15 replies to this topic

[postve]

Can someone help me to understand the overall benefits of running vacuum advance on my SC Mini - and if it’s fuel economy - this is the least of my concerns.

Why do I ask - well I had applied based on my dizzy/vacuum advance and attached to a “T” plastic junction that connects it and the boost gauge line. All I got was a wobbly RPM reading - never was steady and went up and down .5 all the time and car ran with hesitation from time to time especially at low RPM - so decided to remove Vacuum advance and viola, steady idle needle and car runs great? What am I missing here?


Timing set at - 26/4000 so bang on


Thx

Edited by postve, 08 July 2023 - 01:31 AM.

[Spider]

Some Vacuum Advance Units will actually apply Retarded Timing when they are pressurized.  This is seldom desirable. I tried point that out to the guy who makes these kits are reading a string of similar complains, but he seemed to know better

[timmy850]

First up it depends which side of the throttle the vacuum advance is connected to. If it’s pre-throttle you normally get vacuum only when driving and it obviously just adds timing for economy and also for part throttle performance. There should be absolutely no downside to this on any engine if it’s setup correctly.

If it’s post throttle and pre supercharger you’ll have vacuum all the time which will add timing at idle, coasting and part throttle. This can sometimes add too much timing at idle

If you’re using the post supercharger vacuum signal it’d probably give some crazy readings

If you’re mixing the pre and post supercharger signals it’d give a pretty choppy signal (which is what it sounds like you’re reading)

Just for reference my modern Mazda has an OBD port and I can check the timing as I’m driving. At a set rpm like 3000 the timing will vary somewhere between 15 and 55 degrees, which gives you great part throttle performance. If you have it fixed to one advance reading for the rpm (no load correction like a vac advance or map sensor on a Megajolt) then you are missing out on part throttle performance

[postve]

I suspect something is up Spider but not knowledgeable enough to know other than the physical feel I get whilst driving the car.

Tim, can you define “part/throttle” a bit more as I think you may be on to something as the car goes but I have been complaining that N/A minis I find at part-Throttle they seem quicker - off the line? In other words I need to get those revs up before she really gets going but, with a SC I should have a quicker response (in addition torque comes on with my CAM from 1500-6000) u might have solved a major concern I have had.

If you could elaborate more on part throttle performance or is my above explanation what you are referring to?

Edited by postve, 08 July 2023 - 08:21 AM.

[DeadSquare]

The vacuum advance and the boost gauge must be fed from different places.

 

I have had a Shorrock on lots of A series engines, mounted between SU and engine and I have always had the vacuum advance from the SU and the boost from between the blower and the inlet valves.

[postve]

Tim - Yep it's for the suck through vmax Weber kit. The boost gauge is tapped at the bottom of the charger (after the compression).

Dead, I have super charger and configured as above

[postve]

What I read on TMF and elsewhere with the VMAX SC with Vac/boost hooked up as I have done is that you’ll get advance at idle and low throttle, but once you get past 0psi it won't retard any further?

[postve]

Some Vacuum Advance Units will actually apply Retarded Timing when they are pressurized. This is seldom desirable. I tried point that out to the guy who makes these kits are reading a string of similar complains, but he seemed to know better

I reviewed a post you commented on back in 2019 and exactly my experience - runs better without the vacuum advance - strange and I have to chaulk it up to maybe have the r wrong type of dizzy I assume??

[sonscar]

I think that to gain maximum benefits from boosted applications it becomes almost essential to have MAP referenced ignition and have it properly tuned.Just my own thoughts,Steve..

[postve]

I only wish we had a proper tuning shop where I am in Toronto - no such luck and truly on your own

[timmy850]

The vacuum advance canisters were all made to a particular spec:
The vacuum reading the advance starts
The vacuum reading the advance tops out
The total advance degrees applies

Basically the more advance it adds the better the performance gets (up to a point). But as load increases the engine can tolerate less timing

Eg 3000rpm cruising it might want 40 degrees, but full throttle at 3000rpm with 5psi of boost it might only need 20 degrees. That’s where the vac advance helps out

You need to confirm:
Where are you getting your vac advance signal
Where are you getting your boost signal
As they shouldn’t be the same for a vmax style setup

[postve]

Humm, well vacuum displays @17 which is where I expect it to be at idle. the usual when pushing the pedal as goes to zero and under heavy load dies out after hitting 4psi. Note this I am investigating as suspect a leak somewhere as was hitting 9-10 before ?

As to boost and vacuum signal same location as mentioned I split the connection with a “T” - as per manufacturer instruction.

Edited by postve, 08 July 2023 - 01:05 PM.

[postve]

Tim, I checked the install documentation and it does say to connect to the brass fitting for either vacuum or brake servo - that being said I was advised to split the boost line

[NZMini1275]

For a std dizzy the vacuum advance connects to the top brass connector and the boost gauge connects to the connector on the rear
I run a 123 tune dizzy and that connects to the boost fitting as it has a boost retard function
Without vacuum advance your off boost performance would be lacking. Remember that off boost you have a very low compression engine

[postve]

Thank you so much for the information NZ and will connect up today - what connection did you use to connect skinny boost line to much bigger brass connector on top of SC? / can you send a picture. Hooking it up properly will check if I still have the wobbly RPM needle

Edited by postve, 13 July 2023 - 01:03 PM.