Good question.
I took my Dual Diaphragm apart Cuz I really wanted to run it. But there was nothing wrong with it. I played with the pushrod length, but to no good result. I was running a 292/508 cam, with a pretty good tune in it, but the boost was just a bit out of sync for low-rpm use in a stick car with 3.55s.
The larger Swinger booster while not 100%, was huge improvement.
A 475/495 cam could have a buncha different durations and LSAs so the lift spec is not gonna help you make this decision.
Since I never tried that DD booster with any other cam, I can only speculate as to it's suitability.
But I can say this; that 292 barely made 11 inches at 750.If I blipped the throttle a couple of times before I backed out of my carport, it was fine for take-off. Once driving it was pretty good, the manual trans kept the rpm up until right slowing down. But somewhere down at about 7mph, the engine vacuum would become insufficient to maintain assist. So I always had to keep that in mind on long slow slow-downs. At stoplights it was no biggie cuz in's a stick. Some automatic guys have mentioned that it could not maintain assist at stoplights, and the car would begin to creep forwards.
And I can say that the factory put DDs on 340s,lol with 268/276/114 cams . But this cam only has 44* of overlap, compared to 76 on that 292/508
Soooo, somewhere in between, the happy part goes away. I'm gonna GUESS halfway of course,60* , cuz what else can I do? Of Course a cam with 60* of overlap could have a buncha different specs.,too.
But the ever-popular 268/276/110 has just 52*, and this is street-friendly when synced with adequate cylinder pressure.This is especially important with a 318 and manual trans. An automatic with a hi-stall can get away with all kinds of inefficiencies. And a 360 is also more tolerant of low pressure.
Can you go bigger? Sure; there's still 8* left in the speculation.
But I can't see the point with those 255s. A 360 with a 268* and 160psi/iron is just gonna roast those skinny rollers,probably all the way to 60mph with 3.23s, if you're brave enough to keep your foot in it. But if you're gonna do that often, then you should probably look to some oiling mods to keep your rod-bearings happy.And just as importantly, adequate spring pressure.
The 268 is about a 222*@.050. It can make a chitload of low-rpm torque. I had one once. I remember it very fondly.I ran it with Eddies at about 185psi, Daymn! that thing had take-off. And the 106mph in the qtr says it was cooking pretty good.
In a 318 at 8.0 compression, with a factory TC and even 3.55s,it will be IMO a huge disappointment. She want's at least 9.7Scr.
So bottom line, if you're NOT gonna pump it up, I, me personally, AJ, wouldn't put more than a 360-2bbl cam in it, and it will want a 2800 TC and minimum 3.23s with that 2800. With a 2500, it's gotta have 3.55s. Those are my opinions based on my experiences.
When I say a 360-2bbl cam I just mean like-sized in advertised duration. You can put any lift on there that your heads can take and any .050 you can find,the more the better.
For example
the typical ramps for various types of popularly sized street cams are;
Hydraulic roller 50* or more........ hydraulic flat-tappet 46*or a bit less.........solid flat-tappet 40* or less
So a 2bbl 360 cam was listed at 252/2xx/112 I don't recall the exhaust.
Lets estimate it at 252/260/112.....OK the .050s are 202,206, and 212*....... That doesn't look like much difference, now does it? In fact tho, in the engine, each of these is about one size bigger from the previous, and each size will move the rpm of peak power up some 150 or more rpm, and by the formula,more rpm makes more power. But they all three will make about the same cylinder pressure, which on an 8.0 teener is VERY important. In a teener at 8.0 the pressure will fall to about 130psi with any of these, in at 54* ICA. So each will be equally weak at low rpm; and less powerful than the stock teener cam. That's why you need the hi-stall TC, to get the rpm past that doughy part. Then you need a bit of gear, to let her climb up to where the power is, in a reasonable period of time, in accordance with the rate of acceleration that you may be expecting. I mean if a guy sinks 3 or 4weeks wages into an engine, he sorta expects something in return, right?
Anyway,the factory 360, suffers the same problems, but can use less cam for the same expectation.
And the Magnums start from a higher Scr, so they are less sensitive to that crap. You can put a little more cam into them,before the bottom falls away.
Stick streetcars spend most of their lives below 4000, even below 3500; which being 30 to 35 mph, in first gear with 3.55s. So a stout bottom-end is paramount. You don't want to be either slipping the clutch out all the time, or spinning the tires all the time......right? lol.
Anyways way off topic. At your power level, and since you already own it, slam that DD booster in there, and go for a spin. Give it a couple of hours in traffic and see what you think. If you're unhappy,it only takes a few minutes to swap it out.
