I would have though that losing a plug on a Twin Plug engine would reduce peak cylinder pressure and that this should reduce any knocking.

I always thought detonation was more likely with increased advance and than Twin Plugging meant that advance could be reduced and this was one of the benefits.

As losing a plug won't affect the spark timing how does this cause detonation?

Chris

I think it is pretty well established that on early 911 engines the piston dome shape combined with the combustion chamber shape creates an nearly isolated pocket on the exhaust side of the chamber that is prone to detonation when the static compression ratio is above 10:1 or so.

The second plug ignites this pocket and is a must for early higher CR engines. I would expect that if the second plug fails, this region would be prone to detonation even with the reduced advance.

I understand that on the 2.0 litre engine the very high dome causes shrouding but I am not sure that this causes detonation.

We run our 2.0 litre FIA engines at compression ratios of much higher than 10.5:1 without any pinking and with quite a significant amount of advance.

We are currently obtaining around 195BHP from these engines with Solex Carbs . We typically run them for 2 years between strip downs. Several other teams claim up to 210BHP although it is impossible to qualify this.

The later 2.2 engines with the shallower chambers and lower domes should be less affected.

We run a number of 10.5:1 compression single plug 3.0 litre engines in rally cars without any problems and these motors are revving to around 7500rpm and are generally reliable.

I would have thought that any unburnt end gas that caused detonation would have been equally likely to damage single plug high compression engines.

A local tuner has been running a 'Twin Plug' 2.7 engine in a UK Race Series for around 2 years now and he bought the Dizzy of a UK Ignition specialist.

It was a standard Distrbutor modified to run a late Jag V12 Marelli Cap and had been fitted with a Jag V12 Rotor Arm.

Unfortunately they didn't seem to understand the system too well and fitted the rotor with the arms still 180 degrees.

I pointed out the error - everyone looked bank and said it ran just fine and couldn't see the issue.

I also know of an ST Replica that had Twin Plug heasd fitted complete with plugs and ran for around 3 years before being fitted with a Twin Plug Dizzy.

The engine had GE80 cams, 10.5:1 pistons and ran well both before and after.

We are just building a 906 Replica engine and we have a Marelli S121A dizzy and will fit 2 x Ignition circuits with the switches that were used in the factory cars.

It will be interesting to see what happens when we turn one off.

What octane were you running with the single plugs though?

We used normal pump fuel which is requirement of the MSA in the UK.

All Rally/Race cars have to be fitted with a fuel sampling vale arrangement and fuel samples are commonly taken at the end of an event.

Are you suggesting that twin Plugs are a way of using low octane gas?


A typical Euro spec SC runs 9.8:1 and we would run this on a standard 93 RON Pump Fuel.

We Run SC engines with 10.5:i-pistons on 'Super Unleaded' which is 97 RON.

The 'end burn' condition that having a pocket of unburnt gas doesn't seem to cause problems at these compression ratios and I would have thought that this would be the only impact that could cause pinking.

Are we also saying that losing an ignition circuit only matter when it is the lower plug?


I believe that in the US fuel is labelled differently and that the headline number is an average of RON + MON which would typically be about 6 points lower than the same fuel labelled on the basis of RON only

In the US, we often have to live with 93 RON for street cars. With that fuel, you run the risk of detonation even in a 95 mm bore 3.0 or 3.2 engine with high CR.

One also has to ask why Porsche decided to run twin plug in all their early high CR race engines?

I can't speak for twin boxes but I can relay our experience of dual spark.

Extensive dyno testing with our dual spark CDI+ units shows that the second spark burns more fuel and this shows on the AFR plot which goes lean compared to the same engine with the standard Bosch CDI. When the mixture is then 'corrected' on an early stock MFI 2.0 litre we see typically a 10% gain in power across the RPM range. This has been repeated time and time again, most recently by Loren Beggs who bumped his 69S from 110HP to 125HP with a little mapping thrown in.

Results here United States Tuner for CDI+ - Classic Retrofit


On Parr's stock SC engined race car, we saw a factory spot on 204BHP on the standard Bosch unit. On fitting CDI+ AFR read lean so the CIS mixture was adjusted and the dyno recorded 216BHP.

I would imagine then, the twin boxes produce the same (or even better) effect and that mixture adjustment is crucial. We are soon testing the effect of running two of our dual spark boxes.

I am sure they produce more power as a result of eliminating plug shrouding.

We have run Twin Plug engines for 10 years and do agree that they provide a substantial improvement in power output and in terms of the overall power band.

901/-01 engine clearly have the witness for the second plug in the lower valve covers and I believe that the original intention was to run the engine in this format.

I am only questioning the damage that would occur if one of the two plug circuits were to fail.

The 906/20 Engines were reported to have a compression ratio of 10.3:1 whilst the 901/10 engine in a 1969 'S' was 9.9:1 and had a very similar cylinder head/piston configuration.

If damage does occur do we need to fit 'warning' lights to indicate one of these circuits has failed?

In the early days of using Twin Plug engines we ran MSD 6A ignitions with Blaster Coils and we did have some coil failures which resulted losing one of the plug circuits but we never damaged an engine when this happened.

As far as I know, the conventional wisdom has been that the second spark plug per cylinder on the 911 motors gives more power and also resistance to detonation. I would have thought that if you lose one plug in a cylinder on a high compression race motor, that it would potentially lead to detonation and damage. But I don't have enough experience building and tearing down engines to weigh in on it.

