exhaust headers

[bcmc33]

"the best for road cars is a 1.5" OD primary"

Hate to be pedantic here but is this supposed to be ID, if not,what about the header wall thickness?

John

Good point. Missed that the first time around.
I assume he meant I.D. as the O.D. would be meaningless except for fit considerations.

Tube is normally made and specified as O/D. There are exceptions to this, of course, but for welded tube as used in exhaust systems O/D and material thickness are the usual way of specifications.
So it seems to me that a 1.5" O/D tube with 1/16" material would not be that far out for a tuned head with 34mm (1.34") exhaust valves - assuming that the bore of the primary and secondary header pipes should be no bigger than the size of the exhaust valve.

[S2Jay]

Used to work with some of these guys, and if I recall correctly, for 304 Stainless parts, aftermarket Header / Exhaust system manufacturers typically use 18 gauge material, with a wall thickness of 0.049. When using mild steel, they usually use 16 gauge, and I think that I have never seen anything thicker than 14 gauge used for exhaust system tubing.

16 gauge mild steel wall thickness of 0.065 x 2 = 0.130
1.50 O.D. ? 0.130 = 1.370
1-3/8? = 1.375?

Now I?m confused. Are the folks selling or describing Primary tube size talking I.D. or O.D.?
What size is best recommended for a strong road car, say 140 ? 150 HP, maybe a stroker [not that I know of anyone foolish to build a stroker motor]?

Here is a handy reference for gauge dimensions & weights.http://www.engineeringtoolbox.com/BWG-wire-gage-d_508.html

Jay [not building a stroker motor, as far as you know]
26 / 5009

[bcmc33]

1.50 O.D. ? 0.130 = 1.370
1-3/8? = 1.375?

Now I?m confused. Are the folks selling or describing Primary tube size talking I.D. or O.D.?
What size is best recommended for a strong road car, say 140 ? 150 HP, maybe a stroker [not that I know of anyone foolish to build a stroker motor]?

I've always understood that the bore of the primary and secondary header pipes should be the same size (0r perhaps more correctly the area) as the exhaust valves to maintain flow, pressure and temperature of the gasses.
Larger size headers will reduce the pressure resulting in a drop in flow speed and gas temperature, smaller size headers will create a restriction in flow.

That's why I said 1.5" O/D headers is not that far out when using the large 1.34" diameter exhaust valves in a Twink.
The exhaust ports on my 420 head are 34mm (1.34") diameter, that's going on my new tall block engine.

[bcmc33]

I've always understood that the bore of the primary and secondary header pipes should be the same size (0r perhaps more correctly the area) as the exhaust valves to maintain flow, pressure and temperature of the gasses.
Larger size headers will reduce the pressure resulting in a drop in flow speed and gas temperature, smaller size headers will create a restriction in flow.

That's why I said 1.5" O/D headers is not that far out when using the large 1.34" diameter exhaust valves in a Twink.
The exhaust ports on my 420 head are 34mm (1.34") diameter, that's going on my new tall block engine.

I've had some niggling thoughts about the principles of what I said above.
Although the size of the exhaust valves are 1.34" diameter, the actual port size that the valve covers is 1.25" diameter. So to prevent loss of gas flow and pressure - should the I/D of the headers be 1.25" diameter, and not 1.375"?

Informed comment required.

[S2Jay]

I did find some interesting information on-line, although there was less than I would have thought. Several sites preferred to have you buy their software or ?Information Pack? than to giving the information outright.

There were some calculators presented, but the underlining formula was not always available. Even when presented, the formula is only the math for the Theory or science behind it, and many an argument can be launched from that.

I list one formula here, as well as the link to that page, and I have modified the original writer?s formula so that it can be used on a ?per cylinder? basis. It was originally used for American ?Mopar? [Chrysler] V8s. I have included some of the writer?s explanation behind this formula, although I did not find the source of the constant used.

This formula focuses on Peak Torque RPM, and then discusses effects on changes in peak torque RPM by changing the primary tube size.

I have copied some of the explanation that accompanied this formula, just to provide some insight into the original writer. Some of what he wrote is ?Exhaust Flow 101? sort of stuff, but the ideas on Peak torque were interesting. There is also a mention of the effect of the length of the pipe on tuning the peak torque RPM.

Note that the Peak Torque RPM that you want the engine to have is the RPM number that should be used in this formula, not Max engine RPM. There is a significant difference in the tube size result by changing this RPM.

