Teach me about exhaust

[Kalums]

Ok gents... I want to learn something new about exhaust. I've got a 1971 GMC K2500 with a 350/TH350 +NP205... After 6 months of chasing my tail I've finally figured out ultimately/finally why I can not get my engine to run "right". After taking off the intake manifold to swap it and the carb out I found I had a bent push rod on cylinder(s) 7 AND 8 (along with other carnage) *sighs* long story short me thinks my engine needs a complete teardown and rebuild. OR I still have that GM TBI crate motor, (no one bought it yet) may just swap that in... *thinks* hhmmm. Either way, I'm going to keep the stock RAM exhaust headers( provided they fit) ... But why is my current exhaust 2" from the manifold through the "y" pipe to the muffler and then 2.25 to the exit behind the driver's side rear wheel? Taking sound out of the equation, why do alot of after market set ups/ replacements run 2.5" from the manifolds back? If I'm interested in torque vs "go fast" should I keep the replacement as close to my current setup as possible? Why does my current setup have the "cross pipe" going directly under my transmission? Doesn't that effectively cook the fluid? What benefit is duals? What harm is duals? Sorry for all the questions and thank you in advance.

[1970cstblazer]

No harm at all with dual exhausts, but V6/V8 engines do respond better if there is a balance pipe to essentially connect the banks. The factory used mostly single systems because these were not intended to be race vehicles, and it was cheaper to run a single skinny pipe with that quiet, restrictive muffler. That type of system can add some low to mid range torque over a super free flowing system, but the top end really suffers with the single stock system.

[Kalums]

Originally Posted by 1970cstblazer

No harm at all with dual exhausts, but V6/V8 engines do respond better if there is a balance pipe to essentially connect the banks. The factory used mostly single systems because these were not intended to be race vehicles, and it was cheaper to run a single skinny pipe with that quiet, restrictive muffler. That type of system can add some low to mid range torque over a super free flowing system, but the top end really suffers with the single stock system.

I understand the "cheaper" part of the equation, a couple of bucks over thousands of vehicles ads up. Like I said I'm not really interested in Horse power or "go fast" at the top end (definitely not race vehicles here) I'm personally more interested in torque in the bottom to mid range ( I'd like to use this to tow a tailer, possibly a tractor, backhoe and or dozer buckets*grins*) If I did end up with duals, how does that work without a crossover (x-pipe)? I don't really see that as a possibility with the transfer case. Seems like I'd have to have the exhaust system pretty far away from the bottom of the truck/ hanging down pretty far.

[Wootdog]

I know you hear this analogy a lot but think of the engine as an air pump, the more air you can force through it intake and also exhaust wise, the more efficient it is. And that means more horsepower and or torque. It least that is how the exhaust companies explain it. i know they are out to make money. But it makes sense. As far as the pipe between the tubes, it balances out the exhaust notes and provides I believe less back pressure. Hope that helps. I am no expert by any means.
Wootdog.

[1970cstblazer]

Here is a factory exhaust diagram:



Here you can see the factory crossover pipe location. This is about the only spot one could be made to fit..





A competent exhaust shop should be able to make a dual system with a crossover in that location and manage to clear the driveshaft.

[KQQL IT]

Everything I've seen says bigger is better.

H or x pipes don't do anything for power just change the exhaust note.
And different engines will sound completely different with exact same mufflers.

https://youtu.be/kWFNY06vxxM?feature=shared

[Steeveedee]

Have you experienced any overheating or seen any degradation of your transmission fluid when towing things? If not, then your stock system is good enough, and not cooking the trans. One thing you will experience going to duals is loss of low end grunt.

[70STOVEBOLT]

According to most exhaust companies, an x-pipe is for high rpm power and an h-pipe is for low to midrange torque. Big block trucks had 2.5” dual exhaust from the factory with no crossover pipe I believe, but not sure what size on the small block trucks, but they were single exhaust only, no duals, I believe.

[1970cstblazer]

Originally Posted by KQQL IT

Everything I've seen says bigger is better.

H or x pipes don't do anything for power just change the exhaust note.
And different engines will sound completely different with exact same mufflers.

https://youtu.be/kWFNY06vxxM?feature=shared

Not necessarily. If you put a 3" dual system on a relatively stock truck, the low end grunt torque will suffer, albeit no restriction for ultimate horsepower production. These engines need to have some backpressure restriction for better torque production down low where most driving is done.

