It's really hard to build compression with a 318; there's just not enough swept volume. You said you were at 9.8 right? Well at stock bore/stroke that makes a total chamber volume of 74cc. The heads are 63, and the pistons are 5 and the decks are zero, leaving 6 in the gasket and there's no place to go.
That 284 cam is wanting 11.8 Scr to come in at 182psi at your stated Ica of 72*. To get there the Total chamber volume has to be reduced to 60.4cc. Where-O-where are you find 13.6cc? You'd have to cut your 60779s to 49.4cc, and then cut the intake side, and then new pushrods.
The problem is the ~652cc swept volume.
If you take that same 74cc total chamber volume and slide it over a 360 (737 swept), you get a 10.96 Scr, and instantly yur getting somewhere.
If you slid a 3.58 stroke crank into a standard bore 318 you'd get a swept of 704cc and again sliding the 74 over that will get you 10.5scr
If you stuck a 4.00 crank in her, you'd get 786.6 swept and 11.63Scr.
Of course each crank will need zero-deck pistons.
So the deal is that yur about topped out for Scr in your current combo. The 6cc gasket is about as thin as you can go for piston to head clearance. So you have about two choices, mill your heads or get you a faster ramp cam, and make it the fastest you can find, and follow the cam-grinders instructions to the T, as to springs lifters and redline.
BUt I'm assuming that you when you said the Ica was 72*, that you actually measured it. Book math don't cut it when yur pressure is as low as yours, and you have a big mechanical cam.
Lets back up the bus and work with the 9.8. At this Scr that engine wants an Ica of no more than 52* to make a predicted 176 psi. That's pretty small. That Ica comes actually on the factory 360 2bbl stick; that's how small it is. But hang on I got an Idea. What yur after is to keep the [email protected] and the lift, but a shorter Ica. Well is that is doable? I don't think so but, let's see how close I can get..
Here's the 284 cam
284/284/112+2/60 overlap/108 comp(72*Ica)/84power/SFT/28/32
Here is what I created on paper
268/276/102+4/68 overlap/128 comp(52*ICA/116 power This cam is about the most screwed up cam you will ever see, but lets see what's wrong with it
Well firstly, this is a hydraulic, so I haven't converted it to Solid yet, but ignoring that;
Secondly the LSA is down to 102 which IMO, probably most opinions would say is too tight for an automatic, but hang on; What gear are you trapping in with those 3.91s to hit 7200? The math says it ain't third gear nor second. The math says you got 3.73s or a bum convertor, or a bum tach.
The math says; 105 with 3.91s and 28s and a 2.45-1.45-1.00 trans, and 5% TC slip, comes to 7500/5200 in direct. But with 3.73s it would be 7160/4930
So in either case, something is up.
Here's the thing; if you are trapping in second then yur engine won't care about the 102LSA and in the mean time that 102LSA is creating a heckuva 64* Effective overlap, that I'm hoping will make a power difference, to close the gap between this lil 268 and the previous 284 which had an Effective overlap of 56*, by the spec, but after lashing less..
So what else is wrong with it? Well about the fastest hydraulic ramps I have seen is 38* from advertised to .050 making the best possible scenario here of 230/238 @.050, about a size and half a size smaller than your earlier [email protected] and these numbers speak to the power production. So that is gonna bring the power down 250 rpm, which might drop the power 17 hp. Now the increase in effective overlap may blow that out of the water. so we might have an absolute hp number that is extremely close between these cams.
However, the 284 on it's 112 Lsa will make more power on either side of the power peak compared to the 268. However the 268 will make a boatload more power leading thru the midrange.I mean there is no comparison between 175psi that this 268 cam is predicted to make, and the feeble 135/140 you are measuring now.
So with that in mind, this cam needs to be converted to an even faster ramped Solid, which would increase the .050s, and then I could spread the LSA, and, and.....and sometimes less is more.......
If you are trapping in third gear well the rpm drop is to 69%, so if you shift that cam at 6000, the Rs will drop to 3540, which is 2460 rpm, which is a lot. I have no idea what that 268/102LSA /175psi cam would do with that, But I bet it's more than the 284/112LSA/137.5psi cam ever could.
This is an exercise; I made the camspecs up.
BTW if you noticed, the power stroke duration of this 268 cam is 116*, compared to 84* on the 284. furthermore the compression is 128* vs just 108* for the 284 cam.
These two together, conspire to put this 268 cam into excellent fuel economy in steady state cruising, range.... compared to;lousy with the 284 cam.
The totals tell the tale; 244 for the 268, which is excellent; vs 192 for the 284, which is lousy. Street cams are usually in the 230/234 range.
