Header sizing 914-6 Options

[mb911]

So while my 914-6 oil tanks are being machined I going to start working through my header with heat exhanger design out of stainless.. I do not have non compete on these with m&k so that gives me design freedom.. However I will use similar design process as the m&k/ rarlyl8 design. So my question is 1.5" the most common size or allot of you using 1.625

[Steve]

Most common aftermarket is 1 5/8" OD for 3.2 and under and 1 3/4" OD for 3.6 motors.
The factory six concours guys will probably stick with the stock 1.5" OD heat exchangers

[mepstein]

I would like 1 5/8 for my 3.2. I want your first set.

[Dion]

I'll be in for a set. Would be for a 2.4L Right behind ya Mark!

[wndsnd]

If these have heat, I would like to see what they would look like. Probably 1 5/8

[jmill]

From Dempsey's book

2.0-2.4 = 1 1/2 or 1 5/8
2.5-2.8 = 1 5/8
3.0-3.2 = 1 5/8 or 1 1/2 (think he meant 1 3/4)
3.4-3.5 = 1 3/4
3.6-3.8 = 1 3/4 or 1 7/8

Your largest target would be 1 5/8 for 2.0-3.2

Who needs heat? (IMG:style_emoticons/default/rolleyes.gif)

[mb911]

From Dempsey's book

2.0-2.4 = 1 1/2 or 1 5/8
2.5-2.8 = 1 5/8
3.0-3.2 = 1 5/8 or 1 1/2 (think he meant 1 3/4)
3.4-3.5 = 1 3/4
3.6-3.8 = 1 3/4 or 1 7/8

Your largest target would be 1 5/8 for 2.0-3.2

Who needs heat? (IMG:style_emoticons/default/rolleyes.gif)

You and i for sure.. I need to come pick up that engine shortly so I can get working on this..



As to what they look like with heat this will be again a little trial and error.. I probably need a true 914-6 bracket to set height for the muffler and put the engine trans assembly together and start mocking up.. They won't look oem but won't look like b&b either..

Mark I will surely put you on the list for the first set..

[Mark Henry]

1-1/2" up to 2.7
1-5/8" or 1 3/4" for 3.0 and 3.2, stock I'd use the 1 5/8" and use the bigger for HP.

But that said, although there would obviously be a bit of power loss, even a stock 3.2 runs fine on factory heat exchangers with are IIRC only about 1 3/8".

[JmuRiz]

My 2.7 was spec'd out at 1 5/8 by MSDS.

I'm quite interested in your project!!!

BTW if someone has some SSI 911 heat exchangers and/or some stock 914/6 or the new reproduction units, get them to mb so he can use them for setup (IMG:style_emoticons/default/biggrin.gif)

[jmill]

I need to come pick up that engine shortly

Come by and get it. It's on an engine stand right by the garage door.

[Steve]

My 2.7 was spec'd out at 1 5/8 by MSDS.

I'm quite interested in your project!!!

BTW if someone has some SSI 911 heat exchangers and/or some stock 914/6 or the new reproduction units, get them to mb so he can use them for setup (IMG:style_emoticons/default/biggrin.gif)

I also ran 1 5/8" MSDS headers on my 2.7. Ran awesome. It was a stock 2.7 with webers.

[wndsnd]

Is there an accepted ratio between exhaust port size, and header pipe size?

Looking at the chart posted above the 1 5/8 pipe is used for 2.0 to 3.2 head sizes, yet the port sizes vary considerably.

1 5/8 is 41.275 mm.


Exhaust ports run from 32 mm to 38 mm for those engine sizes. I have heard if you oversize, you can kill performance but obviously some oversizing is prefered. What is considered optimal?

[jmill]

Is there an accepted ratio between exhaust port size, and header pipe size?

I'm no engineer but I'd think cylinder volume & RPM would be more relevant.

[mb911]

Is there an accepted ratio between exhaust port size, and header pipe size?

I'm no engineer but I'd think cylinder volume & RPM would be more relevant.

Yes and yes so camshaft could dictate allot. Ssi use to only make one size primary but changed flanges based off engine requirement. I always saw great performance out of 1.5" primary 1.375 id on 2.0-3.0 on 3.2s you have to go 1.625.. I personally liked the 993 heat exchanger the best for this application.. In fact that would be a good fit for 914-6 but useing the Gillette style not bischoff like I preferred for the 911 setup because of flange location and adaption. Though the 993 heat exchanger is not equal length they are 1.625 and are very close to equal length..

I will probably go with 1.625 as my base but I worry about loss of low end torque on the smaller engines

[mepstein]

Is there an accepted ratio between exhaust port size, and header pipe size?

I'm no engineer but I'd think cylinder volume & RPM would be more relevant.

