the heads off and have them milled if its gonna result in marginal HP gains...and I don't totally recall but I don't believe the -8.6cc fished

BJC51984

iTrader: (60)

*dished diamond pistons have valve reliefs...I think its just dished...just worried about p2v clearance with milled heads

LS1-450

iTrader: (33)

If you're looking for minimal "effort/benefit" ratio, just check PTV clearance using a thinner gasket. Going down to .030" will save you from having to remove valves & deck heads .025", based on stock composite gaskets. If there is enough PTV clearance created by having -8.6cc dished pistons, then this yields the most gain for the least amount of effort. It is possible that your cam specs can clear the dished pistons noted, but, you "MUST" check to confirm. 239/242 duration would typically require flycut pistons even w/ a thicker .040" gasket. Best case may be .030" gasket; worst case might be .045"; each an improvement over stock. It will depend on the depth & location of the dish.

It's well worth the time to increase CR. The already noted 3-4% per 1 point in CR is pretty much under the entire curve, not just peak.