As for X pipes, there is dyno proof of power gains. This is an excerpt from a Corvette board:

The car dyno'ed was a SGC 368 with cams and the exhaust used was a new Triflo system with restrictor plates removed.

Before: 441.3 rwhp, 383.9 rwtq

After: 452.6 rwhp, 405.2 rwtq

Difference of 11.3 RWHP, 21.3 rwtq.

Big gain for just a piece of pipe!

The peak numbers do not tell the whole story however. The power and torque gain was throughout the entire power range.

For example at 4600 rpms the difference was 22.9 rwhp and 26.5 rwtq.

Cut and pasted directly from Flowmaster:

http://www.flowmastermufflers.com/?page_id=12583

13. Do I need to install an “H” pipe in my dual exhaust system?

Flowmaster® strongly recommends using a crossover tube, otherwise known as an “H” pipe or balance pipe on all true dual exhaust systems. The crossover pipe equalizes the exhaust pulses and allows the sounds waves to communicate between both banks of the engine. Not only does this usually improve torque in the low to mid rpm range, it also creates a deeper mellower tone both inside and outside of the vehicle and helps eliminate “back-rap” on deceleration.


14. Is an “X” pipe better than an “H” pipe?

Flowmaster has performed extensive testing and determined that the “H” and “X” pipes deliver no significant difference in performance, but do affect tone, the “X” pipe giving off a higher pitched tone. Flowmaster Scavenger “X” pipes however do increase torque through use of their patented
D-Port technology and retains the deep Flowmaster muffler tone desired by most performance enthusiasts.

However, some dyno tests have shown little to no HP gains, yet some have seen pretty dynamic gains. Another case of "your results may vary"....

[KQQL IT]

After they smashed the headers and showed little difference.
Im a firm believer go with what fits.
But recently my kid built a 3 in x pipe system and a 2-1/2 duals on 2 trucks same mufflers similar 6.0 LS motors.
3 in x pipes sounds like an exotic
And the 2-1/2 sounds like.. well you know.

[kwmech]

For a street truck nothing than a single is needed. Even with a big block. Small block trucks I work on get a 2.25 pipe from the ram horns down to the 2.25x3'' flowmaster Y and a 3'' muffler back out the side. Big block trucks I do get 2.5 up front to the 2.5x3 flowmaster Y and out the back 3'' also. Trucks always sound really good and have more than enough grunt and performance. Never had a complaint from my customers or from myself

[Willshook]

For the horsepower range you're looking at, the most important part are the headers; tube size and length has the biggest impact on power.

From there, it's more about noise management than anything else - any kind of crossover helps reduce noise, as does muffler choice and exhaust exit style.

Anything making more than about .8 HP/CID will benefit from at least 2.5" dual exhaust. Higher power, above about 1.2 HP/CID gets very different quickly...

Bent pushrods if there's no evidence of valves impacting the pistons is usually misadjusted lash, assuming the valvetrain geometry was right in the first place...

[Steeveedee]

An "H" pipe also increases the frequency of the exhaust pulses by sharing both sides of the engine. Higher frequencies are attenuated better (have less energy), thus the perceived noise is reduced. I recall reading that it also provides some amount of scavenging.

[Kalums]

Originally Posted by Steeveedee

Have you experienced any overheating or seen any degradation of your transmission fluid when towing things? If not, then your stock system is good enough, and not cooking the trans. One thing you will experience going to duals is loss of low end grunt.

So... I'm finding that my current setup is NOT stock. It crosses over underneath the bell housing and front of the trans pan. And yes my fluid which I changed when I got the truck in June/July already looks dirty and 90% of its life has been idling while I futz with stuff... It may also be that the old fluid was just that gross too. Either way when I swap the engine I'm going to service the trans and transfer case while I'm there.

[Kalums]

Originally Posted by Willshook

For the horsepower range you're looking at, the most important part are the headers; tube size and length has the biggest impact on power.

From there, it's more about noise management than anything else - any kind of crossover helps reduce noise, as does muffler choice and exhaust exit style.

Higher power gets different quickly, but that's the key at these power levels - anything less than maybe 1.5 HP/CID...

