So I measured the primary tube lengths for 4/5/6 bank header using the 90 degree centerline method described in my last post.

To recap: Measurements were taken on the Bursch BX.9258.SS 1.750" OD headers.

Primary tube length was measured from the head flange to the end of the primary tube in the collector. I double checked / remeasured several tubes to check the repeatability of my measurement technique which was within 1/4".

#1: 34 3/8"
#2: 33 1/2"
#3: 33 1/8"

#4: 35 3/8"
#5: 36 1/2"
#6: 31 1/2"

I am not a fan of the Bursch headers....I would talk to William Knight about headers....he knows what works and doesn't work for your build...

Dan
I really like the way you are approaching this evaluation. All factual.

A few years ago, I went through a 'similar' experiment and learned the value of the exhaust gas flow volume vs speed.
My 3.0 itb engine ran a set of 1&5/8 euro headers and I wanted to run a GT3 muffler. So I manufactured the collector pipe from the headers to each side of the muffler.
First set was 2 inch pipe. On first drive, the car simply took off like a rocket! It really felt like I had a turbo down low... but at 5000rpm it seemed to run out of breath.

Second set was 2.5 inch... the sound was nuts. And the torque down low had all disappeared. In fact, it felt like an absolute slug. Top end revved out well to 7k+.

Third set was 2.25 - this gave me most of the bottom end torque back and did not restrict the top end at all.

After I did this, William explained to me why it happened and yes, I could have avoided the hassle by just calling him first, but the first hand experience is worth it.

You are way overthinking this
@~225hp and <7K rpm you are going to be fine w/ SSI type headers 1.5" OD(1 3/8" ID) if this is a race car where you are aways going to be in the 5-7K range then maybe 1.625"(1.5" ID)


over size hurts more at the usual sub 5K range than undersize hurts 6-7k

I know people that run 1.5" OD on street 3.6 because under 5k they enhance performance, they almost never go to 7K and could care less up there.

The primary pipes have a fairly wide range of acceptable differences in length w/o adversely affecting power.

1-5/8" secondary pipes seems small, is this a typo?

Very interesting to me that this made such a noticeable difference. I guess there's a reason why the secondary pipes on street setups are almost always 2". When you think about it it they only need to deliver the flow from one primary pipe at a time.

1000% agree and that was my point on the other thread as well. Dan do the KISS method on this. Buying different cams is fine if you are craving more power but your kind of chasing your tail

Why on earth would you go to the trouble and expense of installing a 3.6L engine in an earlier chassis just to neuter it to 3.0L output levels? A properly designed system will preserve the torque curve AND release the horse power.

Many of the systems we design and build are for racing applications where they are looking for that last 5HP. Even with a stock engine you can't get to the engine's full potential using off the shelf systems. Add modifications and the diminishing returns stack up even more if the flow is not optimized to compensate. Decision becomes how much compromise is acceptable.

Just going to throw this out there, I have run 1.75" headers on my basically stock 2.7 CIS for 5k miles, mainly because I owned them already and I like the sound.

On one hand, I know it's slower than if the primaries were the proper size. On the other hand, this is a street car, not a race car, so if it's a .5 second slower, it honestly does not matter, especially if it sounds glorious at higher RPMs

If you're tracking and driving competitively, all the info above makes sense, but idk if I'm getting old or what, but I just don't care about performance anymore, especially with all the police around that hate anybody having fun. I'm all about experience > metrics now.

All you are doing is hurting your driving experience. 1.5" headers, which is what you should be running, would not sound any different assuming you used the same muffler.

But you go right ahead and do what you think is best....

I agree and I wouldn't do it either other than as a temporary affair, Which I did w/ my first 964 3.6 t/p, around town you never notice the difference, it's only strangled and detrimental up top.

💯 agree with that statement

If someone wants to straight up trade me for a similar system (headers with heat boxes) I'm interested, but until then I don't care.

you're right! I meant to say 2.25.... I'll go up and edit my post.
thankyou!

A collector for a 911 6 doesn't have to be huge as the pulses are well separated
@7500rpm there are ~1.5 to 1.8 pulses in a 38" primary at any given instant, how many depends on the rates of cooling and expansion.

each collector gets 188 pulses /sec
each pulses travels from ~.5 to ~.6m/s
this means that a given pulse clears the collector before the next arrives

there is on average ~.3 to ~.4 pulses in a given collector at a given instant.

That’s cool….but I’d care a lot if the bottom end of my engine was as compromised as yours is…you are tossing away a bunch of low end.

each collector gets 188 pulses /sec
each pulses travels from ~.5 to ~.6m before the next pulse enters the collector
this means that a given pulse clears the collector before the next arrives

I got distracted and didn't correctly type the line underlined

In that part I was solely refereeing to the collector which for the purposes of the above calculation was 10" long

gas velocity was assumed to be 120m/s, 100m/s is probably closer due to the decel of the gasses as they pass through the system, 120m/s when they leave the exhaust port.

As a PS to the above
I never measured any of the collector lengths for the headers I had in hand

all had 26 to 29" long primaries including the collector from 1.5 to 1.75" OD these are all quick and dirty ests.

the outlier was 993RSR header @19" long

bump

I am sorry Dan was there a question? It seems Bill, Brian, Winders shared much valuable information and your car and VE tables support the smaller headers helped your car

No questions.

I'll have dyno data to share in 9 days that should support the conclusion that my 2.8L engine performs better with 1.625" headers than 1.750" headers.

But after nearly 250 miles in the car since switching to smaller headers the one thing that disappoints me most with my 2.8L engine build is that the engine has lost ALL power between idle and 2000 rpm, so driving in town or stop and go traffic on the highway is a royal pain in the butt - revving the engine, slipping the clutch repeatedly to get the car rolling is unacceptable and smaller headers have done NOTHING to improve that situation. Once the car is rolling and out on the open road (or track), it goes great.

The real problem I am having isn't the header size selection, it's the camshaft selection I made - the WebCams 911S MFI cam (p/n 05-062) is not good, according to William Knight it's not enough exhaust flow to balance the intake flow. William guarantees that I will make more power across the rev range and the car will be easier to drive with the K45 cams.

So my next move is replacing the 911S MFI cams with William's K45 cams and a set of his headers. That dyno data comparison will be posted elsewhere when I have it.