Research: Correct exhaust diameters

[Silvermike]

First some definitions...

P- length of primary collectors
P1- length of the 4-2 in a 4-2-1 header
P2- length of the 2-1 in a 4-2-1 header
ID- internal diameter
ID2- diameter of collector inlet
ID3- diameter of collector outlet (TP)
A- angle of taper
IDS- internal diameter of the secondaries
CL- collector
TL- tail
TP- tailpipe
ED- exhaust duration

The basis of individual pipes in a header is to take advantage of the exhaust gas inertia. To maximise this, the first step is to determine the lengths and diameters of the individual pipes. Acoustic tuning is the method used.

Exhaust gas is released at 200-300 ft/sec. but pulses are moving at about 1500ft/sec. As the initial charge of burnt gas exits the cylinder, it creates a wave of positive pressure that travels at the speed of sound through the pipe. As it exits to atmosphere, it produces a negative wave (suction) which returns along the exhaust pipe into the cylinder. That suction wave has a certain amount of "evacuation" power as its pressure is much lower than the cylinder pressure. The following formula sovles the problem of P:

P = [850 x ED/rpm] - 3

Point The optimal length of the primaries is a function of rpm. For the street, it is wise to tune P to the rpm of maximum torque. For racing purposes.. drag- tune at maximum HP, circuit- tune midway between peak tq and peak power.

now... the ID.

ID = sqrt[ cc/{(P + 3) x 25}] x 2.1

in most instances, this works out to roughly the size of the exhaust valves.

for a 4-2-1 setup.

IDS = sqrt[ ID^2 x 2] x 0.93

in simple talk... for 1500-2000cc u will benefit from a 1 3/4 - 1 7/8 primary and a 2" secondary.

Of interest is that P1 should be > 15" and P2 = P- P1 and that a reduction is the primary diameter of 0.125 shifts peak torque down about 500rpm in >2000cc and 800rpm in < 2000cc.

Now collectors are found at the end of the secondaries, where the 2-1 is found. There are two types, baffles, where the primaries terminate abruptly, followed by a 7-10* taper OR a merge type, where the primaries blend into each other followed by a 9-12* taper. High reving (8500+) motors tend to like the merge type collectors with their associated high degrees of taper.

collector length = (ID2- ID3)/2 x cotA

and

ID3 = sqrt[ |(cc x 2)/ {( P + 3) x 25}| x 2

However, the alternative, which would be more practical for the street tuner is to set the collector and tailpipe to the same diameter and use this rough guide...

HP...............................ID3
80-120......................... 1 7/8
110-140.......................2
130-150.......................2 1/8
140-185.......................2 1/4
180-220.......................2 1/2
210-265.......................2 3/4
250-320.......................3


For a 4-1 the ID is constant until the collector. there is no IDS.

However, there is the stepped header. Those familiar with 2-stroke tuning should have heard of this. Two-stroke exhausts are a series of cones, as based on physics, conical follow is the most efficient.

This tech. can be transferred to a steeped header, where for instance the primaries can begin at 1 7/8" and grow to about 2 1/8 at the collector. The "step" is made with a low angle 2* and generally in 1/8" steps. For reference, the first step is made 10-14" from the exhaust port for best results.

In theory, the first section of the pipe is made smaller to improve low end. Exhaust reversion occurs at about 10-11", so thats where the first step should go. The bigger the 1st primary diameter, the more biased the system would be to top end power, and the 1st step should be moved closer to the exhuast port to avoid loosing to much low end power.

The placement of the 2nd step is tricky/complicated. In general, taking the distance from the first step to the collector as D2, then the 2nd step should be 0.2D2 from the 1st step.

This type of header doesnt work for all engines though. High reving engines < 1600cc do not respond well to this setup. Mostly >2000cc benefit. Here is an example from a built Opel 2.0L engine i came across:

rpm...................Test1.................Test2
...........................Hp.....................Hp
4000...................138...................145
5000...................156...................165
5500...................172...................180
6000...................204...................206
6500...................221...................225
7000...................243...................244
7500...................254...................257
8000...................265...................268
8500...................273...................276
9000...................264...................275


Test 1: 2" 4-1

Test 2: 1 7/8 stepped to 2 1/8 4-1

Noted here is that dips in the power curve is smoothed out and the low-mid range is well preseved by the stepped header.


W.R.T. exhaust barrel, there is no clear cut winner, nor is there any clear relationship with backpressure and hp. In some instances it has been found that the noisier, freer barrel netted less power than the more restrictive one, and vice versa. Topping favorable lists are Nur-spec, Hi-power, world sport to name a few. All big brands are within a few hp of eachother.

Happy Motoring
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I need more NOS

[Sanctifier]

Thanks mike, great info...
I think I'll post your follow up as well.
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Walk softly... and carry a BIG stick!

[Sanctifier]