318 REBUILD IN 67 BARRACUDA
OP; you say;
I would like to improve performance The exhaust manifolds are part number 2843953 and 2780946 which I believe are the original 273 manifolds which came with the car........ So my plan with new pistons is to raise the compression to around 9.5 to 9.75:1 (has to run on premium pump gas) and since the heads are going to be rebuilt install larger valves. How large should I go? I have seen as large as 2.02 intakes, is that a possibility or should I go with more like 340 or 360 size valves?
I am going to run the LDB4 aluminum intake which was already on the car.
So that leaves the camshaft and that's where I get a little lost.
I have considered aluminum heads
and this is to be for your wife? ............... Gimmee a break.
I know what it takes to get a 318 up to 9.5 or more, and I know that a 260* cam is no small thing. And I have heard more than a few people complain about California gas. And, depending on your elevation there in SoCal, the cranking cylinder pressure could be over 160psi@ 9.5 Scr, which sorta demands Premium fuel.
IMO, this shaping up to be a recipe for disaster.
Furthermore; the worst part, IMO, is to saddle this log-exhaust engine with 3.23s in the which, the fat part of the torque curve in First gear doesn't arrive until say 30mph, and the powerpeak at say 50mph; and, the top of Second gear will not come until over 85 mph. What's the point of all those engine mods if you never get to where the power is......
IMO, you might as well just keep the long-block stockish (with a slight bump in Scr), and run cheap gas..
IMO, I would let the target pressure dictate the build. I would shoot for something like 150/155psi for California gas, at your local elevation;
which
could be; 9.2 with that 260*cam, or 9.0 with the next smaller cam.
The latter of which is likely to have a stronger bottom end.
Which is, IMO, what the wifey's ~3500 pound loaded convertible cruiser needs, to pull those 3.23s around town, with a typical stall convertor.
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My 68 fastback with a 367/manual trans/8.75/no A/C / but With power steering and brakes,
weighs in at 3450, me Not in it. I lightened it quite a bit with an alloy-cased 4-speed/alloy top-end / alloy wheels/ no rear seat/no console/and lightweight buckets.
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If I was gonna build, (which I never will) a 318/ Second gen Barracuda stuck with 3.23s;
it would have a tight-LSA Solid-lifter modest-period cam (like the 260), and a Commando 4-speed. But if I have to give up the Commando, then it would have the (wide-ratio) 2.74/1.54/1.00 automatic, a slightly wider LSA cam, with at least a 2800convertor. And no matter what cam is in it, I would want to run headers or at least 340-sized logs; even if I was forced to use the stock hydro cam, which really ain't all that bad for a cruiser application.
But if I could run more rear gear, then I would run the Regular (close-ratio) A904 gear ratios, with a lower stall, and a slightly tighter LSA.
For Performance; this combo wants a starter gear of between 10:1, and 9:1. With your A904 and 3.23s the starter is a mere 7.91, leaving you about 20% short of performance on take off. Your cylinder-pressure and stall will have to make up for that deficiency.
But if it really was a convertible cruiser destined for my wife,
I would forget all about such things.
Instead; I would get her a Bug, or a Rabbit, or a Fiat500, or a Smart-Car even; cuz wives tend to drive a LOT!, especially the stay-at-home kind....... which, tooling around on Premium, won't be cheap, and will only get more expensive in the future.
What I want to know is this;
why OP, why would you think of building a hi-compression, big-valve, 4bbl 318, with a big enough cam to worry about how much vacuum it pulls ....... for the Mrs to cruise around in; ....... and then stick it into one of the heaviest of the A's, and finally, choke it with 273 log manifolds and 3.23 gears? IMO, this is messed up.
Does your wife really care about such things? Will she ever even floor it? And if she does; will she hold it there long enough for the secondaries to start working, or to find the power in Second gear, which with 3.23s, doesn't even arrive until well after 60 mph, nor finish until perhaps 80/85 mph?
If the answer contains just one no, then what is the point? IMO, you are very likely leaving out a part of your story, possibly the part about you wanting to go ripping around on weekends ...........
Please notice that throughout this post I never once said a single word about starting with a bigger-bore/longer-stroke engine, which would of course, solve all your problems as I perceive them; including the stall/3.23 thing .
As always;
Happy HotRodding
