Skip to main content

Willhoit is now sell a twin plug dizzy for 356/912 should work for us, doing the heads was no problem it was always the dizzy.  It’s not cheap but it’s from a known brand so hopefully a VW version ( cheaper) is not far behind. Twin plugging has may interesting advantages.  The Willhoit site talks about his hot rod mods for 356 engines, something new and progressive for that world.


Original Post

Replies sorted oldest to newest

Who in the world would ever want such a thing?

I think I'm the only idiot on this site who's attempted this. Getting twin-plug heads may be "easy" but getting twin-plug heads done "right" is less so. There are a lot of ways to screw it up, and a lot more to it than just jamming a second set of plugs in the chamber.

Regardless, you're right-- the heads are the easy part. Willhoit has been doing a twin-plug distributor for his engines for years, but I wasn't aware he was selling them as a part.

It's possible to covert a 009 to work with a twin-plug Datsun capo and rotor (John uses an Alfa cap and rotor), but Willhoit's setup is much more elegant and probably better all around.

The hot set-up is to do crankfire and just drive 2 coils with the ignition controller. My set-up was a hybrid-- I used a single pickup in the distributor, and drove a programmable "Black Box" modified by Mario Velotta ( with two coil drivers. The engine is in pieces now-- but when it goes back together, I'll use a crank pick-up, some sort of ignition controller (the Black Box) or a MSD programmable controller or Megajolt, and just use the distributor to send the spark to the right places.

It's a cool project, and I'm in deep enough to stay committed to it-- but frankly, beyond the "gee-whiz" factor, it's a lot of work (and I do mean a LOT of work) for a smallish gain.

Last edited by Stan Galat

If you need to live right on that razor's edge, twin-plugging a NA motor is a good thing.

It requires way less ignition advance(due to the dual flame fronts making ignition happen more evenly and faster), and you can run more compression, making more power, but not running hotter. Not running hotter is a real benefit with our cooling-challenged aircooled engines.

For me, I'd only do twin-plug with crankfire ignition and smart coils. Smart coils take care of triggering and dwell with built-in circuitry. This way, you only need one ignition controller.

Megajolt using EDIS is a great system, I've run it since 2008. But it uses dumb coilpacks. Dumb coils require an ignition driver for each coil, which requires 2 EDIS(Ford) boxes. Theoretically one crank trigger and one Megajolt should be able to operate two EDIS boxes. But if it was me, I'd use a system designed to accommodate smart coils for a twin-plug application.

With a 36-1 trigger wheel(35 teeth and 1 missing tooth for TDC location) it's accurate to 1/10 of a degree per revolution. That's a FAR cry from the +/- 5 degrees of a typical POS 009.

It's not very well known, but RETARDING advance by 1 degree per 250 rpms above 4800 really helps keep it cool to redline, especially under full load. SO this means that my max advance of 32 ends up at ~25 at redline of 6500. This is really only possible with a computer-controlled system. No way to do it with a distributor. Jake Raby told me this years ago, and I've been running this way with complete success since 2008.

From the Willhoit site:

The WR Twin Plug Distributor is a new product for converting any 356 or 912 engine to two spark plugs per cylinder. The distributor uses a Pertronix Ignitor II unit which delivers a standard 12 volt signal that is not sensitive to interference like many magnetic triggers. There is no advance in the WR Twin Plug Distributor, and it's made to be used with either a programmable MSD 6AL-2 CDI box or an EFI control unit.

Switching to a Twin Plug setup has been proven to significantly boost low and mid range throttle response, as well as allowing a one-half point higher compression ratio. It also adds a distinct look to any 356 or 912 engine.

In addition to just the distributor offered on this page, WR offers a complete Twin Plug Kit for $2,850.  The kit includes this distributor, a preprogrammed and wired MSD CDI box, Magnacor plug wires, two coil options, upper and lower NGK Iridium plugs, and complete easy to follow instructions for installation. The MSD box is mounted under the passenger's side floor board and a period correct wiring harness connects it to the distributor. The entire installation is designed to look vintage and original.  Please email for photos.

WR can drill your 356 heads for twin plugs using an easily available CR7HIX NGK plug. The charge is $600 per set plus shipping.

This is essentially what I did, albeit with different parts and for a lot less money.

It's adequate, but not ideal-- it is fully programmable, but the entire thing is dependent on a Pertonix (or Compu-Fire, in my case) pickup. The problem is accuracy-- there isn't very much space inside the distributor, and some sort of crank pick-up would be a lot better.

Moroso makes a "flying magnet" trigger wheel for V8 guys looking for more spark accuracy, and AJ Sims makes something similar by adapting a Pertronix pickup on a bracket, and then drilling two magnets into the crank pulley. If I do not go with a full EFI/Crank-Fire package, I'll most likely do something similar, but with an industrial proximity switch and magnets.

