Best practical way to Twn Spark ignition 911 engine?

[911hillclimber]

Well, had a restless night about it all in many ways. The 200 is about £300 more but brings in laptops and all that messing about.

On a techie hillclimb forum I know well (and they know the car well) coil on plug is lectured to me, but putting all this together is not easy.
These old Porsche engines are not well known in many circles and few if any understand the type to get, what spec this and that etc where the Clewett is proven, simple and will deliver what I basically need.
In the run of a hill the car it away for about 30 seconds up Shelsley Walsh mostly at WOT, you us the brakes twice and make about 7 gearchanges.

The smallest cock-up will add 2 seconds to the run (bad gearchange).
I'm not chasing max BHP, revs and gearchange are the order of the day.

I doubt I will go to fuel injection so don't need that complexity, definitely will not be turbo charging the thing, I value the life I have left.
I do not want to loose this year of hillclimbing messing about. I've seen too often what happens to people with dicky cars and lost mojo as a result.

Think I will stick to the XDi.

[Gary71]

Cool. I’ll try and be there for it’s first run up the hill

[911hillclimber]

Hope we both leave with a smile!
It will be the usual running-in procedure. Warm it up and balls out.

On a positive run today, got the massive piston pin clips in the engine with no real issues, trantrums or even swearing.
A first.

[hot66]

But with mfi and carbs the ignition doesn’t know throttle position or load etc … On mine neil did bump up the advance at the top end but it made less power .

The only way I can see to do that is using the 3k-8k range setting which is linear so not really a bump at the to end as it were.

The thing I don't know anything about is how sensitive a twin plug motor is to ignition advance. And for Graham's application what's in it for him as it were.

0.5 bhp extra available? Probably not material, 10bhp? 20bhp? At 20 it's worth it, but it won't be 20, probably not 10. Just musing now!

If it's more for engine safety, that's a different matter. So much to contemplate.

admitedly not a bump as such, but if you look at the graphs the 8k knob sets the curve ( line ) from the 3k mark etc so can be advanced or retarded etc .. either way, its not much more advanced in that respect than a dizzy would be, which for me with a MFI is all i required. Maybe there is an argument with my car being a road car for a more mappable ignition curve but i never set out to build a resto mod with modern touches.. mine was always to be old school power from a high comp 2.4S that I could drive even with poor fuel .. its just the engine build costs got out of hand and another £6k on a period looking twin plug system couldnt be stretched to

Graham is probabaly similar as he has a very specific purpose and its pretty much tick over or flat out. He will tune the motor for max horse power and not for emissions or part throttle cruising etc

[911hillclimber]

Graham is probabaly similar as he has a very specific purpose and its pretty much tick over or flat out. He will tune the motor for max horse power and not for emissions or part throttle cruising etc

Spot-on James.

The costs can get silly, this is just a little clubman's race car driven for fun.
I can't keep on pumping money into it, money matters take a different priority when you retire!

Had a great day on the engine today, will sleep well tonight.

[Jonny Hart]

But with mfi and carbs the ignition doesn’t know throttle position or load etc … On mine neil did bump up the advance at the top end but it made less power .

The only way I can see to do that is using the 3k-8k range setting which is linear so not really a bump at the to end as it were.

The thing I don't know anything about is how sensitive a twin plug motor is to ignition advance. And for Graham's application what's in it for him as it were.

0.5 bhp extra available? Probably not material, 10bhp? 20bhp? At 20 it's worth it, but it won't be 20, probably not 10. Just musing now!

If it's more for engine safety, that's a different matter. So much to contemplate.

admitedly not a bump as such, but if you look at the graphs the 8k knob sets the curve ( line ) from the 3k mark etc so can be advanced or retarded etc .. either way, its not much more advanced in that respect than a dizzy would be, which for me with a MFI is all i required. Maybe there is an argument with my car being a road car for a more mappable ignition curve but i never set out to build a resto mod with modern touches.. mine was always to be old school power from a high comp 2.4S that I could drive even with poor fuel .. its just the engine build costs got out of hand and another £6k on a period looking twin plug system couldnt be stretched to

Graham is probabaly similar as he has a very specific purpose and its pretty much tick over or flat out. He will tune the motor for max horse power and not for emissions or part throttle cruising etc

Why have a curve at all? part 1
===================

The proposed system only has the ability to provide a single gradient advance slope from 3k upwards. Correct me if I'm wrong, but I guess we are looking at this engine ALWAYS operating above 3000 RPM. The lower adjustment is therefore a waste of time as it really only helps to get a sensible idle and pull away for road use.

