vac advance or not for FE Cobra

[patrickt]

Originally Posted by fordracing65

What are the pros on not using vac advance??

I went with the 8594 Pro-Billet MSD all mechanical because I didn't care about gas mileage, and not having a vacuum advance unit on it is just one less thing to screw with when you're trying to get your curve just perfect.

[blykins]

And that's pretty much it.

In a Cobra, any benefits would be nominal. If you had an existing distributor that was curved well, I doubt you would see any difference in driving or efficiency.

A vacuum advance requires a vacuum source, which some engines won't have due to intake manifold or carburetor combinations. It also adds another tuning variable to the mix, which may be out of the capability of some users.

[cycleguy55]

Originally Posted by fordracing65

What are the pros on not using vac advance??

It's simpler - one less thing to worry about.

OTOH, distributors with vacuum advance also have mechanical advance.

Mechanical advance is controlled by weights on springs, spinning on the dist. shaft. They advance timing as RPMs increase. Mechanical advance is necessary if you want to rev over 2000 RPM, so it's always there even if you don't think you have it. ALL distributors have it - except those used only as signals to an ignition box with timing curves.

Vacuum advance is there purely for efficiency. It advances the timing based on engine load. High vacuum = low engine load, so the engine can run more timing with no problems and deliver better fuel economy. The round vacuum can on the side of the distributor does this. You can disconnect the vacuum line (and plug it) to do without the vacuum advance if you want.

[fordracing65]

Very informative. This is a great site.

[patrickt]

Originally Posted by blykins

It also adds another tuning variable to the mix, which may be out of the capability of some users.

Like me. And even with the simplest Holley known to man (4160), the simplest cam known to man (SFT), and the simplest distributor that only has two springs and a cam to change, it still took me years to get it all just right.

[RockBit]

I have the MSD billet distributor and no vacuum advance. Since I will need to get an MSD box anyway when I put my car together, I plan to give Lippy's solution a hard look.

[lippy]

Originally Posted by cycleguy55

It's simpler - one less thing to worry about.

OTOH, distributors with vacuum advance also have mechanical advance.

Mechanical advance is controlled by weights on springs, spinning on the dist. shaft. They advance timing as RPMs increase. Mechanical advance is necessary if you want to rev over 2000 RPM, so it's always there even if you don't think you have it. ALL distributors have it - except those used only as signals to an ignition box with timing curves.

Vacuum advance is there purely for efficiency. It advances the timing based on engine load. High vacuum = low engine load, so the engine can run more timing with no problems and deliver better fuel economy. The round vacuum can on the side of the distributor does this. You can disconnect the vacuum line (and plug it) to do without the vacuum advance if you want.

Vacuum advance also makes the engine run cooler and provides better throttle response at most conditions.

[Moriarty]

What's the latest thinking on this?

I have an ERA which I bought used and (knowingly) made the classic mistake of buying a vehicle setup for racing to use primarily for city street driving.

I've been slowly correcting this, and think it's time to swap out the MSD ignition for a distributor with vacuum advance.

My goals:
1) Reduce heat
2) Increase gas milage
3) Look more authentic

Earlier in this thread (2014) it was pointed out that MSD is old tech and Pertronix is the new hotness. Is that still the best today? I would love some pointers on what distributor to pair it with. One thing I like about my existing one is the spring loaded screws instead of clips for the cap.

As for the vacuum advance, the cam in this beast is going to be problematic as I'm only pulling around 8"Hg at idle (~800rpm). I need to fiddle with carb a bit still as I discovered yesterday that turning the mixture screws all the way in on the primaries caused the engine to pull the most vacuum (and didn't stall) so it sounds like I need to adjust the throttle plates to expose less slot maybe. This is all new to me, so I'm learning as I go. I pulled the base timing back to 12* prior to this (I think it was closer to 20* when I started w/ 6-7"Hg).

I've read that there are vacuum cans that apply full advance at 8"Hg, though I think I'm going to need to get the manifold vacuum up a bit more than I've been able to thus far before I'll even be able to use one of those, since the cam is so lopey at idle. Also, I haven't been able to find them online.

For the distributor, I think I'm also going to be limited by space given the coolant reservoir encroachment.

Additional engine details:
- 427 SO stroked and bored to 456 I believe.
- Single Holley 1450, 6.5 power valve, Primary jets: 65, Secondary jets: 80.
- MSD 6AL ignition.
- Cam details unknown. Can someone interpret this for me? Looks like 110* lobe separation maybe.
https://imgur.com/gallery/gRsRLKB

Here's a pic of the engine for reference:
https://imgur.com/gallery/28qRAvL

[Tom Kirkham]

The only reason to not run a vacuum advance on a 427 is to look original. I am guessing that Ford did not put vacuum advance on the 427 and 289 HiPo engined street cars, is because these engines came with dual point distributors. My guess is the dual point distributor did not have enough room for the vacuum advance.

You already have a MSD distributor. I would keep it and tune it. I like the black bushing to limit the amount of ignition advance and then set the idle ignition up around 20 degrees initial. I also like using the blue spring and the light silver spring. These are the two lightest springs. This will give you the second fastest curve. I would prefer to run the fastest curve, but the idle is a little rough because the springs are not quite strong enough to hold the timing constant. The important thing to remember for most 427 with aluminum heads with around 10:1 compression is to not go over 36 degrees total at wide open throttle.