I agree, Robert-- there's nothing like the "bundle of snakes" look of a twin-plug distributor.

almost there_zps4psvojkq


Images (1)
  • almost there_zps4psvojkq

The only place where any accuracy is lost in a flying magnet/6AL-2 is what happens when the coils are fired. The distributor in such a system has no function other than firing a certain plug-wire. It introduces one (extra) mechanical point into the system.

There's no doubt smart coils would remove that mechanical junction and be better by any objective metric. However, we're all making decisions (you included) that are also subjective. It's definitely NOT  purely "function over form", or we'd all be driving something else.

Everybody balances the objective/subjective nuances at a different point. There are no shortage of guys here who just want a "jump in and drive" restomod experience, and are happy to sacrifice air-cooled purity for it. Others are happy to accurately replicate the original 356/550 experience with swing-axle cars and 1600 cc engines. You and I (@DannyP) are splitting the difference-- maybe at different points in different ways, but each of us are choosing where the objective/subjective balancepoint works best for us.

I thought I'd hit my personal bullseye with the modified, locked out twin-plug distributor-- but the size-tiny pickup inside the distributor proved to be the achilles heel of that setup.

Maybe keeping the distributor (for spark distribution) will still not be enough accuracy, but I think it will, and I'm not (quite) ready to take one more step into modernity to get it done.

... but as you know, the twin-plug engine is now for another project down the road. The IM with the 2234 (finally being buttoned up out in Spokane) will have megajolt, and perhaps the full meal deal EFI/crank-fire down the road. My intended use of the car as a long-haul GT demands a different balancepoint than a lightweight mountain flyer.

Last edited by Stan Galat

I want to keep some sort (in my mind) of period authenticity, if not I think a Subaru engine is the way to go. Twin plugging has a history with air cooled Porsche, I only hope it will become popular enough (its very "green") to bring the price down to resonable levels for those of us not Jerry Seinfeld. It also may be a way to deal with lower quality fuels that I believe are in our future.


My ultimate setup would be a Type 4 with a ITB EFI and a Ibrahim Kuzu 547 style shroud.

Mount the injectors low on the manifold and cover the fuel rails with something that looks like

And also twin-plug it. Hide the crankfire pickups and arduino box, use them to drive two four cylinder coil packs. Mount those on the ends of three-inch tubes pointing out of the ends of the fuel rail covers. Then slide accordion style rubber boots over them so just the bundles of plug wires (and the disguised-as-a-coil-wire bundle of small wires from your ECU) flop out the ends. So it looks something like

I believe that would offer the best combo of air-cooled correctness, usable torque, long-term reliability and conversation-starter-at-cars-and-coffee.

Stan, agreed on every point, except the wasted spark/COP will best a single coil for FAT spark, especially near redline.

My ultimate (possible for me) aircooled engine would be twin plug, EFI, with coil-near-plug sequential injection AND ignition, and be 2276cc(82 x 94) with Nickies or JPM cylinders and JE forged pistons at 11:1, with an 86B and 1.5 rockers. Still NA, no turbo. And of course valve seals, piston squirters, and dry sump, natch, because @Stan Galat. Pretty much what I have with 4mm more stroke and more compression and MORE...

With respect to sequential, it requires a cam position sensor. If building from scratch with a new case, It has to be possible to put a wheel/sensor on the cam gear.

With a little time and patience for setup, I'm sure I could build a STRONG jig of steel that would enable positioning a head for that second plug drilling/milling/tapping. Simply clamp the head and clamp the jig to the mill table.

Last edited by DannyP

There is no such shroud to my knowledge. Suspect it would be a diy adaptation of the 4-cam shroud.

My choice of the Type 4 is as much to do with the exhaust configuration as anything else. There's a small weight penalty and a large pocketbook penalty over the Type 1 but the pipes coming out the bottom is a real advantage if you want it to look Carrera-like. Available displacement is just icing.

The budget for this would be ridiculous anyway, much of it wrapped up in that stupid fan shroud.

The problem with the way a lot of machinists twin-plug heads is in the angle of the second plug in the chamber. Most guys come in on a pretty shallow angle (to clear the push-rod tubes) and end up with several exposed threads on one side of the spark-plug. This is inviting problems-- the exposed thread will carbon or degrade by their exposure to combustion, and tear the threads out of the heads the next time the plugs are removed.The wrong way

This is maybe the worst example I've seen, but there's no shortage of people with variations on this theme.

These are mine, or at least how they used to look:Mine


Images (1)
  • Mine
@DannyP posted:

My ultimate (possible for me) aircooled engine would be twin plug, EFI, with coil-near-plug sequential injection AND ignition, and be 2276cc(82 x 94) with Nickies or JPM cylinders and JE forged pistons at 11:1, with an 86B and 1.5 rockers. Still NA, no turbo. And of course valve seals, piston squirters, and dry sump, natch, because @Stan Galat. Pretty much what I have with 4mm more stroke and more compression and MORE...

With respect to sequential, it requires a cam position sensor. If building from scratch with a new case, It has to be possible to put a wheel/sensor on the cam gear.

