I wonder if the 'dip' might have something to do with the large jump from 1.5inch to 1.75 inch headers?

Perhaps a set of 1&5/8 might help?

The 1 3/4" headers are way too big for that motor. He should be using 1 1/2"...1 5/8" is also too big.

I don't recall if we heard how big the ITBs are....and the stacks are short which doesn't help the lower RPM range any.

I have said that at least 2 times in this thread but what do I know? Only made 300 sets of headers for clients over the last 9 years

Simple search http://forums.pelicanparts.com/uploa...1723152881.png

Hmm sorry thought a pictured attached but it didn't hold tight. in the meantime here is a good read.. I can guarantee you as prove the 1-3/4" is to big and 1.625 will be borderline too much and 1.5 would be just right but again as the former owner of M&K exhaust and my aerospace work previous I am not just some dummy spewing ideas from my couch. I interreacted with hundreds of dyno shops and end users over the years. I would encourage you to call Marty and MSDS and see what he says. That is one of the last still USA made Porsche header fabricators amongst many other automobiles. He owns #40 914-6GT that was raced in Le Mans and has many other wild cars. Call Supertec. Talk to Henry and he will tell you the same. Call Brian at M&K/Rarlyl8 he will tell you the same. https://forums.pelicanparts.com/911-engine-rebuilding-forum/487943-sneak-peek-new-stainless-steel-headers.html

simple google.google.com/search?q=header+primary+size+chart&rlz=1C1GCEB_enU S938US939&oq=header+primary&gs_lcrp=EgZjaHJvbWUqBw gDEAAYgAQyCQgAEEUYORiABDIHCAEQABiABDIHCAIQABiABDIH CAMQABiABDIHCAQQABiABDIHCAUQABiABDINCAYQABiGAxiABB iKBTINCAcQABiGAxiABBiKBdIBCTg2MTRqMGoxNagCCLACAQ&s ourceid=chrome&ie=UTF-8

Yes so be careful with OD vs ID . OD is how ALL tubing is measured so when someone refers to 1-3/4 it is usually .0625 wall and that equals 1.625 ID and down the line

I would also suggest looking at Dyno runs on a 3.2 vs 3.0 with SSIs which up until recently were all 1.5” OD and 1.375” ID again .0625 wall is common wall thickness. Yes cam and CR have an effect but you were at such a far extreme that you had no problem up top but low and mid range suffered (think track applications when running mostly higher RPMs) when street driving a car you want the best of both worlds but keep in mind you spend 75% of the time in the lower rpm ranges. You will experience better performance overall stepping down to 1.625 OD (again how all tubing and headers are measured across the industry) but you may leave some TQ down low on the table by not going 1.5”. I made a decision on my car which is a hot 2.4 to use 1.625 (993 exchangers) and sleeved them to 1.5” to maintain as much low end TQ as possible. It works amazing on my setup. Primaries length and collector design play a factor to in exhaust and ideally a 31-32” primary should be used for a Porsche 6 if low end TQ (street driving) is to be optimized. Typed this on my phone hope it makes sense

If you have radical enough cams, you could use 1 5/8".

What cams are you using? What size are your ITBs?

If you still have your SSI heat ex changers maybe you could take them with you and test 1.625" headers vs the 1.5" SSI's.

That is the absolute best way to do it though expensive there is no better way. Anxious to hear the results

40mm ITBs do not match up with 1 3/4" headers....

So much converstation about the dip in the dyno plot I questioned.-- great posts by all that offered their thoughts. When you see that dip, you need to ask yourself "what is the cause?"

I think you are getting some good feedback. However, it is not always "this size header goes with that displacement".

This thread is LONG, which is great! But, can you refresh us on CR, Cam choice, ITB size along with the timing curve and AFR plot? Many vairables.

Thanks for keeping up the contributions and answering the questions being posted.


Cheers

Well, if you had a high output 3.6 to 3.8L race engine with 1 3/4" headers, you would expect to find ITBs with an effective diameter of 50mm to 53mm.

My current race engine which I have not dyno'd yet, is 3.746L with 48mm AT Power "shaftless" ITBs which effectively flow the same as standard 52.8mm ITBs. The engine has 1 3/4" headers and should make over 440 HP at the crank. The 3.6L version of this engine made 431 HP with 45mm AT Power ITBs (49.5mm effective) which were too small for the engine.

I have increased displacement 4.1% and ITB size 13.8%. I would not be surprised to see 450 HP.

That is double the power you are making with the same header size.

While I was wishing William "Happy Birthday!" today, I asked him about what cams he liked for a your motor. He thinks you don't have the right cams and your headers are too big. He did say you could get away with the 1 5/8" O.D. headers if you used a better cam. He would use the K45 cam. He sells them...I am not sure who makes them.

How long are those primaries from the head to the collector? 30-32" ? If so those are known as long tube headers and have a sweet spot tune point in the 5000-6000 RPM range and that's good for the 3.0 but 1 3/4" are a bit to large. Would have been better if 1 5/8" header.

But I'm most interested in the runner lengths.

Hi Sal,

I'll measure those runner lengths tomorrow and post them on the new thread I started to focus on this header size discussion.

http://forums.pelicanparts.com/porsche-911-technical-forum/1165423-exhaust-header-size-my-rebuilt-2-7l-motor.html#post12300226.

Dan so back on topic. I also embarked on EFI this spring and went with the poorman solution and used my carbs as TBs and MS2 with a base map. It has been a good learning curve but I very happy with the results. Easy starting, great power, easy adjustability. So far the biggest hiccup was mismatched banks from throttle linkages and I have corrected that now. My question for you is do you think the dyno was worth the time and money or did autotune and big hills/foot on the brake seem effective enough? I have found the latter to work really well for me.

Ben, my experience so far has been that I can use recommendations for the AFR targets in the AFR table and then use the driving techniques you describe (plus ET's at the track) to get the VE table dialed in with auto tune, but I don't know what the ignition timing should be especially when I build an engine with non-standard parts.

So what I have read is ignition timing is where we can unlock hidden torque / power and to do that I have to run timing sweeps for various combinations of engine speed and load to determine the timing point that yields MBT (maximum brake torque) for each of those combinations and then I can build an optimized timing table for the ECU to use. That's where the dyno and a skilled calibration tech come in.

Long answer, short: I think dyno testing is good value for the money especially since I am not an experienced tuner of EFI plus I like having hard data (even if the data is relative and not absolute).

Hi Ben,

In the last post I said I thought a dyno session is good value for the money, then I reread your post and I want to add that if you are using the ECU to control the fuel injection only, then tuning the EFI with the big hills/foot on the brake technique is adequate if you are happy with your results.

I bought Al Kosmal / X-Faktory's Option#2 conversion package that added ECU ignition control to the ECU fuel injection control, so dyno tuning made sense for me to optimize the timing settings.

Here is a photo of the ignition table before dyno tuning using timing values from the characterization of the distributor mechanical advance curve before I started the CIS to ITB EFI conversion project:

http://forums.pelicanparts.com/uploa...1723307773.jpg

Here is a photo of the ignition table after dyno tuning using timing values derived from timing sweeps to determine MBT (max brake torque) at various combinations of rpm and load:

http://forums.pelicanparts.com/uploa...1723307773.jpg

Interesting and thanks for sharing. I am using 123 ignition so yes correct no ignition added. It has worked really well. I was lucky enough to @jpnovak as my remote advisor for tuning. It really has gone well. I actually am surprised more people don’t use MS and opt for big dollar setups. It doesn’t make a lot of sense to me as MS seems pretty easy to work with.