Well I've spent the last few hours reading through archives on here and classicalpontiac and have seen many opinions. Figure as long as I'm sitting here avoiding actual work I might as well get some more.

It's time for the exhaust system and I'm wondering what size tubes to use 2.25 or 2.5"? It'll be dual exhaust. Just about every posting I've read through is a discussion of 2.5 vs 3" but I haven't heard many people say anything about 2.25. In reading on here I found that Bjorn ended up with 2.5 and I think Vikki has 2.25. Most postings I've read recommend 2.5 but I'm not sure that I need it. I believe the 400HO and Ram Air IV used 2.25" so I'm thinking that would likely be fine for me as well.

Here's my situation. 67 coupe with a 400 bored 0.060" and a summit 2801 cam. Stock 1.5 rockers, #62 heads with no porting. TH400 with whatever torque converter was on there when I bought the car; all I know is it's pretty large and I'd be suprised if anything much bigger would fit in there. Not sure what rear gears I have but can check once I get home if need be. I have the headers that were on the car when I bought them but I don't know much about them other than they're the 3 tube design. I'm not planning on doing any racing or sustained high engine speed running; this thing will be a weekend driver (once it actually moves...). Of course I'll push it a bit but I'm not gonna be going to the strip. Or at least that's what I say now.

I'm just kinda looking around and have seen a few systems on summit's website. I can add in an H or X crossover later if need be. They have a 2.25" and several 2.5" systems. I'd like to order a system and put it on there rather than having a shop do it just because I like doing things myself, but I have no means of doing much more than fit and weld.

so anyone have an opinion? Maybe 2.5 won't hurt so I might as well go with it? Maybe 2.25 would give better low end which is probably where I'd do most of my running anyhow? Maybe I wouldn't notice the difference so I should quit wasting your time and just rock-paper-scissors myself for it?

thanks,

Dan

Dan , from my stock exh manifolds I have 2,25 down to where it turns backwards, there I went to 2.5, then a H pipe then to two 40 flowmasters, no transverse...maybe you already got all that ..but I thought I better post it...I may even have pics if you want....email me if thats the case... bsefeldt@mhinc.com
I also have a rebuilt 400, not sure if bored ,but would guess so...I bought it installed in the car....I do have 5C heads milled to 88CC....1.65 rockers...holley 650 carb...think thats it...

If using headers, use 2.5 inches.. -plus the X-over will make more horsepower than H-over. more than open headers, in fact.

Actually I made stainless 3 inch with x-over, with 'spin-tech' mufflers... -the x-over really helps the scavanging../low/mid/top end..

I'm sure you wouldn't go much wrong with 3 inch.. -also pretty good results with 2.5, but with headers 2.25 is simply a bit on the small side..

Stock non-ram air was 2" and the cars ran pretty well. I run 2.25 with long branch manifolds and it seems like plenty of pipe.

Unless you are going to change the three tube headers to 4 tube than the 2-1/4 will be big enough.

from my understanding, 3 in muffflers are wayyy too loud

my car will be performance, not exhaust noise

2.5 is fine with 1 3/4" headers or manifolds. 3" absolutely not recommended or headers bigger than 1.75". Run a restrictive muffler.

Or change pipes to 2.25 with a decent flowing exhaust.

You go too big, the motor will fall flat on it's face. I have 3" on my 67 that had a 600 HP motor..and that was just right.

2.5" would be my choice. Any exhaust restriction reduces performance and fuel economy.

Any muffler can be too loud regardless of the pipe size. With a larger pipe size and muffler, you can reduce the sound level more without restricting the exhaust flow. My car had dual 2" with glass packs that sounded like a machine gun at high revs and attracted police just as fast. I switched to dual 2.5" pipes with Sonic Turbo mufflers and was almost disappointed at how quiet it was. An H pipe also reduces the sound level as do full length tailpipes.

As a point of interest, I ran a single 4" exhaust (equal to dual 2.8" pipes) on a fairly stock Buick 350 4 bbl in a circle track car. All the cars had open exhaust systems and it was the quietest car out there. The loudest? Dual 2" systems. The larger volume has a natural sound dampening effect. And yes, it made more power than the other cars and yes my exhaust was eventually banned!

