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Discussion Starter · #1 ·
After reading here, articles, youtube, etc., it looks like a 3" exhaust would work for me. One article had a chart recommending 3 1/2" for a 400-450 ci 400 hp. Mine is a SBC 350 421 ci 10.9/1 comp EFI 619 hp @ 6300 565 tq @ 5100 90% street 10% strip.

Which would be better for me, the 3" 3 1/4" or 3 1/2"?
 

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Which would be better for me, the 3" 3 1/4" or 3 1/2"?
:cool: An exhaust system that's got some backpressure is desirable for street use. Back in the day a guy in Minneapolis constructed "Headers by Ed." They weren't the biggest, but they were the baddest. They were superbly engineered, tuned headers and as a result they made more horsepower than headers with bigger primaries/collectors, etc. Are you running headers? If so what size are the primary tubes? What size are the collectors? Are you contemplating a crossover pipe?

I suspect that anything over 3 inches would be overkill...
 

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:cool: An exhaust system that's got some backpressure is desirable for street use. Back in the day a guy in Minneapolis constructed "Headers by Ed." They weren't the biggest, but they were the baddest. They were superbly engineered, tuned headers and as a result they made more horsepower than headers with bigger primaries/collectors, etc. Are you running headers? If so what size are the primary tubes? What size are the collectors? Are you contemplating a crossover pipe?

I suspect that anything over 3 inches would be overkill...
its amazing to me how this back pressure myth wont go away. back pressure in an exhaust is never good and allways costs power. going to big on the collector and exhaust will hurt power due to the velocity and reflective tuning wave that helps scavenge the exhaust. simply put ,any time pressure goes up power goes down but you are correct in that to big is bad and that erring on the small side is better as the car will spend the majority of the time in the 2 to 3 k range
 

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I built a 355" for a friend he stuck some Borla 4" mufflers on it had full length headers into 3" collectors and then 4" pipe into the Borla's.
This was in a 76 stepside chevy truck and dumped 2 feet before the rear end.

I was thinking this thing will have no low speed TQ will have soggy throttle response.

The cam was a single pattern 252 @ .050" 1.6 LSA .571" lift with the 1.6 rockers.
Hurricane intake and 800 double pumper.
3500 stall summit converter and TH35 then into an 8" ford with spool and 411 gears.

Darn thing revved like a KZ1000 super quick throttle response 11.5" vacuum at 1000 rpm.
You could not hear pinging from too much timing and you would go deaf driving it without ear plugs.. but the very short 4" did not hurt the performance.

I have seen 3" exhaust on mild 350" say 420-440HP that picked up power when dumps were used before the mufflers.

More kinked bends the larger the tubing needs to be.
I like mandrel bends.. say you can't find 4" bends.. hit the salvage yards and get some factory pipe No kinks.
Another good place is a school bus type salvage.. You can find some really large stuff with nice flowing bends.
 

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BUT I would like to possibly look into the NASCAR style oval pipes too. That way we could route them away from the car in fornt of the rear tires. Not always the best idea, but dumping at the rear axle is also noisy at 9400RPM.[/QUOTE]



At 9400 it would be the parts coming thru the hood that I would be hearing.....Lol
 

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Discussion Starter · #9 ·
:cool: An exhaust system that's got some backpressure is desirable for street use. Back in the day a guy in Minneapolis constructed "Headers by Ed." They weren't the biggest, but they were the baddest. They were superbly engineered, tuned headers and as a result they made more horsepower than headers with bigger primaries/collectors, etc. Are you running headers? If so what size are the primary tubes? What size are the collectors? Are you contemplating a crossover pipe?

I suspect that anything over 3 inches would be overkill...

I am running headers. All I figured out so far was to plug my numbers into a formula to come up with primary tube diameter. If they're 16 gauge (.065 wall thickness) my OD comes to 2 3/32" that is either 2" or 2 1/8" headers. Other than that I don't know if I can specify collector diameter and length. I was thinking X for the crossover.
 

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Given your engine size the 3 inch would work well and fit well. I think the formula is like 475 or so hp you need to go to 3 inch and that is good to like 700 hp and 3.5 inch is good to over 900 hp. The basic formula 1 hp needs 2.2 cfm of flow and a 1 inch square tube can do 110 cfm. So 50 hp per square inch of tube. A 3 inch round pipe has 7 inches square and times 2 pipes is 14 square inches so 14 inches x 50 hp = 700 hp. This is a pretty good idea what you need there may be some other factors when you get to really big engines over 500 CID.
 

