I have two in progress, one EFI and one not, both have very similar issues I have to contend with for the builds.
You said ICL of 104, really need to know LSA to determine overlap (or if cam card says that too, even better)
Vacuum will be affected by overlap, intake valve centerline, and timing (and of course ring seal, valve seal, elevation, air pressure, etc, but those won't change with a cam change)
1. Wider LSA = more vacuum and better mileage
I don't think your lobes are bad, likely fine for what you are using it for, although if it was originally picked for a smaller engine. it's likely tighter LSA than you'd need anyway
If its 108 LSA overlap = 74 degrees
If its 110 LSA overlap = 70 degrees
I'd like to see you in the low 60s, so the same cam ground on a 112 would be 66 degrees, on a 114 would be 62 degrees.
Spreading the centers will be dramatic, idle will mellow out, likely lose a little torque at WOT, (not necessarily at part throttle, it might even feel stronger) but also will likely be more pleasant to drive.
The issue then comes down to how mellow do you want it and what else could you do while you are in there? Depending on what the cam is now I'd make my decision on 112 or 114.
This won't only help for braking though, it'll also help mileage and make it happier in OD
2. Later intake centerline = slightly less vacuum. However, you can't really go any earlier with that lobe or compression and I don't think you want to add cam
Sticking with the ICL at 104, you are smack on what a normal hot rod should be in a 8.20 DCR range. So after the 112 or 114 gets you some vacuum you'd still be fine on pump gas, however, you could also rock the cam back a few degrees, and end up with a 106 or 108 ICL. Seems backwards, but it could gain you a little more on top and give you some room for a little more initial timing and room to play around a bit. Not to mention, it will likely allow you to get that thing to run on regular gas. FWIW, I can run my 489 EFI FE on anything at all, and only the worst fuel will rattle. I usually run mid grade or premium, but no fear if 100 miles away and I need gas, anything works.
I don't think you HAVE to run a later ICL but I would if buying a new cam for this application as it has evolved
3. Ignition timing = More gets you more vacuum
Less overlap NEEDS less ignition timing, but with an ICL change, you might be able to add a bit more and get even more vacuum
Lots of overlap wants lots of advance, less overlap needs less, however, by retarding the cam a couple degrees, you get to a DCR of about 8.06, still good, but gives you some room. Not sure what your curve is now, but I'd like to see a minimum of 20 BTDC as soon as it fires, then have it come all in by about 2700. Nice thing is you can change it every 15 minutes until you are happy, easy to do electronically. FWIW, my EFI 489 is at 12 degrees for crank and adds 20 when it fires. This is how Ford does it in the 5.0 Mustangs as well.
So, would like to hear where you are now with the timing curve, idle timing, and LSA but, my initial gut feeling is:
288/292 (same lobes), 112 or 114 LSA, on 106 (or maybe even 108 ICL), and a timing curve that pops a bunch of advance in after it fires.
You could also temporarily try adding some lash, not sure if you lashed it tighter than designed (I almost ALWAYS do) but less lash should drive up vacuum too, unfortunately it can hammer a solid roller pretty badly when too loose. Neat test though
One question though, why not hydraulic roller next time around?
FWIW
- on my EFI 489, I run 286/294 110 LSA on 105 (70 degrees overlap), but no power brakes
- on my EFI 461 I am going 280/286 112 LSA on 106 (59 degrees overlap) with power brakes
- on the carbed 461 with power brakes we are doing, I am trying to make a little more power, and likely going 288/292 114 LSA on 106 (62 degrees of overlap) or a single pattern 292/292 114 LSA on 106 (64 degrees of overlap)
Hope this helps, if you can give more info on the things I asked above, will dive in deeper after work
