Engine and Transmission Tech

Question for the brain trust (Dan & Co are you there)?

Are there any benefits of having equal length headers on a 90 deg V8?

I ask this because cylinders 5 & 7 are physically next to each other and also in order of firing sequence so need more exhaust pipe length to compensate.

David

According to Ed Hennegan it makes a dramatic difference when it comes to tuning, and therefore in power output and economy as well. Ed's been making headers about as long as anyone alive and is very knowledgeable. (Headers by Ed) He has some very helpful tech articles on his site.

JB

> Question for the brain trust (Dan & Co are you there)?

I'm here for a few more hours then back on a plane. Got over
700,000 miles just on Delta in the last couple of years.

> Are there any benefits of having equal length headers on a 90 deg V8?

Vizard's testing on bank separated 4-into-1 headers is they are relatively
insensitive to primary length. They are more sensitive to primary diameter
and especially to collector length and diameter. 180 degree headers are
much more sensitive to equal length. I'm not sure about bank separated
tri-y headers. I've recently purchased a exhaust system design tool called
Pipemax and run it in conjunction with Dynomation. Vizard says it's been
pretty accurate in his testing. Pipemax is a complete exhaust system
design tool, specifying all headers dimensions (for 4-into-1 and tri-y,
with or without mufflers), H or X-pipe location, tail pipe length, etc.
It computes the best dimensions, along with their harmonics as well as
the worst dimensions and their harmonics.

> I ask this because cylinders 5 & 7 are physically next to each other
> and also in order of firing sequence so need more exhaust pipe length
> to compensate.

The problem is usually handled by increasing the diameter of the collector
to allow the flow from both cylinders to pass more easily. If you look
at the collectors of an optimally designed set of 180's, you'll find
they use much smaller collectors.

Dan Jones

In trying to resolve the seemingly opposing views of Vizard and Hennegan, both justifiably considered experts, I've come to the conclusion that they are pursuing different goals. I suspect that Vizard's work is more race oriented while Hennegan's is more of a wide-band approach. He most certainly has been involved in racing for a great many years, but has very likely built headers for as many street machines as for race cars and perhaps more. In racing, a significant portion of the engine's operating envelope can be and is disregarded. Between idle and the lower limit of the powerband the only requirement is that the engine not die, and what the exhaust does in that range does not matter in the least. Only powerband and higher speed concerns are addressed, and I expect this is the realm Vizard concerns himself with, and rightfully so.

Hennegan however, by his own admission is greatly concerned with the area below the powerband, as it is an area of considerable concern with any street driven engine. The tuning and characteristics of the engine in this range can make or break a conversion, and I have had cause recently to more fully appreciate the quote that there is no substitute for cubic inches. Here is why that applies here: Let's take the MG-Roadmaster as an example. This car has an overabundance of power and torque. It also has a somewhat lumpy idle due to nearly .5" of valve lift, I don't recall the duration. The idle is OK, we can live with that and in fact I have very few complaints with the car. However, if we had chosen a milder cam the idle would be smooth, the exhaust would be cleaner, and there would have still been an overabundance of power and torque due entirely to the massive displacement, even with the mildest cam imaginable. This is where the displacement comes into play for a conversion. Now, with that as background, and looking at Hennegan's work, it seems apparent that for the best driveability we need the most displacement we can reasonably get, matched to the best state of tune between idle and the powerband as well as shifting the lower limit of the powerband downwards as far as is practical. This is directly at odds with maximum performance (and Vizard) where we are raising the upper limit of the powerband as much as possible and disregarding and usually killing performance below the powerband. Roller cams help us to extend the powerband in both directions but even in the best of circumstances we must choose one or the other.

It may very well be, as Vizard says, that primary length is not very important in a maximum performance application. That makes sense. You are generally running rich anyway and a rich mixture is relatively forgiving. But once you begin tightening up the tuning parameters like you have to do for transition performance and economy, small variations have an ever larger effect and I think Hennegan is correct in his statements that exhaust pulsations affect intake tuning. Logically enough, the pulses from a short tube would be different from those of a long tube. With EFI it is possible (though expensive and difficult) to individually tune each cylinder but with a carb it is exceedingly difficult. Interestingly enough, Ed (Hennegan) recommends smaller (but longer) collectors than what are widely used, but also pioneered a number of important features in header and collector design, has extensively tested virtually every design feature that has been used in headers such as stepped ports, and is very highly regarded for his performance work. We just don't hear about him much here because most of his work is oriented towards drag racing, hot rod and street use. Would a stagger in the length of the 5-7 pairing be a benefit? Only at a specific rpm range determined by the dimensions. So for racing that could possibly make a small difference. But equal length allows the use of smaller collectors because the sequencing into the headers is consistent at any speed. With unequal length, due to the massive variations in not only the speed of the pulse but the volume of the gasses the sequencing into the collector is all jumbled up and a larger collector is required. This slows down the flow of exhaust gasses (reducing and releasing energy in the form of heat) and creates a backpressure at the transition to the exhaust system, explaining the popularity of extremely large diameter exhaust systems. So to summarize, with shortie headers a large collector and a large diameter exhaust system should be used, whereas with equal length headers a smaller collector and smaller diameter exhaust system is appropriate. In essence you are trading space needed for equal length for space needed for large diameter exhaust pipes.

So the point is, you have to decide what goal you are chasing. If all out performance, use Vizard's recommendations. If street use consider Hennegan. I would suggest that for a LBC conversion virtually all of the car's use will be in the range below the midpoint in the powerband, particularly one with a large increase in displacement. Once past the initial driving stage where the driver reasonably enough wants to see what it will do, use of the upper powerband only happens on track days, passing (possibly but not necessarily), and showing off. Other than that it's just not necessary, is a waste of gas, and most drivers will not regularly indulge it. So for a car that will be driven regularly, maximum displacement, a mild tune and conservation of fuel is a real concern. In those cases equal length headers can give real benefits while also performing as well or better than non-equal length headers on the top end. But in all practicality, it is extremely rare to find a set of equal length headers in one of our conversions. There simply is not enough room and they have to be hand made. Of those who have taken on that challenge I've not heard any say they regretted it. But I've heard a lot who have "shortie" headers claim that they are, "Just As Good". Personally, I don't buy it.

JB

If you have a cam made with the 5/7 swap= non issue ? roverman.