Here's some guidelines..

http://www.flowmastermufflers.com/faq.html

http://www.pypesexhaust.com/faq.html


2 inch up to 100 per pipe (UP to 200 hp dual)
2.25 inch up to 160 per pipe (UP to 320 hp dual)
2.50 inch up to 200 per pipe (UP to 400 hp dual)
3 inch up to 300 per pipe (UP to 600 hp dual)

So depending on your hp, -that's something to keep in mind

With your combo, 2.5 is 'around' enough.

-also, X-over is really recommended, for quieter AND better flowing exhaust.

http://www.pypesexhaust.com/xpipes.html


On the 'note' of loud exhaust, I must agree with TOHCan. -My dual 3 inch exhaust is more quiet than the dual 2,5 inch that original was on the car.
-Mainly because my current setup have a x-over, -AND the 'spin tech' mufflers are really better flowing/and more quiet than the flowmasters on the 2,5 inch setup. also a point is the fact that even if my current setup is stainless steel, which has a higher note than 'normal' steel, -it's still more quiet. PLUS this engine has around 450-460 HP vs. the old one that had around 340-350.

3" is too big for the everyday 400 motor. Trust me on this. I have a dual 3" setup with a 9.7-1 67 670 headed 400 est. 300 HP motor and it was overexhausted.

Same motor, same rear gears, same trans, different exhaust 2.25 vs 3"

Thats why I said run restrictive mufflers if you are left with 3" and have no other option.

The motor ran like crap and all that was changed was exhaust from one car to another.

Zero or minimal back pressure is not good for a Pontiac.

thanks everyone for the advice. I checked it out and looks like my headers are 1 5/8" on the outer two pipes and 2" in the middle.

I'm not really thinking about the sound right now. Will tackle that later once I hear it.

I'm also not really considering 3" because I think that's just too big for me. I'm not planning on going anywhere near 600hp. I think for me an argument could probably be made either way for 2.25 vs 2.5 inch. Sounds like 2.25 is sort of on the edge for me right now but I might want 2.5 if I'm planning on doing upgrades in the future. I'm looking more for driveability and lower engine speed torque than all out peak hp.

Thanks for the links Claus. They recommend 2.5 but don't mention 2.25. Everyone recommends a crossover so I'll throw one in there.

Tom what exactly is a 'restrictive' muffler?

I drove my 69 to the race track and removed the exhaust when I got there. Ran open headers. You can't get much less back pressure than that and the 428 ran perfectly. I've got it all on video if there are any doubters. I had a good pit crew that time! I really don't think you need much back pressure.

There is a difference between back pressure with an exhaust system bolted up versus back pressure with open headers.

Look it up...

Good point.

Check out the stock 1967 gto dynochart.. -they test 2.25, AND 2.5 plus open headers, headers w. x-overs..

Sorry I didn't see this earlier, but good info..

www.pypesexhaust.com/dynotests.html

2.5" and an x-pipe is the only way to go. A 3" will also cause fitment problems. Too me, some exhausts look as if the car is dragging around a set of stainless steel balls. I guess people like the look because I see a lot of steel ball systems on the road. Unless you have a super artist fabricating the system, the car will have the steel ball syndrome. Also, the exhaust starts to get out of porptorion with the car, and the plumming will start to look like the fart can rice burners, only twice as worse.

I see ricers, with 3.5 or 4 inch tips.. -the look is awful...

-Actually even if I have 3 inch, -people have asked where the exhaust ends ? -It looks as if it doesn't have any. (well sorta) -Compared to the flowmaster 2.5 that it came with, -it clears 2.5-3 inches MORE than that. -and keep in mind it's 3 inch.

-I think (from the looks of it) Pyres exhaust is one of the best fitting I've seen for firebird/camaro..(mine is nicer, ofcause ;-)...)


here's a pic of start of fabrication..

Tom, I'm going to have to disagree with you on this one. Back pressure is a bad thing regardless of the engine make (talking about 4 stroke engines only). The engine has to do work to move the exhaust through the pipes. Extra resistance means less power and less fuel mileage. The engine is an air pump; more air in and out means more power. Even with 2 strokes, a lot of people confuse back pressure with proper tuning of the sizes and lengths of the pipes.