My understanding is that if running 108 octane or better, you can get away with high compression like 11:1 or 12:1, so perhaps if a twin plug motor running race gas loses a plug (or the second plugs' CD box, coil, etc.) and the compression isn't above that threshold, you can get away without damaging the engine.

I was always under the impression that the early Porsche race motors used twin plug to achieve the higher HP associated with high CR without the risk of detonation.

Two things:

1. Damage potential depends on CR and octane, two big variables.

2. The difference in optimal ignition timing, 26-28 degrees vs 35-38 degrees leaves a twin-plug engine with a severely retarded spark if one ignition system fails. Depending on CR & octane, one may experience overheating and all the consequences from that.

Wheredid my week go? oh yeah ...to the Pelican search function
 

John H - any update on using twin CDI+ with twinplug?

I spent all my spare time this week reading the ignition timing sticky threads and what not here on Pelican. So much great info buried in never ending debates.....

Built 3.4 turbo with twin plug and bosch CDI boxes and coils. Recently engine builder added the J&S Boost Control unit.

The car runs fantastic mostly but I ask a lot of it and worry if the 40 year old ignition system is up to the task. Last week I ruptured the WUR diaphragm. Sent out to Brian at Rarelyl8 to rebuild. I have a ton off fuel so while I fantasize about a state of the art EFI conversion with Turbocraft or have Peter Dawe design me a conversion, its a lot of money and time. I really dont need more HP.

What I want is reliable and optimize what I do have,. In reading all the discussion on imprioving the ignition timing sytem on a 930.....my head is spinning and I seem to feel that adding a multispark style box like the Classic Retrofit CDI+ makes most sense.

I think I can pair as a plug and play for better spark and let the J&S piggy back and run the boost retard feature.

I like that Classics CDI+ is lap top programmable....which may help the timing curve be optimized for better below 3000 RPM crispness.

I get crazy every time the car is down for repairs.....

Anybody using CDI+ with twin plug??

The addition of load-based advance/retard to the CDI+ would be a plus.

Loads of twin plugs using CDI+, here's a recent one:

https://www.youtube.com/watch?v=s_Ae_cmUAHk

I emailed Alex. We have been chatting about AC / Heat ......

Glad to hear it. Seems like I get what I need with much less complexity than a complete EMS just for igniition,


Quote: The addition of load-based advance/retard to the CDI+ would be a plus.
__________________
1983 911SC Coupe

Not sure I follow what you are saving 100%....but I guess you like the idea too. Are you saying use the dizzy and programmed safegaurd for ignition control and the CDI+ for spark?

Regarding load-based advance/retard:

Ok, I may be all wet, but here's my take. Load based advance/retard would be the equivalent of vacuum advance on standard distributors. Vacuum advance moves timing ahead under low load, lean cruise conditions. It dramatically improves cruise drivability and tip-in response. However, none of the twin-plug distributors have vacuum advance.

I understand that the twin-plug distributors are meant for highly modified, race type engines. However, many of these highly tuned engines running a twin-plug distributor are running strictly on the street.

So, for instance, the centrifugal advance would be locked out on the distributor, as one would expect when integrating a programmable device such as the CDI+. Even with a stock distributor, locking out the advance plate, also eliminates vacuum advance. Adding a MAP sensor capability to the already programmable CDI+ would give you the ability to run much higher, light throttle, cruise timing advance.
Take a look at this stock timing map, grabbed from Steve Wongs site, for a 964 (twin-plug engine). You can clearly see the mapping of the low load, light throttle area, with timing as high as 45 degrees advanced!
http://forums.pelicanparts.com/uploa...1633910943.jpg

thanks for that explanation. I absolutely follow you. I missed that important detail in the Ignition Timing Sticky Thread- that twin plug dizzys dont have vacuum advance. Whether that be all or some, I can say definitively that I have a single pot dizzy.

And yes mine was built for the street. Everything about my engine package lives 3k and up. It is paired to the 8:41 R&P which helps in low RPM grunt...but the rebuilt 7006 turbo spools in the mid 3's....and its a touch soft in throttle 1-3k. I initally thougt getting my hand on one of Charlies Evergreen K27 7200 Raptor turbos would give me the grunt with spool starting at 2500 RPM.....but then my attention turned to ignition and wondering what I was leaving on the table with the basic 6 pin CDI boxes vs a CDI+ or MSD set up.....interesting to note ...both k27 7200 raptors i drove were on essentially stock 930s but with CDI+ ... they both felt torquee off the roll on easy throttle. I am not talking about all out acceleration but feel....they felt good......as evidenced by your attachement.

thanks

I like the idea of locking out the dizzy and using the programming of the CDI+ but not sure how they alll work together...

That will be in part due to increased spark energy (dual spark) of the CDI+. Cleaner burn, more torque in the mid range makes for a 'smoother' drive.

Max HP increase is probably there too but it is harder to feel an increase at max power vs low/mid range.

To quote one of the top UK engine builders:

"If I gave you an extra 10HP at max power, you would hardly notice, if I gave you 10HP in the mid range, you would say it's a different car".

Great. Does that statement still ring true for a twin plug?

My sense is the advantage of CDI+ over CDI is not only multi spark over the RPM band but also having the ability to lock out the distributor and program in the timing curve and pull timing as the boost spools.

Sure, any twin plug ignition offers a flame front that ignites both 'sides' of the mixture. I do believe we 'mop up' any unburnt mixture pockets on the second spark. Effect possibly less noticeable with a with a twin plug.

Yes, having a tunable timing curve has been used to good effect on NA and Turbo applications.