Oddball engines and prototypes

[husaberg]

The valentine article mentions the banning of twin plug heads (as well as 5 valve heads) - I bought a J.A.P speedway motor from Bruce Abernethy (aka Abbo) many years ago - it had a twin plug head, the additional plug hole was between the pushrod tubes. I didnt get the special timing case/magneto platform though. It used to run two magnetos, one timed fractionally ahead (or behind) the other. Abbo said it made that motor unbeatable out of the starting gate (This would have been in the late 40's early 50's I guess). Sold all the J.A.P stuff to Harold Sergent when i switched to Jawa. Harold has a fairly good speedway museum in Fielding and he has a knack of finding rare things so he may now have the complete Abbo engine. He may also have the Drake / SRS /Offy engine as well.

The Gold Top spec JAP's had twin cylinder slip rings on the mags and ran at 1/4 engine speed, they gained a HP (from memory) just from this mod. they needed careful stoning to achieve the same timing on each stroke.
They also had different cams ands springs compression Mahle pistons that outlasted the bores and they used to sleeve them and use the same pistons.
IMO Twin plugs are a bandaid for what was a poor initial combustion chamber design.
Bevel Ducatis, when converted to twin plugs, appear to have the second plug in the Cylinder (due to the acute angle)

[WilDun]

Bevel Ducatis when converved to twin plugs appear to have the second one in the Cylinder (due to the acute angle)

Husa - Not quite sure what you mean there.

[husaberg]

Husa - Not quite sure what you mean there.

Have a look at the picture, plus my spelling correction

[Grumph]

Have a look at the picture, plus my spelling correction

That's a 250 I think - and the plug is almost horizontal but still in the chamber.
On the 450 there's enough room to slide a 10mm plug in around the side of the bevel tower.

And yes, that 450 had to use twin plugs as it was on alky and the combustion chamber shape was badly compromised...I was stuck with a piston the owner had ordered and had no input into crown shape.

[husaberg]

That's a 250 I think - and the plug is almost horizontal but still in the chamber.
On the 450 there's enough room to slide a 10mm plug in around the side of the bevel tower.

And yes, that 450 had to use twin plugs as it was on alky and the combustion chamber shape was badly compromised...I was stuck with a piston the owner had ordered and had no input into crown shape.

Yeah i realise its just the look, but the first time i seen one of Bruno Spaggiani bikes in pics i did wonder where the heck it was.
those Gold top JAPS had the valves and the cylinders filed to prevent them clashing on overlap and with the outside edge of the cylinder. (Like a big valve commando)
The Speedway JAPS are a prefect example of the poms resting on their laurels, failing to move with the times and spend a tiny little bit of the money they made on retooling and redesigning a better engine instead they gave their directors new Jags every year and just tarting up what was a 1930's design.

1960's JAP and Ducati combustion chambers were not a patch on the BSA combustion chamber design.

i was looking for pic and found this.
As i am pretty sure he is currently dead though, so the pricing and waiting times might be a bit off.
http://www.mezporting.com/page2.html

[WilDun]

Have a look at the picture, plus my spelling correction

Ah yes, ahem - maybe I should go to Specsavers (where everyone gets a bargain) - or is that The Warehouse? and get another pair of glasses or even look for my other glasses (lost)!

[Grumph]

Historically, the big problem with going to twin plug for a race engine was that if you fouled one, you lost a lot of power.
This is because with twin plugs the ignition can be pulled back as much as 4 - 5 degrees. Go back to single plug at that timing
and you've lost a lot of power...
As better ignitions became common it was a viable proposition again - then head shape development made it largely redundant.

The engine that has me puzzled though is the BMW/Rotax 650 which as std has 2 plugs - and most tuners add another one as well.
But it's a very basic 4 valve chamber with no odd cavities to explain the need for more than one plug...
I did a big bore version for a local BEARS racer in a Bimota chassis. Left it at 2 plugs but would have liked to try it with just one.

[Frits Overmars]

the big problem with going to twin plug for a race engine was that if you fouled one, you lost a lot of power.

Wouldn't you lose even more power when you foul the plug in a single-plug engine?

[husaberg]

Wouldn't you lose even more power when you foul the plug in a single-plug engine?

Historically, the big problem with going to twin plug for a race engine was that if you fouled one, you lost a lot of power.
This is because with twin plugs the ignition can be pulled back as much as 4 - 5 degrees. Go back to single plug at that timing
and you've lost a lot of power...
As better ignitions became common it was a viable proposition again - then head shape development made it largely redundant.

