Vacuum Advance :scratchchin:

[MPO]

I'm trying to identify at what ‘inHg’/'mmHg' the Standard Dizzy Vacuum advance is before it starts to add advance and what the ‘inHg’/'mmHg' is when it stops adding the advance. I’m also interested in identify the start and end advance values.

Can anyone advise?

Also, does anyone have the equivalent ‘Optimised’ Vacuum Advance info for after market solutions like the Amethyst, 123Ignition, Canems, MS, etc…?

Cheers
MPO

[DaveG]

Check this out:
http://www.pistonheads.com/gassing/topic.asp?t=153...

May help.

[blitzracing]

Basically any reading is simply a pressure below atmospheric- 15 psi- which is the equivalent to 30 inches (or mm equivalent) of mercury (this is how much of a column of mercury you can suck up with a pure vacuum) so as you drop below 1 atmosphere the inches mercury will drop accordingly. Problem is sometimes its displayed as a positive or negative value that confuses things. The metric equivalent is kpa (kiloPascals ) where 100 = 1 atmosphere, but you can get conversions on the web. There is nosuch thing as a generic optimised map, you need to set it up on the rollers for maximum torque through the RPM range. In your case Id start by taking the advance the current dizzy by removing the vacuum pipe, and then measuring the amount of advance in 500 rpm intervals with the timing strobe, as a safe starting point for the 123's centrifugal map without vacuum. Adding vacuum is then a case of adding up to about 10' in addition to the centrifugal advance at minimum manifold pressure,(above about 2500 rpm RPM, very light engine load, almost closed throttle- light cruise) and ramping it down to 0' at WOT.

My base settings for the Amethyst are here:

http://www.g33.co.uk/ignition_system.htm

Edited by blitzracing on Tuesday 3rd May 10:56

[MPO]

DaveG said:

Check this out:
http://www.pistonheads.com/gassing/topic.asp?t=153...

May help.

Hi Dave

Thanks for your reply

Funny enough, I stumbled on your link last night and it answered quite a few of my questions , I also found your formulae interesting… “Nice Work!”

You also refer to your Standard TVR Dizzy as having:-

Vacuum:
Hg Advance
5". . . .0
17". . . .20

At17 inHg you recorded 20 Degrees of Vacuum advance and other internet references suggest no more than 15 Degrees is added. 20 Degrees does seem quite high and was this the maximum you recorded?

I’m trying to determine what a standard TVR Vacuum Advance curve would be so that I can build on it to create one for my recently purchased 123Ignition Tune.

It will be starting a New 123Ignition Topic very soon showing my progress for anyone interested...

Cheers

MPO

[MPO]

Guys

This is what the Vacuum Advance is set as for an 'out of the box' 123Ignition Tune for a RV8

Hence my questions...



As you can see, it is miles off what Dave has identified with his Standard TVR Dizzy. Dave is showing 20 Degrees Advance @ 17inHg compared to the same point in 123 that shows 10 Degrees...

My current Static timing is 14-15 BTDC + approximately 20 Degrees "All In" above 3500-4000 RPM

Also, I'm not convinced my STD Dizzy Vacuum is working properly so I could not record the Vacuum Advance in 500 Steps prior


I know I could go with the 123 standard Vacuum settings (as is) but feel it would make sense to see what others are using as a reliable TVR base line and if necessary amend the static config.


What Dave has done is really useful and may be what I'm looking for but I need to be sure the Vacuum advance combined with the Static isn't too much and cause cross-over sparking with too much Advance (if you know what I mean?)...


Has anyone got any other good Vacuum examples?


Cheers

MPO

Edited by MPO on Tuesday 3rd May 15:33

[blitzracing]

Nice easy way to see if the vacuum is working is simply to pull the pipe out of the plenum, and suck on it whilst watching the timing marks and see them move. I would not go too far as its adding advance, so just check its working. You cant test vacuum against RPM, as its manifold pressure so it depends on how fast the engine is turning against how wide open the throttle is. With no load on the engine you cant generate the same level of vacuum that you get with the throttle part closed and a load on the engine to stop the engine racing (as it will in neutral). Don't get too hung up on the vacuum advance, its only there for light load to try and maximise engine efficiency when the compression is low and flame front travelling slowly. Adding more and more advance is not a good thing as the torque just drops off as the explosion peak gets nearer to TDC, whilst the ideal point is about 12' (from memory) after TDC.- Id say 20' of vacuum advance on top of centrifugal is pushing it somewhat. The 123 curve is a safe setting simply that will work perfectly well. To be honest these engines are so bad fuel wise I doubt you would even see and measurable results with an extra 10' shift in vacuum advance anyway.

