Ported vs Manifold vacuum reading

[Robert T.]

Working on a stock 66 427/425 with a 4140/3247 carb. Initially had the vacuum advance line connected to the "normal" ported source on the metering block. After getting an education here, I changed it over to a full time manifold source. I noticed at the time that even though it was a ported source, I had a 8-10" reading at idle (compared to 14-15 manifold). I haven't rechecked those readings since readjusting everything after the switch to full time vacuum. Just about to.

Question is, should the ported vacuum reading be 0" at idle. I would assume that there should be some ported value at idle. If I find that it still reads a "significant" amount, is that indicative of having the throttle plate open too much at idle. Is there a typical delta between manifold and ported readings?

Thanks

bob

[Edward J.]

Re: Ported vs Manifold vacuum reading

Bob, ported vaccum works only on accelaration, there maybe a possiabilty that you may have the primary butterfly partly open because of the idle speed screw maybe actually tapping into that port which is generally just above the butterfly, once the butterfly starts to open, the engine vaccum starts to pull a vaccum at that port.if you put a vaccum gage on that port and lower the idle you'll see the vaccum drop more.

hope this makes sense to you. Ed

[Duke W.]

Re: Ported vs Manifold vacuum reading

Theoretically a ported source should read zero vacuum at idle, but if you've made the switch to full time, so what.

If you haven't already done so with the new full time vacuum advance configuration, go through the idle speed/mixture adjustment and target an idle speed of 850-900, which should yield 14-15" Hg manifold vacuum.

Then when you check total idle timing it should be rock steady, and the idle should be very stable with just a slight lope.

Duke

[Robert T.]

Re: Ported vs Manifold vacuum reading

Thanks for the reply guys. I was away this weekend and didn't get to play with the car. Next step is to replace the vacuum can. Car came with the stock one. 201? I think. I think I may have been compinsating for the "wrong" can by turning up the idle instead of the timing. Right now I have

10 init (@ 750 rpm)
28 mech ( starting at about 1000 and all in at 3500)
but only about 8-10 vacuum advance at idle.

I was afraid of going up any higher on the initial for fear of going over 38 total.

Once I get the right can I should get a little more advance at idle.

Thanks for taking the time to reply.

bob

[Duke W.]

Re: Ported vs Manifold vacuum reading

No, the 201 was not used on L-72, but it was used on L-71.

What is the ID number of the currently installed VAC?

Why do you fear going over 38 total WOT advance?

Duke

[Robert T.]

Re: Ported vs Manifold vacuum reading

Hello Duke,

Thanks for the clarification on the 201 can. I was going by my poor memory. I'll check it to be sure and post again. That may have been part of my problem if it is a 210. I'm changing in anyway to the recommended B2? (forget my memory, back to the archives)

I thought the 38, maybe 40, total timing was the target to shoot for. I know some of the postings recommend advancing till it knocks and then back off "a little". I just thought this was a safer way. With my hearing and side pipes I wasn't sure that I could actually hear the knocking. I guess if it was bad enough I would.

This all kinda started when I felt that my idle screws weren't having much affect when adjusted. Then I remembered the ported reading and thought that I may be compensating for a poor idle by have the idle speed set too high and maybe exposing too much of the transfer slot.

I need to change that can and readjust everything again.

Thanks for taking an interest, and for the reply. Any advice you can offer is always appreciated.

bob

[Duke W.]

Re: Ported vs Manifold vacuum reading

I don't recommend advancing the timing to the detonation limit without the use of a timing light, which is an "old school" "timing by ear" recommendation.

Based on actual test data I have 327s typically produce best power at about 38 total WOT advance. I have one dyno test on avgas where a 10.4:1 CR "327 LT-1" made best power at about 45, but that would be way too much for pump premium, so my general recommendation for 327s is 38 if the engine will tolerate it without detoantion.

My understanding is that the big block closed chamber heads like a little more - say up to 40-42. I don't have my own test data, but I wouldn't hesitate to run a BB this high if it will tolerate it without detonation.

You want a B22 or B26 VAC, which is 16 @ 12". The OE L-72 VAC is 12 @ 12", so the extra four degrees in the B22/26 will add four to total idle timing and get it up to the mid 20s with 8-12 initial.

When converting a L-71 to full time advance the OE ...201 must be changed to a B22/B26 because the ...201 doesn't meet the Two-Inch Rule.

Converting a L-72 doesn't require changing the VAC to a B20/26 because the OE VAC meets the Two-Inch Rule, but I think the extra four degrees in the B22/26 is advantageous.

The Chevrolet intial timing spec and centrifugal curves were generally conservative because they had to set them up for worst case situations.

Even with today's slightly lower octane premium many OE engine configurations will tolerate both a more aggressive centrifugal and more total WOT timing.

Every engine and operating environment is different, so if an owner wants to optimize the spark advance map, it takes some experimentation. Always has, always will.

The L-72 tuneup specs published in Corvette News list max centrifugal of 30 @ 5000, with a "nominal" initial timng of 8 and a range of 8-14, which results in a maximum total WOT advance of 44 degrees with 14 initial.

Of course, it's implicit that 44 is okay as long as there is no significant detonation.

Duke

[Robert T.]

Re: Ported vs Manifold vacuum reading

As always, a tremendous amount of great info!! Thank you. And thanks for giving me the correct can numbers.

I'll get that changed out and do the readjusting. I'm anxious to see what happens.

Thanks again! This is great info.

Bob

[Robert T.]

Re: Ported vs Manifold vacuum reading

Duke,

FYI, it does have a 201 can now. So... sounds like this is the first thing to get rid of.

Thanks,

bob

[Joe C.]

Re: Ported vs Manifold vacuum reading

Here's a decent explanation of the dynamics of spark advance:

http://www.team-integra.net/sections/articles/showArticle.asp?ArticleID=235

[Robert T.]

Re: Ported vs Manifold vacuum reading

Hey, thanks Joe. I hadn't seen that one. Trying to learn everything I can about this stuff.

Thanks for taking the time to respond.

Bob

[Joe C.]

Re: Ported vs Manifold vacuum reading

Hey, thanks Joe. I hadn't seen that one. Trying to learn everything I can about this stuff.

Thanks for taking the time to respond.

Bob

After studying that article, answer the following question:

Will an engine whose spark advance is optimized (ie: cylinder pressure peaks at the optimal point ATDC to deliver maximum down force, at the longest duration, to the piston crown), and running 93 PON fuel, produce more torque if 104 PON avgas is used, and the spark is advanced an additional 8 degrees, or, just prior to the detonation limit?

[Duke W.]

Re: Ported vs Manifold vacuum reading

Duke,

FYI, it does have a 201 can now. So... sounds like this is the first thing to get rid of.

Thanks,

bob

Keep that 201 in a safe place. It was OE on several engines, but was discontinued from service parts in the late sixties.

Being as how it is neither original or of proper spec for your engine with full time vacuum advance, I doubt if it's of any value to you, but there are probably plenty of guys who would like to take it off you hands.

As the matter of fact, you should check the number on the distributor band if it is still there. The 201 VAC along with the centrifugal curve you reported tells me it's a '67 L-71 distributor.

Duke