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My thought are size the header & collector to what you need HP & RPM range & make the rest as free flowing as possible. My system is 1 3/4 -1 7/8 -2 with a 2 1/4 choke & 2 1/2 final collector. 421 headers & 3 inch the rest of the way.
 

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Exhaust Pipe Size Estimate: A good section of straight pipe will flow about 115 CFM per square inch of area. Here’s a quick table that shows how many CFM each common pipe size will flow, as well as the estimated max horsepower for each pipe size:

Pipe Dia..Pipe Area..CFM.. Max HP Per Pipe..Max HP For A Dual System
...1 1/2......1.48.......171.........78................155
...1 5/8......1.77.......203.........92................217
.....2..........2.76.......318.........144..............289
...2 1/4......3.55.......408........185...............371
...2 1/2......4.43.......509........232...............463
...2 3/4......5.41.......622........283...............566
.....3..........6.49.......747........339...............679
...3 1/4......7.67.......882.........401..............802
...3 1/2......8.95.......1029......468................935

NOTE: These numbers are just estimates. All pipes are assumed to be 16 gauge steel.


I like this chart, I have done the math side from different sources and this chart seems to be about right, so your 619 hp small block would fit good with 3" dual exhaust system giving you some room for growth.

I would like to say a little about pipe sizes and the effect it has on the engine from a street driven stand point on what it does on a street machine.


I have been toying with a base model challenger, yes a little v6, it's a high revving engine, even tho it my be lower hp than your 421 ci engine the concept is still the same.

I played with the exhaust tubing size to increase low end torque on this v6 engine to help throttle response for around town driving. Since the v6 really lacks torque down low the smaller 2 1/4 inch pipe on my 350 hp engine sure helped vs the factory 2 1/2" it had before the 2 1/4" catback.

Anyways I had a full mandrel bent 2 1/4" exhaust under my car, I then rebuilt the entire system out of 2 3/4" pipe using the same style of straight thru muffler (but larger) to match the new exhaust size.

The difference down low was apparent, the smaller higher velocity 2 1/4" pipe allowed my engine to spin the tires on launch from an idle for about 5 to 10 ft in a controlled environment, I would have to lay down some black marks, back onto them and launch off those black marks to hook and run my times. I tried different launch techniques at higher rpms to try and reach full potential of my runs.

Now I went and put on my 2 3/4" exhaust system which was ~40 lbs heavier overall, going back to the same location in the same density altitude to rule out weather effects, the car from a stand still from idle would not spin the tires- just general driving I felt the engine was sluggish down low and not near as responsive. As far as times go, I had to leave at a much higher rpm to achieve my same times and was just slightly better than the baseline runs.

To my surprise I didn't loose e.t. as I thought I would loose, my 60 ft times was the same but required different launch technique, I did loose some "torque" down low just from everyday driving habits which made the car sluggish in the very low rpm range of driving.

The v6 magnifies what most would say, you loose low end torque going to bigger pipe, this is true, but I think the bigger v8's with much more torque available the losses of low speed torque is almost invisible because for one to miss maybe 10-15 ft lbs ( pulling this number out of my azz) at the lowest level of the rpm scale may not be felt or even missed, specially getting on it. But on an engine like mine where you really lack low speed torque in a smaller ci engine it can be felt and missed to a degree. Like I noticed.

Why did i change my exhaust, I'm preparing my car for a supercharger that will push the power to around 550 hp. 2 3/4" inch is just big enough to give me room to move that much air for my hp potential and yet small enough to keep what ever I can down with out hurting the low end even more.

So that moves me on to my thoughts of mufflers, one goes thru all the trouble of picking the right exhaust pipe size, keep in mine the muffler choices, what good is picking the right exhaust pipe size then slap on some junk muffler that chokes the engine up anyways. Make sure to pick the best flowing mufflers, straight thru design mufflers promotes velocity and even good mufflers like Magnaflow will help increase velocity improving performance.

Not all straight thru mufflers are the same, some are smaller inside diameters, others have restrictive vents inside that decrease pipe size/ flow potential. Just keep that in mine.

