Unless something has changes, they are the best headers available, for those who only want to buy a set of headers or do you sell yours stand alone now? Last I heard you only sold the headers as part of the completed exhaust. As beautiful and well thought out that whole system is, it doesn't work for everyone (like me) who would just like to buy the headers then make their own exhaust.

I recall Mike (aka Z) telling me years ago at the Wichita OCIC, him, Louie and Tom spent the better part of a day dyno tuning Mike and Tom's car. Mike's had Devek L2 headers with an exhaust of his design, Tom's was the one shown in the photo above. At the end of the day both made very similar power across the board. Mike was very happy with the results since him and everyone else in attendance expected the cross over snakes to be better. He showed me a copy of the dyno charts years (like 20) ago, wish I had a hard copy.

 

Louie Ott tested a lot of exhaust stuff and found out those Devek headers were the best things available at the time.

In my opinion, it is not that important for 90-degree cross-plane V8 header to have equal length primaries. Because of the firing order, the pulses are never right so there is no single primary length that is perfect. Therefore, prioritizing a straight flow path over exact equal length primaries makes sense to me.

The 180-degree headers are not a good design because the primaries end up being way too long.

My theory (not practice) is that for 122mm bore spacing, redline at 8000 rpm or less, and cats far away from the engine, tri-y with short primaries and longer secondaries would be the best.

 

hmmm is that what you had made for your monster !!!

 

I've always offered my headers as a completely separate item, from the exhaust system (which is an incredible pain to ship, BTW.)

Now that my new clutch project is wrapping up, I'm in the middle of a major redesign/update of headers to better suit my current needs, for all the different varieties of S4 style engines, all the way from 5.0 to 6.5 liter variants. Working on a set of 1 7/8" pipes to either 3" or 3.5" collectors.....with the "target" application for the 5.8 Liter and street 6.5 liter variants. Completely new collector design, with 12 degree merging. Equal length within 1/16", with the firing order correct in the collector. Frankly, pretty crazy to design and have fit in the chassis.

Also have designed a set, for the really high output engines, with 2" primary pipes, reducing to 1 7/8" along the way. Also with a super "evolved" 12 degree collector and equal length primaries. These are complete insanity to make fit in the chassis!

Certainly, the 1 3/4" "street" headers will also be "updated" to this new collector design and various details improved.

I'm always searching for the "next step", never sitting still and just assuming I'm done with anything....always looking for evolving technology to apply to the 928 platform, if it improves what I'm building or doing.

 

Very cool looking concept. Crazy long primaries!

 

and it fried the underside…. i guess no need anymore for a cabin heater, it will be hot enough…..

 

Yep, heard the same story, and remembered being disappointed in the results because those 180's were so cool, they HAD to make more hp - LOL!

I wonder how long it took to weld them up?

 

I'm interested in the header design theory, so a couple of questions.

Why exactly equal length primaries? The pulses are never correct in a cross-plane V8, so I can't figure out why exactly equal length primaries would produce a benefit. I would prioritize straight flow paths instead of exact equal length primaries, and I'd accept 2" or so length differences if that would allow me to reduce the number of bends and increase the turn radii.

The S4 stock exhaust port is 40mm (1.575") diameter round hole at the exhaust port flange. The area of that hole is 1256 mm^2 (1.95 sqin). With performance cams that have a lot of overlap, that is quite a large exhaust port that's ideal for hp that almost none of the 928 engines will ever see. According to PipeMax software, good velocities for the exhaust port (when tested at 28" inches on the flow bench) are in the 210-310 fps range. With a lot of camshaft overlap and you're probably better off at the 310 fps end of the range while with less camshaft overlap you're probably better off at the 210 fps end of the range. Better shaped ports can take higher velocities, over 310 fps. According to PipeMax, the recommended cross-sectional areas for the exhaust port for a stock S4 engine are 1.049-1.554 square inches -- the stock exhaust port is way outside of that range that is best for producing power. In fact, if the recommended sizes produce 310-210 fps velocities, the stock port will produce 167 fps velocity. Another way to ask the same question is what's the crank hp range for normally aspirated engines for which the stock port size is appropriate. According to PipeMax, the least powerful hot rodded 928 S4, baby girl cam engine for which the stock exhaust port is appropriate at the maximum size of the recommended range makes 425 hp. The most powerful engine, with nice high-overlap sports cams, for which the stock size exhaust port is at the small end of the range (with velocity of about 310 fps) makes 660 hp.

