Stock 318 bolt on performance modifications
Second dual exhaust, don't waste the money on the "340 manifolds",
Wow....Where do I start ?
While you are entitled to your opinion,and that of a magazine.
I rely on real world experience,and to say that a HiPo 340 manifold is only
3-4 hp over a 318 log is hogwash. If that's the case I have a set of HiPo 318 manifolds for you,cheap !!!!
Precisely.
Cheap. Cheap as in free. Free is a great price, more on that later.
I use that article largely because there's so much hyperbole and so few numbers about exhaust system performance except at the very high end of the spectrum, there are very few "we bolted up five things to this exact motor and here is what we got" articles out there. I have yet to find an article that is similarly controlled that claims amazing 75-100 horsepower gains for exhaust headers. I see "bolt on 150 horsepower" articles which usually involve cylinder heads, intakes, carbs, cams, headers, and the whole shebang, but very little gets said about exhaust other than it is mysterious, bigger is always better except when it isn't, something about "backpressure" and then a voodoo hex. But here's my reasoning, which is separate from that article but informed by some possibly wildly understated assumptions those numbers give us.
340 Manifolds around these parts in "as removed from car" condition go for as much as a set of new ceramic coated full length headers ~$300. Not TTIs or SanDougersons or anything but a coated set of speedbump draggers. I will admit maybe there's a VAST gap between a 318 and 340 manifold's flow at the cruisin'-the-strip-end of the spectrum, wouldn't there also be a similarly sized bump from a set of headers? Why not go with that, if it's cheap, bolt on performance you're looking for versus period correctness. But I would bet my eye tooth on the right side (the _pretty_ side) that for a no-cam-swap (which, honestly, if you have a motor out of a car on a stand, I consider a "bolt on" item, unless you're not even gonna do the timing belt in which case I don't really know what to say other than "do the timing belt while you got it out dummy") engine from a 19
75!! !!!
!! car, the difference between using the free stock manifolds connected to a dual 2 1/4" exhaust run to the back of the car and a TTI small tube header to dual 2 1/4" exhaust run to the back of the car is going to be both _under_ 25 horsepower and _over_ $500. But both will be significantly more than the motor put out through the single exhaust.
In this scenario dual exhausts to 318 manifolds only cost about $300-400 (a spitball price for the cost of a local shop to hook up a dual exhaust for you or a jegs kit shipped and installed and downpipes fabbed. Dual exhausts anything else costs that same $300-400 plus whatever you want to pay the header. For me, doubling the cost better net me a TON more horsepower, and I just don't believe the hype about a 44 year old exhaust casting. If you're gonna buy an exhaust system, and are gonna change from the stock manifolds, it doesn't make a ton of sense NOT to just go to headers which offer a LOT of cheap, pre-pop options in the "after the crossmember" system instead of springing for expensive original 340 manifolds and repro 340 downpipes.
This is a little tl;dr, but that's the upshot. I think if you're gonna BUY an exhaust manifold in the name of performance, you should just get headers and be done with it. If you wanna cruse the burger shot and make good burbly V8 noises, get a muffler shop to hook up a dual exhaust with a crossover pipe and get back to your life. If you want a "period correct 340 style 318 kinda sorta throwback" whatever then buy them there 340 manifolds. They're totally fine, I just don't think they're worth the scratch when headers _start_ cheaper, and once they hit price parity with the 340 mani, they even have a nice pretty coating on them.
