Primary Size?

[Prodigal]

1 3/4”
1 7/8”
2”

Kooks sells all 3 for 21 mustang. For a NA E85 car what’s my best option?

Sponsored

 

[GregO]

1 3/4”
1 7/8”
2”

Kooks sells all 3 for 21 mustang. For a NA E85 car what’s my best option?

Couple of rules to follow.
Typically the primary diameter should match the port diameter with about a 6” radius off the header flange. The S550 strut towers does not allow for a 6” radius bend so in the case of the S550 the primary needs to go up in size due to the tight bend radius (exhaust gas flow rules, deep in the weeds stuff)
Not many options for step primary header designs so your stuck with single continuous tube sizes.
There’s virtually no lower rpm power loss with properly built 1.875” primary tubes vs. 1.750”.
2.0” is getting into the high rpm, high CFM, Very high power levels.
At the end of the day 1.875” IMO

I’ve wondered why Kooks builds step tube GT350 5.2 VooDoo headers and not step tube 5.0 headers especially now that the Gen3 has been out for awhile.

Hope this helps

 

[Prodigal]

Couple of rules to follow.
Typically the primary diameter should match the port diameter with about a 6” radius off the header flange. The S550 strut towers does not allow for a 6” radius bend so in the case of the S550 the primary needs to go up in size due to the tight bend radius (exhaust gas flow rules, deep in the weeds stuff)
Not many options for step primary header designs so your stuck with single continuous tube sizes.
There’s virtually no lower rpm power loss with properly built 1.875” primary tubes vs. 1.750”.
2.0” is getting into the high rpm, high CFM, Very high power levels.
At the end of the day 1.875” IMO

I’ve wondered why Kooks builds step tube GT350 5.2 VooDoo headers and not step tube 5.0 headers especially now that the Gen3 has been out for awhile.

Hope this helps

Very much so. Thanks for the education.

 

[NGOT8R]

1.875 also gives you room to grow if you ever decide to make more power.

 

[K4fxd]

1 7/8 is what I'd run but 2" works well also. Coyote likes larger tubes than say a comparable push rod engine. Maybe it is the radius but I've seen it hold true on engine dyno"s also.

 

[Mspider]

1 7/8 is what I'd run but 2" works well also. Coyote likes larger tubes than say a comparable push rod engine. Maybe it is the radius but I've seen it hold true on engine dyno"s also.

years ago when I had a LS3 camaro just about everyone ran 1 7/8 including myself. Then 2" was the way to go for boost. I think they were just as popular on the LS pushrod motors.

 

[Bahndvr]

Couple of rules to follow.
Typically the primary diameter should match the port diameter with about a 6” radius off the header flange. The S550 strut towers does not allow for a 6” radius bend so in the case of the S550 the primary needs to go up in size due to the tight bend radius (exhaust gas flow rules, deep in the weeds stuff)
Not many options for step primary header designs so your stuck with single continuous tube sizes.
There’s virtually no lower rpm power loss with properly built 1.875” primary tubes vs. 1.750”.
2.0” is getting into the high rpm, high CFM, Very high power levels.
At the end of the day 1.875” IMO

I’ve wondered why Kooks builds step tube GT350 5.2 VooDoo headers and not step tube 5.0 headers especially now that the Gen3 has been out for awhile.

Hope this helps

I wonder that too. I would love 2" to 1 ⅞". That would even give a little space in there.

 

[Angrey]

There's a lit of old school myth and disinformation on the interweb about primary sizing.

The bottom line is above a certain cross section diameter, all increasing the primary size does is move the peak torque higher in the RPM range.

So if you're someone who already has big cams, your flow/torque occurs at a higher peak.

This is why motor applications that want LOW END torque (like 4x4 and offroad crawlers) want smaller primary sizes.

Generally speaking, 1-7/8 is about perfect for a naturally aspirated coyote. 2" primary will flow better, but the result is going to be more to suffocate lower end torque and give you the full sauce way up high.

On very very high flow applications (like 4 digit +) a 2" primary is beneficial for the added flow.

So in summary, unless you're going 1xxx rwhp with 800+ rwtq, OR you want a screaming track monster that lives above 5k rpm, go with 1-7/8. It's a good medium between high flow and not totally destroying low end torque.

 

[K4fxd]

The bottom line is above a certain cross section diameter, all increasing the primary size does is move the peak torque higher in the RPM range.

You want to size the tube for max HP or peak flow. We are looking for velocity, but we don't want the gasses moving too fast. The length affects torque much more than diameter. Pulsing, you want the return pulse to hit the valve just as the intake charge tries to escape. You want this to happen at peak torque. longer tubes makes this happen at lower RPM. You can use length to rock the curve. Longer will raise torque below the peak and lower it above.