The original formula also added a wall thickness number at the end to provide an O.D. value if it is used for 18 Ga. [.049 x 2 = .098]. This can be omitted to read Tube I.D., or changed to use for 16 Ga. [0.065? x 2 = 0.130?]. Also this formula is set up for cylinder size in Cubic Inches, so you can do the appropriate conversion for cc.

Area of Primary Pipe = RPM ? Cylinder Size ? 88,200

Or, Pipe [I.D. * I.D.] = [RPM ? Cylinder Size ? 88,200] ? .7854

http://victorylibrary.com/mopar/header-tech-c.htm

Using this formula for a basic +.030 TwinCam with a Peak Torque RPM of 5500 [just to pick an arbitrary point] produces a recommended Primary I.D. of 1.37?. This agrees with the 1.50? tubing with a 16 Ga. wall of .065? that was mentioned earlier in this thread. If you try this formula at higher RPM such as 7,000 or 8,000 RPM, then the primary size is much larger [obviously].


Quote from Original Writer:

?The concept that maximum power is obtained by zero pressure in the exhaust is only partially true. There should be absolutely no back-pressure from the collector rearward, but the diameter of the system beginning with the exhaust valve is a compromise. The highest efficiency for the system requires a minimum speed for good exhaust gas velocity to insure that gas does not ?back up? into the chamber during overlap at low engine speeds, and that the ?suction? (negative pressure pulse) effect of a resonant (tuned length) and/or collector (overlapping exhaust pulses) system is optimized.
To predict what primary size will be best for a specific motor, you must know where you want the engine to develop peak torque. If the existing torque peak is at bit lower RPM than you prefer (typical in under-cammed or stock motors), it can be ?bumped? a bit by increasing the primary diameter. If the torque peak is too high (motor is ?peaky?, with no range and poor recovery from gear changes), the peak can be adjusted down by using a smaller pipe. A change of 1/8? in the primary diameter will raise or lower the peak torque RPM by 500 or so.
This factor slightly overlaps the effect of primary pipe length, but the pipe length generally will not change the peak torque or the RPM at which it occurs. A length change has the effect of improving the torque on only 1 side of the peak by ?borrowing? it from the other side. A shorter pipe improves the torque after the peak (reduces it at lower RPM), preventing the curve from flattening out so quickly as speed increases. A longer pipe extends the torque curve backwards to improve the engine's flexibility, at the expense of after-peak torque.
For best effect, the gas speed in the primary tube at the peak torque RPM should be about 240 feet per second."

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In the process of scanning through the net, I also found a couple of sites that provide several ?Calculators? that can be useful, in case anyone would like to access them. Wallace Racing in particular, presents many potentially useful engine related calculators, from speedo correction to HP estimators.

Calculators:

http://www.wallaceracing.com/Calculators.htm

http://www.bgsoflex.com/auto.html

Oh, yes the disclaimer. I have nothing to do with any of the above sites or people involved.


Jay

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[StressCraxx]

Installing the header was painful. In the end I had to relieve the frame a bit to get
the header to fit in. This was on an S2 with Weber head. Once on the car however, I have had no issues
with it and Ray is always great to deal with.

Do you think this is still an issue with the Spyder frame with the removable cross member? I'm trying to decide between Ray's header and Dave Beans header at the momment. Dave Beans is SS, more expensive, but with a little larger bore with a slip fit to a 2 inch exahaust.

,Russ

Russ,

I bought a set of the Dave Bean mild steel version more than 20 years ago. They give very good performance for a street elan with sprint cams and compression. They really come on about 5000 RPM like an afterburner all the way to 7K. They have a unique long collector for good torque along with the 4 into 2 merge.

When I rebuilt the water pump in my car last year, I noticed the tubes were mudcracking on some of the bends and the welds were breaking down. I found a set for sale last fall on Craigslist and had them ceramic coated. Will put the new set in when its time. I will also put in a muffler like Sarto's.

The easiest way to install is to hold the engine up with a sling/hoist and remove both motor mounts. Push the engine over to the carb side and fit them up. Reinstall the motor mounts. If the one of the tubes are too close to the frame, slot the motor mount bolts to allow a little side positioning.

There is no comparison to running the old cast manifold or the OE header.

Regards,
Dan Wise

[Johnfm]

The OD on my headers is only 1.24" - about 31.5mm.

Seems a bit on the small side...