With a little time and patience for setup, I'm sure I could build a STRONG jig of steel that would enable positioning a head for that second plug drilling/milling/tapping. Simply clamp the head and clamp the jig to the mill table.

You're most of the way there, Danny.

If your engine is 94 mm x 78 mm and it runs 10.2:1 CR, then an increase of stroke to 82 mm using the same everything else yields 10.6:1, which is what I was running in my twin-plug engine.

All else being equal, you've got a better cam and I believe your heads are better as well (which I hope to correct if we can ever ship anything with confidence again). If you really want 11:1, just fly-cut what you've got a bit.

I've driven your car with haste and malice. I don't think you are leaving much on the table right now.

Also, regarding a cam sensor:

@edsnova posted:

Oh man, that does look like trouble: a potential hot spot that could effectively undue the benefit of the second spark.

Could more material be welded in there, then drilled and tapped and ground back even to reduce the sharp edges and keep chamber volume consistent?

This particular head is a mess in so many ways that it's hard to know where to start. There's no quench pad on the side the 2nd plug comes in on. My guess is that this particular setup was for an aircraft engine, for ignition redundancy. There's no way that thing is an improvement in any way over a stock 043 VW head.

But if you recall me saying that a guy who said he'd done "dozens" of twin plug conversions wrecked a set of Super Pros for me about 7 or 8 years ago-- this was what he did. The angle he came in on left 2 or 3 spark-plug threads exposed in the chamber on one side. More significantly, he broke through the quench pad.

In theory, the chamber could be welded up and reshaped so that no threads were exposed, but that messes up the swirl in the combustion chamber which is the entire point of the exercise.  The chamber above (and my Super Pros) is a lost cause because if the head had a proper quench pad, the plug would come out right in the middle of it.

It's harder than it seems at first blush.

Last edited by Stan Galat
@Stan Galat posted:

You're most of the way there, Danny.

regarding a cam sensor:

I am aware of this sensor. I use the distributor hole for a 3/4" diameter hose to my breather, and removed the dizzy drive and washers. I'm not keen on putting anything back in there. That's why I was looking at doing the cam sensor like modern cars.

But hey, if I could get REALLY good cylinder sealing I guess I might not need my big breather hose. (Hint: JPM cylinders/JE pistons, plus valve seals)

And I'd definitely do 11:1.

@R Thorpe posted:

Strange, must be a one of

The guy that posted the original video is Ibrahim Kuzu. He makes fan shrouds that replicate the original 4 cam shrouds and maybe others of which I'm not aware. Edsnova was talking about a 4 cam looking shroud for a Type 4 and he ran across this shroud made by Ibrahim Kuzu on an S90 motor. Ibrahim Kuzu also made the shroud on @arajani's 550. I don't know much about him other than he's a master with metal and making shrouds.

I am not an expert, but the "nose piece" with the dual distributors on that vid does not look like the ones I have seen on Type 547 and later 4-cams. Those appear to put the disties at about a 90-degree angle from each other, like

This thing, on the other hand, appears to set the disties apart by a 140, maybe 150 degree angle.

I suspect Mr Kuzu has cast and machined new parts to turn Type 546, 528 and 616 pushrod engines into dual-plugged, dry-sumped beasties.

Prices available on inquiry, no doubt.

Last edited by edsnova

It does look quite a bit different from any that I've seen, too.

All the Spyder engines I've seen in the Northeast (even at Al Alden's shop in the 60's) looked like this, with a disti on each cam mount.  That's what made them so cool to me.  This is what the Spyder Factory is running, too, AFAIK.  these guys suffered from spark bounce all over the place caused by wear in the cam drive gears - That's why the Spyder Factory went to hidden crank fire ignition and just use the distis to move the sparks to the right cylinder.

I saw one at Paul Russell's shop that had the type of disti setup below, but it wasn't installed in a car so I don't know if it went into a Spyder or Carerra.  I think this is more like what Ed's seen in the past:

And apparently there is yet another drive style, but if both distis are driven off of the same drive point I don't suppose it matters a lot whether they're 30º, 90º or 120º apart, other than clearance to other engine components.  Once they're timed they stay put (except for spark bounce from wear in the drives).


Images (3)
  • mceclip0
  • mceclip1
  • mceclip2

I think your guru buddy is mistaken, though I have no doubt that racers adapted the crank-driven system asap.

The way I heard it, Porsche developed the front mounted distributors in order to ameliorate the spark scatter and timing adjustment issues inherent in the original design. The early cars—both 550s and 356s—originally all got the early outboard distributer treatment, as seen (barely) here.

Ed, you are correct, the Spyders had the distributors on the opposite ends, 'cause mid-engine. But both the early Speedsters and Spyders had them on the camshaft.

The factory did indeed move distributors to the crankshaft with an auxiliary housing on later engines due to spark scatter AND vibration problems. I have some books to look it up if anybody cares for references. It wasn't a plain bearing only thing either.

Add Reply

Post Content
Link copied to your clipboard.