RV8 - Distributor Vacuum Advance

[MendipWursel]

Chaps

I have a 3.5RV with Edelbrock 1404 Performer Series Carburettor (Webber 500) and Offenhauser 360 manifold.

The distributor needs to be fully advance at approx 35 deg before TDC on decent revs.

With the vacuum attached I get approx 50 deg advance BTC running at STEADY revs (ie 3000rpm or in excess).

This should not be !!!! I should be at 35 deg under these curcumstances.

Does the Offenhauser/Edelbrock setup create too much vacuum giving me too much advance?

Do I need to disconnect the vacuum advance?

I'm not worried about mpg as this is a kit car.

[DEVONMAN]

Chaps

I have a 3.5RV with Edelbrock 1404 Performer Series Carburettor (Webber 500) and Offenhauser 360 manifold.

The distributor needs to be fully advance at approx 35 deg before TDC on decent revs.

With the vacuum attached I get approx 50 deg advance BTC running at STEADY revs (ie 3000rpm or in excess).

This should not be !!!! I should be at 35 deg under these curcumstances.

Does the Offenhauser/Edelbrock setup create too much vacuum giving me too much advance?

Do I need to disconnect the vacuum advance?

I'm not worried about mpg as this is a kit car.

If you are not worried about mpg then you can remove the vacuum advance.
Realistically when on the road at say 3000 rpm, the extra load of the car will effectively reduce the vacuum advance (different butterfly position)and total advance will be less than 50 degs.
Unless the engine is pinging under light load I would not be too worried but, yes the weber 500 does give high vacuum advance. I suspect it's not ideally matched to the Rover vacuum capsule.

Regards Denis

[MendipWursel]

Chaps

I have a 3.5RV with Edelbrock 1404 Performer Series Carburettor (Webber 500) and Offenhauser 360 manifold.

The distributor needs to be fully advance at approx 35 deg before TDC on decent revs.

With the vacuum attached I get approx 50 deg advance BTC running at STEADY revs (ie 3000rpm or in excess).

This should not be !!!! I should be at 35 deg under these curcumstances.

Does the Offenhauser/Edelbrock setup create too much vacuum giving me too much advance?

Do I need to disconnect the vacuum advance?

I'm not worried about mpg as this is a kit car.

If you are not worried about mpg then you can remove the vacuum advance.
Realistically when on the road at say 3000 rpm, the extra load of the car will effectively reduce the vacuum advance (different butterfly position)and total advance will be less than 50 degs.
Unless the engine is pinging under light load I would not be too worried but, yes the weber 500 does give high vacuum advance. I suspect it's not ideally matched to the Rover vacuum capsule.

Regards Denis

This is my worry. The standard carbs are the twin SU's. Do the SU's create a different vacuum to the Edelbrook 4 barrel carbs. I think it does hence the need to maybe disconnect the vacuum advance otherwise you will have a massive advance under real load and possible damage to the engine.

[katanaman]

Where are you taking the vacuum from? there are two points on the 500

[bigaldart]

When you drive on the road you do not have any real vacuum advance when under load. The vac advance comes in on the overrun or a trailing throttle. At this point there is no real load on the engine. Any time there is load the throttles are open hence no vacuum.

Alan

[sidecar]

I've done a shed load of work on several ignition systems when using this carb. Basically the carb does not suit the Rover vac system well at all.

You would get better performance by disconnecting the vac system, running 14 degrees static 36 all in at around 2750-3000 RPM.

You can buy springs from Real Steel to change the curve, from memory one silver spring and on copper spring is about right. If you can not get the total advance required the dizzy can be modified.

[MendipWursel]

I've done a shed load of work on several ignition systems when using this carb. Basically the carb does not suit the Rover vac system well at all.

You would get better performance by disconnecting the vac system, running 14 degrees static 36 all in at around 2750-3000 RPM.

You can buy springs from Real Steel to change the curve, from memory one silver spring and on copper spring is about right. If you can not get the total advance required the dizzy can be modified.

