[dlloyd]

Ok I got it. You guys are pulling my leg.
Big laugh at my expense. Hahaha
You guys really had me going.

[Jack Gifford]

Nobody's laughing.

[Tom Vaught]

I gave you factual info.
No pulling your leg on any of my posts.
Just solid advice.

Tom V.

[Greg Reid]

No jokes. Try it again...maybe even try it on one of the direct manifold fittings like at one of the 'F' fitting ports. Don't forget to plug the leg that you pull off if you do it there.

[dlloyd]

I apologize for my pulling my leg comment.
You guys have always steered me correctly on everything I have asked. It’s just that I was so sure of what I thought I saw on the run stand and I kept getting message after message that contradicted it I thought maybe you guys were having fun. I should have known better and I apologize for it.
Mike Wasson restored my tripower I am sure he know what he’s doing and Butler did my motor and I am sure they know what they are doing. It’s me that doesn’t know what I am doing!
I will put a vacuum gage on F fitting or power brake and get myself straight on what is happening. I will let you guys know what I find. I am sure you already know what the results will be.
Thanks to all for setting me straight.

[Murf]

Originally Posted by dlloyd

When I had the motor on the run stand and had my vacuum gage on the power brake hose, I had 12” of vacuum. So when I reved up the motor it should have went to zero?
I am totally confused again.
I saw it go over 20” while reving.
I hear what you guys are saying and I am not doubting what you say. Because I know what I saw on my run stand it is making me confused.
I guess I should disconnect the power brake line and put a vacuum gage in it and see what I get now.

If by "when I reved it up" you mean going to WOT, your vacuum should drop.

If you increased the RPM but were at a constant throttle position, like during high speed highway cruising your vacuum could be higher than idle, I think.

12 inches at idle is low tho.

Sent from my Z936L using Tapatalk

[dlloyd]

Is 12” at idle low for a performance cam?
I was told that it was going to be in that range because of the cam.
I was not going WOT on run stand. That would have hit the rev limit in distributor. I had rev limit set at 5,500.
Maybe 4,000 to 5,000 RPM is where I thought I saw 20”.

[Tom Vaught]

Originally Posted by dlloyd

Is 12” at idle low for a performance cam?
I was told that it was going to be in that range because of the cam.
I was not going WOT on run stand. That would have hit the rev limit in distributor. I had rev limit set at 5,500.
Maybe 4,000 to 5,000 RPM is where I thought I saw 20”.

MAKING A LOT MORE SENSE NOW.

1( You are on a engine run stand
2) Very easy to have 12" of vacuum at idle with a decent camshaft.
3) You do not have any load on the engine when you rev it up on the engine stand. No Torque Converter drag/Trans drag.
4) With no load on the engine you basically can be at 4000 rpm and have 20" of vacuum because you are now above the Cams reversion point (due to overlap) so the engine shows 20" vacuum on the gage.

The info is valid but only for a reference point if you put the engine back on the same test stand with a different camshaft profile for example.

The run-In stand basically puts a light firing load on the pistons/rings and helps get the initial timing close and the carb idle close, before engine installation.

Tom V.

[Greg Reid]

That's a new one on me Tom...not doubting you, I just never knew that. I guess what's happening is that, unloaded, you can get the higher RPM with the throttle plates being relatively barely cracked open?

[Tom Vaught]

Originally Posted by Greg Reid

That's a new one on me Tom...not doubting you, I just never knew that. I guess what's happening is that, unloaded, you can get the higher RPM with the throttle plates being relatively barely cracked open?

You are correct. High load transportation vehicles might be in a lower gear (think Bus or Delivery Truck) and be under a high load condition (wide open carb barrels) in every gear.

A 1200 pound street rod Kit Car might rarely be into the throttle unless he was playing around.
Basically little load to get to a given speed.

Tom V.

[Greg Reid]

Good info, thanks Tom.

[Tom Vaught]

One comment, most high load engine Holley Carburetors had High Number Power Valves installed. 8.5 or 10.5 on the valve.
This was because the engine needed more fuel quicker under the high load condition so the 10.5 PV wouold open sooner as the vacuum dropped from cruise vacuum on its way to 0 vacuum, (actually very low vacuum) on a gage.

Tom V.