Board index Flathead Power-Technical Questions, Answers, and Suggestions 45 Flatties Exhaust formula (long)

Exhaust formula (long)

Moderators: Curt!, Pa

Post Fri Oct 11, 2002 4:47 pm

I've come to a dead end. Let me begin with the assumption that some people here are familiar with "tri-y" (a.k.a. 4-2-1) headers? I'm trying to figure out:
1. a reliable formula for determining pipe lengths, both absolute and proportionate (primary/secondary)
2. a reliable formula for determining relative diameter of secondaries (other than the "classic 20%" rule)
3. any reason why this should not work on 2 cylinder motors
Before you answer, yes, I've already read A. G. Bell and D. Vizard without finding an answer in either.
Any comments, links to good site, etc. most welcome as always.

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Post Sat Oct 12, 2002 4:42 am

Posts: 1319
Location: State College, Pa.
Damn, I had a whole page written up, and then lost connection. Rats!

Check out:

Scientific Design of Exhaust & Intake systems
by Philip H Smith and John C Morrison

I always thought that the formulas from Bell's book were pretty straight forward. Used them for lots of engine planning on numerous 2 into 1 nad single pipe systems.


Post Sat Oct 12, 2002 1:30 pm

Bell's reference to 15" as the minimum for primary length is mystifying - Vizard has used lengths down to 8" on Mini motors. Bell's formula for secondary diameter (IDS = 93%*((ID2^2*2)^.5)produces a pipe much larger than is commonly used. I have a feeling it's a typo - that it's his formula for the collector of a 4-1 pipe, not 2-1, in fact it returns a secondary diameter larger than his collector formula. It also gives a figure for the center pipe of an LCB much larger than any made. Do you have a figure from your edition for secondary pipe size?

[This message has been edited by panic (edited 12 October 2002).]

Post Sat Oct 12, 2002 3:48 pm

Posts: 896
Location: Bixby,OK, USA
On of the most efficient systems for later model stuff claims to use an exhaust pressure wave analysis to determine the length of the primaries. this dismisses the wave into a collector or past a reversion dam prior to the negative pressure/piston dropping at overlap occurs.

Post Sat Oct 12, 2002 5:08 pm

Posts: 582
Location: meridian, id. usa
I just had this vision of you guys setting around an old pot belly stove in your "FLATHEADS RULE" blasers drinking corn squizzens out of a mason jar deliberateing on the virtues of the proper leanght of steel tubing. Image

Post Sat Oct 12, 2002 5:12 pm

My blazer says "Free Silver 16-1", if you go back that far...

Post Sat Oct 12, 2002 7:43 pm

Posts: 376
Location: Lebanon, CT USA
Hey dotman...I like the Flatheads Rule blazer thing...Maybe in a Tee shirt that looks like a blazer...Geezer

Post Sat Oct 12, 2002 8:51 pm

Posts: 1644
Location: Interlaken, NY USA

Hey Panic, William Jennings Bryan "Cross of gold"??

Post Sat Oct 12, 2002 9:49 pm

Posts: 1319
Location: State College, Pa.
I have an August 1983 Bell's

P. 112 has the primary pipe length:

P = ([850xED]/rpm) -3

Then you can work on the ID of the primary pipe. after that work out the secondary pipe formula. Secondary length is P + 3

ED =180 degrees plus the number of degrees the exhaust valve opens before BDC

Now here's the stickler. I haven't found a reference as to what lift the valve opening measurement is at. (.015, .020, .053)?

Working with his book makes me believe that his valve opening measurements are taken at the low range.


Post Sun Oct 13, 2002 12:36 am

Winner! Bryan it is.

Post Sun Oct 13, 2002 12:40 am

Agree - may be at .00" or lash for duration. I have the same formula, but where does secondary = P+3 appear? Mine says after you get the total length, subtract primary, and P2 is remainder. Does yours have the 93% reference for secondary diameter (bottom of p.113)?

Post Mon Oct 14, 2002 5:27 am

Posts: 1319
Location: State College, Pa.

All those diagrams show P+3" on pages 112,113
Primary length = P
P1 + P2 = P. That's used on the 4-2-1 system.

page 115 P2 = P - P1. If P = 36" and P1 = 15" then P2 would = 21"

You'll need a four cylinder engine to utilze the 4-2-1 sytem.


Post Mon Oct 14, 2002 1:24 pm

I think the "93%" is a mistake; it's either:
a. that's the formula for the collector diameter of all 4 pipes, or
b. it's a typo, and the correct percentage is 63 or 73.
93 returns a figure much bigger than anyone else's ideas.

Post Tue Oct 15, 2002 4:53 am

Posts: 1319
Location: State College, Pa.
Read your '93%' as 9396. I was going HUH? Then I realized my eyes were fooling me.

Nothing wrong with the 93 %, if you plug 63% into the formula you would get a secondary that was smaller than the primary (bottleneck). Remember, the secondary takes the volume of both primaries.

Who elses ideas?

I built a 2-1 system for my Rat Shovel, 74", A grind. 2" intake valves and used 1 5/8" exhaust pipes (to fool the engine into thinking it had 1 5/8" exhaust valves). Used Bell's formulas. Worked extremely well, straight thru system, no muffler, and it was quiet compared to straight pipes.


Post Tue Oct 15, 2002 6:10 am

Posts: 896
Location: Bixby,OK, USA
Out here in the `bojacks we up the size of the primary 1/8 over the exhaust valve size, build a nice system with equal length primaries and a long collector made of a roll of stovepipe (very thin). Paint the collector with cheap paint, go out and run the hell out of it for half a day, then cut the collector off at the point where the paint stopped burning......or push a valve spring into the pipe a little at a time until it runs the best....sho do...jb

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