Posted by Brian in Oregon on Monday, September 09, 2002 at 11:00PM :
In Reply to: Re: 318 2barrel to 4barrel posted by doyle on Monday, September 09, 2002 at 0:18AM :
For a late 60's - early 70's 318 you need to get an intake manifold that has SMALL INTAKE RUNNERS.
Many of the performance manifolds are made to fit the runner size on 360's, which later became the norm for 318's (1978 on).
Some companies call these "early 318's", which to us is confusing, as we consider an early 318 to be the poly engine. It gets more confusing when an intake is advertised as fitting a 68-72 318 as well as the 340 and 360. This is because it usually has small runners. But it pays to check the runner size to make sure. If in doubt, call the tech support numbers and have them actually measure a port.
If you use a 360/'78-on 318 intake manifold on pre-78 heads, you'll have a large intake runner abruptly reducing size. Not good. I do not know if you can take enough material off to port match, but even so it is a lot of work and the runner narrows anyway, though more gently.
The intake I went with was the Holley Street Dominator, which despite the awe inspiring name is really a near clone of the stock 2-bbl single plane, but the runners are a bit better designed and it is a 4-bbl. It is also a low riser, so it works nice for hood clearance. Alas, Holley discontinued it, and I got one of the last 12 they had in stock when I ordered it in spring of 2001. P/N is 300-7, but good luck finding one. Power band is idle to 4800, which I considered perfect for the M37, since I don't expect to exceed 3200-3500 rpm anyway (3200 is 55mph with stock 5.83 gears and 900x16 tires).
The nearest direct replacement is the Holley P/N 7545 Team G Intake Manifold
http://www.holley.com/HiOctn/ProdLine/Products/AMS/AMSM/7545.html It is likely the direct replacement for the 300-7, looking nearly identical but the power band is 1500-7000.
Most of the dual plane intakes I've found are strictly high perfomance. An exception may be the new MOPAR M1 intake. My understanding is that it has the smaller ports so that it will work on pre-78 318 heads. This needs to be verified. If this manifold had been available when I was searching for one, I'd have probably gone with it instead, once I'd verified the port size.
Edelbrock makes a dual plane that might work. Here is the info from their web page:
Performer 318/360 (Idle-5500 RPM)
Designed for street 318-340-360 c.i.d. Chrysler V8s. #2176 is stock replacement/street legal part for above V8s with OEM 4-bbl. carb.; 1966-72 (1973 non-CA); except stock equipped EGR. #3776 is stock replacement/street legal part for same V8s with OEM 4-bbl. carb.; 1972-86. These manifolds are not for use on marine engines used in salt water. Will not fit 1992 and later Magnum engines. See Power Package Graph.
Performer 318/360 (non-EGR) #2176
Performer 318/360 (EGR, 1972-86 only) #3776
You can get to the Edelbrock info via http://www.edelbrock.com/automotive/index.html
Again, I cannot stress too much that you need to verify the intake port size on whatever mainfold you buy. The stock intake manifold ports are only 1-3/4" tall by 7/8" wide. I don't have a 360 intake manifold or head to measure the port size on. Maybe someone who does can add that info here.
If you buy an intake, it might be best to order one from PAW (Performance Automotive Warehouse). I got an intake that had too large of intake ports, and they took it back no questions asked and refunded my money. They will also do special orders, though they aren't returnable. That's how I got my 300-7. PAW ordered it directly from Holley and shipped it to me (Holley does no direct to consumer sales).
Oh, three other things to consider. Hood clearance height and the crossover heat riser. I don't know if hood clearance is an issue on your truck. The crossover heat tube helps prevent carb icing, which surprisingly can happen even in summer under the right conditions. Chryslers are rather cold blooded, so this is a good feature. If you don't want full heat, weld up the runner with a bead just inside both crossover ports, but don't seal them. The old sheetmetal seal trick usually doesn't last long as the shim burns through anyway. If the intake lacks the heat passage I'd really consider passing on it. The third thing is related to hood clearance. Some of these manifolds will not allow air conditioning. If you have air conditioning on your truck, read the fine print.
If you do want to go to the big intake ports, you'll need '78-'91 318 heads, or 360 heads. The '92-on heads won't fit earlier 318's. The big drawback is now the cam no longer has the profile to make these heads and the intake work. By the time you do all this, you might as well go to a 360.