true true for the most part. But the collector is what helps transition out of the primaries and into the rest of that system and does in fact have impact on lower rpm scavenging. I've witnessed it firsthand at dyno sessions and have talked personally with many big name exhaust and engine builders about the subject. The basic laws of physics show that any airflow in or out of an engine will lose velocity/energy as it's required to slow down and speed up. Even 1/4 of an inch can and does have impact.. especially when it's too large to begin with. Also consider the cross section decreases in the smallest diameter cone section of merge style connectors as this is some of the secret to improving scavenging and reducing harmonic reversion which is a real problem on too large a collector applications. And there's not much to argue about when comparing a 3 inch header collector(connected to longer primaries) to an overly starved factory manifold(with an actual collector opening of about 1.75 inches!.. I know because I've ported them many times in low budget builds).. is like comparing a sapling to a full grown tree when it comes to improving the shade in your yard. Obvious improvements will be made and it's kiwi's to coconuts. Anywho. I stand by my original response in hopes that some of it may rub off on the OP as I was only trying to share. He may also consider using a 2.5 inch design to begin with and then simply mod'ing them for larger collectors down the road when the compression/power levels go up. 1.625 primaries are more than enough for 350HP 302's. I've also paid for and done the collector mod many times as well and you can actually adjust power curves quite substantially if the rest of the combo is tuned for it. Good luck with it all.
There is a science to headers and each motor has a totally different profile when you look at selecting headers for your motor. The most important thing to remember is that bigger is not always better.....................merge (angle of the merge) collectors/primary length/collector size and length can make a huge difference in performance at WOT....................for street cars this is less important as usually you don't have your foot planted on the floor all of the time. Here are a couple of pictures of what Chris at SPD did to my collectors to try and bring the torque curve down a little............and coupled with a looser converter I'm hoping to get the 60' times from the 1.4's to the 1.2's.
that's really what I was talking about in the first place. The 3" collectors will be an improvement for him over the manifolds he's replacing so it really doesn't matter if a 2.5" collector will give him 5HP more on a dyno. So worrying about them being too big now versus when he makes performance improvements to his engine later on isn't really something to be worried about in the scheme if things. Especially changing collectors twice would be kind if a waste of time and money when you think of it from a practical standpoint. Like I said, not trying to argue just saying the 3" collectors aren't really worth being overly concerned about because they're still going to make more power than manifolds.
I completely get what you were saying in the above and to many/most people "average run of the mill bolt-ons'" are good enough for their money. I don't look at building and mod'ing motors that way since I was about 18 and NEVER run parts right out of the box since there's just too much potential being left on the table AFAIC. Cleaning welds, casting flash, slight mismatches/gasket alignment.. it all adds up. And my point was that they will hurt him at engine speeds right off idle(not a huge deal).. and ALL the way through 2,500 rpm.. or higher. This is AVERAGE HP/TQ I'm talking about here.. not dyno numbers after yoy spin the motor up. And with stock type motors it can even extend upwards of 3,000rpm when too large an exhaust is used(such as a 3" collector mated to a 2.5" exhaust. Of course the stock cam diminishes the effect/loss somewhat compared to a larger profile as that would just add insult to injury for the bootm of the engines power curve.. but there will be losses without a doubt. I too have installed modified exhausts on stock motors and unless you have gears and start to drive em' like you stole em'?.. the motor is definately going to be softer down low where you would typically drive it anyways. You can tune some of that loss out.. but not all of it. So, I still stand by my input for the OP to stick with a smaller header collector all the way towards a 350HP 302. If it were 50 cubes larger?.. no doubt that a 3" collector would be better all around. BTW.. my latest 383 powered toy went from 2.5 inch long style collectors to 3" merges like olerodders.. and even it lost some slight bottom end out of the deal with 11.5 compression. Granted.. I didn't care because I had 3.06 first gear combined with 4.11's and was eventually swapping from the RPM manifold to the Brodix SP-1 and a larger camshaft.. but it actually did hurt throttle response and torque up to about 1,600 or so rpm. @olerodder. No doubt about that.. but we still have thousands of others experiences combined with our own to go off of when giving generalizations for design parameters. If you hop on over to speed talk or yellowbullet + many others on occassion.. you can actually see some pretty serious street motors.. and far more competent builders than us.. that nearly defy the laws of physics with seemingly too small exhausts. Mufflers or not.. on the street it's always better to have too small an exhaust than too large as average power across the usable range goes up.. and the driver get's a more enjoyable experience from such a design. Which was where my advice stemmed from in the first place.
Headers I have Flow Tech headers # FLT12100 on my '74 Maverick. The Headers that Summit sells, that they say fit a multitude of cars including Mavericks and Comets, will not fit without cutting the shock towers. The Flow Techs have 3 inch collectors. I am running 2 1/2" pipes. Installation was easy. I had to raise the engine about an inch to get the driver side in.
Headman 88400 Short Headers Hi all, Just last night I installed Headman 88400 Headers. They went in flawlessly, no rubbing, no issues whatsoever. I attempted to go with the Summit Full Length ones at first, and what a pain in the ass they were. Had to beat a couple of the tubes in so they wouldn't rub on the shock tower. The driver side ran into steering issues with the pitman arm that would have limited my turning radius so I bailed. The 88400 shorties are a great way to go unless your looking for full length. Hope this helps anyone who is learning by doing (like me)!
How far back do the shorties get ya once they are in? Are there any issues at that point to be concerned about?
is that bracket mandatory for PS cars? I have the slightly used 6901's loose that came with my GT and they have no previous dimples or clearancing damage from trying to make them fit right. So, I can only assume that it wasn't an issue on mine. But is that a fair assumption though?
