[ta man]

Originally Posted by Cliff R

Just goes to show how compression is your friend with these things.

We did one 455 here with the OF cam and 10 to 1 compression and it was only making about 9-10" vacuum at idle speed. The same exact build with 10.5 to 1 compression will make 11-12" and my engine at 11 to 1 compression makes 13.5".

The same logic applies to the larger RP cam, as it's basically the OF on steroids.

You abilities as a tuner play a role here as well. I probably have just a little bit more experience in that area than most folks, so can easily get these engines to idle smooth in and out of gear and make decent vacuum WITHOUT running the initial timing clear off the scale.....FWIW......Cliff

I notice the difference going from 10.2 to 10.75 in the new build. With a slightly more aggressive OF 2.0 cam vs the original OF it actually has a better idle. I'm also using my backup distributor so I only have 8 degrees initial currently..my old setup I was running 14 degrees initial. I use a basically box stock 950hp Holley with untouched metering blocks. I find the four corner mixture screws are very sensitive and turn them very slightly when fine tuning. I also tune for strongest cleanest in gear idle.

[AG]

Originally Posted by ta man

I notice the difference going from 10.2 to 10.75 in the new build. With a slightly more aggressive OF 2.0 cam vs the original OF it actually has a better idle. I'm also using my backup distributor so I only have 8 degrees initial currently..my old setup I was running 14 degrees initial. I use a basically box stock 950hp Holley with untouched metering blocks. I find the four corner mixture screws are very sensitive and turn them very slightly when fine tuning. I also tune for strongest cleanest in gear idle.

What is your in gear idle vacuum and RPM?

[ta man]

I will check again this week..I have 2 gauges both read different..one shows 13/12 the other 11/10 if I remember correctly.That is at 850rpm/900rpm.

[72blackbird]

My issue is fuel- living in Kommiefornia, I'm currently limited to 91 octane and Dave advised me to set the SCR to 10.25-10.3:1. I have his 325 cfm CNC e-heads. Eventually I will run a stick, but the get the engine in my car sooner, I'll use a TH-400 and a converter over 2500. So vacuum will obviously be more important.
The rest of the engine is a .020 455, T-II, and a 950 Holley.

Geno

[steve25]

The size of your combustion chambers and anything that then effects the motors compression like deck clearance will have a effect on the vacuum a given motor can pull!

[Larry Navarro]

So there are two versions of the OF cam? What's the difference between them?
I'm running the early OF cam (hybrid) with 1.65 solid rollers. Running alum. KRE cnc 290cfm heads, Mighty Demon 850 with Jeff's(Shaker455) version of "idle-ease", 474, 10.25:1 CR.....WITH 4 speed. I was checking tuning just the other day and I cannot remember what the initial timing was set at.....ARRRG, but I can remember that at 900rpm idle the vac. gauge was "searching" between 14-15hg. I thought that was pretty darn good.

[AG]

Originally Posted by steve25

The size of your combustion chambers and anything that then effects the motors compression like deck clearance will have a effect on the vacuum a given motor can pull!

Combustion chamber is the Edlround port CNC heart shape at 68.5cc and the deck height is zero to 0.002".

[ta man]

Originally Posted by Larry Navarro

So there are two versions of the OF cam? What's the difference between them?
I'm running the early OF cam (hybrid) with 1.65 solid rollers. Running alum. KRE cnc 290cfm heads, Mighty Demon 850 with Jeff's(Shaker455) version of "idle-ease", 474, 10.25:1 CR.....WITH 4 speed. I was checking tuning just the other day and I cannot remember what the initial timing was set at.....ARRRG, but I can remember that at 900rpm idle the vac. gauge was "searching" between 14-15hg. I thought that was pretty darn good.

Advertised duration is about the same the OF 2.0 has more valve lift.

[65CatCoupe]

I buy that higher compression yields more vacuum but I don't see why. Most of the vacuum signal is produced by the piston moving down the bore plus the inertia of the air stream after BDC when the piston is inches from the deck so why would a couple of CCs in the chamber make a difference?

[steve25]

Because if you care to look at things for the sake of simplicity as in the decending piston must first suck down whatever size the chamber is before it sucks on the Intake runner.