Ok guys,

Here is a question for you. Full out small block(350-400cid) race motor, 13:1, roller cam 2x4 the whole ball of wax. Say in the 7-8k rpm range. top of the line heads, fully ported.

What size headers would you look to run? would you use step headers? Roughly how long primary tube length and over all length? what size collectors.

Just trying to get some food for thought. Have a buddy doing a small block, non stroked brand X and I am trying to tell him the 1 5/8 primary, no step and 3" collectors are going to kill the motor.

Thanks,

PD

P.S. Vegas has the over under on replies to this thread at 7, with 4 being answers and 3 being sarcastic.

1 7/8" to 2" but you a lot more info than you gave

My 405 is getting 1 7/8ths " primaries......shooting for 32 inches with a coin toss between 4 into 2 into 1 collectors in 3" or around 16 " of 3 1/2 ". I'll try them both on the dyno and see which I like best.

is there a point where too big will kill hp?

Yes... if your engine cant rev high enough... big
tubes need more volume... big bores/stroke or more revs

2 in to 3 1/2 coll 422 w9 motor around 700hp.

I don't know how to predict it, but the strength of the returning wave is very important. Larger diameter reduces the density of the wave, therefore less strength, pulls less gas through the chamber on overlap.
If the chassis has limited room, a larger diameter than predicted may help. The pulse lost is less than avoiding the restriction of a small radius right at the port.

Thanks for all the replies guys!!!

From what I can guess on his motor is looking like .800ish lift roller, valves I am going to guess are 2.2 and 1.6-1.65. rod length, I am guessing here but I would say 6.15-6.20 but I have no idea.


No worries about the headers, there are no inner fenders.

thanks again guys!!! Once this is hashed out and I will show this to him so he can see that he is going to kill the motor with 1 5/8 primaries. Why the heck he is skimping on the headers I have no idea.

PD.

some ok reading that may help.

How To Calculate Muffler Size and Exhaust Pipe Diameter | Exhaust VideosSkip to
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SubscribeHow To Calculate Muffler Size and Exhaust Pipe Diameter
If you’re a math wiz and/or an engineer, you’re probably going to like this
article and the resources we’ve linked to. However, if you find yourself getting
stuck (or bored) with the info below, here are the key take-aways:
1. The factory exhaust pipe diameter is usually a good choice for most vehicles.
2. The muffler manufacturers are doing all the math for us – no need to reinvent
the wheel. If they say it will work for your vehicle, it will probably work for
your vehicle.
3. We’ve got an easy-to-read exhaust system size table that is good for quick
calculations.
Breaking Down The Problem
While we’re not going to go through and list out all the formulas and
calculations you need to figure this exactly, we will break down the problem,
explain how you would go about figuring things out scientifically, and then
leave you with some good quick-and-dirty exhaust system math as well as some
interesting links.
The science goes like this…
1) Mass of air that the engine breathes in + mass of fuel = mass of exhaust
gases
Conservation of mass, right?
2) To calculate the volume of air the engine takes in, we multiply the
displacement of the engine by the engine RPM and then divide by two (it takes
two full revolutions for the engine to exhaust it’s entire air volume). We then
convert that to volume to mass.
3) To make the calculations easy, you want to assume that combustion is perfect,
i.e. there aren’t any byproducts, any unburned fuel, etc. It’s easier to assume
perfect combustion and then “back in” to the actual numbers using an estimate
after the fact.
4) Since you’re assuming perfect combustion, it’s easy to figure out how much
fuel mass is added to the exhaust.
5) Once you know the mass of the exhaust gas, you just figure out how much
volume that mass would occupy. Of course, you have to adjust for expansion due
to the high exhaust gas temperature.
That’s it! Of course, when you sit down to figure it, you’ll find that getting a
good scientific estimate takes a lot of work (which is why we don’t bother with
it here).
Quick and Dirty Exhaust System Math
Easy Way To Estimate: Your intake system needs to flow 1.5 CFM per engine
horsepower, and your exhaust system needs to flow 2.2 CFM per engine horsepower.
Good Way To Estimate: Take engine RPM x engine displacement, then divide by two.
This is the intake volume. Use this same volume of air for the exhaust system,
but then correct for thermal expansion (you need to know exhaust temps to figure
things out).
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 Diameter (inches)Pipe Area (in2)Total CFM (est.)Max HP Per PipeMax HP
For A Dual Pipe System
1 1/21.4817178155
1 5/81.7720392185
1 3/42.07239108217
22.76318144289
2 1/43.55408185371
2 1/24.43509232463
2 3/45.41622283566
36.49747339679
3 1/47.67882401802
3 1/28.951029468935