A good exhaust system will improve the scavenging and can quite effectively change what the engine wants for jetting. I suspect that your car might have been running on the lean limit and when you changed from the restricted exhaust to a freer flowing one that it went over the edge. Three or four jet sizes up usually cures the problem. I'd really advise trying it again, there's power to be gained!

As with any major change, cam, air cleaner, heads, intake manifold, carb, you have to start with a clean slate and tune the car to what it wants after the change. Sometimes it isn't much of a change but make sure anyways. Adjust the fuel mixture with the jets in the carb, not by restricting the exhaust. Would you install a smaller air filter to richen up the mixture? Some people do!

This is from many years of experience solving other people's tuning problems for the street and the track. Also, most dyno operators don't tune the low to mid throttle and rpm ranges and leave that up to the owner. Most dyno tuning is done at full throttle and higher rpm; I tuned the engines for proper street manners after the engine was installed. Been there, done a lot of that!

Build an unrestrictive but not unmuffled exhaust system and tune for it.

Then why such a performance loss from 2.5 to 3", same motor?

And why did Pontiac assert the RAII heads were overexhausted to a point to where in order to correct, restrictive manifolds or modifying the heads for an approximate 70% I/O ratio?

The rule of thumb with any pontiac head is 70% I/O flow ratio..

The improvements were made with the 722's the following year.

I'd like to know the answers to those questions too! Describe how the car ran after the exhaust change with some indication of a measured power loss, and give an explanation why you think the exhaust caused a power loss. I'm a fact-based person!

What did Pontiac claim would happen if the ratio strayed very far from 70%? What were the flow numbers of the heads in question, and what do they say now about the "ideal" ratio?

Check out this article by David Vizard: http://www.superchevy.com/technical/engines_drivetrain/exhaust/0505phr_exh/

The idea is to get the bad air out and the good air in. By using negative pressure in the exhaust system (not to be confused with back pressure) more power is made. Tried, tested, proven, repeatable, transferable to all street engines regardless of make and port flow ratio. Yes, you can have less back pressure than open headers by tuning the exhaust system with the right size and length of pipes.

Claus, I would attribute the 10 hp loss shown in one of the dyno tests from using a particular muffler to a change in effective length of the pipes that the engine "sees", but they don't describe the different mufflers so I don't know for sure. The above article explains what happens with different muffler types.

I've worked full time building race cars and hot street cars for most of the last twenty years so the number of cars that I've worked on is likely well into the hundreds. I have yet to witness a single car that didn't respond to a better flowing exhaust system and have yet to see one that liked a more restrictive exhaust - including Pontiacs! Of course, I haven't seen everything yet either! And I'm always willing to learn!

I did say restrict the exhaust in one of my posts if you are using an existing 3" exhaust and have no other option. Reasoning, my motor as stated prior..ran 13.30's with a 2.5 and ran 13.80's with 3" This was the Black 67 drag setup in garage. I changed nothing to the motor other than the exhaust. When I changed mufflers, I scaled down to 2.5 Inlet/outlet and went with a muffler that a friend didn't like because his ET's dropped 3/10's with them. He in turn went to Flowmaster 50 series and gained back 5/10's total. The 3 he lost, and 2 he gained. If motor A is happy with a 2.5 exhaust and nothing changed but the 3" exhaust, I restricted the flow with a different set of mufflers and gained back my ET. The original exhaust is a 3" dual Flowmaster offset 3", dumped over the axle. There is such a thing as having too big of an exhaust..right? If not, why then, other than pipe size and mufflers is there such a gain or loss across the board with 12 or so different systems for our cars..? If you install a 2.5" system on a 550-600 HP motor, would you see an improvement or a loss?

I really can't get into the why aspect, other than motor A ran 13.3's with 2.5" and ran 13.8 with 3"...I can't remember the trap speeds, as that was 7 years ago.

