BuckeyeBuicks, Do you know that for a fact? I've never heard that a 350 exhaust manifold or header will fit the 340. Have you done this yourself?
340 headers do NOT fit a 350. Different exhaust pattern. 340 pattern matches the 300 but the 340 is a taller deck block. Greg
Is my face red or what? I ment to say that 300 headers would fit 340's. Didn't see my screw up til I read it again. Sorry , didn't mean to mislead anyone. I really ain't that dumb.
I'd like to talk to anyone who has a set of headers that would fit my '65 Sportwagon 300 4V. Thanks in advance, Steve.
Poston's is the only place with 300 headers. After examining the stock manifolds though...I can't imagine the headers making a huge difference on a stock engine. They cost something like 300 dollars new and uncoated...the stock design looks pretty efficient for a factory manifold. Just my 2 cents, maybe somebody else will have something to add or personal experience with the headers. Aaron
I recall reading a post three or four years ago on another board by a guy who specialized in building 340 engines. He also stated the opinion that the stock exhaust manifold was very efficient, and headers would be useful only for all-out racing builds.
300-340 Headers at Postons......if anyone has used these on a 65,66,67 how good do they fit?.....I've heard not very good. http://www.postonbuick.com/eshop/10Browse.asp Click the drop down and go to Skylark/ headers....for some reason I can link that page and it still goes to the camshafts...go figure ou:
I have a set of the Poston headers on my 64 skylark convertible. These headers will bolt right up & I didn't notice any clearance problems around cross-member, steering box, etc. Poston also includes a bracket for the auto shift linkage (assuming you have a column shift) - which I believe it was intended for. However in my case, I switched to a 4 speed from the automatic, which required modification of the rear header exhaust tube on the drivers side. The clutch z-bar's path was directly in that of the header tube. I paid a local guy a few bucks to cut the offending pipe, & weld in a new section curving around a little different to make some room. Bought some 1 5/8 tubing from an online retailer. Since I did a rebuild with some high-perf parts at the same time, I cannot comment specifically on perf improvements from the headers alone, I never fired up the newly rebuilt engine with the stock exhaust. I can say that the overall performance is much improved over stock, but I also changed many things including 65' heads with larger intake valves, a cast iron 65' 4bbl intake, new carb, high-comp pistons, Cam, valves, valve springs, etc - and it sounds really mean!
I only really am interested in the headers because I have now had two rounds of cracked stock manifolds. Wondering if the headers would be less susceptible to the same problem, and therefore a more definitive fix. Don't want to keep doing it! Aaron: It's a Roadmaster. Best, Steve.
Steve, I had the same problem as you did...I scoured the rural northern Michigan junkyards and found a '67 Special 300 and plucked the manifolds for 30 bucks a pair (!). You might check ebay too. Most of the time they are around 40 bucks a piece used. Some on this board might have some they could sell you. Oh...couldn't see the fourth porthole...nice Roadmaster! Aaron
Oh I just reread your post...you've replaced them twice. I guess the headers could help that. I have read that bad motor mounts put strain on the exhaust manifolds by letting the engine move around too much and straining them. Who knows if that's true but why not try it. I found a crack in my left one...they are 9.99 a piece at Auto Value...Just brainstorming here...
What I've heard over the years about cracked exhaust manifolds on the 300/340 engines is that they are tightened too much. Think the specs were for about 14 ft-lbs of torque, which is pretty mild. If tightened more, they could crack when expanding under heat. Interesting that some folks use gaskets, while mine never came with, nor has had, exhaust manifold gaskets.
Before you do anything else, check your motor mounts and transmission mounts. If these are bad, you'll crack the manifolds... almost guaranteed, especially the right side. As others have mentioned, they are also torque sensitive and will crack if over torqued. JMC
Headers Hey guys I put a set of poston headers on my 67 and they are some of the best fitting headers i have seen. They took up close to the floor boards and really make a diffrence on how the little 300 sounds. By the looks of the stock manifolds they probaly do flow pretty well but every one i have talked to has had a pair crack I truthfully think that they are bad castings because my engine had never been apart and the driver side manifold developed a crack. My dad and i both seem to think the headers have made the 300 rev easier but as of yet i haven,t driven it except in the drive way with these headers. One thing you could do though to improve on the manifolds is port match the openings to the heads and also open up the outlet as much as possible, then remove any casting flash with a die grinder in side. Also in my driver side exhaust maniifold their was a valve that was designed to keep heat in the engine on start up then open up when the engine got warm. Some times these can get stock closed hurt exhaust flow drasticly. Even if the do not get stock closed the have a very tiny opening in them which hurts exhaust flow to. Another thing you guys can do is design your own X-pipe There is a guy on here that has a 65 with the 300 in it and he picked up 9 rear wheel horse power on the chasis dyno just by adding that. thanks Adam