Bent pushrods if there's no evidence of valves impacting the pistons is usually misadjusted lash, assuming the valvetrain geometry was right in the first place...

So... It started with bent push rods and then it advanced to broken exhaust valve springs... I haven't pulled the heads yet or looked in the spark plug hole with my bore scope for either piston... But one of the exhaust valves is seized (#7)... So I'm thinking the swap would be my best bet... Then use my core as a rebuilding platform for the future... Or maybe just for pack-ratting... *laughs*

[jhe.1973]

Originally Posted by 1970cstblazer

Not necessarily. If you put a 3" dual system on a relatively stock truck, the low end grunt torque will suffer, albeit no restriction for ultimate horsepower production. These engines need to have some backpressure restriction for better torque production down low where most driving is done ......... However, some dyno tests have shown little to no HP gains, yet some have seen pretty dynamic gains. Another case of "your results may vary"....

I would like to go into a bit more detail with the points mentioned above.

If an engine need a 3 inch exhaust and you install a 2 pipe this would obviously create backpressure. However there is another type of restriction that isn't as obvious.

If the engine is efficient with a 2 inch system and you go to 3 inch with no other changes the exhaust does not have enough energy to maintain its speed when it hits the larger pipe. This is why the torque is reduced as the exhaust is forced to slow down from the restriction posed by the volume of the too large of a pipe.

The effect can be seen when a fast flowing stream meets a larger body of water such as a lake. The stream doesn't have the energy to 'move' the lake and it immediately slows down.

With dyno testing a vehicle or engine is usually sped up, a load applied and the torque measured. With several tests like this at various RPMs you can plot a power curve.

The problem in the real world is that when a vehicle accelerates the load is constantly multiplying with increasing speed. It does not increase in a nice neat straight line. There is no running up to a speed and then applying a load.

This last point is why Ford built two computer controlled dynos for their GT40 project that were hooked to each driveshaft from the transaxle. They could control the loads as the acceleration increased and actually program an entire 24 Le Mans race with all the changing loads on each driveshaft.

I bring this up as a possible way to explain the different claims for dyno gains.

Plus, when you include the power of advertising for companies reality often takes a back seat!

[PbFut]

This may sound harsh but here goes. It sounds like you may have a lot more problems than worrying about configuring an exhaust I'm sorry to say. Take a step back and see that your motor is self destructing for some reason. Find that reason first and fix it. As for exhaust, kwmech provides the best advice on the thread. Its a street truck. Configure and use accordingly. If you want a race car, buy one. You can't properly duel purpose either.

[Willshook]

Yeah, that's bad valvetrain geometry for sure.

Pull the heads; if there's no impact marks and no serious metal in the oil do a head/cam swap. No reason to overhaul the bottom end unless there are other issues.

Of course, if the same guy built the bottom end that assembled the heads...then I would have low confidence it was done right.

[LS short box]

So maybe this won't help but?
I have a 98 Wrangler with a LS6/4L60E swap. About 420 hp at the crank. Due to the front drive shaft on the left hand side I couldn't run the left hand exhaust past it.
So I brought the left hand side (2.5") around the front of the oil pan and then using a 2.5" Flow Master 2:1 merge with a 3" output. Then 3" to a large 3" Dyno Max muffler and then a 3" tail pipe. Made the whole system out mandrel bends from Summit.
One comment on the pic above the exhaust pipe is coming into the exhaust pipe at 90 degrees. Can't think that would be good for flow.
Pic of the Wrangler in progress.

[truckster]

Engines do not need backpressure. That's a myth.

However, proper exhaust design can promote scavenging.

I built my own 2.5 stainless exhaust with 2.5 rams horns. I put the crossover (H pipe) after the transfer case and before the mufflers.

[Hidesertchevy]

I have blueprint sbc 400 that I had dual 2 inch exhaust and thought the engine ran good but kinda expected a little more. After having it a year or so I finally took it to the exhaust shop for a 3inch tucked inside frame. After picking up the truck from the exhaust shop I couldn’t believe how much better the truck ran. If someone would have told me this about the exhaust I would have probably took it as b.s. I truly can’t believe what a difference it made. Long story short the small pipes were not letting it breathe like it needed to.