Big Cam (to me) and Low Vacuum - Is this Correct?

[furious70]

Timing and idle screws need to be adjusted at the same time at hot idle. Tweak one then the other then go back to the first, searching for the highest idle - after triple checking for leaks.

 

[70RoadRrunner]

Furious70,

Whereabouts do you live in Chicagoland?

 

[furious70]

Hanover Park

 

[70RoadRrunner]

Nice.

Naperville here.

 

[66Satellite47]

The timing comment 66Satellite47 made was about installed cam timing, not ignition timing. That’s the number of crankshaft degrees past TDC that the intake reaches max lift. If that’s 4 degrees late your vacuum won’t be as good as you might otherwise expect.

Nate, thanks for the clarification, that's what I meant, the installed centerline makes a difference. Advancing the cam should also help getting the idle a bit better at a little lower RPM.
EDIT: BTW a lower power valve would be OK. On my big cam motors(much bigger than OP's) I used 4.5 PV's all the time.

 

[Ranger16]

When I bought my Road Runner (383), it had a big cam in it (don't recall the numbers). It idled around 2000 RPM. When I got it home I found out why when I lowered it to 1000. The car would fall flat on it's face when I let out the clutch unless I had the R's up to about 2000. It was not fun to drive and hard to start. I was lucky if I had 3" of vacuum. I replaced the cam with a much milder one and now have about 18". Based strictly on my experience (and limited cam knowledge), I'd say, yes a bigger cam with greater duration and overlap will mean lower vacuum.

 

[70RoadRrunner]

Ok guys.

I had some time to work on this issue a little today.

First, I capped the vacuum hose that leads to the PCV valve.

Vacuum actually dropped - when opening the hose to atmosphere the idle went up.

I then reset the two idle mixture screws and have a very solid 1000 RPM idle (I might need to increase as the RPM drops to around 700-800 RPM in gear and is lopier than I prefer.

I then rerouted the PVC line to my air filter.

My son and I did some shake down testing and filmed the vacuum gauge through the gears three times.

The car is running great and fulls strong - I still think there are improvements to be made with timing and fuel.

I will link in a moment.

 

[70RoadRrunner]

Test #1

 

[70RoadRrunner]

Test #2

Pay no attention to my dorky commentary - I am trying to get my 13 year old son interested in the hobby.

 

[70RoadRrunner]

Initial timing is around 16-18 BTDC - car starts right up without much trouble.

While I am still going to play with this a little I am looking for carb power valve suggestions.

Current idle vacuum is 5-6 inhg.

It currently has the 6.5 (stock) unit in it.

Rule of thumb is 1/2 idle vacuum but it was suggested I see how my vacuum changes while driving.

Thank you.

 

[artr8]

What is your 60 mph cruise vacuum? Use this as a baseline to make your power valve choice. 1/2 of the vacuum reading at cruise speed. As you drop the pedal from cruise, the power circuit opens and adds more fuel to help the transition. 1st change for drive-ability.

 

[70RoadRrunner]

What is your 60 mph cruise vacuum? Use this as a baseline to make your power valve choice. 1/2 of the vacuum reading at cruise speed. As you drop the pedal from cruise, the power circuit opens and adds more fuel to help the transition. 1st change for drive-ability.

This is great info.

I will try to check the 60 mph cruise tomorrow.

Thank you very much.

 

[furious70]

Good advice from all, if you want local eyes on it sometime for suggestions PM me, always cool to meet people local.

 

[70RoadRrunner]

Good advice from all, if you want local eyes on it sometime for suggestions PM me, always cool to meet people local.

Thank you for the offer.

 

[Wietse]

I have around 6" vacuum as well at idle and am using a 9.5 power valve.
When cruising i get much higher vacuum readings, so when you floor it the vacuum drops, so the higher the PV number the sooner it kicks in.
With getting some vacuum readings as you did, you can pinpoint the required PV much easier.
Just avoid the recommendation of using a PV with 1/2 your idle vacuum reading, with a cam like that you can ignore many "basics".

Also, to avoid your throttle being open too far at idle, taking too much transfer slot you could use the PCV valve to add more idle air without using the throttle.
I just installed the adjustable Wagner PCV valve and could not be happier, these are fully adjustable and are a real great add-on for engines with camshafts like you and me have.

 

[66Satellite47]

Wietse, you make a good point, the power valve should consider the actual vacuum during the operating range, not just idle. I'd use one below the idle vacuum. But off idle cruise may have a higher vacuum level. Guess I never considered the PV opening point part of the "transition" when flooring it, always considered that the accelerator pump job & tuned that accordingly. But ....
Your point of not using up the transfer slot(too much idle adjustment screw) to get idle speed is a real good point also. PCV???

 

[70RoadRrunner]

Thanks for all the suggestions.

Here are my recent findings:

Vacuum Speed
12 inhg 40 MPH
13 inhg 50 MPH
15 inhg 60 MPH

To be honest, I very seldom drive at 60 MPH or quicker.

Suggestions on power valve sizing (6.5 currently)?

Sounds like the current valve might be in the neighborhood.

 

[artr8]

Agree that your current PV may be good. Next, go to the primary jets. Your A/F meter is a great tool to get the final tune in. Based on your video, I would drop 2 sizes from your current.
I am assuming that you have a Holley carb. Somewhere around 750 cfm. Likely not a double pumper.

 

[70RoadRrunner]

Holley 770 cfm, vacuum secondary.

Not a double pumper.

I am ordering jets now.

 

[Wietse]

Suggestions on power valve sizing (6.5 currently)?

You can see on the AFR gauge what is going on, when you stomp the throttle and it jumps very rich for the first part, it means it opens too early and you can go lower on the PV number.
If it jumps too lean for a certain time it opens too late and you can increase the number.

Your point of not using up the transfer slot(too much idle adjustment screw) to get idle speed is a real good point also. PCV???

I played around for some time with a orifice in the PCV hose to the carb, using different size holes to see what worked best.
In the end i decided to go with a Wagner PCV, this allows me to adjust the air feed on the fly with just a little allen key.