As far as I remember, one of them is here http://www.wallaceracing.com/ultimatehead.htm

The issue was that the RA II heads were the best exhaust port, but with the smaller non RAIV specific intake port size (these RAII heads ran stock 9794234 intake, Not the aluminum RAIV with taller intake ports) thus one would conclude that the air intake ratio versus the exhaust was insufficient.

When I assert that the RAII heads were overexhausted, the intake port sizes were insufficient for the size of the exhaust ports. Thus Pontiac changed the intake port size by 1/8" taller and I believe 1/8" wider for the 69 model year with the 722 heads. I can not be certain of the sizes, but when I put a stock, non RA intake gasket on my stock RA II heads, there was quite a bit of material..furthermore, I have a HO 455 71 aluminum intake, exactly same as RAIV ports. The RAIV gasket matched exact.

Did you try tuning the carb when the car went slower? Any other changes to the car? Having the mph and sixty foot times might help pin down the range that the loss occurred in. Based on this, I can see why you would say that your car went faster with a specific amount of back pressure in the exhaust system. However, I feel that more investigation and testing would likely show another less restrictive exhaust system and setting the engine up accordingly would result in a power gain - like your friend experienced.

Certainly one case isn't enough to condemn all Pontiacs to restrictive exhaust systems! Not to argue but you also said that Pontiacs need back pressure, specifically zero or minimal back pressure is not good for a Pontiac. That's what I disagreed with.

To reiterate, I'd advise dual 2.5" pipes with good flowing (non-restrictive) mufflers, an X-pipe and full length tailpipes. Pointing the ends of the tailpipes down and slightly in will also cancel some of the noise, allowing slightly better mufflers, which will make more power and better fuel economy. The main point is to design the system, not just grab the cheapest parts and stick them on.

I think that a well thought-out 2.5" dual exhaust would out-perform a poorly designed dual 3" system. It may not make the same peak horsepower but should have a better average output through the speed range. As with any pursuit of performance, the whole package must be considered and sized accordingly.

I will clarify that zero or no back pressure as in my case. With my specific comment, it was personally related and motor specific. Perhaps I should have clarified.

We spent better part of a weekend on a test and tune. Not that I wanted to keep a substandard 400 in my track car, we went through everything..from timing to the carb. I had 4 Pontiac guys, with probably 70 years combined working on this car. Two of them consistantly run in the 11's and Pete runs in the low 9's, so there is no question that the motor was tuned potentially.

I have had maybe 2 or 3 different exhausts in my 67. Am I an exhaust expert as in Jim Hand? no. Do I have the money and sponsorship to try 20 different systems? no as well..

I changed two things on the car..switching the motor to 3" and changing the mufflers from 3" to 2.5. The motor ran better with the mufflers installed. These mufflers were slowing John's car down and sped my car up. Unless I am totally missing something untechnical, restrictive mufflers, car ran better.

A less restrictive exhaust is great for a motor that needs it. I do not think a 300 HP motor, as in my case, needed or required 3" exhaust.

Without getting technical, a 428 lets say has heads which flow a max of 300 cfm per intake. That is potentially 2400 cfm total. What exhaust requirements are needed for this motor? Lets assume correct carb, cam, headers and intake.

Then you have a stock 400 which usually runs in the 170 cfm per intake for 1360 cfm potential. Lets assume the same as above regarding the cam, intake etc.

The exhaust requirements are dictated by complete potential of the motor at "X" lift, rpm, shift points...etc.

By allowing a stock motor free room to expel, there is no consequence to performance without changing any motor parameters?

Sorry if I fragment..I am between cases at work and needed a breather.

I guess from Pete's description that I'm a cynic! Yet your results are hard to ignore - not that I would, anything different is an opportunity to learn! Can you PM me with as much info as you have on the engine specs, exhuast details and muffler brand/part number/design? As I said, I haven't witnessed a car that wanted back pressure and I'd like to understand why yours did.

Another interesting thought I had..

Built a 428 stroker motor..305 CFM E heads, 72 CC, Victor intake 1050 Dominator. CC 285C-R6 roller. Dynoed 628 HP. What torque, somewhere in the low 600.

