Thread: When is a motor supposed to have the highest vacumn

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I have a 351-w that has its highest vacumn reading with the motor idling about 1200 RPMs,ever since I put it in my 55 sunliner the trans slids in third gear at light throttle and the motor needs to be revved up for the vacumn wipers to work and a slightly rough idle. When I had the motor built I requested a rv type cam since it was going to have a tall gear and a highway cruiser,I was supposed to get a cam card with the motor but did not and it has a 302 firing order so who knows what it is. I am leaning towards the cam or the cam timing being retarded as the problem and would like to get the issues repaired before heading to Daytona in november but its been like this for 6000 miles and could wait untill I get back.

I am in the process of installing a vacuum guage in my 53 F100. With an HO 302 and AOD tranny I am getting about 12 mpg at highway speeds. I do have a stock F100 rear with 3.92 gears, but I also have 30" tall rear tires. Anyway, as I understand, the vacuum reading at highway speed will be mid range [15 hg] and up. I am taking this info from an article by Jim Linder of Bubba's Hot Rod Shop. I believe your are familiar with him from the HAMB. Joe

I have herd it some times helps to add a second vaccum can.
I am having problems with a 72 Vette with Vaccum door for wippers the wippers and head lights. I am going electric with this now though after pouring money in to this thing and it's still not working right. Kurt

I am going to install a vacumn can for the trip and worry about the motor later,the 55 that its in is going to get a 292 Y block built for it next year and I will be putting the 351 in my 65 custom 500. When the 351 comes out and on the stand I will change the cam,intake and heads and hopfully wake it up a little. I would like to fix it before the trip but dont want to tear open the motor so soon before leaving so its best left as it is.

Jeff, it should have it's highest vacuum at idle, throttle closed. When you get into the pedal, it should drop, and then regain some as the engine equalizes. Methinks you need to put a gauge on it and see just what is going on; the symptoms don't sound right to me.

I had the gauge on it when checking it out and at idle it was at 17 and slowly revving the motor up some the gauge went up to 24 and then started to drop the higher it went,for the trip I installed a vacumn can and installed another set of valve covers so I could get the PCV to seal up tighter and will see how it does once I get it back on the ground. I think the problem is related to the cam some how.

A bigger cam can drop vacuum to the point where stuff like power brakes don't work right, etc. Summit makes this electric pump that might help you out:

http://www.summitracing.com/parts/SUM-760150/

Don

Instead of a rv grind I think the builder stuck in a radical 302 cam which I cant feel since its in a larger motor but is causing me the vacumn issues.

those early cars with vacume wipers used a special fuel pump with dual diaphrams --one side pumped fuel--one side worked the wipers

You actually have two issues here--one the engine, the other one is the wipers

It did originally have a dual diaphram fuel pump which it will get again when the Y block goes back in,years ago I had a 53 olds that would beat the wipers to death at idle but this motor does not have enough vacumn to do that at idle.

your wipers need some attention