In my opinion based off what I have seen just in design, magnaflows are very good at promoting velocity, any of your chambered mufflers restrict velocity and slows the pulses down.

On my car I'm running Corsa extreme mufflers same design theory as Magnaflow, straight thru perforated pipe to keep velocity high.

Hope this helps
 

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Would RPM play into the HP/pipe size equation? You have very respectable numbers on the high end of the RPM range. Now what about a setup pulling 1400 HP with 500+ cubic inches at 7-8K?
 

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Its called the law of the circle gentlemen. The are is compounds by a squared number. If a 3 inch exhaust is good 700 hp then the 4 inch is good for 1250 hp - 700 x ((4x4)/(3x3)). I think these charts are here to give people a generalization. Also going with too big exhaust can be harmful too when dealing with part throttle issues (so I think chart also looks into all around numbers). I think also if you have a theoreticial 750 hp motor and are running a 3 inch exhaust not too say it is going to only put out 690 hp (because of chart) maybe it is only down 5-10 hp from its true maximum output. Now at 1500 hp and 3 inch system OK we might have an issue here.
 

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Look into Dynatech 1 3/4 to 1 7/8 . Sure that would be enough unless your 7000+. 3 in exhaust .

Dynatechs great for fitment for a street/strip car , like the collector seal as well .
 

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I posted a good example "I think" of what an exhaust can do going from one size to the next with no other changes and taking into the account of weather ( density altitude).

Also making the case between the 2 1/4 and 2 3/4 inch size being all mandrel bent and using the same design of mufflers to match the exhaust size. In my mind you can not get any closer to perfect on what each pipe size does to your performance.



In my example after I installed the 2 3/4" I did fail to mention I did take my car to the dyno to make sure it wasn't running lean, I carried right at 12.5 to 1 air/fuel ratio during my dyno time, so it was right on, at the track I did play with air fuel ratio some with no extra gains going little richer and little leaner. The 2 1/4" I had it maxed for 1/4 mile performance so I know the tune was right as I did as well try richer and leaner in previous outings

Just from past theory's I thought for sure my car would have slowed down making such a jump in exhaust size on a small 220 ci engine. Was really surprised that I did not.

So does this change things from what I learned growing up, it changed my mind a little, that a bigger exhaust may not be all that bad, as I was told growing up it will slow a car down. Did the bigger exhaust change my car for the worse, not really just a little different feel down low on the very bottom of the torque curve for general driving, but the over all e.t. and mph was not hurt.

Maybe one can look at what I did as a small scale test, keeping everything as fair as one could for a-b comparison, I did it mostly for my own pleasure and learning.

Now I know some will say they went faster/slower with bigger exhaust, but how was it bent, what mufflers was used because hp and torque will vary from muffler to muffler and DA- not everyone tracks DA numbers for a-b comparison, what about the tune of the engine, to fat, to lean, was the combo maximized for the tubing size.

Just going from one exhaust to the next and ignoring the other factors to me has no legitimate value. I think one has to keep this in mine, when one hears I went slower or faster doing this or that on the exhaust..

At the end of the day, your hp levels require so much air, whether 350 hp or 1350 hp, and the exhaust should move that air as efficiently as possible so one does not loose your hp potential. For me it's in the math and if you know what kind of hp your making one should be able to at least figure out a minimum size required.

Header test from the past:

Now I would like to point out a test I did back in the 90's on a 402 bbc, my car ran with open headers 12.50 at 110 mph using some cheap small tube headers, which I believe was 1 3/4" primary tubes. I bought a pair of hooker headers from a friend that was 2" primary and I stuck them on my car thinking I was going to go faster, I went to the track with only a header swap and my car went mid 13's at just over 100 mph. I was totally surprised by this outcome, my car was dead lazy off the line. I went home that day a took those headers off and I put my old headers back on and next weekend my car was back into the mid 12's at 110 mph.

Of course I did not keep track of DA back then, I did not tune the engine again, I just knew my car was a lot slower, even today I can't even think tuning and DA would ever bring that combo back down to what the small tubed headers was able to do.


Just thought I would share.
 

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back in the 90s my 70 race car had a 460 inch bbc that ran high nines with 2 inch headers. i tried 2 1/8 with no gain maybe even a little loss in 60 ft
 

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Discussion Starter · #20 ·
Thank you all for your detailed and helpful replies. I'm going with 3"
 
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