I think sidecars response has answers my question. As I thought the Edelbrock creates too much vacuum for a Lucas distributor. Even under no load conditions I am getting 50 deg of advance. Without the vacuum I am down to the optimum max of 36deg and about 15deg at tickover. H&H Ignitions have set up the distributor to be fully advanced at about 2750 to 3000 rpm.

Thanks Chaps

[sidecar]

I've done a shed load of work on several ignition systems when using this carb. Basically the carb does not suit the Rover vac system well at all.

You would get better performance by disconnecting the vac system, running 14 degrees static 36 all in at around 2750-3000 RPM.

You can buy springs from Real Steel to change the curve, from memory one silver spring and on copper spring is about right. If you can not get the total advance required the dizzy can be modified.

I think sidecars response has answers my question. As I thought the Edelbrock creates too much vacuum for a Lucas distributor. Even under no load conditions I am getting 50 deg of advance. Without the vacuum I am down to the optimum max of 36deg and about 15deg at tickover. H&H Ignitions have set up the distributor to be fully advanced at about 2750 to 3000 rpm.

Thanks Chaps

50 degrees total advance when the throttle is just cracked open is probably OK as the VE and therefore effective CR would be really low.

The problem is that the vac system does not shut down quick enough with the Eddy carb so the total advance remains too high for too long. The result on my engine was that the engine would kick back as the throttle was opened. I actually think that the vac system did not work too well even with the standard SU's. Rover got round this by setting the static advance really low, like 6 degrees.

There could be ways to get round this by using some sort of bleed system that bleeds off some of the vacuum via a tee piece and a jet which acts as a restrictor.....I could not be bothered!

[DEVONMAN]

Sidecar wrote:- There could be ways to get round this by using some sort of bleed system that bleeds off some of the vacuum via a tee piece and a jet which acts as a restrictor.....I could not be bothered![/quote]

Or you could engineer an adjustment screw into the end of the capsule to limit the total movement.

Cheers Denis

[MendipWursel]

I've done a shed load of work on several ignition systems when using this carb. Basically the carb does not suit the Rover vac system well at all.

You would get better performance by disconnecting the vac system, running 14 degrees static 36 all in at around 2750-3000 RPM.

You can buy springs from Real Steel to change the curve, from memory one silver spring and on copper spring is about right. If you can not get the total advance required the dizzy can be modified.

I think sidecars response has answers my question. As I thought the Edelbrock creates too much vacuum for a Lucas distributor. Even under no load conditions I am getting 50 deg of advance. Without the vacuum I am down to the optimum max of 36deg and about 15deg at tickover. H&H Ignitions have set up the distributor to be fully advanced at about 2750 to 3000 rpm.

Thanks Chaps

50 degrees total advance when the throttle is just cracked open is probably OK as the VE and therefore effective CR would be really low.

The problem is that the vac system does not shut down quick enough with the Eddy carb so the total advance remains too high for too long. The result on my engine was that the engine would kick back as the throttle was opened. I actually think that the vac system did not work too well even with the standard SU's. Rover got round this by setting the static advance really low, like 6 degrees.

There could be ways to get round this by using some sort of bleed system that bleeds off some of the vacuum via a tee piece and a jet which acts as a restrictor.....I could not be bothered!

Sidecar.... My engine does kick back as I open the throttle, just like you mentioned. This kick back problem I have been trying to sort for ages. I seems to me it is probably better to disconect the vacuum advance, regardless of set-up.
Thanks

[DEVONMAN]

Hi Kevin,
The kick back can be due to 2 factors relating to the vacuum advance.

The first as already stated is due to too much advance as the throttle is opened.
The second can be due to the fact that the vacuum capsule moves the baseplate in the dizzy as the vacuum increases. This effectively moves the point where the rotor arm fires in relation to the plug lead post inside the dizzy. Too much advance and the rotor may no longer be opposite the post.

If you examine your rotor the main scorch mark should be near the trailing edge. As vacuum timing increases the spark then happens closer to the leading edge. Too much vacuum advance can put the fire point in front of the leading edge and cause a weak spark at the plugs or a misfire or even a spark jump to the adjacent post inside the cap.

The centrifugal advance does not affect the rotor firing position but vacuum advance and any electronic timing control which has been added to the system will change the rotor firing position.

