CVH Vacuum Advance on Twin DCOE Vs. Omex 3D

[cerealsurfer]

I'm still working on my Morgan CVH for next season's racing and it's occurred to me that the old Carb and ignition had a Vacuum advance capability which could now cause a problem.

Having moved to Twin DCOE's the vacuum facility is not available... yet the ignition (stock Ford) still expects it. I've looked at various bits of info which indicate to block off the vacuum pipe then move the advance curve to the best compromise position, potentially altering the bob weights to create a new curve (which again would be compromised).

I'm worried that the options above could lead to piston damage? as I now run High Compression pistons.

Alternatively, is there a cheap option to sort this out

OR

Do I need to move to something like Weber Alpha or more likely Omex 200 ECU + DCOE trottle Pot Sensor & Coil pack and move to full 3D ignition map?

Any advice/help... info on this is pretty limited but I've already looked at various web sources and tuning books.

[GavinPearson]

cerealsurfer said:

I'm still working on my Morgan CVH for next season's racing and it's occurred to me that the old Carb and ignition had a Vacuum advance capability which could now cause a problem.

Having moved to Twin DCOE's the vacuum facility is not available... yet the ignition (stock Ford) still expects it. I've looked at various bits of info which indicate to block off the vacuum pipe then move the advance curve to the best compromise position, potentially altering the bob weights to create a new curve (which again would be compromised).

I'm worried that the options above could lead to piston damage? as I now run High Compression pistons.

Alternatively, is there a cheap option to sort this out

OR

Do I need to move to something like Weber Alpha or more likely Omex 200 ECU + DCOE trottle Pot Sensor & Coil pack and move to full 3D ignition map?

Any advice/help... info on this is pretty limited but I've already looked at various web sources and tuning books.

Not having vacuum is not going to damage the engine - it is fail safe.

As you are racing much of your time will be on full throttle so not having vacuum advance won't make a difference to your straight line performance.

You can just obtain a brass fitting from a hardware store and drill and tap the inlet manifold to get a vacuum tapping. I would then have the engine on a dyno. Don't worry about absolute power numbers, just get the engine to the point where it doesn't detonate on part throttle. Then evaluate the torque curves of full throttle on the distributor setting you have just obtained, then with increasing levels of advance. Then put the distributor setting back to the level where detonation is avoided.

If the difference justifies the cost, then put a mapped system in, if it doesn't then don't.

If you need to persist with using a CVH your options are very limited, if you can use another engine then put as big a Zetec in as you can because they cost about the same from a scrapyard as your ignition system will cost and generate a lot more power with potential to develop even more for very little effort.

[Pumaracing]

I wonder if you understand exactly what the vacuum advance actually does. It increases ignition advance at part throttle and reduces it again at full throttle so that only the centrifugal advance is then operating. It therefore makes no difference at all to full throttle performance as Gavin had said. Clearly the engine would not have such good part throttle performance, or economy, without it but all that really means to a race car is maybe having to use a bit more throttle to balance the car in corners than otherwise would be the case. There's always going to be enough actual power available to do this so in practice it's irrelevant.

BTW, I find it imposssible to believe that "info on this is pretty limited" given it's one of the most commonly asked and answered engine questions. There are countless web sites showing how distributers and/or mappable ignition systems work.

Edited by Pumaracing on Monday 15th March 08:23

[cerealsurfer]

Pumaracing said:

I wonder if you understand exactly what the vacuum advance actually does. It increases ignition advance at part throttle and reduces it again at full throttle so that only the centrifugal advance is then operating. It therefore makes no difference at all to full throttle performance as Gavin had said. Clearly the engine would not have such good part throttle performance, or economy, without it but all that really means to a race car is maybe having to use a bit more throttle to balance the car in corners than otherwise would be the case. There's always going to be enough actual power available to do this so in practice it's irrelevant.

BTW, I find it imposssible to believe that "info on this is pretty limited" given it's one of the most commonly asked and answered engine questions. There are countless web sites showing how distributers and/or mappable ignition systems work.

Edited by Pumaracing on Monday 15th March 08:23

True, I have found/read many general articles... just not that many available regards the CVH in particular with problems and benefits discussed. Plenty about x-flow or Zetec.

I did understand the point about the vacuum system and what it actually does. I do BTW.. want to be able to drive it on the road as well so having part throttle accuracy would be very handy.

I have also read the Omex documentation inc. manual as well as the Weber Alpha documents, so understand how they work.

It seems to be, I can get a compromise out of fixing the advance curve to a set position using the system I have, again at a mid rev point that avoids detonation but possibly giving up accuracy at the top end, alternatively fix the top end and live with the issues.. OR... put in something like an Omex 200 and get it right at all positions whilst getting the most out of the engine I've now built.