2" primary will flow better, but the result is going to be more to suffocate lower end torque and give you the full sauce way up high.

Except this does not hold true with the coyote. The coyote likes the larger diameter tube and it loses nothing to the 1 7/8 on the bottom.

 

[Prodigal]

You want to size the tube for max HP or peak flow. We are looking for velocity, but we don't want the gasses moving too fast. The length affects torque much more than diameter. Pulsing, you want the return pulse to hit the valve just as the intake charge tries to escape. You want this to happen at peak torque. longer tubes makes this happen at lower RPM. You can use length to rock the curve. Longer will raise torque below the peak and lower it above.


Except this does not hold true with the coyote. The coyote likes the larger diameter tube and it loses nothing to the 1 7/8 on the bottom.

Are you suggesting 2” is the better choice in a gen3 coyote?

 

[K4fxd]

I would stick to the 1 7/8 unless going FI simply due to the amount of space available for the header to fit. The larger tubes can hit things and cause false knock.

N/A you don't lose anything with the 2 inch but you don't gain either unless FI.

 

[Angrey]

You want to size the tube for max HP or peak flow. We are looking for velocity, but we don't want the gasses moving too fast. The length affects torque much more than diameter. Pulsing, you want the return pulse to hit the valve just as the intake charge tries to escape. You want this to happen at peak torque. longer tubes makes this happen at lower RPM. You can use length to rock the curve. Longer will raise torque below the peak and lower it above.


Except this does not hold true with the coyote. The coyote likes the larger diameter tube and it loses nothing to the 1 7/8 on the bottom.

Primary tube diameter moves the peak left or right in the rpm range. This isn't controversial or debatable at this point, there's countless mechanical engineering experiments and published pieces (ranging from media to academic/scholarly papers). The result is formulas and rules of thumb.

Primary diameter moves the peak torque point (related to rpm). Primary length "rocks" the curve and adjusts whether torque lingers or falls off sharply after the peak or flattops or ramps highly before the peak.

You can literally do a google search and find hundreds of sources that support all this.

How to Pick the Right Header (motortrend.com)
Header Primary Tubes (summitracing.com)

The endless myth is that primary size results in more or less torque, when in fact it simply adjusts the curve (in the ways I outlined above).

Sizing the headers (both length and cross section) has to do in what application/use you're trying to get from it. A big diesel motor that redlines at 3500 rpms and is used for pulling stumps is going to want shorty headers with relatively small cross section (to create good torque and right away).

A Formula One car that revs to 20k rpms is going to want very LONG tubes and the diameter depending on how it's geared (again, bigger = higher rpm for peak torque, smaller = lower rpm for peak torque).

 

[GregO]

@Prodigal
You putting the headers on a MACH 1 ?
It might be worth a call to Kooks to see if they’ll build you a set of their GT350 step tube headers (1.750” to 1.875”) with 5.0 flanges in place of the VooDoo 5.2 flanges.

Don’t lose to much sleep over OTC headers, every set out there is a compromise. A real set of headers would be step tube 4-2-1’s. If done right those will create the highest intake system depression with power to be had across the curve. Unfortunately that header doesn’t exist for the S550 and if it did it would be priced north of $3k to $4k possibly more with current material costs.

 

[bankyf]

Couple of rules to follow.
Typically the primary diameter should match the port diameter with about a 6” radius off the header flange. The S550 strut towers does not allow for a 6” radius bend so in the case of the S550 the primary needs to go up in size due to the tight bend radius (exhaust gas flow rules, deep in the weeds stuff)
Not many options for step primary header designs so your stuck with single continuous tube sizes.
There’s virtually no lower rpm power loss with properly built 1.875” primary tubes vs. 1.750”.
2.0” is getting into the high rpm, high CFM, Very high power levels.
At the end of the day 1.875” IMO

I’ve wondered why Kooks builds step tube GT350 5.2 VooDoo headers and not step tube 5.0 headers especially now that the Gen3 has been out for awhile.

Hope this helps

Kooks does make a step header for the 5.0, but for some reason it is not on their website and seems to be exclusive to Lethal Performance.

Edit: I no longer see them listed on Lethal's site. Perhaps they were discontinued.

 

[4V Mayhem]

You might be NA for now but in a few years you might want to go forced induction. So my suggestion is to try to predict your future desires and try to build towards that. Therefore I say go with either 2inch or 1-7/8. Those are the only 2 options I would consider.

Sponsored