LQ4 Exhaust Question

[jackyl9999]

I am looking to get my exhaust setup on my 70 GMC done this week.

Currently the engine is stock.

I have a set of 1-7/8's doug thorley headers, I plan on running a x-pipe, true dual setup with dynomax bullets. I am planning on 3 inch pipe.

My plan in the future is a TBSS intake, a cam upgrade and stall.


Is the 3 inch pipe overkill or am I thinking correctly allowing for future upgrades? I have done research but have found conflicting info on 3 inch dual setups being overkill with the upgrades I am planning.

[littlevictories]

it's noticeably louder for sure. i stick with the 2.5" stuff nowadays coz old.

also, in before brew loses his **** over "x-pipe, true dual"

['68 Newtricks]

Originally Posted by littlevictories

"x-pipe, true dual"

thinking the same thing lol

2.5" is plenty
my 400sb runs 2.5" Y pipe into single 3". I can't recommend dual 3", that would be a lot regardless of what intake/cam/snake oil you run.

[DjS71]

Originally Posted by littlevictories

also, in before brew loses his **** over "x-pipe, true dual"

If you've got parts and are averse to using reducers (more leak points?) and NOT averse to loud, then 3" is the right choice for you. I'll be going 2.5" w/X with my LQ4 ( no headers).

[BR3W CITY]

Originally Posted by littlevictories

also, in before brew loses his **** over "x-pipe, true dual"

..and yet they never learn

[Jon01]

It'll be fine, don't think too much into it.

[jackyl9999]

Thanks for the replies.


I tend to over think things like this. The only thing worse than doing a exhaust system is doing it twice.

[Andy4639]

I was told to stay with 2.5" pipe with anything under 500HP.Take a look at the link for sizes of piping.

http://www.exhaustvideos.com/faq/how...pipe-diameter/

[chuckmc8]

I have 2 LQ4's running 2 1/2" and they are fine. If your truck is lowered, (mine are 5" drop spring in rear) its tough clearing the panhard bar knuckle over the diff without it bumping.(if you are going past the rear end) Larger tube size doesn't always equal more HP or Torque-it may pay to research pipe size as you make up your mind. HTH

[Hart_Rod]

I have 600+ to the wheels with 2.5" pipes to the rear.

[jackyl9999]

I have the parts for 3 inch. (mufflers,x-pipe, collectors)

Has anyone ever done a 3 inch system then down to smaller pipe after the crossmember to save space when dealing with the rear suspension?

[69gmcc10]

I have a computer program (PipeMax Pro) and based on engine specs it will calculate your ideal exhaust system specs. Based on stock engine secs an LQ4 should have a 2.606" dual exhaust system. That being said, that is based on a peak HP output around 5200 rpm. If you go down to 2.5", in "theory" it should give you better throttle response and move the curve lower and would be better for daily driving.

[BR3W CITY]

....does that take into account a dual exhaust? Pipe volume and scavenging will change drastically on a dual exhaust when changing diameter. Much more so than a combined or "single" exh.

[69gmcc10]

Originally Posted by BR3W CITY

....does that take into account a dual exhaust? Pipe volume and scavenging will change drastically on a dual exhaust when changing diameter. Much more so than a combined or "single" exh.

Sorry, I should clarify more!

The 2.606" diameter a dual system that is for shorty headers 1.54" in diameter that are 17.9" to 20.4" in length and merging into a 2.606" collector tube that has an H in the system at 10.4", and continues to 20.9" or 41.7" from the collector into a "resonation" chamber or chambered muffler. That should meet all the volumetric efficiency and harmonics requirements in the system perfectly, but we don't live in a perfect world and I have found it better to go down to keep the exhaust velocities up than go large and create loud drones and scavenging issues. I use the shorty header specs when considering manifolds because it is as close a to a manifold as the program can get.

Interestingly there is a dual exhaust with Tri-Y headers system calculation as well. Tri-Y specs are primaries of 1.668" that are 17"-18.4" long merging into 1.793" the secondary Y of again 17" to 18.4" in length. The secondary Y should merge into a 2.606" to 2.856" collector tube that is 20.9" long where it flows into a "resonation" chamber with an H pipe again at 10.4" behind the meeting pint of the second Y and in front of the Resonator.

In both cases the distance for an H-pipe to be installed in an LQ4 was nearly 7 feet back of the header merge point, before the resonation chamber and in my opinion at that distance packaging would become an issue and this system would be more work than benefit. If one was so inclined to still use an X I would install it around the distance of the 3rd or 4th harmonic (20.9" or 41.7").

Resonation chambers are supposed to be 2x the c.i.d. of the engine, so if you are using a dual system as we are there should be a resonation chamber on each side equivalent to the c.i.d of the engine. In this case you should be running a Flowmasters, Super Turbos or any "chambered" mufflers (no glass-packs or Magnaflows) on each side that has a internal volume of 364 cubic inches or more. To the exhaust gasses the chamber represents enough atmosphere that most any harmonics or scavenging down stream of the chamber has almost no factor in performance as long as the pipe is large enough to exhaust the gasses of the engine. It is at this point that sound, noise and drone will become a factor. Allot of successful systems will add another muffler (glass-packs and magnaflows usually) down stream to keep noise to a minimum and sound smooth. That being said over sized pipes will have a higher chance of being loud or causing a drone so tail pipe length will become a factor. Drone can be canceled out with a J pipe, but that gets to be tedious and technical.

[aknovaman]

Why do you say j pipes are tedious and technical?

I have described here how to simple eliminate any drone.

http://ls1tech.com/forums/conversion...ps-solved.html

[69gmcc10]

Originally Posted by aknovaman

Why do you say j pipes are tedious and technical?

I have described here how to simple eliminate any drone.

http://ls1tech.com/forums/conversion...ps-solved.html

Thanks for posting that. It isn't that it is that difficult, I just didn't want to write an explanation about it. nice write up!

[jackyl9999]

Thanks for the info guys. Very informative.