Edited by blitzracing on Tuesday 3rd May 15:53

[MPO]

blitzracing said:

Nice easy way to see if the vacuum is working is simply to pull the pipe out of the plenum, and suck on it whilst watching the timing marks and see them move. I would not go too far as its adding advance, so just check its working. You cant test vacuum against RPM, as its manifold pressure so it depends on how fast the engine is turning against how wide open the throttle is. With no load on the engine you cant generate the same level of vacuum that you get with the throttle part closed and a load on the engine to stop the engine racing (as it will in neutral). Don't get too hung up on the vacuum advance, its only there for light load to try and maximise engine efficiency when the compression is low and flame front travelling slowly. Adding more and more advance is not a good thing as the torque just drops off as the explosion peak gets nearer to TDC, whilst the ideal point is about 12' (from memory) after TDC.- Id say 20' of vacuum advance on top of centrifugal is pushing it somewhat. The 123 curve is a safe setting simply that will work perfectly well. To be honest these engines are so bad fuel wise I doubt you would even see and measurable results with an extra 10' shift in vacuum advance anyway.

Edited by blitzracing on Tuesday 3rd May 15:53

Hi Mark

Fair point about the manifold pressure and not being able to test against RPM… Doh!

Looking at the STD 123 Advance Curve again, yes you have a good point “It’s safe” and given I can tweak it (and the static if necessary) on the fly maybe it’s a good starting point… But, given all the Brains and Experience on this Forum it’s good to talk/ask given these were designed for cars weighing twice as much as ours .

To be honest, it’s not ultimately the economy I’m looking for with the Vacuum Advance, it’s trying to understand how I can smooth out the engine on light town throttle and maybe work with it to manage higher performance Cam’s at lower RPM and Load when the time comes to upgrade (My tick-over is pretty lumpy at the moment and I have already extinguished the shunting with better Fuel Mapping).

Am I right in my thoughts that Vacuum Advance Management is going to help with High performance Cam Management at lower Load and RPM?



Cheers

MPO

Edited by MPO on Tuesday 3rd May 17:10

[blitzracing]

A high performance cam with breath badly at low RPM, so not much vacuum, so I dont know how much scope that part of the timing will have. Id of thought "centrifugal" would make a bigger difference.

[MPO]

I need some clarity…

If we go back to DaveG’s really good post on the standard Vacuum he refers to 5 inHg giving 0 Degrees BTDC and at 17 inHg giving 20 Degrees BTDC. I’m assuming the increase of it is linear (makes sense) but what about the decrease after 17 inHg is reached? Does the advance really drop off suddenly (like off a cliff!) or taper away (as the inHg continues to increase with throttle depression) until WOT is reached?



Anyone know?

Cheers

MPO

[blitzracing]

You have your wires crossed- as hg value drops as you open the throttle, not the other way around- so no cliffs involved- the additional advance simply peaks out and remains level, so even if the vacuum was to increase more you would not add more advance. Dont forget that a very high vacuum is closed throttle on the over run anyway, so there will no fuel to burn worth speaking off, so the timing is of little relevance as you are not producing any power.

Edited by blitzracing on Friday 20th May 12:19

[MPO]

blitzracing said:

You have your wires crossed- as hg value drops as you open the throttle, not the other way around- so no cliffs involved- the additional advance simply peaks out and remains level, so even if the vacuum was to increase more you would not add more advance. Dont forget that a very high vacuum is closed throttle on the over run anyway, so there will no fuel to burn worth speaking off, so the timing is of little relevance as you are not producing any power.

Edited by blitzracing on Friday 20th May 12:19

Hi Mark

Thanks for your reply.

Funny enough I was originally working on the principle that the inHg was at it’s highest with the throttle fractionally open and decreasing when opened further. Just like you say….

What threw a spanner in the works was my passenger was monitoring my inHg real time using the 123Tune interface. On the flat in 5th at 60 MPH I was showing 15 inHg and increasing the speed to 70 MPH in 5th and still on the flat my Hg increased to 25 inHg. This repeatedly happened… So to me; it shows an increase of inHg when the throttle is depressed. To add to the confusion, I pushed on with the throttle and we saw the inHg increase further before dropping to 0 inHg just before WOT

Now I’m scratching my head!

Thoughts?

Cheers

MPO

[MPO]

Well,

It looks like I am getting to the bottom of the incorrect Vacuum readings when accelerating, it transpires that the when the USB lead is connected to the 123 Tune it must have an adequately shielded Mini B 5 Pin USB lead. Otherwise, it picks up electrical interference and throws the readings. This doesn’t affect the distributor it’s self, just when interfacing with it for real-time tuning and data logging. By coincidence, I also changed my Plugs to the resistive type and this helped too.



Headache gone, Sanity restored! and thanks for your responses Mark

Cheers

MPO

[davep]

MPO said:

Well,

It looks like I am getting to the bottom of the incorrect Vacuum readings when accelerating, it transpires that the when the USB lead is connected to the 123 Tune it must have an adequately shielded Mini B 5 Pin USB lead. Otherwise, it picks up electrical interference and throws the readings. This doesn’t affect the distributor it’s self, just when interfacing with it for real-time tuning and data logging. By coincidence, I also changed my Plugs to the resistive type and this helped too.



