Does anyone have access to Engine Analyzer Pro or other software that will allow me to estimate what my engine vacuum will be at idle? I have all the cam details and I believe all other pertinent factors to plug into the program.

 

It's all about the cam and timing specifications. Why not just tell us what cam your running?

 

Can You quantify with a high degree of accuracy what the vacuum is just based on cam specs?
Looking to learn here...

 

Compression and intake valve size can make a difference also.

JIM

 

Chances are one or maybe several of us are already running that cam. Or one very similar. So people can answer with experience. Not some computer formula.

 

I called Edelbrock (they acquired Comp Cams in 2020) and talked to Bruce Huggins who provided me with a link to a program they use for this purpose.

Engine Idle Vacuum Estimator

Here's a screenshot of the tool (not my cam specs)

 

Now that is COOL.

I double checked that against my engine dyno results and they were almost spot-on!

 

Mine is pretty far off. The model says I will have 15.1" of vacuum and a fair idle, but reality is I have over 18" of vacuum and a smooth idle.

Thank you Mike Jones.

EDIT: I went back in and tried it by taking out some of the .050" duration because I have a solid lifter cam and it did work right that way. The advertised .050" lifter rise will work fine for a hydraulic lifter cam and this software but the advertised .050" duration for a solid lifter cam needs to be adjusted down about ~10 degrees due to the running lash. Once I figured that in, it's pretty darn accurate.

 

The estimator was on the money, when compared to my 1970 L46 while at 800 rpm and normal operating temperature.

 

Play with the rpm, and it shows you how dramatically the vacuum picks up from idle, say ~13" at idle, to 20" at 2000 rpm cruise speed.

And it pretty correctly predicted an L88 idle vacuum at around 8.5", as well as my LT-1 with 2 different cams, 10.7" for the 30-30 cam, or 13.5" for the stock cam.

For solid camshafts, you do need to account for the valve lash. SWDuke previously measured several cams both ways, and if you take the lash in thousandths, say 24, cut it in half, and subtract that number of degrees from the .050 duration, you get actual net duration. Then you can even compare a solid cam to a hydraulic cam. I have done this for many years. The 242* duration LT-1 cam, @ .050" turns into 231* after you account for the 24 thou lash. That makes it 7*larger than a 224* L46 Hyd cam, and that is the way it runs. This one he actually measured. That spec applies to the BBC L71/L72 solid cam as well. Same cam lobe. And 230* has been recommended forever as the max for a HiPo street engine with decent street manners. Having driven them that way, and 10* larger, I would agree. 12-13" idle vacuum seems to be the "edge" as well, and about where 230* cams wind up.

Anyway Useful Vacuum tool!

Play with all the parameters, and you can see how several other items affect the idle vac level as well.

 

Wow. Mine was spot on as well. Cool. Thanks for link.

 

I live at 6,000 feet and was wondering why my idle vacuum on our L71 was low 'compared to normal' (I knew it would be on the 'low' side due to the cam, but...). I had one of those dooh moments about our altitude. Our idle vacuum is actually spot on for our altitude. This is another thing to take into account depending on where you live. The 'estimator' listed above by 69L88 above takes that into account - cool! Best, Paul

 

WOW!

An inch vac per 1000 feet.
And Denver is at 5000 ft, down 5 inches!

 

Yep, surprised me too. Our new home sits at 6,000 feet and I felt much better when I realized this after doing some research. Makes a difference! Best, Paul

 

Just an observation:

Same engine - 383,10.7 CR, idle at 750, alt 400 ft, LSA 110, ICL 106, ADV 280 deg seat duration. Compare a hydraulic cam vs a solid cam of the similar size.

With a Comp 280H - ADV 280, 230 @.050", .480" lift, - 14.3" Vac, Fair Idle

With a 280 solid lifter cam - ADV 280, 228 @.050" (need to remove ~10 deg for solid lifter lash ~ use 218 deg), .498" lift with 1.6 Rockers - 17.0" Vac, Smooth idle

Pretty big difference in idle quality and vacuum for the solid lifter cam vs the hydraulic lifter cams of similar size. Faster ramps also help reduce critical .050" overlap in the solid lifter example. Same LSA and cam advance.

The solid lifter vacuum numbers are very close to what I actually measured.

Model shows you loose 14 ft-lbs torque and 11 HP using the hydraulic cam vs sold lifter cam AND it idles worse too.