Head/exhaust question

[B4C]

So my car has an LQ9 with 241 heads and 9.8:1 comp ratio. It also has a 4in qtp y pipe and 4in cat back, with qtp stailess headers. I will be adding magnaflow bullet style high flow cats in 3in to add alittleback pressure and pass inspection. Now heres my question, what would the compression ratio be with ls6 heads? Should I get a smaller exhaust system to add back pressure? My car made 407 rwhp what would it make at around say 11.1:1 copm ratio?

[BarneyMobile]

The factory compression ratio on an LQ9 was 10.0:1. With stock 241's you'd be just under 10.7:1 and with LS6 243's, you'd be at 11.0:1. The 243's will likely give a 10-20 hp bump with just the increase in flow over the 241's, and they'll raise compression so you may see even more power so if you can get them cheap, they're worth while. With a more restrictive exhaust, you'll likely lose a few ponies.

[emolineaux]

unless its really an LQ4 which is 9:1 compression which the 241s would bump that up a little over the stock 317 heads.

[B4C]

Could be lq4 the paperwork I have says 9.8:1 but idk it's so general it just says 6.0. Are you shure the exhaust isn't too big everyone says it will run better if it has alittle more backpressure

[emolineaux]

yea im not sure how to tell the motors apart without pulling a head off. the LQ4 would have dished pistons. as far as the exhaust goes. that im not sure. im running kooks long tubes tsp catted y and magnaflow and i dont have a problem what so ever with it.

[B4C]

Idk what to think about the exhaust some people say its too big some say its fine IDK.

[NJ Torque]

Whats the collectoe size? Is it a standard style Y or a merge?

[Tru2Chevy]

I think you are fine with the 4" system (which muffler / tip setup is it?) since you are trying to evacuate quite a bit of air from a 6.0 with a big cam.

Even if it is a little on the large side, switching back to a 3" catback would most likely take away a little upper RPM horsepower, although it would also give you some low speed torque as well. If you are really worried about the 4" catback, I would look for someone with a 3.5" setup that wants to upgrade and see about a trade. I think jumping back to a 3" is the wrong idea, especially if you are thinking of more power in the future (like your aforementioned head swap).

- Justin

[B4C]

Couldnt tell ya the muffler but the tips are stock looking quad tips so they dont give anything away

[Pampered-Z]

Unless you’re running some type of competition header ( 1+3/4 – 2” Primaries with 4” collectors) or have small exhaust ports I wouldn’t worry too much about what you have. The cats will give you a bit more BP, after that the exhaust pulse is less of an issue.

I won't be all that concerned about the cat back. The weight of the system is probably more of an issue then the actual pressure, it is heavy!

Above 500 HP you really want a 4" system, the the key is actually in the muffler more then the size of the intermediate pipe being 3.5 or 4". You probably have either the Flowmaster or Spintech, the Spintech does have more back pressure then the flowmaster ( reason it's quiter too ). Both actually do produce a fair amount of BP, common for racers to switch to a moer bullet type stright thru design which will gain HPs.

So don't be too concerned about back pressure. In general terms: You need to worry more about too much back pressure then not enough. Not enough would be addressed starting at the exhaust ports where you want to focus on exhaust gas scavenging and velocity. Issues you could run into with small exhaust ports, cam with very tight LSA, or excessively large primary tubes.

Or address it with step tube headers. With a standard street type comp header it’s a none issue unless you want to spend a ton of money trying to find 5 more HP but tuning your headers? If you were maybe running some class that mandated none ported heads or stock cams where a few more HPs could be made by tuning your headers to get the exhaust scavenging perfect. This is where the exhaust pulse/velocity from one cylinder is helping pull the exhaust gases down from another cylinder. This continues down the entire exhaust system to some extent, the large the exhaust, the less velocity your have due to the volume, leading to the exhuast needing to be pushed out.

[B4C]

Same size header you just describe 1&3/4"in primaries 3"collectors just looked at my paper work lol. Car made 407 hp and 379 ft-lbs at 6480 RPM (that was the peak) Made that at 9.8:1 comp ratio. Wonder how much it would wake up with 243's and how much the curves on the dyno sheet would change? Maybe make peak power lower? The car dosent really come on until about 4500rpm but it pulls like a freight train if it hooks

[sweetbmxrider]

Originally Posted by B4C

Same size header you just describe 1&3/4" 3"in primaries 4"collectors just looked at my paper work lol. Car made 407 hp and 379 ft-lbs at 6480 RPM (that was the peak) Made that at 9.8:1 comp ratio. Wonder how much it would wake up with 243's and how much the curves on the dyno sheet would change? Maybe make peak power lower? The car dosent really come on until about 4500rpm but it pulls like a freight train if it hooks

is that an extra 3"?

[B4C]

Heres what the paperwork from the shop that did the work says...1&3/4in QTP stianless headers 3" merge collector lt's no AIR/EGR QTP ORY (I was mistaken). All the clamps on the system past the headers say 4" on them. Also LS1 Billet core custom grind camshaft, HD dual spring kit titanium seals/seats/retainers. PSI Chromeoly 7.400" pushrods, and some more stuff relating to the 6.0 swap...

[sweetbmxrider]

alright you miss typed before then. i thought you meant you had 3 inch primaries, that would be a little excessive

[B4C]

Yeah I wanna get that compression up not see if my HP and TQ comes on alittle sooner anyone got some bare 243's?? Are 243's my best bet?? IDK much about ls1's still a newb