Some cheap rotors are not made well and the location peg is not always as OE. Also the width of the brass bit on rotor is only just adequate for the standard set up.
Hope this helps with the problem.

Regards Denis

[sidecar]

Hi Kevin,
The kick back can be due to 2 factors relating to the vacuum advance.

The first as already stated is due to too much advance as the throttle is opened.
The second can be due to the fact that the vacuum capsule moves the baseplate in the dizzy as the vacuum increases. This effectively moves the point where the rotor arm fires in relation to the plug lead post inside the dizzy. Too much advance and the rotor may no longer be opposite the post.

If you examine your rotor the main scorch mark should be near the trailing edge. As vacuum timing increases the spark then happens closer to the leading edge. Too much vacuum advance can put the fire point in front of the leading edge and cause a weak spark at the plugs or a misfire or even a spark jump to the adjacent post inside the cap.

The centrifugal advance does not affect the rotor firing position but vacuum advance and any electronic timing control which has been added to the system will change the rotor firing position.

Some cheap rotors are not made well and the location peg is not always as OE. Also the width of the brass bit on rotor is only just adequate for the standard set up.
Hope this helps with the problem.

Regards Denis

Denis,

All of the things that you have described I have come across!

It can be quite tricky to get the HT post and rotor arm relationship right across the whole rev range but unless this sort of thing is checked then all sorts of cross fire issues can happen. I ended up grinding out a location peg on a rotor and making a new one in the end.

I'm actually ditching my whole setup now even though it is working well.

I'm in the process of fitting a locked out dizzy and a programmable MDS 6530 unit. I've spent all morning spinning the dizzy with my pillar drill whilst moving the base plate mounting holes so that the rotor is EXACTLY where I want it when the MSD fires. (I've hooked up the whole system out of the car with a spark plug in my vice, my strobe gun freezes the rotor arm so that I can see what going on). The rotor has a now has a bolt at its tip acting as a pointer. (Old duff rotor).

As an example this is what my new system will be...

Static timing set to 32 degrees via the dizzy, the MSD will then 'add' 20 degrees of retard making 12 degrees advance at tickover. The rotor centre is 10 degrees before the centre of the HT post when the pickup 'tells' the MSD to fire. The leading edge of the rotor and the edge of the HT post are actually in line as the the pickup sends its signal. The MSD then 'counts' 20 degrees of retard, or 10 degrees or rotor retard. This places the rotor centre and the HT post centre in line when the actual HT pulse is fired. However the MSD then fires another 5 pulses over the next 20 crank degrees. (10 at the dizzy). This puts the trailing edge of the rotor and the edge of the HT post in line as the last spark fires).

At high RPM the MSD 'adds' no retard so the single high RPM spark will fire when the rotor is 10 degrees before the post. (centre to centre) The edges should actually line up....Working that lot out was quite a brain strain!




Kevin... My Cob was a pain when it kicked back, it ruined pulling out of corners hard and fast. You will lose BHP if you just ditch the vac system unless you fit the correct springs to get you advance in by 3k at the latest.


I wrote a shed load of stuff on dizzies, near the end I explain how to check the HT post / rotor relationship. I'm sure I've bunged the link up before but just in case....

http://how-to-build-a-pilgrim-sumo.wiki ... by-members

Cheers,

Pete

[DEVONMAN]

Kevin and Pete,
We seem to have drifted off the original problem that Kevin has and probably in the wrong section also. But, let's continue.

Pete, It's good to know that others mess about with their set up's as much as I do. I have in the past experimented on the bench with the dizzy set up and eventually got things as good as you can get them. I have 2 twin turboed RV8 road cars to play with, so getting the timing for both cruising and full power was important. (Even more so now with the price of fuel)

The Factor I initially didn't consider was the multi sparks period of 10 dizzy degrees produced by the MSD units. This really didn't show up in practice as the rotor was still just within range of the post at the final spark.
The biggest factor to dial in was the boost retard which brought the spark point very much to the trailing edge of the rotor. But with a bit of playing I got things right. The engines are 5.0 and 4.6 so the max all in advance is slightly less than in your case and this gives a better chance of optimising the rotor position.

