Hot Rod magazine just had an interesting article and chart about choosing exhaust system size. I will tell you one thing from experience, 3" pipe will be a lot louder over an identical system made from 2 1/2" pipe.

 

AFAIC there is NO such thing as "too large". Too small, yes. The only thing about larger is larger=louder.

 

i disagree-- too large can be as bad as too small --

i had headers built for an 'ed pink' motor and it was specific to tube sizes and merge collector-- and lengths of each pipe--.. if bigger was always better then you would use as big as could fit.. as i understand it - larger will lose HP if the motor doesnt have enough back pressure.

i put 2.5 on mine with 3" tails and its REALLY loud.. HP = decibels...
granted mine is all stainless and mandrel bends and V bands.. 2.5" is more than enough for 500hp.. i didnt read the article but i would think 3" would support 700hp.. this is all assuming NA engines NOT turbo
i have used the same exhaust on other cars and no where near as loud.

 

I understand there are trade offs. But will I lose allot of Low end grunt jumping up to 3"?

 

Here is the link to the Hot Rod article!

What Size Exhaust Pipes Work Best?

 

caliscrew, yes i would use 3'' exhaust

 

Three inch won't be too big, but ground clearance might become an issue, depending on how low the car is and how the pipes are routed.

 

Texas, get to driving jackass....? Doesn't have the same ring to it.

Mine is not a max effort build, so you need to consider the purpose of the car. I know I'm leaving a little on the table going with 2.5" but mine is pretty slammed and routing the 3" is a little more difficult. Plus, i didn't want it "too" loud that I didn't enjoy driving it as much.

Cruiser or taking it to the track some? Most of the gains you would see I would (not an expert here) think would mostly be in the upper rpm range...I doubt I would ever notice it...I bet only a dyno would be able to tell...or clock, it knows all.

 

Texas, get to driving jackass....? Doesn't have the same ring to it.

That, and Rodney Carrington never said it

 

will 3" help you in any way ?

No, and they're going to be more expensive, and more difficult to route, etc.

Will they hurt you performance wise.

No.

Z.

 

3"

I don't think you'll leave anything of consequence on the table using 2.5" exhaust if you have a crossover

 

Thanks for info guys, but I think I'm going to run dual 3" since there is room for growth.

 

You can always go with 3" ovals if there is a clearance issue.

 

I run 3" all the way with an X-pipe. Just switched from Magnaflow muffs to Dynomax turbos. It's loud but I can carry a conversation and the low end of my stereo and subs can now be heard above the exhaust. The magnaflows ate all my low end stereo sound.

 

On my 70 with a 460 SBF, I have 3" pipes from the headers back with an H pipe and Magnaflows. Cruising down the highway at 65mph I can easily carry on a conversation in 5th gear (about 2200 rpms). When I get into it, then it is very loud!

The 3" size was based upon displacement (460 ci, and HP 550), as well as the dyno runs.

 

I agree with jdub. If you want 3", then go for it! The worst thing that might occur is that resonance will change and likely go up with larger pipe and less velocity.

I really think the "back pressure is good" came from large or open pipes on OEM tuned cars! A car HAS to be TUNED for exisiting exhaust. MFI cars are VERY, read EXTREMELY, sensitive to exhaust changes. To open the exhaust on a currently tuned MFI car or any non feedback FI car without a retune will effect the AFM and therefore the performance. Carbs and O2 sensored EFI both "self tune" by reading air flow (over simplified, yes).

I am in the camp that the importance seems to lie in primary diameter, sometimes length, and most importantly in collector size and length. If the collector volume is appropriate, I find it hard to believe any increase in exhaust length or "backpressure" is going to increase power. I've seen drag cars drop .3 in quarter by changing and sometimes shrinking the collector diameter. So bigger is NOT better here. jmho

 

I just put a 3" x pipe kit on my 66 with a 347. Jeg's chambered mufflers, it's pretty loud and runs good.

 

I just remember that beast 55speed built...had to pull the oval exhaust because of all the complaints in his neighborhood.