In this application, you may as well run fixed timing set at the optimal advance for high end power. It will idle high but who cares? An example that springs to mind is the BMW CSL race cars - they run fixed advance at 28 degrees - no curve.

Why have a curve at all? part 2.
===================

What you really want is the ability to have MORE advance between 3000 and (probably) 5500, then to REDUCE the advance up high. That will improve the power in the lower RPM range (say 3000 - 5500) and not grenade it at high RPM (6000+).

The proposed unit can only offer a downward slope from 3 to 8k which isn't quite what you want. You need to be able to quickly pull down the advance and then level out (see 964/993 graphs on my previous link)

[911hillclimber]

Thanks for your detailed comments.
I have the dilemma that Ineed to get a system that will Drive twin plugs and can be readily tweaked just as a distributor system can.
The options Ihave are restrained by cost and time and my expertise.
The latter is in short supply as you all know.

I do not want to chuck money at it as it is not worth it.

So, I have to select something that will do, and the Clewett system seems right at a price I can afford.
Clewett have this simple system and it goes to 8k so I presume they have proven it to work or it would not be on the market for years.

It may not be perfect but it is there to buy.
I am not aware of any other system that comes as complete as this and even if it is little more than a glorified distributor that may be enough to control this twin plug engine.

[hot66]

Why have a curve at all? part 1
===================

The proposed system only has the ability to provide a single gradient advance slope from 3k upwards. Correct me if I'm wrong, but I guess we are looking at this engine ALWAYS operating above 3000 RPM. The lower adjustment is therefore a waste of time as it really only helps to get a sensible idle and pull away for road use.

In this application, you may as well run fixed timing set at the optimal advance for high end power. It will idle high but who cares? An example that springs to mind is the BMW CSL race cars - they run fixed advance at 28 degrees - no curve.

Why have a curve at all? part 2.
===================

What you really want is the ability to have MORE advance between 3000 and (probably) 5500, then to REDUCE the advance up high. That will improve the power in the lower RPM range (say 3000 - 5500) and not grenade it at high RPM (6000+).

The proposed unit can only offer a downward slope from 3 to 8k which isn't quite what you want. You need to be able to quickly pull down the advance and then level out (see 964/993 graphs on my previous link)

I cant deny I would be interested to see what gains could be got on my motor with a more map-able ignition. Id be pretty sure a the crudeness of MFI might over ride any gains in efficiency etc though

[gridgway]

Why have a curve at all? part 1
===================

The proposed system only has the ability to provide a single gradient advance slope from 3k upwards. Correct me if I'm wrong, but I guess we are looking at this engine ALWAYS operating above 3000 RPM. The lower adjustment is therefore a waste of time as it really only helps to get a sensible idle and pull away for road use.

In this application, you may as well run fixed timing set at the optimal advance for high end power. It will idle high but who cares? An example that springs to mind is the BMW CSL race cars - they run fixed advance at 28 degrees - no curve.

Why have a curve at all? part 2.
===================

What you really want is the ability to have MORE advance between 3000 and (probably) 5500, then to REDUCE the advance up high. That will improve the power in the lower RPM range (say 3000 - 5500) and not grenade it at high RPM (6000+).

The proposed unit can only offer a downward slope from 3 to 8k which isn't quite what you want. You need to be able to quickly pull down the advance and then level out (see 964/993 graphs on my previous link)

Thanks Jonny, very interesting. Some questions about what's going on (basic, sorry!)
1 overall why do you need more advance with higher revs? Is it that the combustion takes essentially a fixed amount of time and with higher revs, you thus have to get it started earlier in the cycle?
2 what happens at higher revs (6k+) that damages the engine so you need to start the combustion later in the cycle?
3 in the link you pointed at, I think the advance reduction in the 3.2 at higher revs was less pronounced than the 964/993. If I got that right, why is that?

Cheers
Graham

[911hillclimber]

I cant deny I would be interested to see what gains could be got on my motor with a more map-able ignition. Id be pretty sure a the crudeness of MFI might over ride any gains in efficiency etc though

I suspect PMO carbs are cruder still!