Given that the S4 exhaust port is large, wouldn't it make sense to start with a primary pipe that matches the 40mm/1.575" id of the port and continue that for about 20cm/8" distance and then step up the diameter? My understanding is that headers are stepped up in size because the step generates a rarefaction wave that is timed to arrive at teh exhaust valve when the exhaust valve pressure ratio drops below the critical value that is needed to keep flow sonic.

 

This is an extremely complex subject....with more "theories" about what it correct and what is wrong than stars in our galaxy. The reality is that even though many very smart people have studied "headers", there's no "quick" answer to the problem. The only real answer is trial and error, with a a scattering of success tossed in.

You ask 10 professional custom header makers a question....and you are going to get 11 different answers. Doug Thorley lived across the street from my best friend and we had many "fireside" discussions about headers and what works and what doesn't work. What I came away with, from those discussions, is that you set a goal and try see if you can meet that goal.

Regarding what I'm doing, I'm not trying to increase bottom end torque...I've got more than I want. I'm trying to increase horsepower on the "big end".

Relating specifically to this post, my 1 3/4" headers made more horsepower (on an actual DTS engine dyno) than the "Stage 2' Devek 1 7/8" headers on Andy's engine....10+ years ago. I've made 2 major "evolutionary" changes to those headers, since then, both of which increased torque and horsepower.

This brings up the obvious question....when were Devek Headers designed and built...and what has changed in "header science", since then?

 

If I’d have to guess, I’d guess that the smaller 1 3/4” primaries are more appropriately sized for a 600 crank hp engine seeking over 100% VE.

As a point of reference, the 700++ crank hp Simard engine uses 1 3/4” primaries. So it might be that even the 1 3/4” primaries are too large for a 600 crank hp engine.

In my opinion, if one is giving up on the 100%+ VE and significant camshaft overlap, then going too large on primaries doesn’t hurt much. Sizing the primaries right is only critical when the target VEs are significantly above 100%. That’s an opinion.

 

I'm going to guess that there are stacks and stacks of thesis papers that were done on exhaust/header design, right?
I'm just trying to see if I can improve what I'm already doing, since I'm "freshening" Andy's old engine for use in a different 928 with a 5 speed and it will go onto the dyno.
Great opportunity, for me, to test some ideas I've had.

I'm guessing that the change in collectors I'm going to use, alone, will change the results.

 

and they will grind the road.
Åke

 

Process simulation packages are only as good as the quality of the underlying programming methodology. I take the point of view that such packages help iterate to a "good solution" quickly but a perfect result first time [if such were expected] would be somewhat optimistic or so I suspect. If things were that precise there would be one correct solution then practice and theory would be perfectly aligned.

At the end of the day what counts is what the dyno says- how one gets there may well take many iterations- it is a complex multi faceted problem given every engine speed has an optimal solution transposed onto one manifold configuration.

 

Software needs to be calibrated to experiments, for sure. It’s a symbiosis in which the two together add up to more than the sum of parts.

Usually, in internal combustion engine, things work at one rpm and then don’t work at another rpm. If one changes lengths and diameters of pipes, one helps torque at one rpm and hurts it at another. Most of the time, it’s just picking one’s poison.

In terms of headers, I think there are two exceptions to this, things that just help at a wide rpm band. (This is an opinion, not a fact.) The first is to come off the properly sized exhaust port with the same ID pipe for about 8” so the port ID size section continues up to about 11-12” from the valve, before stepping it up (sometimes a lot if looking for peak power). The same length works across rpms for this section if the gross torque is about the same, so this is one of the few things that isn’t directly tuned off the rpms. The second is to do this port-matched about 8” pipe section as straight as possible, with maximum bend radius that’s possible. This is based on logic and reading, not personal experiments.

Other things that help at lower rpms without hurting at high rpms is optimizing the collector lengths. If the collector length matters much at peak power rpm or higher rpms, then the collector is probably too large of a diameter. If the collector diameter is right for the power level, one can help the mid range rpm torque with tuned collector length without hurting the top end. The same for combining the pulses from the banks with a crossover.


But, as they say, everyone has an opinion...

 

So, let me ask a question for you to think about/research:
At 6,000 rpms (each cylinder firing is 50 times a second), how does the sonic "wave" get back into the exhaust port/cylinder in the engine? (Asked another way, what does the flow, inside the actual primary pipe, look like, in cross section?)