I need the help of the exhaust system Gurus

[derotoreut]

I am getting ready to have a new exhaust system installed on my '69 C20, with 383 stroker, and I want to ask a few questions before I do it. When ready, I will be using the services of a custom installer.

1. What size exhaust should I run? I have a 383 stroker engine that was dyno tested at 465 HP/465 TQ. I am running Doug's D371Y Tri-Y full length headers, which have a collector size of 2-1/2". I am thinking of running the entire system in 2-1/2", plus using either "X" or "H" pipe (probably MagnaFlow mufflers). I want to make sure the exhaust pipe is properly sized.

2. Can I open up the rear crossmember to route the exhaust through it? My truck isn't lowered, but I think it would make for a cleaner install. I have seen some threads here where other members have done this, without putting any additional support structure on the crossmember. I don't really want to install one of the aftermarket ones if it's OK to open up the original. I just want to be sure I won't have a problem with the engine I'm running.

3. Is there a way I can route the exhaust in anticipating putting in a frame mounted gas tank at a later date? I don't want to tackle the gas tank issue now, but maybe sometime in the future. If there is some way I should route the exhaust, it would be good to know.

[So.Cal.Super]

I'm not a Guru but I will share what I have learned and a few things to read.

http://www.carcraft.com/techarticles...ion/index.html

http://www.magnaflow.com/07techtips/faq/question10.asp

I have the doug tri Y set up with X pipe and 2.5 dual flowmaster out the rear. but about 150 less hp when I pulled the motor. The Magnaflow is a better performance muffler to go with. it's a straight through vs baffled and my engine builder can go into which flows more cfm.. etc. I just really love the Flowmaster sound. I'm worried just how loud its going to be After my new motor goes back in. I probably will swap them. I would call and talk to Magnaflow about the 3" exhaust, I think it would probably work great, but I have seen too much (3.5 on a mustang) Kill performance also.

just my .02

hopefully someone can chime in with more help on question 2 & 3

[derotoreut]

Thanks. I just don't know if it makes sense to increase up to 3", if it's already 2-1/2" coming out of the header collector. It seems like there is already the restriction of the 2-1/2" at the collector, so will going to 3" help? Hopefully someone can chime in.

The MagnaFlow guide states for 250-550 HP to use 2-1/2" - 3" dual exhaust. I realize this is only a general guide and not cast in stone.

[treveiger]

If your header is 2.5 then going 3 would be pointless because you already have a 2.5 restriction. I'm assuming your running duels? Where are you running them and do you have a coil or leaf spring?
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[cdowns]

the larger the pipe the more t will kill the loend torque is what i've always been told

have had and know of alot of 500plus horsepowered vehicles that went real good with 21/2 exhaust systems

[suspect]

2 1/2" piping will be good. get mufflers that flow 500 + cfm each.

[derotoreut]

Originally Posted by treveiger

If your header is 2.5 then going 3 would be pointless because you already have a 2.5 restriction. I'm assuming your running duels? Where are you running them and do you have a coil or leaf spring?
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That's what I thought that there would already be a restriction. Yes I am running dual exhaust. I have a coil spring truck with trailing arms and I'm not sure how to run the exhaust. That was the reason for my question #2. Even though it is not lowered, I like the idea of a clean look. I thought if I could run the exhaust over my CPP transmission crossmember and through the rear crossmember it would be easier and cleaner, but I don't want to jeopardize the structure of the rear crossmember by opening it up. It appears that other members have done that successfully without reinforcing it.

[treveiger]

When I took my truck to the guy who did them said if I run them out the back that he'd have to bring my 2.5 exhaust to 2 over the rear end because it could hit the rear end over big bumps.
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[derotoreut]

Originally Posted by treveiger

When I took my truck to the guy who did them said if I run them out the back that he'd have to bring my 2.5 exhaust to 2 over the rear end because it could hit the rear end over big bumps.
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So far it sounds like 2-1/2" exhaust would be good. I also spoke to the custom shop who's going to do the work. I told him I would like to have a stainless exhaust system. He told me that he would have a tough time bending a 3" stainless exhaust and recommends 2-1/2" also.

I would like someone to chime in on my questions #2 and #3. My previous exhaust with my 305 cid was 2" and it fit over the rear end, but close. That's why I want to see about modifying the rear crossmember. If this is possible I think it would make it easier and cleaner to route a 2-1/2" exhaust.

[GuyO]

Dan, 2 1/2" when bent will reduce to 2 1/4" unless you buy a mandrel drawn exhaust system. But going up to 3" from a 2 1/2" collector won't help anything. On that note, long headers drag especially if you drop or slam later or have any off road applications. I recommend you stay with short headers and call Flowmaster and tell them what you have, what you want and ask what they suggest. I run 50 series with 2 1/2" over the rear with absolutley no problems. On question 2, cutting reduces strength. Strength you might need when you light up the rears with a posi or load it heavy. My experience is you won't need to, you will have enough clearance with a 2 1/2" system. On question 3, I would go with a tank or tanks between the frame rail and sheet metal. Dual tanks on both sides with ethenol free gas would be a very cool touch. My 2 cents. G.

