# 1.8T AEB ideal manifold primary diameter



## Truckinduc (Apr 24, 2011)

Hey guys im in the process of designing an equal length twin scroll turbo manifold for an AEB. Ill be shooting for 300 whp at as low rpm as possible. Smaller turbo, bigger than stock of course. I still havent decided on a turbo yet so feel free to suggest one, doesnt have to be twin scroll either. 

My main question is primary size. The stock exhaust ports in my AEB head are 1.410" at the gasket surface. If I run 1.5" OD 16 gauge primarys the ID of the manifold will be 1.370" nominal. So a .020" step at the flange that i can easily port match. 


Stock port cross sectional area is 1.56145 square inches

1.5" OD primaries would be 1.47411 square inches - I THINK THIS WOULD BE IDEAL

1.625" OD primaries would be 1.75538 square inches 

1.75" OD primaries would be 2.0612 square inches

Since im shooting for boost as quick as possible i think it will be more important to keep high port velocity as a trade off vursus total flow.

Id love to have some opinions on this and discuss it to make sure I dont overlook anything. Im going to the effort to build a custom manifold so I dont want it to be a waste if I get something wrong. 

Thanks for taking the time to read this.


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## leebro61 (Nov 7, 2001)

I wouldn't go any larger than the 1.5" OD you are considering. We use 1&1/4" SCH40 pipe (~1.380" ID) on most of our manifolds and our experience is that this outperforms 1&1/2" pipe (~1.600" ID) for most applications.


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## Truckinduc (Apr 24, 2011)

Thanks for the response. Im glad someone finally chimed in.. I was 95% sure thats the size ill run but i wanted to ask someone who knows alot more about these engines than I do.


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## franz (Jan 30, 2000)

the size that you want to use will change depending on camshaft, rpm and manifold pressure.

-f


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## slowgti (Jul 15, 2003)

Have you run the math on stepped primaries? 1.5" to 1.625" sounds good by my math.


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## Truckinduc (Apr 24, 2011)

slowgti said:


> Have you run the math on stepped primaries? 1.5" to 1.625" sounds good by my math.


I'm oblivious to the formulas with stepped primaries. Anyway I thought stepped primaries were only beneficial in naturally aspirated setups.


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## Truckinduc (Apr 24, 2011)

franz said:


> the size that you want to use will change depending on camshaft, rpm and manifold pressure.
> 
> -f


Very true. I'm basing my calculations off of the stock port size. I'm not building a high rpm motor. Mostly looking for quick spool and bottom end grunt over top end power. The car will be used for autocross and track days. Stock cams and a smallish turbo.


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## leebro61 (Nov 7, 2001)

franz said:


> the size that you want to use will change depending on camshaft, rpm and manifold pressure.
> 
> -f





slowgti said:


> Have you run the math on stepped primaries? 1.5" to 1.625" sounds good by my math.


Would either of you guys mind sharing how you would actually use this information quantitatively? Like, show the actual inputs, formulas, etc. you would use for a sample calculation...

Thanks


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## slowgti (Jul 15, 2003)

leebro61 said:


> Would either of you guys mind sharing how you would actually use this information quantitatively? Like, show the actual inputs, formulas, etc. you would use for a sample calculation...
> 
> Thanks


http://www.bgsoflex.com/bestheader.html

This will get you in the ballpark for non-stepped headers. 
Google and a pen and paper will take you where you want to be


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## Truckinduc (Apr 24, 2011)

Hey slowgti do stepped primary's really offer any advantage over single size primaries on a turbo manifold? I'm thinking no.


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## leebro61 (Nov 7, 2001)

slowgti said:


> http://www.bgsoflex.com/bestheader.html
> 
> This will get you in the ballpark for non-stepped headers.
> Google and a pen and paper will take you where you want to be


I can appreciate your comment about them being "ball-park" tools, so absolutely none of this is a criticism of your advice. 

