Vacuum with a XR280HR cam?

[mungus]

Hi,

Anyone running a COMP CAMS XR280HR camshaft? How much idle vacuum are you getting? And at what idle?

By comparison for the sake of vacuum. Mine is a 68'Mustang 390GT. Bored +60 (401 CID), CJ valves, 10.5 SCR, SCAT H rods, FPA headers, 730CFM Holley (CJ), Petronix etc. Just wondering what is typical for this HYD roller cam in a 390/428.

Thanks in advance.

[blykins]

Not a fan of their off-the-shelf hydraulic rollers, but with only 60° of overlap, you should have over 15 inches of vacuum.

[mungus]

Yes I remember you saying some time ago you weren't a fan. IN 282/ EX 288 & 110 LSA = 65 degrees of overlap. Not a huge amount I suppose.

I have seen a fresh mildly worked 390 (+30) with a XE262H (a lot less overlap) that only gives 14"Hg at 850-900 RPM. So will mine really do 15”? But of course who knows if everything is sorted on that motor. Maybe it will give more vacuum when its run in etc. Anyway I'm hoping you're right, as 15" should be enough for my factory Midlands brake booster. I asked as some web folks are saying they were barely getting 10" using this cam. But who knows what the other specs were (or leaks etc.). COMP Cams own dyno sheet for the XR282HR (same specs as the 280HR) on a Chev 350 with Victor Intake etc, only showed 9-11", and that is not enough, although a high single plane intake possibly hurts vacuum a bit.

In the end I'll eventually run it on my test stand and the truth will be revealed!

Thanks for the reply Brent.

[My427stang]

If that cam is installed on an early enough centerline, it may be OK on a 390 with power brakes, but I think you are right on the edge. 

You could advance it a little more if you had clearance, and it would help, and more initial timing often helps crutch it too, but if your compression is a measured 10.5:1, you don't have much, if any, room to play.

I would expect closer to 11, a stroker would pull harder on the port, but I don't think it'll be quite there with a 401

[blykins]

I just went off of the 280.  I should have looked closer because it's not in traditional naming convention.  It's actually 282/288 advertised, 110 LSA.  That's 65 degrees of overlap.  I'm with Ross, I think you'll be marginal there.  I try to not go over 64-65 degrees overlap when I do cams for power brake applications. 

[mungus]

No problem Brent, it all helped the discussion.
When I’ve got it running properly on my stand I’ll reply with what I got, then we will all know. 

Whilst I don’t want the work and cost, if necessary I’ll hide a canister somewhere or I could even abandon this set up altogether, and use some unmolested date correct GT heads (Deeper chambers) and a new, still boxed up XR270HR cam (53 OL) I have on the shelf. As I wasn’t aiming for a lopey idle with this build, and only used this cam to fix DCR issues due to a PO shaving the heads too much The other standard heads came from a late friends estate after I had already built this motor...

Anyway we’ll see how it goes, I’m trapped in a hotel on corona virus lockdown for a while but I will put the vacuum results on the forum sometime soon.