[mseifried]

I have a SBC 350 Motor, Nothing special built .030 over with headers, I ordered my exhaust pipes from Summit the universal X-pipe in 2 1/4, I received it and its 2 ½. My question is can I use these? I will have a few clearance issues but I feel I can work these out. Will the size difference in the pipes hurt or help me?

[Landshark928]

You should be fine with the 2.5" pipes. You can reduce it after the X if you need to prior to the cats or mufflers. If anything you'll see a increase in performance. IMO 2.5" is not too large for a mild 350.

[mseifried]

Great thanks,

The drive is long and I dident want to make if I dident have to.

Thanks

[Duntov]

Originally Posted by Landshark928

You should be fine with the 2.5" pipes. You can reduce it after the X if you need to prior to the cats or mufflers. If anything you'll see a increase in performance. IMO 2.5" is not too large for a mild 350.

I'm curious as to what is the ill effect of having oversized exhaust tubing on a vehicle, like 3" pipes on a 283 for example? I never had anyone explain it.

[Landshark928]

Originally Posted by Duntov

I'm curious as to what is the ill effect of having oversized exhaust tubing on a vehicle, like 3" pipes on a 283 for example? I never had anyone explain it.

The larger diameter exhaust diameter causes the exhaust gas to cool too fast and slow down. Lower velocity exhaust gas negatively affects scavenging at low rpm. And therefor torque. It's a balance. Generally bigger will be better, but only up to a point.

Overlarge piping will also allow our exhaust pulses to achieve a higher level of entropy, which will take all of our header tuning and throw it out the window, as pulses will not have the same tendency to line up as they would in a smaller pipe.

At least that's what Bill Nye told me Saturday morning.

Of course there much more to it than just diameter. Over all length, bends and material make a difference. Where your X or H is placed etc..

[sickolds]

so what it u have a 383 with 2 1/4 is that to small

[Aerotech]

You can run a small tailpipe after the muffler which will will produce a little back pressure which is good for low end torque. The larger pipes become in effect a long collector and mellow out any pulses. I did this on my 350 C10 and it worked great.

[Duntov]

Originally Posted by sickolds

so what it u have a 383 with 2 1/4 is that to small

//////Red Alert////// The quote contains a serious question. I vote "Not a Problem" but 2 1/2" wouldn't hurt, but I may no longer be qualified to answer the question. I'll know in a couple o' days.

However, I still have questions re my earlier inquiry..
Ok, Suppose I had a 283 with big tube stepped headers, and instead of 3" tubing I had open headers?? What then? Still bad?

Part of the reason is as you may know there are a couple of other threads where a Dyno Sim knows more than all of us put together; and I want to find out if my virtual Dyno is lying when it insist open headers beats pipes every time.. Seriously, If I were a hundred I doubt I would profess to know it all. The reason I enjoy this is that I still learn things often. If I didn't I would have lost interest long ago.
Plus I like these little smart axx faces> Duntov

[Duntov]

Originally Posted by Aerotech

You can run a small tailpipe after the muffler which will will produce a little back pressure which is good for low end torque. The larger pipes become in effect a long collector and mellow out any pulses. I did this on my 350 C10 and it worked great.

Ok, AFP guy,
Then is a small exhaust valve also a good thing? Bigger ex valve will almost always register more torque on virtual dyno. Or is it better to have an ample ex valve and then after the fact add in a measured amount of back pressure.....for improved torque> another smiley face coming up>>>>

[Aerotech]

I'm just guessing it's a street car in which case torque is more important than HP. I run 1.72/1.50 Power Pack heads on my T-bucket even though I have a set of 1.96s. It launches like a rocket and chirps 2nd and 3rd but would fall flat at high speed if I ever drove it that fast. My truck had 1.96 valves and improved noticeably with big pipes and mufflers and smaller tailpipes. These are just general rules, of course there are exceptions. Just offering personal experience that may help someone make a decision.

[Duntov]

Originally Posted by Aerotech

I'm just guessing it's a street car in which case torque is more important than HP. I run 1.72/1.50 Power Pack heads on my T-bucket even though I have a set of 1.96s.

I buy every set of 882s with 1.72 valves I see. Hard to find. Use them on 334s with high vacuum despite the port work, but I have had a bit more tq on actual Dyno with the 1.6 ex. I don't know where the top is on 334s cause we don't build for that, but it's up there better than I would have thought. Thinking back to some of 'um the stock exhaust doesn't seem to hold them back. They pull Race car trailers in the foothills in the Charlotte
NC region, a few tough grades. Every body likes them. 16 18 mpg with total gross weight on the scales of 10,700ish.

You AFP guy?

[Aerotech]

You mean airframe/powerplant? Avionics (instruments) in USAF, hydraulics (engine-driven pumps) in real life.

[Landshark928]

Originally Posted by Duntov

//////Red Alert////// The quote contains a serious question. I vote "Not a Problem" but 2 1/2" wouldn't hurt, but I may no longer be qualified to answer the question. I'll know in a couple o' days.

However, I still have questions re my earlier inquiry..
Ok, Suppose I had a 283 with big tube stepped headers, and instead of 3" tubing I had open headers?? What then? Still bad?

Part of the reason is as you may know there are a couple of other threads where a Dyno Sim knows more than all of us put together; and I want to find out if my virtual Dyno is lying when it insist open headers beats pipes every time.. Seriously, If I were a hundred I doubt I would profess to know it all. The reason I enjoy this is that I still learn things often. If I didn't I would have lost interest long ago.
Plus I like these little smart axx faces> Duntov

Then why use headers at all on a dragster? Even the best engine dyno can't account for all of the physics involved. The point where a pipe is too big varies so much there is no stock answer.

Which engine dyno do you use. I have engine Analyzer Pro and it really doesn't get into detail on exhaust. Collector size, primary size and length, that's about it. It then asks if you use mufflers or what the rated CFM of the system is. The issue is simulator programs assume less back pressure is always good. There's just too much info to input to be accurate...8.73 inches then 45* mandrel bend followed by 12.36 inches connecting to "X" piep with 37* crossover and X amount of internal volume followed by 36.76 inches to FM Turbo 44 muffler (With it's own set of specs) followed by 16.55 inches and a turn dow...chrome..

And for most of us the difference in performance is small. If you have money on the line, it's worth it.

[Duntov]

[QUOTE=Landshark928]
Which engine dyno do you use. I have engine Analyzer Pro and it really doesn't get into detail on exhaust. /QUOTE]


I have a bootleg copy of something which is still unreleased. It's designed for 2mile+ asphalt but the results are almost exactly the same as my old Dyno2000< which still isn't a bad program but doesn't address exhaust systems in real detail.

So far I haven't seen a version which truly addresses exhaust in detail and I suspect they all started out like my new copy. Crew chiefs get them first.. From what I've been seeing on other threads there are some toy versions out there now. I was originally being a bit facetious about the Dyno sims being smarter than all of us put together.. From another thread<<you'd have to have been there..
My question however was sincere, I really don't know much about back pressure being beneficial.. When I was a kid I experimented with flat track 2 cycles and adjusted chambers but never really understood it completely.

Hey Areotech.. I wasn't all that far off. USAF anyway... I was ATC

[Landshark928]

Great read on exhaust theory by Vizard.

http://www.superchevy.com/technical/...xh/index1.html