The engine that has me puzzled though is the BMW/Rotax 650 which as std has 2 plugs - and most tuners add another one as well.
But it's a very basic 4 valve chamber with no odd cavities to explain the need for more than one plug...
I did a big bore version for a local BEARS racer in a Bimota chassis. Left it at 2 plugs but would have liked to try it with just one.

Pretty sure one of those big bore singles rotax engines they made in the mid 90's had three plugs std.
The Rotax/BMW that David Morris developed for some reason also had an offset combustion chamber. I think that version was only 2 plugs though.
https://c2.staticflickr.com/4/3920/1...218361fc_b.jpg

[guyhockley]

The Speedway JAPS are a prefect example of the poms resting on their laurels, failing to move with the times and spend a tiny little bit of the money they made on retooling and redesigning a better engine instead they gave their directors new Jags every year and just tarting up what was a 1930's design.

1960's JAP and Ducati combustion chambers were not a patch on the BSA combustion chamber design.

One of those "what might have been" articles the british press love. I've read elsewhere that the BSA was miles off the pace.

[guyhockley]

Historically, the big problem with going to twin plug for a race engine was that if you fouled one, you lost a lot of power.
This is because with twin plugs the ignition can be pulled back as much as 4 - 5 degrees. Go back to single plug at that timing
and you've lost a lot of power...
As better ignitions became common it was a viable proposition again - then head shape development made it largely redundant.

One of my first jobs as an apprentice was helping to twin plug a couple of cylinder heads. Not for performance, though, it was a VW 1200 for a plane!

The engine that has me puzzled though is the BMW/Rotax 650 which as std has 2 plugs - and most tuners add another one as well.
But it's a very basic 4 valve chamber with no odd cavities to explain the need for more than one plug...
I did a big bore version for a local BEARS racer in a Bimota chassis. Left it at 2 plugs but would have liked to try it with just one.

Wonder if the second plug is something to do with emissions? Adding a third one is probably just psychological - people like symmetry. Think Aprilia used the same basic engine with a 5 valve head - that could do with all the plugs you could squeese in?
Saw an interesting french supermono that took a bit of head scratching. Rotax motor, air-cooled cylinder, water-cooled head but it had the intake cam lifted and very down-draught inlet. No idea if he changed valve angles. Bike also had an airbox feed through the headstock, first time I'd seen that.

[Grumph]

Wouldn't you lose even more power when you foul the plug in a single-plug engine?

Quite true of course Frits - but a twin plug 4T single will keep running on one plug albeit a lot slower...

On the British race engines of the time at somewhere like the IOM, the rider then had the dilemma, do I stop and change plugs or carry on at a slower pace...

[Grumph]

One of my first jobs as an apprentice was helping to twin plug a couple of cylinder heads. Not for performance, though, it was a VW 1200 for a plane!

My father decided to take up flying in his mid 60's. At one of his first post flight social cuppa's he had to explain to a group of tutors exactly why you did a mag drop pre flight...They'd always done it because they were taught to.



Wonder if the second plug is something to do with emissions? Adding a third one is probably just psychological - people like symmetry. Think Aprilia used the same basic engine with a 5 valve head - that could do with all the plugs you could squeese in?
Saw an interesting french supermono that took a bit of head scratching. Rotax motor, air-cooled cylinder, water-cooled head but it had the intake cam lifted and very down-draught inlet. No idea if he changed valve angles. Bike also had an airbox feed through the headstock, first time I'd seen that.

The big bore BMW eventually cracked the oversize liner. Even though we'd made the top flange locate on the inner diameter of the water jacket and thus a stiffer barrel than the OE arrangement. I'd heard that they gave head gasket probs as a big bore cos the unsupported liner moved around...
It did make very good power though.

[T.W.R]

A slight digression from the context of the thread but a worthwhile read to where the minds were in 66.
A discussion panel comprising of Phil Vincent, Jack Williams, Steve Lancefield, & Bob Gayler and what the put forth to an audience at Bromley Technical College.

Wont upload in order

[Frits Overmars]

a worthwhile read to where the minds were in 66. A discussion panel comprising of Phil Vincent, Jack Williams, Steve Lancefield, & Bob Gayler... at Bromley Technical College.

Nice reaction to the question, what sort of engine the above specialists would like to work on, given a free choice.
Jack Williams, best known for his development of the 350 cc AJS 7R four-stroke single, named a 250 cc two-stroke four .