For your MSD distributor, I would also recommend running an MSD 6530 or something equivalent with a MAP sensor. As has been pointed out by several people there are lots of advantages to running a manifold pressure referenced ignition advance system.

1. cleaner running spark plugs,
2. cooler running engine and exhaust system (cooler side pipes)
3. better fuel mileage
4. better throttle response
5. smoother running engine, especially at idle

Here is a link on ClubCobra on how I was able to solve some of these issues with a Weber equipped engine, with an MSD 6530.
MSD 6530 with throttle postion sensor
The instructions also say you should lock out the mechanical advance. I wouldn't but you will have to account for the mechanical advance in your programming. This will help with rotor phasing...

I also suspect with more ignition timing, your engine will calm down somewhat at idle. Easy to check...

And yes your engine has a big cam.

[Tom Kirkham]

My guess on your cam:

Advertised Duration is 294* In, 304* Ex
Duration @.050" is .256* In, 266* Ex
lift .600" In, and .626" Ex
Lobe separation angle is 110*

*=degree

[twobjshelbys]

Originally Posted by Moriarty

What's the latest thinking on this?

My goals:
1) Reduce heat
2) Increase gas milage
3) Look more authentic

Just so you know 1&2 are practically impossible to achieve in a Cobra with any engine. The engine is huge and sits right next to the un- or poorly insulated firewall. It's going to get hot.

Mileage is also likely to be chasing rainbows. First, those engines even in the 60s only got 10-12MPG. All the magic you do is going to add 10%.

If you want a cool operating engine with good gas mileage then the Cobra was not your right choice.

Another one that people complain about is the gassy smell. That's a badge of honor.

[olddog]

I'm not sure if 1450 is a model or the CFM rating of the carb. If it is CFM, it is too big in my opinion.

456 cid spinning 8000 rpm at 100% efficiency will only suck in 1050 CFM. Most engines cannot achieve 90% pumping efficiency. That said it is possible to tune cam, ports, intake and header lengths to achieve way over 100% in a narrow rpm window, but race teams spend countless hours on a dyno to do it. Assuming your engine is not at this level, I would think a 1000 cfm carb would be plenty. 850 cfm would be enough for 6800 rpm.

[Moriarty]

I'm not sure if 1450 is a model or the CFM rating of the carb.

Oops, I meant 4150. I believe it is 750 CFM. Thanks!

Excellent info, Tom. I forgot I was going to ask about programmable options like that as well. Would never have occurred to me to use a TPS in place of a MAP sensor. Someone mentioned the CB Performance Black Box in the thread you linked to which sounds interesting, as it would allow me to keep the existing MSD setup, and provide redundancy if either ever failed. Plus it's half the price. Unfortunately, they are out of stock. Not sure what the deal there is. I'm curious if the same TPS trick will work with that unit (seems like it should) or if the wider range will enable using a MAP sensor with my setup.

Re: heat - I'm just looking to get from my current situation of creeping up to 110C in city traffic down closer to 100C.

[Gaz64]

Originally Posted by Moriarty

Oops, I meant 4150. I believe it is 750 CFM. Thanks!

Excellent info, Tom. I forgot I was going to ask about programmable options like that as well. Would never have occurred to me to use a TPS in place of a MAP sensor. Someone mentioned the CB Performance Black Box in the thread you linked to which sounds interesting, as it would allow me to keep the existing MSD setup, and provide redundancy if either ever failed. Plus it's half the price. Unfortunately, they are out of stock. Not sure what the deal there is. I'm curious if the same TPS trick will work with that unit (seems like it should) or if the wider range will enable using a MAP sensor with my setup.

Re: heat - I'm just looking to get from my current situation of creeping up to 110C in city traffic down closer to 100C.

CB still has them for $199, not sure about this out of stock.
I run a map sensor input on my CB Black Box, completely tuneable, and can use timing to help stabilise idle speed, which is half of how EFI does it.
You can't do that with fixed timing like a TPS at idle.
Gary

[6TNCRZY]

256/266 @ .050" is quite a lot of camshaft if you want a great street driver. A cam with about 15 degrees less duration will do a lot more for your enjoyment of the car than any changes you make to the distributor.

[Tom Kirkham]

Originally Posted by Gaz64

...can use timing to help stabilise idle speed, which is half of how EFI does it. You can't do that with fixed timing...

Gary you nailed it.

By having the ignition advance increase below the desired idle speed, and increase above the desired idle speed, a V shaped ignition advance curve is formed. At the bottom of the V is where the desired idle RPM should be. A V shaped ignition advance curve will form a extremely fast, self correcting feedback loop that will help lock down the idle speed. We had an over-cammed 427 powered GT40 come into the shop to be sorted out. One of the issues was if the engine was idling, and if the AC was turned on, it would stumble. By advancing the timing below the desired idle speed, the idle stumble was eliminated.

[wolf k]

https://progressionignition.com/

This looks interesting, built in MAP sensor in the distributor, fully adjustable ignition timing via phone app.

[Moriarty]

Originally Posted by wolf k

https://progressionignition.com/

This looks interesting, built in MAP sensor in the distributor, fully adjustable ignition timing via phone app.

That looks perfect! Sadly, also out of stock for the 427FE version. I emailed and will give them a call tomorrow to see what's up there.

[wolf k]

Cool, let us know if you get one, I have been eyeballing them for awhile.