Isn't 3" piping better than 2.5" piping on these trucks????

I don't have one, but it's been reported previously that the Magnaflow narrows to 2 1/2" inside at the crossover, and is not a true 3" exhaust. Frankly I do not know if this makes any difference in performance. MF may even be accellerating velocity at the X-over for a reason?

I am not an engineer, but I do not think it is so simple as to say your exhaust is only as big as its narrowest point. Theoretically, larger 3" piping could help in curved areas that may offer more restriction than straight sections; larger cats could mitigate restriction due to the substrate internals, etc.

Just some thoughts...
 
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I'll try to get Joe to put me on the dyno again soon and see if I picked anything up by cutting out the 2 1/2" neck down.

I have no other mods since the last dyno besides the 3" pipe replacement.
 
As other posters have mentioned,Its more than just the size. Its a whole fluid flow system.It has to do with the head loss through the whole system. All the bends the changes in size, pipe diameter, frictional losses/ scavenging losses. Without actual in situ testing everything is speculation.
 
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ViperTruck2933 said:
I'm not sure what the limiting factor is regarding the rpm drop off. PBJ ported my heads, larger valves, ported intake runners, ported the throttle body and installed the larger cam. Looking at the dynographs, the hp and trq both still fall off at 5800 rpms. :dontknow:

As far as the exhaust goes, I have Belanger long tubes and Belanger mids with high flow cats and Magnaflow catback. Belanger mids DO have a neck down to 2 1/2" to create the slip joint into the catback. Justin cut the neckdown off for me and welded in a 3" section to create a true 3 inch exhaust. This was done after my dyno tuning, so I don't know how much of a difference it made.

The Belanger high flow cats have almost no restriction. My truck is just as loud as a catless truck.
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Well hopefully the sudden drop off at higher r.p.m. ranges is not port stall on the ported heads. When I spoke with PBJ on the phone, he seemed to think it was juggling cam timing/lift a bit to solve the problem. No doubt he will figure it out.

A 2-1/2" restriction further down the exhaust path with a 3" pipe coming off the manifolds is NOT the same as using 2-1/2" pipes off the manifolds.

I proved it again tonight (as my truck is still at the track as I type this). The lost torque is back with the 2-1/2" pipes (factory 4 cat system) instead of using the B&B 3" pipes off the manifolds, through the Hi-Flow Cats. The m.p.h. is back where it used to be BEFORE the 3" system was installed. There was a large increase in bottom end torque and traction has once again become an issue.

I mentioned rarefaction with headers in a previous post: As the exhaust pulse leaves the cylinder it travels down the primary pipe until it hits the collector. When it hits the collector a rarefaction occurs and a pressure/sonic wave returns to the exhaust port. IF the primary is sized correctly, it will arrive during overlap and prevent (block) the flow of fuel and air (new charge) from overscavenging and losing fuel that should have been burned for power production. This of course is not 100% effective but is a big part of sizing primary pipes correctly and why big bucks are charged for equal tube length headers. And of course they are harder to build. It is also why long tubes produce torque at a different r.p.m. range than shorties.

I am not suggesting the stock manifolds act in this manner (like headers) but a sudden drop in exhaust velocity (like the 3" pipes) will cause a similar effect. But with exhaust systems, it is far more involved than just (the overused term) relieving "backpressure". Again lower altitudes or certainly supercharging like with a Roe (which is like adding over 190 more cu. inches to the 505) can use the larger diameter pipe.
 
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The size of pipe effects the flow of gasses right? Well I noticed my throttle sensitivity increased immeadiatly after I deleted my cats. Big difference. Cats were slowing down the flow of gases......correct?

Like Nowwhat said..........."big picture"..........what is the end result you want with all your mods?

patrick
 
So Is my System Going to make sense? Hi flows In the front, rear cats delete, 2 and A half Inch piping, Catback:rock:
 
Delete the all cats or leave them in my opinion, going half way is of little use when it comes to power production. It will sound different but you will still have a huge restriction.
 
rottenronnie said:
proved it again tonight (as my truck is still at the track as I type this). The lost torque is back with the 2-1/2" pipes (factory 4 cat system) instead of using the B&B 3" pipes off the manifolds, through the Hi-Flow Cats. The m.p.h. is back where it used to be BEFORE the 3" system was installed. There was a large increase in bottom end torque and traction has once again become an issue.
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Ronnie:

just another possible angle here. Is it possible that with exhaust mods you were running lean and getting KR? I see you have a Stage 2 PCM. Were you dynotuned after your mods or just using an off-the shelf flash?

Again, as you have noted you are at higher altitude. I have no idea how the computer may react or compensate...

Just a thought.
 
hunter_angler said:
Ronnie:

just another possible angle here. Is it possible that with exhaust mods you were running lean and getting KR? I see you have a Stage 2 PCM. Were you dynotuned after your mods or just using an off-the shelf flash?

Again, as you have noted you are at higher altitude. I have no idea how the computer may react or compensate...

Just a thought.
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Thanks for the response.
I started with a B&B catback and that helped. The A/F was 12:1 at this point. I installed the 3" B&B Hi Flow system and the engine leaned out slightly to 12.4 to 12.7 (same dyno and operator). I then contacted DC, faxed them the dyno pull and asked them if the addition of the mids required a reflash. They said no.

Our air is really poor here. Our altitude is 3750 but DA is almost always above 5000. Often 5800-6500 feet.

I did get a chance to drive the truck at sea level and it made an enormous difference. The track was packed and I didn't get any runs in but it really responded well to the denser air.
 
Prof said:
Delete the all cats or leave them in my opinion, going half way is of little use when it comes to power production. It will sound different but you will still have a huge restriction.
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In good ole California. If you get caught without cats. It's a minimium $2500 fine. So deleting the two rear cats and going to two hi flow cats up front would be playing it safe. Not to mention the possabilities of them towing your truck. 30 day impound. etc... it's not worth it. So that is why we are tempted to go with two high flows on and delete the rears. Beside having no cats on makes the exhaust stink.

P.S. who has the best high flow cats?
link?
 
i've always deleted my cats on my performance rigs, they've always choked the motor back....... the delete was like adding another gear....... now that deq is such a pain, i wanted the ability to swap in my hi-flows for inspection, then swap back out to catless....... this is why i went B&B for my exhaust -- flanged with headers for shorter mids..... it was a pain to get it where i wanted it but i'm happy with the end result........:D

http://www.vtcoa.com/forums/showthread.php?t=13367&highlight=b&b
 
I just did the Bassani shorty headers, Justin's 3" cat-less mids, and Magnaflow catbacks.....Justin put a 3" joint where the mids meet the Magnaflows....I cut the 2.5" joint off the Magnaflows so I have 3" at least into the mufflers......

The power increase was amazing...feels like 30+ horses have been unleashed..about like the change I got when I install a cam & ported heads on the SRT8..
 
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