Exhaust Header Size for My Rebuilt 2.7L Motor

[mb911]

Originally Posted by dannichols1474

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 in unacceptable and smaller headers has 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.

Solex cams would have been a great choice as well. I did that on my current engine and it has ridiculous amount of TQ for being a 2.4S pistoned engine. Correct me if I am wrong as I don’t know much about his headers but aren’t they a copy of Brian’s (M&k) headers? Either way if his are 1.625 you will have an improved collector design but a much shorter primary? Again if they are copy’s of Brian’s? And to be honest I could be wrong on that. I am not really involved in that scene any more.

At the end of the day this and your other thread should be sticky’s as incredibly helpful for the next guy with lots of knowledge and information shared.

[PeteKz]

Originally Posted by dannichols1474

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 in unacceptable and smaller headers has 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.

As you discovered, at lower RPM ranges the headers don't matter. You are way below the resonant frequencies. That's why cast-iron "log" manifolds work fine on big Detroit v8 engines in passenger cars and trucks. Your cam is the main factor in lack of low end torque. That's the tradeoff. Use what4ever cam William recommends for your application.

[dannichols1474]

Here is the head to head dyno data, same engine (2.8L 9.8:1 CR 911S MFI cams) 1.750" headers versus 1.625" headers.



And before somebody asks about the scale labeling, it's 0 to 250 either ft-lbs or hp full scale with 25 ft-lbs or hp divisions. See previous dyno graph for reference.

[mb911]

Dan those are good numbers and good data thanks for sharing. I would be happy with that setup

[KMoore68]

Great to see these results and I really appreciate the transparency!

I'm curious to see how the new cam affects these numbers if you decided to dyno again.

+1 for a vote to sticky this and your other thread, I believe they are a big positive to the community!

[Showdown]

More impressive than the power gains of 6hp, the smaller headers seems to have helped shorten the flat spot at 3500RPM. Wondering is smaller headers or a bit of back pressure from the muffler would smooth that out even more.


Dan, do a whole bunch more dyno pulls...Let's spend more of Dan's money!!

[jj911c2]

This is great data - thank you for sharing. Nice to see you filled in that drop around 3,500 a bit and picked up some top end as well! Looking forward to the next test.


Sent from my iPad using Tapatalk Pro

[Bill Verburg]

Originally Posted by Showdown

More impressive than the power gains of 6hp, the smaller headers seems to have helped shorten the flat spot at 3500RPM. Wondering is smaller headers or a bit of back pressure from the muffler would smooth that out even more.


Dan, do a whole bunch more dyno pulls...Let's spend more of Dan's money!!

That 3500 rpm dip looks like a resonance issue to me, it is least bad in your first run, what was different about that one

[RarlyL8]

Need to make one change at a time so effects are not blurred. Change the cams and dyno that then change the headers and re-run.
What is the difference between the 1-5/8 headers you have now and the set to be tested?
We just designed and built a complete system for a similar 2.8L engine. The headers make the power but the muffler has to stay out of the way. The system is not designed properly if adding back pressure enhances performance. The muffler has to complement the flow and velocity produced by the headers best it can. That is why you cannot slap on a one-size-fits-all muffler and expect anything good to happen. OE dual inlet mufflers have 1-7/8" inlets. Inlet size and internals need to match the header secondary size. Ours start at 2" and increase incrementally to 2.5" depending on engine flow requirements. Muffler internal flame tubes are also sized to match (no baffles).

[Jonny042]

Originally Posted by dannichols1474

Here is the head to head dyno data, same engine (2.8L 9.8:1 CR 911S MFI cams) 1.750" headers versus 1.625" headers.



And before somebody asks about the scale labeling, it's 0 to 250 either ft-lbs or hp full scale with 25 ft-lbs or hp divisions. See previous dyno graph for reference.

Very surprising (to me at least) that the 1-5/8" headers actually make MORE top end power than the 1-3/4". After driving it for a while, what are your seat of the pants impressions?

Thanks for sharing this, it will be a valuable addition to the knowledge base here for a very, very, long time.

I do look forward to the dyno plot with the new cams, will be an equally interesting comparison.

PS - That's a healthy 2.8! Do mustang dynos still have a reputation of being "heartbreakers" that under-report power numbers when compared to Dynojets, or have they tweaked the calibrations to bring them more in line with the competition?

[dannichols1474]

Originally Posted by Jonny042

Very surprising (to me at least) that the 1-5/8" headers actually make MORE top end power than the 1-3/4". After driving it for a while, what are your seat of the pants impressions?....

With the 1.750" headers, I could feel the engine sort of let up from 3000-3500 rpm then feel it really start pulling again around 3800rpm. With the 1.625" headers the engine just pulls without let up.

Regarding the increase in top end max HP, Gary said the engine wanted 2 degrees more timing at the top with the smaller headers so that may be where we got the increase in max HP from 229 to 236.

[RarlyL8]

The larger headers lost velocity which impacts scavenging. The pulse timing gets diluted as the flow slows then power falls off. That is why the smaller headers needed a bump in timing, to better synchronize the pulse timing with the scavenging events.