As for the Engine it's self.. yes I do have to stick with the CVH if I want to stay in the class I'm in... "3" Modified up to 1701cc Carb. a 2.0 Zetec would take me to class 7, which is unlimited mods up to 2000cc.... that would be even more expensive as a whole job as there are other areas of the car that would need sorting. I do intend to get there though so trying to invest in kit that I could use on a different engine later such as a Zetec or Duratec.

[stevieturbo]

He's maybe entering a mpg race, where he's tip toeing around the track to achieve the best fuel economy :P

[Pumaracing]

If you also want road tractability and economy then it's very easy to add vacuum advance to what you already have. Again as Gavin has said you need to add a vacuum takeoff to the inlet manifold but I suggest from long experience that a single tapping will generate very strongly pulsating signals which will confuse the hell out of the vacuum canister. You need to drill and tap more than one runner, ideally all four, and connect them together with small diameter tubing which then goes to the vacuum can. You'll then get continuous and steady vacuum signals regardless of which cylinder is firing.

I've actually run DCOEs on a road CVH with vacuum advance and done properly you can get very good economy and tractability but basically a CVH is no different from any other 4 pot so I see no need to only look for CVH specific advice.

I'd also suggest that track jetting will be a mile away from what you really need for economy for road jetting. However this can be as easy as two sets of chokes and jets you can swap over after a race meet. You'll want to go maybe 2mm or 3mm smaller on chokes and with correspondingly leaner jetting, especially idle jets. This will transform the car from 20mpg tops in road driving to over 30 mpg but you'll need a couple of rolling road sesssions to establish the right jetting for each use.

Putting mappable ignition on won't solve the jetting problem so I see little point unless you really want to invest lots of money in future plans.

[stevieturbo]

Best advice is go full EFI/Ignition and dump the carbs lol

I'd iamgine it could be done on a pretty sensible budget selling the old carbs.

Or does the race class dictate carbs ?

[fatjon]

Take a vacuum tapping of each of the 4 runners between the throttle plates and the head. Join all four together in a little pot of about 50cc capcity then take one pipe from this pot to the vacuum capsule on the dizzy. Using one runner only will cause problems as a previous poster mentioned. With a vacuum from a single pot you will probably hear the capsule clicking to and fro each time that cylinder sucks and the capsule may well fail quite quickly. The pot damps the pulse to give a smoother vacuum signal. Racers often run without vacuum advance as they are wide open most of the time but road manners are improved if you leave it in place and you may well find that it bangs and lobs flames out of the back on the over run without sufficient advance.

[cerealsurfer]

Two good quality papers I found this evening:

http://65corvette.nonethewiser.net/technical/vacuu...

http://www.lbfun.com/warehouse/tech_info/timing%20...

Neither the Haynes CVH Engine nor the Pat Braden Weber Carb book were much help...

It seems to me that I would need to calibrate the tap'd holes in the manifold to match the in.Hg specification of the vacume system on the Disty, which is not going to be easy I think.

Also are the CVH vacuum advance running of a venturi or manifold method... as I would also need to match their characteristics in DIY fix.... One paper also talks about the problems of using a vacuum Distributor without the advance capability working, which is what I'm concerned about. Not gas mileage.

[Mr2Mike]

It's an ancient clockwork ignition system which can only ever give a compromised advance curve anyway; plugging the vacuum into the DCOE manifold via a small vacuum chamber to damp out pulses will be fine.

Popular mythology says that leaving a vacuum advance disconnected can cause spark scatter due to the timing moving around, and welding or otherwise fixing the base plate inside the distributor is essential. Said mythology doesn't attempt to explain why this wouldn't also be a problem with a vacuum hose connected however, so I don't believe it.

[stevieturbo]

cerealsurfer said:

Two good quality papers I found this evening:

http://65corvette.nonethewiser.net/technical/vacuu...

http://www.lbfun.com/warehouse/tech_info/timing%20...

Neither the Haynes CVH Engine nor the Pat Braden Weber Carb book were much help...

It seems to me that I would need to calibrate the tap'd holes in the manifold to match the in.Hg specification of the vacume system on the Disty, which is not going to be easy I think.

Also are the CVH vacuum advance running of a venturi or manifold method... as I would also need to match their characteristics in DIY fix.... One paper also talks about the problems of using a vacuum Distributor without the advance capability working, which is what I'm concerned about. Not gas mileage.

None of what you just wrote makes any sense at all. SO either the papers are FOS, or you grossly misunderstood.

[cerealsurfer]

stevieturbo said:

cerealsurfer said:

Two good quality papers I found this evening:

http://65corvette.nonethewiser.net/technical/vacuu...

http://www.lbfun.com/warehouse/tech_info/timing%20...

Neither the Haynes CVH Engine nor the Pat Braden Weber Carb book were much help...

It seems to me that I would need to calibrate the tap'd holes in the manifold to match the in.Hg specification of the vacume system on the Disty, which is not going to be easy I think.

