Vacuum Source

[bdcvg]

Any body ever try connecting vac line to manifold source vs ported source and adjust timing as needed?

 

[WHT]

Hi bdcvg,

First, I have not tried to run our BJ8 25D distributor’s vacuum advance using a direct manifold source, yet.

However, I am looking at trying direct vacuum also and purchased two vacuum gauges with 1/8 connections. When they arrive, I will connect one gauge to the ported nipple and the other to the manifold nipple to better understand the difference between both sources on OUR engine (direct information about our engine, not just general information from technical papers). We have only had our car for 6 months and want to learn as much about the engine as possible.

Watching both gauges and documenting the differences in vacuum at various rpm should help predict the differences in timing that will occur before making the actual change. I intend to graph both gauges and make both vacuum advance plots based on the advance information printed on the distributor vacuum canister. You could do the same with one gauge, but watching both at the same time is a more interesting (fun) experiment.

There should not be a major difference in vacuum except at idle. Several technical papers stated many engines will run slightly cooler with direct vacuum, but the idle might not be as stable. I believe a stable idle is one reason Austin-Healey uses a ported vacuum advance.

Hopefully, this will initiate a flow of more useful and educated responses.

Regards, Bill

 

[bdcvg]

This will be interesting info. In My other car experimentation yielded best results with manifold vac. over ported.

 

[Bob_Spidell]

This will be interesting info. In My other car experimentation yielded best results with manifold vac. over ported.

Agreed. But, using manifold vacuum at idle makes no sense to to me; that is the point of max vacuum and slowest engine speed (i.e. least advance needed). My understanding of vacuum advance is that it's most desirable at part-throttle cruising, when additional advance can be used without risk of detonation--for fuel economy--above and below that speed centrifugal advance sets spark timing. Lots of discussion on the interwebs; looks like I've found (another) time-waster.

 

[bdcvg]

In My other car the most improvement was start-up and cold driveability. As soon as the throttle plates move ported and manifold vac are the same.