View unanswered posts | View active topics It is currently Wed Jun 28, 2017 11:56 am



Reply to topic  [ 3 posts ] 
 Relation between a vacuum advance can and a power valve 
Author Message

Joined: Fri Feb 08, 2013 5:52 pm
Posts: 78
Is this right that when the can is active, the PV has to be closed, and vice versa? Consequently, PV opening vacuum rating must be about the same as the can's release rating?

Does this make any sense?

Thanks!


Mon Apr 27, 2015 6:29 am
Profile

Joined: Wed May 29, 2013 6:12 pm
Posts: 33
webermaniac wrote:
Is this right that when the can is active, the PV has to be closed, and vice versa? Consequently, PV opening vacuum rating must be about the same as the can's release rating?

Does this make any sense?

Thanks!


Perhaps this will help.

Tuner wrote,

"There is no reason to run an engine at part-load any richer than as lean as it will run without misfire. Of course for each engine just what that AFR may be depends on the particular engine’s characteristics, cam, compression ratio, headers, mufflers, etc. The biggest factors are uniform AFR distribution to the individual cylinders and exhaust reversion through the valve overlap. At part-throttle, distribution is affected by throttle angle and other carb geometry as well as the manifold, manifold heat, fuel distillation curve, etc, etc.

Between about 10% to 20% and 70% to 80% load some engines will run well at part-throttle with 17/1 AFR or leaner and others start to turn bitchy at 15/1.

You need to realize that the leaner the AFR, the larger percentage of oxygen in the exhaust. When running at part-throttle the high intake vacuum is drawing hot exhaust back into the intake manifold. Leaner than stoichiometric the excess oxygen in the exhaust is returning to the cylinder in the reversion gasses and the hot oxygen improves combustion.

Obviously, the leaner it is, the larger the proportion of unburned oxygen in the exhaust will be, and up to a point (unique to each engine) the lean running usually noticeably improves the part-throttle combustion.

Depending on your particular engine’s nature, you will be surprised how much part-throttle torque improves as it is operated progressively leaner on the lean side of stoichiometric. The limit is usually reached when the leanest cylinder misfires.

Because lean part-load mixtures have a slower combustion rate, they require additional spark advance, compared to rich maximum power WOT mixtures.

Correct vacuum advance tuning is as important as carb tuning. If the timing isn’t correct, you can chase the carb tuning into a box where a rich cruising AFR is wasting gas, because, without vacuum advance, a faster-burning richer AFR compensates for retarded timing.

You have to use common sense applied to your particular engine to determine at what amount of load the engine should be switched from the lean economy AFR to the rich power AFR. The power valve opening point and the vacuum advance starting point (fully retarded vacuum amount) should usually be near the same intake vacuum.

A high compression engine that is close to stressing the available octane, such as a 10/1 mild-cam street engine on pump gas, like a muscle car engine in the late ‘60s, may need to be rich and retarded at 9 or 10 inches Hg.

A low compression engine may not need to richen up until much closer to WOT and may tolerate the vacuum advance not fully retarding until as low as 5 or 6 inches Hg.

Because lean mixtures burn slower they require the correct amount of additional timing, compared to rich WOT mixtures. The correct vacuum advance is a key to lean part-throttle tuning."


Mon Apr 27, 2015 3:08 pm
Profile

Joined: Fri Feb 08, 2013 5:52 pm
Posts: 78
Quote:
The power valve opening point and the vacuum advance starting point (fully retarded vacuum amount) should usually be near the same intake vacuum.

Thank you, that's what I was looking to confirm.


Mon Apr 27, 2015 5:10 pm
Profile
Display posts from previous:  Sort by  
Reply to topic   [ 3 posts ] 

Who is online

Users browsing this forum: No registered users and 1 guest


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
cron
Powered by phpBB® Forum Software © phpBB Group
Designed by ST Software for PTF.