Also are the CVH vacuum advance running of a venturi or manifold method... as I would also need to match their characteristics in DIY fix.... One paper also talks about the problems of using a vacuum Distributor without the advance capability working, which is what I'm concerned about. Not gas mileage.

None of what you just wrote makes any sense at all. SO either the papers are FOS, or you grossly misunderstood.

I would hope as a person of reasonable intellect that I have in fact been grossly miss-understood. Which is probably more of a reflection of my ability to get my thoughts accross. Which bit does not seem to make sense for you?

BTW.. I am prepared to conceed that I'm still learning (hence the initial post) and want to do a good job and get it done right. Whilst there have been some posts that have helped I still have questions, such as:

1) Assuming I were to keep the existing system without vacuum, I would experience the following:
a)Sluggish performance at low revs (ie. on pickup, High Engine load at low revs)
b)Potentially high engine running temps.
c)A reduced top end power
d)For the reasons above, impact on the longevity of the pistons.

2) If I went for the tapping methond of attaching the vacuum to help with the issues above
a)What size holes to tap into the manifold
b)Does the size of the holes matter? - evidence seems to suggest they do... ie. need to ensure accurate and full operation of the vacuum cylinder on the distributor.
c)If this method is chosen.. whilst cheap, how much of a compromise is this actually? (probably only answerable after the fact.. ie. direct comparison)

3) Full ECU ignition
a)The most accurate
b)The most expensive
c)How much benefit do you actually get? Cooler running engine, longevity, responsiveness etc.

Edited by cerealsurfer on Thursday 18th March 20:13

[stevieturbo]

cerealsurfer said:

1) Assuming I were to keep the existing system without vacuum, I would experience the following:
a)Sluggish performance at low revs (ie. on pickup, High Engine load at low revs)
b)Potentially high engine running temps.
c)A reduced top end power
d)For the reasons above, impact on the longevity of the pistons.

No, No, No and No.

Have you readn anything posted in this thread ?

cerealsurfer said:

2) If I went for the tapping methond of attaching the vacuum to help with the issues above
a)What size holes to tap into the manifold
b)Does the size of the holes matter? - evidence seems to suggest they do... ie. need to ensure accurate and full operation of the vacuum cylinder on the distributor.
c)If this method is chosen.. whilst cheap, how much of a compromise is this actually? (probably only answerable after the fact.. ie. direct comparison)

Small holes.
What evidence ?
Compromise ?? Again, you've been told you dont need the vac advance. its for economy. So what are you compromising on your track car ?

cerealsurfer said:

3) Full ECU ignition
a)The most accurate
b)The most expensive
c)How much benefit do you actually get? Cooler running engine, longevity, responsiveness etc.

They are all accurate enough to do the job. How accurate do you need ?
Why would you want the most expensive, when something relatively cheap would do the same job ?
Why would it offer a cooler running engine ? That makes no sense.
Longeivity...again ? what have you been reading ?

Responsiveness, potentially some gains there due to more optimal timing when mapped properly over a dizzy based system.

Edited by stevieturbo on Thursday 18th March 22:14

Edited by stevieturbo on Thursday 18th March 22:15

[Pumaracing]

Talk about over-thinking a very minor problem eh.

[dellow]

Andy,

Forget about the vacuum advance, chuck the pipe in a box in your garage and don't worry about it, trust me you won't even know it's not there. Vacuum-less dizzys are common place on race cars, my 4/4 has an Aldon dizzy with no vacuum and it's fine on the road, the only issue on the road is the cam and the light flywheel mean you need to rev it a bit more when pulling away.

Jim

P.s. if "pumaracing" is this chap http://www.pumaracing.co.uk then I'd take his adivce as he knows more about engine tuning than I ever will. If it's not the same guy the site is well worth a read, lots of good well founded info.

P.p.s whats your first event this year ?

Edited by dellow on Thursday 25th March 00:06

[Pumaracing]

stevieturbo said:

cerealsurfer said:

1) Assuming I were to keep the existing system without vacuum, I would experience the following:
a)Sluggish performance at low revs (ie. on pickup, High Engine load at low revs)
b)Potentially high engine running temps.
c)A reduced top end power
d)For the reasons above, impact on the longevity of the pistons.

No, No, No and No.

Have you read anything posted in this thread ?

Apparently not which mildly irritates me after trying to help in so much detail. Horses and water though I suppose.

[Shippers555]

Aldon automotive in the West Midlands have a nice mappable ignition for a reasonable price,
Worth looking at

I’d probably just stick with the above solution and tap each inlet trac with a 1/8 nipple and join each together leading to a small vac chamber. Vac chamber feeds the vacuum advance on the dizzy.

[Mignon]

What possible benefit do you think anyone can get from you replying to a post 7 years old? Bugger off.

[Little Pete]

I thought there’s a thread I’ve not seen before. I read it all too!!
I’ll look at the posting dates next time.

[Shippers555]

Yeah it may be old but it’s still relevant to a new reader . Mignon obviously knows everything though !