340 cam in a 318
I have a question about putting a 340 cam in a 318. From what I've found out, the 340 cam has a ivc point of 69* and this brings down the dcr to like 6.63:1 ( 18 psi lower cylinder pressure than a stock 318 cam ). Wouldn't this hurt performance rather than help? I based this on a 1973 318 with a 8.6:1 cr. Did I figure something wrong or have the wrong information?
Did I figure something wrong or have the wrong information
Yes and no.
The 318 won't make 8.6Scr with the stock heads, and
You are 100% correct that the Later Ica of 69* compared to 48* of the stocker, is gonna drop a ton of cranking pressure.
As compared to the stock cam;
this is gonna KILL low rpm power.
Then the power will increase due to the " better breathing" with the later Valve events, but still be down because of the reduced pressure.
Eventually the reduced pressure will be offset by the greater cylinder filling.
and at high rpm the power will be considerably stronger as the engine comes up on the cam.
Except;
With 318 heads still on it, it will never really wake up.
This leaves you with a wheezing , gasping POS engine, that suddenly comes alive and just as suddenly hits the wall.
If you've been paying attention to the guys; they are, for the most part, telling you to install a higher stall convertor and something like 3.91/4.10s. The higher stall will overcome the low pressure, and the gears will overcome the laziness. But you are still stuck at the top when the heads choke.
So what you started with was; a reasonably peppy combo that is well able to rev and pull from the factory stall near 2000 up to at least 5000, with ANY gear over 2.76s
But with the 340 cam, firstly you have the added expenses of the TC and gears, and then your powerband is from 3000 to say..... still 5000, cuz of the heads.
But consider that ;
With the 318cam and say 3.23s, your shift points for 5000 are 52/88/127, and
65=2614 at zeroslip say 2700 at 3%
With the 340 cam and 3.91s, your 5000 rpm shift points are 42/71/103, and
65=3165 at zeroslip, say 3260 at 3%
But also consider what happens at the various speeds at which you will be cruising at. I'm gonna give all speeds at zeroslip.For instance,
take 32 mph.
With 3.23s, I get; 3150/1870/1290 And you wonder why the 318car is a dog?
With 3.91s, I get; 3820/2260/1558 Ok now show me why you would chose a low-dcr 340 cam at these rpms. It's pure madness.
Lets move to 50mph, another common roadspeed.
With 3.23s I get 4930/2920/2010
With 3.91s I get 5964/3530/2430
So you will have to be in second at 50 with either of these gears. Obviously now, the 3.91s have the edge in second gear. But with the 340 cam, the engine is just waking up, whereas the 318 cam is right in it's sweetspot.
The point that I am trying to make is don't neglect the importance of gearing and stall.
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As for me; I have at least 4 maybe five low mileage 340 cams on the shelf here, And I'd have to be pretty desperate to install one in any low-compression engine.
About swapping stock parts around;
My favorite odd-ball combo was a hi-compression 340 short with a complete 69 318 top end on it including the 2bbl cam. With nothing more than fenderwell headers and 2" duals. In a 65 V100 wagon, that was a pretty good ripper.
If I only had a low-compression 318LA;
and mechanically it was A-OK, then;
I would do NOTHING to the Long block except;
install some good valve springs to allow at least 5200 rpm, and
the 273 valve gear. , and
any old 4bbl set-up,
plus headers and I would hang
the biggest dual exhaust I could find in front of the mufflers.
And that is all.
I would run whatever gear gets me into the powerband at the roadspeed I spend most of my time at. If I would run an automatic, I would install at least a 2800TC, and this car would be a citycar. But more likely is that I would run a manual trans with a minimum 4 gears, because this makes it way easier to select a rear gear.
What I mean is this; I spend a lot of time at 30/35 mph. So that is where I want to motor from. With a 4bbl 318 and a 904 I'm gonna need the rpm to be about 3500rpm on the 2-1 downshift. So that will take 3.23s for 32=3470 at 10% And 65=2700
but with a Commando 4 spd (3.09low), I would need 2.94s for 32=3620 and 65=2380
Both combos will hit about 45@5200.
Obviously the manual trans combo is more suited to hiway use.
The Low-compression318 does pretty good with the Very-wide ratio A833od. With this trans you can run up to 3.91s but after that the starter gear gets to be a wee-bit low.
I have first-hand experience with ALL of these combos.
I have had two low-compression 318s pumping 135psi with 2bbl cams, that after the 340 cam went in, and with a stock stall TC, they were bar none the doggiest POS engines I ever drove.......... until the rpm got up. They had pretty good passing gears tho,lol. It took 4.10s and 2800s to wake them up. One of them got 340 heads.
Knowing what I know now after all these year; if I had to be content with a 318LA,
the first thing I would do is strip it down and throw those 8/1 pistons away and
the second thing would be to toss the heads and
the third thing would be to install a custom solid lifter cam/kit.
Oh wait, is it still a 318 lol.