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Discussion Starter · #1 ·
After doing some searching on 3" exhaust systems, I've found it abundantly clear that there is one general consensus on header size for cars running either ported eatons or twin screws running under 625rwhp: stay with the 1 5/8" collector size with a 2.5" exit. HOWEVER, there is little to no empirical evidence (that i've found) suggesting that 1 3/4" collector sizes cause detrimental effects to the power band for the ported or lower boosted cars. It seems to me that word of mouth and the generalization "1 5/8" under 600rwhp" has relentlessly plagued this discussion. I'd like people to chime in here with technical information supporting either side of the argument, hopefully with dyno graphs and actual #'s to support their claims. It seems to me that higher boosted ported cars trapping 126+ with the proper setup (gears, pulley combination, tune etc) could benefit from running 1 3/4" primaries (1 7/8" is probably too big). Let's start this thing.
 

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The problem with proving either size is more beneficial to the other is that there is limited information supporting these claims, just as you said. That kind of limits the whole discussion before it can even go far.

I too have searched this in depth and formed my own conclusions with what little information is out there. On subjects like these, that is pretty much all we're left to do without any proven data of support.

With that said, considering the power levels most Eaton/Ported Eaton cars run at, it is of my opinion that they'll be fine at 1 5/8'' size. Would 1 3/4'' be hurtful to those with Eaton/Ported Eaton guys? I would think absolutely not. There would likely be more power left on the table for those guys who chose 1 5/8'' over 1 3/4''. As I've said before, I think 1 5/8'' are so popular on these car because of price and availability of larger headers. This was surely the case for me when I bought my Mac headers when back when I had my Ported Eaton combo.

As I've also said before, ideally I like to see 1 3/4'' headers to maximize performance. Sure a 1 5/8'' header will work for some set ups, but at a cost of how much power over the 1 3/4''? Again, there is limited data there. I have seen someone state that in Sean Hyland's book the difference between 1 5/8'' and 1 3/4'' is 5-7RWHP. Maybe someone with the book can quote exactly what the book states?

Then there is 1 7/8'', which I don't feel is too large for our cars. I feel that a header this size will have some compromise, where as a 1 3/4 size will not. As I've posted before, a member here has posted his gains of going from Bassani Mids to 1 7/8'' American Racing Headers. The gains were substantial. However, he did compromise what looked to be a small amount of HP and TQ in the lower RPM's. Although his set up is not as common as others (3.4 Whipple at 20lbs), this is one of the only proven claims I have come across of a larger header making more power over a smaller header. Here's the link, scroll to post #7. http://www.modularfords.com/showthread.php?t=92326

I think our cars would definitely benefit from 1 5/8'' up to about Ported Eaton level. After that, a 1 3/4 set up should be more beneficial to those of us running a Twin Screw. Again, these are just my personal conclusions from what information is out there.
 

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Discussion Starter · #4 ·
Great post! The reason I mentioned the 1 7/8" headers being "too big" is because i feel this conversation pertains to 95% of cobra owners: eaton/moderate boosted twinscrews. Are there ANY tuners out there who have had customers that go from stock manifold to 1 3/4", or someone who goes from mid-lengths or 1 5/8" headers with 2.5" collectors to a 1 3/4" 3" design? I've also heard the argument about the increased heat displacement with 1 3/4" headers. Is there much validity to this (actual temperature drops in cylinder heads), or is this just a wash?
 

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I have the old 1-5/8's Kooks with a 2-1/2 collectors. They hang low like a mo-fo, and are loud as hell with cats. Did they give me any power? I don't know. They got some heat out of the engine compartment, and your feet don't feel like they are on fire anymore inside. I think most of that is due to having them coated. After reading that it got me thinking though. IMO go as big as you can or don't bother...
 

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I don't see how running 1 3/4" can hurt you in any way. You might loose some TQ but you can probably gain more HP. BTW i'm going 1 3/4" this summer.
 

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With twenty+ cars in my current stable and way more years of experience than the age of the average mustang owner, I must say that with more cubic inches you can use bigger primary tubes. With almost everyone running some form of power adder to their cars/trucks, it would be logical to go with bigger tubes. But, if your car/truck is not run typically between 5-7,000 rpm, the larger tubes are not warranted nor necessary.
With my 477 cubic inch Oldsmobile, I run 2" primary tubes into a 3" X-pipe dual exhaust.
With my 409 cubic inch AMC Wagoneer, it has 1.625" primary tubes into a 2.5" duel exhast.
With my 360 cubic inch Chevy, it has 1.75" primary tubes into a 3" x-pipe into 2.5" tails.
With my 410 cubic inch Pontiac, it has 1.775" primary tubes into 3" X-pipe duals out the back.

The point is that each one has a thought out exhaust SYSTEM to compliment the application.

The Olds is a large capacity motor that is revved to 6,300 rpm.
The AMC is made for Torque and seldom sees or needs 4,000 rpm.
The Chevy is made to rev to 7,000 rpm.
The Pontiac is a bit more track than street and is revved to 6,500 rpm often.

My 281 cubic inch Saleen is Super Charged and sees 7,200 rpm often. It has 1.75" primary tubes and 2.5" X-pipe duals.

It is all relative. Choose wisely.

JMHE,

Eric
 

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I Would Say This, If You Have 500 Bucks For The Headers, Then 1 5/8 Inch Macs Are Great. Perfect Fit And Ceramic Coated. If You Have 1,500 Bucks To Spend On Just Headers Then 1 3/4 Kooks,arh,stainless Works Are Good Headers. Just As An Added Note, Primary Tubes Do Make A Difference On The Way The Motor Revs, How Fast It Revs, Hp And Tq Curves. It May Be Small But It All Goes In The Word "combo." Pick The Best "combo" For Your Application.
 
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