Headache gone, Sanity restored! and thanks for your responses Mark

Cheers

MPO

So how are you getting on with 123 ignition set up Glynn? Are there any big gains to be had?

[MPO]

davep said:

So how are you getting on with 123 ignition set up Glynn? Are there any big gains to be had?

Hi Dave

Its been an interesting couple of weeks, with my focus on installing the 123 Tune, producing an ignition advance curve similar to my standard TVR dizzy and producing a Vacuum advance curve that reflects the original Lucas (thanks to all that shared their vacuum info for reference). I wanted to get a base that I was familiar with and therefore see what benefits I could gain.

After installing the 123 Tune with my original setup, it started first turn of the key and drove pretty much as expected but a little smoother… To install it, I spent most of my time (an hour or two) finding TDC with a DTI and removing my old dizzy cog and installing it on the 123 Tune Dizzy shaft. The rest was easy!

When using the strobe on the 123 Tune I noticed the advance timing scatter was gone at higher RPM, I assume this is what made my initial drive a little smoother.

The next step was to stabilise the tick over as it has always hunted and I put this down to the performance cam (whatever it maybe). This was quite simple really, I stopped the vacuum advance kicking in until it reached 1800 RPM (I later reduced this to 1600) (not something you can do on a Std. Lucas Vacuum), I then proceeded to increase the ignition advance (at idle only and with a small dwell) until the engine stabilised and I actually saw a very small increase in RPM too. I assumed this was the sweet spot and I was happy (not something you can do on a Std. Dizzy). The result was fantastic and I found I was able to crawl along at very low revs in 1st and 2nd gear without a hint of the classic shunt . Sadly most of my low RPM over run pops and bangs disappeared as a result of the ignition advancing I put in place. I proved they could be reproduced by decreasing the ignition advance.

Next, I spent a little time working on improving the engine cranking. Again this was quite easy, I removed all ignition advance below the idle speed and the engine fires up effortlessly

So, as you can see, I got some real benefits and that was before I started looking into the vacuum and ignition curves in more depth.

Setting the ignition advance curve further up the RPM range is probably going to need the use of a Dyno but I am experimenting at the moment, it will be interesting to see what I can achieve prior to testing it at the Griff Growl .

What has become very evident is that my simulated Std vacuum Lucas advance setup doesn’t suit my driving style and/or my TVR. It is pretty scary to see so much advance at ¾ throttle and accelerating. Consequently, my 123 Vacuum Advance Curve has changed multiple times and I now feel confident and safer with it. IMO, I really don’t think the Std. Lucas Vacuum advance was ever right for a TVR (mine was the same as used on a Range Rover Classic) and if I was to use a standard dizzy again I would certainly consider an after market vacuum solution or maybe disable it completely… Obviously having WOT would eliminate the vacuum advance but I don’t drive with my foot buried to the floor, I’m progressive with the throttle and I’m sure I’m not on my own!

I have also setup the vacuum curve so that it works better between gear changes.

This is just a quick overview at the moment and there plenty more detail I could talk about and I’m studying the data logs at the moment (collated every 1/10 of a second) and merging it with 14CUX and AFR data.

So, at this time I’m reasonably happy with the purchase and results. However, there are still a few niggles with interference when logging and the dizzy cap clips need bending to secure the dizzy cap better????.

I’m not advocating that this is the best solution available but it is easy to install and I’m loving the smoothness and sharpness of the engine. Weirdly I’m missing the roughness when I had the clockwork dizzy in… Regardless, I’m having fun and look forward to some Dyno time to see if there are any BHP improvements


Cheers

MPO

[blitzracing]

A bit people forget is getting the RV8 emission compliant was never going to be easy, so the stock timing is not ideal, as its emission based, not performance.

[davep]

blitzracing said:

A bit people forget is getting the RV8 emission compliant was never going to be easy, so the stock timing is not ideal, as its emission based, not performance.

Blitz, I shouldn't think MPO will be too worried about emissions compliance since his is a 4.3 pre-cat, open loop only, so more performance focused - not that TVR would have taken the trouble to tune the ignition/distributor for that, probably.

[blitzracing]

My point indeed.

[MPO]

This was particularly interesting when I researched the Lucas Vacuum:-


Rub a little chalk on the nose of your Vacuum and see what numbers show up…


My Pre CAT vacuum from a dizzy with a 1992 date stamp:-



Vacuum:
Hg Advance
5". . . .0
17". . . .16 (8*2)


and a spare Vacuum from a interim range rover dizzy - 1996:-



Vacuum:
Hg Advance
5". . . .0
17". . . .16 (8*2)


Both the same, so it looks like it made no difference having a Pre CAT or not…



Everyone I asked had the same (5 17 8)...


Anyone got anything different and want to post it along with your dizzy and year details?



Cheers

MPO