Car was running low 11's until I changed the exhaust from 2" primaries, 3.5" collectors to 1-7/8" 3" collectors. Motor then ran consistantly into the low 10's. 3" mandrel bent, Flowmaster 40's 3" in/out. 60' times I believe were low 1.6's, high 1.5's

Last time that car was run was 2000. I believe my times were around 129 and 10.31 with a 3.73 gear.

Too bad I gave up racing... I may have been good at something.

I guess Ill throw my 1½ cents in here (I need a short break from working in the garage anyways ). Otay, heres my limited-knowledge take on exhaust sizing...a fast-flowing, but unrestrictive exhaust is ideal. This may sound like a contradiction in terms, since larger pipe is less restrictive but flows slower, while smaller pipe is faster moving but restrictive. Its my opinion that most peoples exhaust pipes on their cars (street use) are way oversized, with the reason being that they watch too many shows or read too many magazines that tell them bigger is better, cuz after all, its less restrictive and can flow more than smaller pipes. For the dragstrip this is probably true, but for a car that has a street engine and sees street use, I would take an undersized pipe over an oversized one any day of the week. Simply put, too large of a pipe certainly can cause an engine to run less efficiently because of the slow-moving gases travelling through the pipe. The best analogy I can think of right now is that its similar to trying to siphon fuel from your tank...using a hose thats maybe 1/4-1/2" in diameter works far better than using one thats 1-2" in diameter. Why? It just does! lmao Just kidding...its because the larger hose is too [censored] big and the fuel would not travel fast enough to create the vacuum needed to maintain a strong flow. Another misconception, in my humble opinion, is that the terms "back pressure" and "restriction" are interchangeable. Sure, they are related but mean two different things. To me, a restriction is a flaw that occurs, whether it be by poor design, an operational malfunction, contaminant build-up, etc, and can CAUSE back pressure, and back pressure is simply the resistance of the gases to further move along the pipe, if that makes any sense (I know what I mean but am not very good at putting it into words, remember? lol). And heres where, for the first time ever, Ill hafta repectfully disagree with my buddy TOHcan...while back pressure IS usually undesirable, a certain amount of back pressure is beneficial sometimes, especially on street rides (and probably never on a strip car), and is another part of my reasoning of why oversized pipe is worse for an engine than an undersized one. Well, back to work for me...this concludes my 1½ cents worth on exhaust sizing

I think the reason Tom got better with smaller pipe size is not because of back pressure but scavenging of the exhaust through the better use of the exhaust pulses.
The larger head pipe size could have slowed the pulses enough before the collector to not scavenge the hole system efficiently.
Switching to smaller size head pipe and collector would more efficiently (faster) pull gases through system hence more power.

I used 2.5 collectors, -and welded them into 3 inch pipes..

-thereby creating sort of a funnel, -into a bigger pipe.

Some speciality headers are made this way..

- this way it speeds up gasses, just before entering the X-over, -and then into the 'spin tech' mufflers..

just like I did , but I went from 2.25 to 2.5, and no X , but an H

This is all "muddying the water". Brent/TOHCan had it nailed. The carb is tuned for the exhaust. Change the exhaust configuration and your car may not run as good. If you put a less restrictive exhaust on your car and it runs worse, then you have potential to gain more power by retune. It's a simple sign that your 'flow' is suffering because of the restriction.

Every motor will have different exhaust requirements, and for the most part, trial and error. While changing one or more of the following has an effect +/- a certain percentage... (pipe size, crossover, crossover type, headers, manifolds, muffler type, muffler size, collector size, primary size, muffler inlet/outlet position) Each has it's own positive or negative effect on a combination, the key is either having the knowlegde on what works where, or the money to have it known.

There are certain instances where a "non restrictive" or zero backpressure exhaust may be required..Nascar?

I am sure there is a huge difference between the exhaust system from a superspeedway track at 2.5 miles (Atlanta, Michigan, Talladega) versus 1/2 mile tracks like Richmond, Bristol and Martinsville. And I am sure there is a difference between those two exhaust set ups and a road course such as Watkins Glen. Each track has a specific range where the motor needs to operate at peak HP/torque, thus one would believe the exhaust system would be modified in one detail or another. There may even be a different exhaust used for restrictor plate racing.