Yes and yes so camshaft could dictate allot. Ssi use to only make one size primary but changed flanges based off engine requirement. I always saw great performance out of 1.5" primary 1.375 id on 2.0-3.0 on 3.2s you have to go 1.625.. I personally liked the 993 heat exchanger the best for this application.. In fact that would be a good fit for 914-6 but useing the Gillette style not bischoff like I preferred for the 911 setup because of flange location and adaption. Though the 993 heat exchanger is not equal length they are 1.625 and are very close to equal length..

I will probably go with 1.625 as my base but I worry about loss of low end torque on the smaller engines

Smaller engines already have the stock he's. The large engines have been waiting on a solution.

[mb911]

Is there an accepted ratio between exhaust port size, and header pipe size?

I'm no engineer but I'd think cylinder volume & RPM would be more relevant.

Yes and yes so camshaft could dictate allot. Ssi use to only make one size primary but changed flanges based off engine requirement. I always saw great performance out of 1.5" primary 1.375 id on 2.0-3.0 on 3.2s you have to go 1.625.. I personally liked the 993 heat exchanger the best for this application.. In fact that would be a good fit for 914-6 but useing the Gillette style not bischoff like I preferred for the 911 setup because of flange location and adaption. Though the 993 heat exchanger is not equal length they are 1.625 and are very close to equal length..

I will probably go with 1.625 as my base but I worry about loss of low end torque on the smaller engines

Smaller engines already have the stock he's. The large engines have been waiting on a solution.

Good point.. Well guess I will have to solve that.. ..

Anyone know what the difference between a 1.7/ 1.8 muffler bracket? I remember they can simply be redrilled but been 15 years since I have done that..

[jmill]

Smaller engines already have the stock he's.

At $3K a pair. (IMG:style_emoticons/default/shades.gif)

[eitnurg]

Many (too many) years ago I had a Euro 3.2 and standard 914 HEs on a rolling road. From memory peak power was down about 20hp and 800 rpm compared to mild steel headers.
Also for a Q&D solution a friend who messed about with engines said a good rule of thumb was the "pipe should be the same size as the valve". Without looking it up I've no idea what that equates to, but at the time the comment was "hmmm, he's not wrong...".

[Mark Henry]

One thing I haven't seen discussed is the other half of the exhaust equation...what are you going to run for a muffler?

1 3/4" pipes on a stock banana? then you may as well stuck to a smaller tubes anyways.
Zero backpressure then you are likely very loud and may lose some bottom end torque.

Like everything in the high performance engine world there is no perfect combo, everything is a trade off. Too much of a good thing is usually bad for street drivability. Getting the drivability back for the street and you will lose the peek HP.
Hot street with driveability is a balancing act and with this type of engine I'm looking more at torque curve than peek HP.


One thing I was always seeing was peeps building full race tuned engines, then not being happy with the end result, because they only ever drove on the street.
Also here's a rub, in the 90's friend and I built same size 2.0 bug engines (78mm X 90.5). His had 044 big valve head's and mine had ported and polished stock valve heads, his had a w-120 cam and mine the slightly smaller w-110, his a 1 5/8" header and mine a 1 1/2", other than that I'd say everything else was equal. We drag raced (yes illegally) countless times, he never could catch me, I beat him off the line every time. He would be catching up to me, but he could never match my bottom end torque.

Now this is more experience with Type 1 vw performance engines and I build T4 engines with the same premise, but at 2.0 (78X 90.5mm) each cylinder has the same volume as a 3.0/6.*
I always noticed smaller (1 1/2") tube headers provided more bottom end grunt, where bigger tubes gain top end at the cost of the bottom end torque. As many know this is also true with cam selection, mild to wild what you gain with one you lose with the other. Same when added a bit of backpressure with the muffler compared to a megaphone straight out pipe stinger.

Since 95% of the engines I've ever build were for the street, I've always strived for the balance of an acceptable idle (a little loppy is OK as long as it will idle at 900rpm) with good bottom end torque and acceptable top end.

What I'm getting at is a bit of loss in exhaust flow may be worth it, you have to be truthful to yourself as to what are you using the car for.
Racing? you want the biggest baddest straight through loud pipes.
Quiet or semi quiet? with street drivability? Then the muffler is likely killing (at minimum effecting) any larger tube advantage anyways. Plus personally I'd be more interested it bottom end torque then the top end HP.

*Even with VW t1 big valves they're are still smaller valves then /6, etc. I know VW t1 references it's not a true apples to apples example but IMHO and just an FYI I've build a lot of T1&4, plus a few /6 motors, so it's close enough.

[wndsnd]

Good information, those heads have 40mm exhaust valves, so that would make sense.