I've seen these calculators before and it seems like they are largely (if not entirely) empirical. I ran a couple of cases where I varied exhaust valve diameter, for example, in 0.25" increments with all other parameters fixed. The change in optimal runner length for each case was exactly 2x the change in exhaust valve diameter... to the hundredths decimal place (the optimal pipe diameter and collector diameter did not change). I can't think of anything in the "physics" that would suggest that exact 2:1 ratio between those parameters (if there is any correlation at all). If anything I would expect that optimal runner length is really only a function of perhaps ~cam duration and local sound speed, where the latter is very, very weakly related to pipe diameter. In a practical application, even with radical cams and other engine changes, I just can't imagine going with any pipe diameter that is more than one pipe size different then the existing exhaust ports in the cylinder head.

When you say you are doing "the math" on stepped primaries, would you mind elaborating on exactly what you are calculating and why? I'm not trying to be lazy here, I've just seen so many poor explanations offered in the past that I have no faith in what google will turn up. For example, what made you choose 1.5" to 1.625" (targetting a specific area ratio, pressure rise for wave reflection, etc. ?). Thanks


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## slowgti (Jul 15, 2003)

leebro61 said:


> I can appreciate your comment about them being "ball-park" tools, so absolutely none of this is a criticism of your advice.
> 
> I've seen these calculators before and it seems like they are largely (if not entirely) empirical. I ran a couple of cases where I varied exhaust valve diameter, for example, in 0.25" increments with all other parameters fixed. The change in optimal runner length for each case was exactly 2x the change in exhaust valve diameter... to the hundredths decimal place (the optimal pipe diameter and collector diameter did not change). I can't think of anything in the "physics" that would suggest that exact 2:1 ratio between those parameters (if there is any correlation at all). If anything I would expect that optimal runner length is really only a function of perhaps ~cam duration and local sound speed, where the latter is very, very weakly related to pipe diameter. In a practical application, even with radical cams and other engine changes, I just can't imagine going with any pipe diameter that is more than one pipe size different then the existing exhaust ports in the cylinder head.
> 
> When you say you are doing "the math" on stepped primaries, would you mind elaborating on exactly what you are calculating and why? I'm not trying to be lazy here, I've just seen so many poor explanations offered in the past that I have no faith in what google will turn up. For example, what made you choose 1.5" to 1.625" (targetting a specific area ratio, pressure rise for wave reflection, etc. ?). Thanks



I don't take offense to any of it, no worries there. I'm by no means an expert, I'm just fascinated by the actual physics of how everything works upon each other in a running engine. 

Without the data for exhaust speed and gas density at the exhaust port, it's all just an educated guess. I don't know anyone that has those numbers to share. As far as stepped primaries, I talked to several high end circle track header builders and what I got from the conversation was 1. A larger tube acts as a longer tube. 2. The step, if placed correctly, acts as a second anti-reversion device. So your basically tuning two different sets of primary's in one. I'm on my cell phone now, I should have time tomorrow to give some links and examples. And yes, everything is based off of cam duration/timing and cylinder pressure. Bore size and piston stroke length play into it as well.


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## slowgti (Jul 15, 2003)

Truckinduc said:


> Hey slowgti do stepped primary's really offer any advantage over single size primaries on a turbo manifold? I'm thinking no.


On an all out, no budget build. Yes. Will 99% of the cars we build benefit from a stepped header? No


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## Truckinduc (Apr 24, 2011)

I've decided to run 1.5" OD 16 gauge since I'm not building a high rpm high power engine. 

My next question is about runner length. I've been doing a ton of reading an research and I want to build a long tube manifold. As I understand it if its designed correctly it can actually spool faster than a comparable setup with a short runner manifold. Equal length is obvious, and I'm fairly sure the tuned length will depend on the size and a/r of the turbine. 

The engine will be using stock cams, very slightly bump up in compression. 

I know this is far from an exact science and I'm not looking for someone to give me magic number. I'm looking for a little discussion on this topic and to pick some of the more knowledgeable members brains a little, that is if they have the time to try to explain it.


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## Truckinduc (Apr 24, 2011)

I have chosen to run a Borg Warner EFR 6754 turbo with the .80 AR twin scroll housing. 

Tonight I designed the manifold head flanges and rapid prototyped one. They will be .625" 304 stainless.


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## Truckinduc (Apr 24, 2011)




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## Truckinduc (Apr 24, 2011)




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## Truckinduc (Apr 24, 2011)

flanges made


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