Got to move on.
Can anyone detail a twin plug system proven to work that costs £1600 + HMG's cut and I can buy now?
Clewett have been in this business for some time and have worked closely with PMO on carbs and fuel injection.

I appreciate the tech debate, that's good in general just as Graham's questions and Jonny's expertise, but the subject has come to an end for me.

This engine was a gem to drive on PMO's, a 2.7 dist and a Halford coil with a condenser.
Electronic points fitted last year did nothing to improve things, engine still gave 260 bhp.

Any advance curves of whatever shape came from the bob weights in the distributor. I accept this was only to 6300 rpm.

Probably irrelevant, but the tick over is set at 1200/1500 rpm to allow easy moving of the car around the paddock.
Starts are 5500 rpm and dump the clutch, never less unless raining.

[Jonny Hart]

Why have a curve at all? part 1
===================

The proposed system only has the ability to provide a single gradient advance slope from 3k upwards. Correct me if I'm wrong, but I guess we are looking at this engine ALWAYS operating above 3000 RPM. The lower adjustment is therefore a waste of time as it really only helps to get a sensible idle and pull away for road use.

In this application, you may as well run fixed timing set at the optimal advance for high end power. It will idle high but who cares? An example that springs to mind is the BMW CSL race cars - they run fixed advance at 28 degrees - no curve.

Why have a curve at all? part 2.
===================

What you really want is the ability to have MORE advance between 3000 and (probably) 5500, then to REDUCE the advance up high. That will improve the power in the lower RPM range (say 3000 - 5500) and not grenade it at high RPM (6000+).

The proposed unit can only offer a downward slope from 3 to 8k which isn't quite what you want. You need to be able to quickly pull down the advance and then level out (see 964/993 graphs on my previous link)

Thanks Jonny, very interesting. Some questions about what's going on (basic, sorry!)
1 overall why do you need more advance with higher revs? Is it that the combustion takes essentially a fixed amount of time and with higher revs, you thus have to get it started earlier in the cycle?
2 what happens at higher revs (6k+) that damages the engine so you need to start the combustion later in the cycle?
3 in the link you pointed at, I think the advance reduction in the 3.2 at higher revs was less pronounced than the 964/993. If I got that right, why is that?

Cheers
Graham

1. The engine is turning faster but the fuel takes a set time to ignite. Therefore, you need to light the fire earlier to get the explosion coincident with the piston downstroke. Too much and you'll cause the explosion before the piston is on the downward stroke. Too much pressure, heat, mechanical stress - detonation.

2. Risk management. Everything turning faster, higher temperatures, pressure etc.

3. The twin plug engines need less advance overall as there are two flame fronts, resulting in less time needed before the explosion. The 3.2 is a single plug so needs more advance overall as it takes long for the explosion to happen. Other characteristics may come into play (different power outputs) which a probably a factor.

Up until the arrival of electronic ignition on the 3.2, no 911 distributor could reproduce a mid range 'advance bump' since physical bob weights once flung out, cannot retract at a higher RPM. The mechanical distributor therefore is a compromise. Mid range advance can make a huge difference on the older engines, particularly the E, S and RS engines. I know of one RS that was rolling road mapped and runs 9 degrees over stock advance mid range without any hint of knock. IIRC this yielded another 15 or so HP in the mid range which gives the car epic pace.

(end of thread hijack - sorry)

[gridgway]

Thanks Jonny and apologies Graham for the year hijack.
Once I get my T back and running with its 2.4s mfi motor, I'll have to have a go at what can be achieved with clever ignition!

[911hillclimber]

I don't read any of this as a hijack, just expands the topic interestingly.
Buying XDi today.

[RobFrost]

Will be dead easy to dial in the standard 3.2 curve to start with an advance it from there.
Get some ‘99 octane in stock

Standard 3.2 curve should leave plenty of room for improvement because at lower revs the intake flow is restricted by the air flow meter despite being at wide open throttle, whereas this car's on PMOs so has no such restriction.

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[RobFrost]

Can anyone detail a twin plug system proven to work that costs £1600 + HMG's cut and I can buy now?

Megasquirt includes ignition timing for just a grand, right? Tim, user impmad would be able to advise what it can do. I think there are a lot of 911 users on it.

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