I agree that an open exhaust (headers or dumps) produce the most HP. The key is finding the right combination that works for the vehicle. The exhaust requirements could be the same for the street or track for the same car, but at the track, more HP could be achieved with different mufflers, size etc.

Maybe there is a grey area with terminology. There is and can be many variations to increase or decrease a system +/- 1-2 HP or more per change. You could possibly gain up to (as example) 40-60 HP (over a good system) with the proper setup if you have the time and money to have changes made. I have heard guys buying a certain exhaust system from Pypes and making exhaust mods at the track and over time and reducing their E.T's with other recalibrations.

The systems out there, for the most part appeal to the general class. You could squeeze more performance out of it, sure.

Here is a good read..

http://www.pontiacstreetperformance.com/psp/exhaust.html

well I think I'm gonna go with 2.5. Sounds like mixed reviews and perhaps 2.25 but perhaps 2.5. Only way to know for sure, without some simulation software, would be try both out. Since I have way too many other things to do on this car I'll just pick 2.5 and be done with it. Maybe in the future I'll try out some things but for now this'll do.

here's another question. How far down do you put the crossover? I assume it'd the be same point whether you use an X or an H since the wave reflection is gonna happen at the point where the pipe opens? Or perhaps there's more to it than that? Seems to me that this would be a critical point

The exhaust system that you are going to purchase will have the pipes pre measured for the crossover. If you are having one built at a shop, I recommend looking at a few articles by Jim Hand and looking at..

http://www.pontiacstreetperformance.com/psp/exhaust2.html
http://forums.performanceyears.com/forums/forumdisplay.php?f=422 and
www.classicalpontiac.com search Q&A keywoords H and X pipe.

Some guys in PY and CP are really good with the exhaust systems out there for Firebirds. I remember one having tried 4-5 over a few years.

I had mine done at a shop, heres pics of the 2.25 going to 2.5 and a H crossover point in opposite order...hope You can tell from these where those points are...note that these are on a vert, so therefpor You`ll see the X plate where they tuck in over

I think one of the most amazing facts about exhaust and raw horsepower is that you do not gain much over original 1st gen exhaust. I've seen outragous claims but dynos will tell you it's like 20 HP difference between original and custom. Not a big factor that's why I put the 'logs' back on my cars.

Banshee, you beat me to the Jim Hand articles on Pontiac Street Performance, but the content is generally what is presented in Jim's book, "How to Build Max Performance Pontiac V8's".

Seems to me that CDJr and 8point hit on the heart of the issue. Depending on the application, whether street, drag racing, or Nascar, the exhaust requirements differ greatly, but the key is the negative pressure or vacuum that is created in the pipe. It's the vacuum created from the hot exhaust gases expanding in the pipe that creates the vacuum necessary for efficient scavenging. (I believe that is why having an x-pipe enables you to take advantage of both vacuum pulses instead of just a single pulse from true dual exhaust.) If the available expansion volume (the pipe diameter x length of pipe) is too great, then the vacuum generated would be weak and the reduced performance would be seen. Reducing the pipe diameter in Banshee's applications apparently strengthened the vacuum pulses and generated more power. I think the required expansion volume would be related to the engine displacement and operating rpm - the rate and volume of air you are putting through the 'air pump' that TOHCan mentioned. I think this explains why the high revving, big displacement engines of Nascar and many drag racing applications would warrant bigger exhaust pipe diameters.

IMHO-

Todd

I don't believe that there's any vacuum created in an exhaust system. That defies the laws of physics. No vacuum, just varying amounts of pressure. Unless you want to count the tiny bit you might get when the air is flowing past the tip at high speeds.

Just take one bank of cylinders and think of number 1 firing. As the exhaust pulse goes down the header tube and passes the collector the expanding gas causes a vacuum in the other 3 tubs witch helps scavenge those cylinders.

If this is a game of syntax, the comment doesn't apply.

It seems that vacuum referes to the low pressure following a column of gas in motion. Then again, it might be a vacuum: The overlap allowing transfer of the low pressure of the manifold.