NOTE: These numbers are just estimates. All pipes are assumed to be 16 gauge
steel.
The table above is probably over-estimating pipe size, but you can see that a
400 hp vehicle with a dual exhaust system only needs 2 1/4 – 2 1/2 inch pipes.
Anything larger is overkill.
Useful Links
Great forum discussion that really discusses the details of the scientific
calculations: http://www.eng-tips.com/viewthread.cfm?qid=104735
An interesting discussion of header pipe designs:
http://victorylibrary.com/mopar/header-tech-c.htm
A good general article about designing the perfect exhaust system:
http://www.popularhotrodding.com/enginemasters/articles/hardcore/0505em_exh/index.html23
Comments Post a commentonyeh
Aug 16 2011it was a very good article. i red and i was impressed. it solved my
problem to make more efficiently exhaust for racing need. do you have any
article about header? size of pipe diameter and how long it can be to give an
optimum performance. i’m atuner from indonesia. an in my country, something
like this was very rare. i have searched on the web, but found no article that
make me delightfull to solve header problem. if you have some, it will make me
very gratefull. thanxs before.
Replybriankmizell
Aug 31 2011This is in regards to this:
“Good Way To Estimate: Take engine RPM x engine displacement, then divide by
two.”
My question is… for the engine displacement do I used cubic inches or CC?
thank for an informatic article,
Brian Mizell
ReplyJason
Sep 19 2011Brian – It can be either. Just remember that whatever units you use
(cubic inches or cubic centimeters) must be converted to a unit of mass. My
suggestion? Stick with metric. Conversions are much easier.
ReplyGeoffrey Moulton
Nov 21 2011Hello
Great table but it may not be as accurate as you may think as i have a very
quiet 3″ system on my 3.0L turbo Nissan skyline with 315kw@all 4 wheel, the
problem is it’s 315kw on 15psi boost and 335kw on 19PSI boost, so did a google
search for pipe size vs horse power
Kind regards
Geoffrey
ReplyGeoffrey Moulton
Jan 10 2012Just a quick update
I removed the rear muffler and increased power to 355kw@ all four wheel.
New rear muffler fitted dropped the power to 345kw@ all four
made up a 3″ mandrel bent pipe to replace the rear muffler 350kw@ all four
Now it’s just too noisy
Destroyed my clutch before Christmas so have not played since
best so far is a 11.66 quarter at 201km with street tyres
Hoping to up the power to 400kw@ all four
Great site and helpful
kind regards
Geoffrey
ReplyBuz
Jan 8 2012Very informative! I do not understand “overestimating pipe size”. Do
you mean I can make more power with each size pipe in the chart? I have a non
mandrel 1.75″ system on a Daihatsu Rocky that originally made 94hp with 1.5″
from the cat back. Am I properly sized, it needs all the HP and tq it can get.
ReplyJason
Jan 8 2012Buz – The size of your exhaust system pipes should be matched to
the power of your engine. So, if you have an engine that generates less than
100hp, the proper single exhaust tube size is 1 3/4″ . Smaller (like 1 5/8″)
is probably just fine (might actually help with low end torque to got to 1
5/8″), but if you go much bigger you’ll actually rob power from your engine.
Also, when I wrote that our chart was over-estimating size, it means that
the diameters listed aren’t exact. Instead, they’re rounded to the common
exhaust pipe sizes so you can buy pipe…if we just used math to arrive at the
diameter, we’d end up with some funky pipe sizes that would be impossible to
buy. Hopefully that makes sense (the article is a bit cryptic).
ReplyMichael Walton
Jan 10 2012Hi there Jason, Great reading, Im running a 572ci big block chev on
a twin 2.5 inch system with an x pipe and flowmaster mufflers(620hp)primary
pipe size is 2 inch going into 3 inch collectors i have been thinking about
going to a twin 3 inch system but am wondering if i would see any noticible
difference in the car as I seldom go beyond 4500rpm on the street(Max rpm
6000)the system is ceramic coated so cost a bit what are your thoughts?
ReplyJason
Jan 10 2012Michael – If I’ve done the easy math right, your big block could
pump out nearly 1,100 CF of exhaust per minute at 4500 RPM. If you’ve got a
section of pipe that’s only 2″ in diameter, than you’ve got a bottleneck
that’s probably restricting flow on your vehicle.
However, dual 3″ is a lot..I’m thinking dual 2.5″ or dual 2 3/4″ is probably
enough. Based on all my readings and conversations with people who would
know first hand, it’s better to be a little on the small side (2.5″) than
too big (3″).
ReplyMichael Walton
Jan 10 2012Jason, Thanks for the reply the only 2 inch pipes in the system
are the primary header pipes(this is quite a big size primary pipe size as
I understand) I think i will stick with the dual 2.5 system as its only a
street driven vehicle (I do drive it hard often though)People comment on
how awesome the car sounds it currently has Flowmaster delta 40 series
mufflers on it but im thinking of trying out the super 44s as i understand
they have a deeper tone than the delta 40s(Can you comment on this?)
ReplyJason
Jan 10 2012Michael – I’m sorry I misunderstood – 2″ collectors are quite
big. No restriction there.
I can’t comment on Delta 40 vs Super 44’s from experience, but I’ve
heard the same thing. My honest opinion, however, is that you won’t
notice enough difference to justify the expense. They’re both going to
sound great.
ReplyMichael Walton
Jan 10 2012Jason, So basically unless im going to run the car at max
revs the dual 2.5 inch system will lose nothing to a twin 3 inch
system ? and may even be better for low down torque (Although the car
makes massive torque anyway) thanks Michaelpatrick holt
Jan 13 2012thanks in part to the information you have provided ihave decided
to go with a single 3 lnch exaust instead of a 2 or 2.5 inch dual setup all
the information i have been able to find says i will gain torqe with a single
3 inch system thank you much
ReplyJason
Jan 10 2012Michael – You’re going to lose something at 6000 RPM using 2.5″
pipes – the CFM requirements are higher than your current exhaust will allow
at that RPM. However, in low to mid RPMs (4000RPM or less) you’ve got more
than enough pipe capacity, and just as you say it’s actually a good thing to
have smaller pipes in terms of off-the-line performance.
So, if you use the vehicle as described, than you’re better off with your
current setup than a set of 3’s…assuming all the assumptions we’ve made are
correct.
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Thanks Clanton.

I am going to read up on that and let my buddy know and then show him the thread. I guess I know what I am doing the rest of the day....lol

thanks

PD

well i jus thaught it would help to better understand.i have a chart from a hot rod artical that makes it more plane.i will try to find it.

cool thanks!!

this was it.
http://www.exhaustvideos.com/faq/how-to-calculate-muffler-size-pipe-diameter/