 

66resto:

I'm hoping its the Upper class NY citizens and not the exhaust in full as I bought his (55Speed) old exhaust. Trying to decide if I'm putting behind a 347 crate motor or my monster 427" in the 70. With the craftmanship, I'd like it in the "pro-touring" type car, but fear my 427 will do the same as his and be nearly useless in traffic.

 

Be sure to post up some vids when you fire that beast up.

 

Is there anyone making 3" tailpipes for the 64-66 cars? Even if they needed some tweaking it would be a great start for my 65 versus having to scratch build them.

 

I am not sure about 64-66, but I could not find any for my 70, so I custom made mine by starting with some nice original style repops and cutting them all up. It was not all that hard and look stock except for the size.

Its in my build thread page 4 showing all of the steps.
http://forums.vintage-mustang.com/m...ustang.com/mod-custom-forum/725585-1970-mach-1-460-sbf-boss-build-thread-4.html

Just need to be able to weld.

On the X pipe or H pipe, you definitely want one. I found that with my trans and everything else I have under the car, including the 3" pipes I could not fit an X-pipe without it hanging way low, so I went with an H-pipe.

 

App01,

Is that a steering stabilizer on the the gearbox steering setup?

 

Cali_Screw, if you are referring at the picture with the two end links and the round turnbuckl/jam nuts, that is the torque link to keep the engine in place if a motor mount fails. No room at the front of the engine due to power steering, so mounted in back with a custom welded bracket on the frame below the steering box. If you are referring to something else let me know.

 

Kits won't fit with aod swap.

 

I have close to the same question with my 70 mustang. About ready to put together
An exhaust system, but dont know if I should use 2.5 or 3?! I'm building a boss trans am
Style street car, so I'll be dumping the exhaust out the sides under the doors. I want 3's but I don't know if that's too much. I also want it loud!!

I'm running a 351w 4v heads 4v stock autolite carb, and a mild cam, I just bought headmen shorty headers with 3 collectors. And plan on running slp loud mouth 2 mufflers

 

I think we confuse back pressure with constriction or reduction of pipe diameter as you exit the exhaust. So following the theory behind Huski, as the exhaust gets farther from the source (I.e. Leaving the headers entering the exhaust pipes) you generally get smaller in diameter in order to maintain or increase exhaust gas speed which helps the gasses continue to their eventual exit. The reduction in tube diameter effectively helps keep the pressure elevated (read back pressure)along with the temps and the gas speeds. So this is also where crossovers like h-pipes and x-pipes come into play to help scavenge those gasses by balancing the harmonics from the left bank to the right bank by equalizing the "back pressures" and smoothing out the gas flows.

 

the exhaust pipe itself does NOT help any. the pistons pushing the gas out is the only force behind the gas flow. the longer the pipe length = more resistance. the smaller the tube = more resistance. the more tubing & bends = more surface area to slow the gasses down. that is why full exhaust systems & smaller diameter pipe is quieter. the slower the gas flow = quieter the noise it makes. the exhaust pulses are banged around cancelling each other out, this is what is considered backpressure = no good for performance, harding working engine & hotter engine operating temps & poor gas mileage.

anyone who does not get this needs to look at it in a more simple form.... sump pumps, their design mimics car exhaust. a 3/4 hp pump can flow 5000 gallons per hour through 10 foot of 3'' pipe. now make that same pump push through 50 foot of 2.5'' pipe and it will only move 3500 gallons per hour. the only difference is air vs fluid

 

Mostly true but we do not have a sump pump. We have two banks of cylinders firing in a specific pattern that means that the suction created by one bank can create a vacuum in the exhaust pipe of the other bank if you hook them together. This is true of H/X pipes but also the header collector itself for individual cylinders.

 

This thread reminds me of the Farmer who put a square exhaust pipe on his car because the grain silo had square pipes to pass the corn / wheat, etc.

"...if it works good for the grain, it'll work good for car exhaust..."


Z

 

:yoho:

 

Exhaust Science vs. Myth

Here is one article that puts some science and testing behind exhaust design.

Auto Exhaust Science