[So.Cal.Super]

I did call magnaflow today and talk to them.(currently I have flowmaster super 40's) Your questions got me curious on my own setup. They also recommended the 2.5 system. He explained as Cdowns Stated the 3 inch will drop your low end power but increase your top end HP. He said unless your just going to track it the 2.5 will work best for the street. I have a pic. its slightly higher then side view. the engine and headers are out of the truck currently so I can't get a perfect side shot. but it does stick down a few inches past the frame. you have to be full on side view to see them, this is the best pic I currently have and even this low you can't tell, they are dougs full length tri-y But, like GuyO said if your going to drop the truck it could create a problem down the road.

[oklalawnboy]

Originally Posted by treveiger

If your header is 2.5 then going 3 would be pointless because you already have a 2.5 restriction. I'm assuming your running duels? Where are you running them and do you have a coil or leaf spring?
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If that was the case, why would you run anything larger than the diameter of the header tubes which are what 1 1/2 or 1 3/4 just my 2 cents. What stall convertor are you going to use or use a stock one. I agree if your not going to race it much and will be running it at lower rpm's, 2 1/2 should be fine. The 3 will give it better sound though.

[treveiger]

Originally Posted by oklalawnboy

If that was the case, why would you run anything larger than the diameter of the header tubes which are what 1 1/2 or 1 3/4 just my 2 cents. What stall convertor are you going to use or use a stock one. I agree if your not going to race it much and will be running it at lower rpm's, 2 1/2 should be fine. The 3 will give it better sound though.

Because for each header you have 4 tubes coming from the exhaust ports into one which is the 2.5 tube that mounts onto the exhaust pipes. If you ran that small of tube that would restrict flow sense you have 4 tubes going to one the same size if that makes sense. How would a 3 inch exhaust make it sound better? Unless you have a seriously healthy motor 3inch would be useless.
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[oklalawnboy]

Originally Posted by treveiger

Because for each header you have 4 tubes coming from the exhaust ports into one which is the 2.5 tube that mounts onto the exhaust pipes. If you ran that small of tube that would restrict flow sense you have 4 tubes going to one the same size if that makes sense. How would a 3 inch exhaust make it sound better? Unless you have a seriously healthy motor 3inch would be useless.
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Dan said his 383 was producing 465 hp with 465 tq. That in my opinion is a healthy motor. My 383 produces 425 hp with a GM performance Z-28 cam and it runs better with the 3 inch but I do run it at higher rpms. I also run a 4200 stall to get the heavy Monte moving off the line. Several people have commented they thought it was a big block. That is why I said it would sound better with 3 inch. I seriously doubt most people including me would FEEL the difference between 2 1/2 or 3 inch exhaust.

[derotoreut]

Originally Posted by GuyO

Dan, 2 1/2" when bent will reduce to 2 1/4" unless you buy a mandrel drawn exhaust system. But going up to 3" from a 2 1/2" collector won't help anything. On that note, long headers drag especially if you drop or slam later or have any off road applications. I recommend you stay with short headers and call Flowmaster and tell them what you have, what you want and ask what they suggest. I run 50 series with 2 1/2" over the rear with absolutley no problems.

I already have the Doug's full length Tri-Y headers installed. I will never be lowering or slamming the truck so that won't be an issue. I don't like block hugger or short headers if they can be avoided. The only reason for using them is to improve ground clearance. I wanted the Tri-Y headers because they're designed for muscle cars, trucks and motorhomes; and they yield maximum power from off idle to 4500 RPM making them well suited for applications demanding maximum mid-range torque with maximum fuel economy.

Originally Posted by GuyO

On question 2, cutting reduces strength. Strength you might need when you light up the rears with a posi or load it heavy. My experience is you won't need to, you will have enough clearance with a 2 1/2" system. On question 3, I would go with a tank or tanks between the frame rail and sheet metal. Dual tanks on both sides with ethenol free gas would be a very cool touch. My 2 cents. G.

Thanks for your input concerning the rear crossmember. I will take it into consideration. Here's a couple of threads I was looking at for this:

http://67-72chevytrucks.com/vboard/s...d.php?t=526532
http://67-72chevytrucks.com/vboard/s...d.php?t=338103

[GASoline71]

Originally Posted by treveiger

When I took my truck to the guy who did them said if I run them out the back that he'd have to bring my 2.5 exhaust to 2 over the rear end because it could hit the rear end over big bumps.
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It's prolly because he doesnt have the capability to mandrel bend any tubing over 2".

I've seen lots of systems on these trucks with 2.5" and 3" tubes over the axles with no probs.

Here is a pic of a '69 C10 with a BBC with a full 3" system mandrel bent over the axles.



Gary

[derotoreut]

Originally Posted by GASoline71

It's prolly because he doesnt have the capability to mandrel bend any tubing over 2".

I've seen lots of systems on these trucks with 2.5" and 3" tubes over the axles with no probs.

Here is a pic of a '69 C10 with a BBC with a full 3" system mandrel bent over the axles.



Gary

Thats good to know. Since my header collectors are 2-1/2" it seems to make sense to stick with 2-1/2" throughout. Hopefully it's big enough to work with my engine.

I still would like some feedback from those who have modified their rear crossmember to see how it worked for them. It seems like it would be easier to run the exhaust through that X-member.