On your new set up, are you retaining the vacuum capsule or does the MSD unit allow you to dial in some cruise advance? I know it programmable for boost retard but can that be used for advance also or do you just set up the run curve to some effect.

As all of the problems we have had on the turboed engines seem to be related to the dizzy I am now planning to move to coilpacks in combination with an EDIS8 and a programmable MSD DIS-4 unit.

The trigger wheel and VR sensor is already on my 5.0 lump so it won't be long before I will be playing again. The plan is to set the VR pick up 2 notches forward of the normal position thus giving 30 degrees advance and programme the MSD curve to reduce this at cranking and low revs.

Hope your project goes well.

Regards Denis

[sidecar]

Kevin and Pete,
We seem to have drifted off the original problem that Kevin has and probably in the wrong section also. But, let's continue.

Pete, It's good to know that others mess about with their set up's as much as I do. I have in the past experimented on the bench with the dizzy set up and eventually got things as good as you can get them. I have 2 twin turboed RV8 road cars to play with, so getting the timing for both cruising and full power was important. (Even more so now with the price of fuel)

The Factor I initially didn't consider was the multi sparks period of 10 dizzy degrees produced by the MSD units. This really didn't show up in practice as the rotor was still just within range of the post at the final spark.
The biggest factor to dial in was the boost retard which brought the spark point very much to the trailing edge of the rotor. But with a bit of playing I got things right. The engines are 5.0 and 4.6 so the max all in advance is slightly less than in your case and this gives a better chance of optimising the rotor position.

On your new set up, are you retaining the vacuum capsule or does the MSD unit allow you to dial in some cruise advance? I know it programmable for boost retard but can that be used for advance also or do you just set up the run curve to some effect.

As all of the problems we have had on the turboed engines seem to be related to the dizzy I am now planning to move to coilpacks in combination with an EDIS8 and a programmable MSD DIS-4 unit.

The trigger wheel and VR sensor is already on my 5.0 lump so it won't be long before I will be playing again. The plan is to set the VR pick up 2 notches forward of the normal position thus giving 30 degrees advance and programme the MSD curve to reduce this at cranking and low revs.

Hope your project goes well.

Regards Denis

Hi Denis,

My engine is a stage III 4.6, I do run a bit more advance than the 28 that some people recommend, it seems to run a bit better around 32.

I did not run a vac system with my old setup and I don't run one with my new MSD. It does have some boost retard function but as my engine is not blown I'm not using it. I do run the NOS retard function, at the moment it knocks off 3 degrees when the NOS kicks in.

I've just got the whole thing running today, I did test it all using my pillar drill running at 150 RPM (300 engine RPM). All seemed well but when I fitted the whole lot into the car it would not spark!! I ended up counting how many times the dizzy goes run in 15 seconds, it turns out that the dizzy only turns at 80 RPM during cranking. This was too slow for the MSD pickup when used with the Rover reluctance wheel (20" air gap). The only thing I could try was reducing the air gap to 10".....it worked!!!

I've done some other stuff with the timing curve to. From 0-300 RPM the timing is only 7 degrees BTDC which gives the starter an easy time. From 300 to 700 timing goes up to 14 degrees. From 700 to 900 the timing goes back to 12 degrees, from 900 onwards the timing advances as normal. The reason for all of this up down malarkey is that when my fans cut in the engine revs drop due to the load on the alternator, now when the revs drop below 700 the timing actually advances which picks up the revs to compensate. I could run the timing at 14 static rather than 12 and as the revs drop back this up to 16 degrees. I'll mess about with it over the next few weeks.

Anyway I'm quite pleased with the programmable MSD unit, I've order some Magnecore leads from RPI as I'm just using the stock jobbies that moment.

Making a new base plate and mounting the MSD pickup was a real pain, I reckon that it took me 20 hours to sort out!


All good fun! (but a bit cold!)

Pete

[DEVONMAN]

Pete.
Yes indeed, all good fun.
Glad to hear it all worked and that the msd pickup worked with the Rover reluctor. Sometime I intend to try the Rover pickup/reluctor on one of the projects and get rid of the ign amp.
Regards Denis