Central Spark Plugs

Purity Vendetta · Sep 8, 2016

I'm thinking of machining my downdraught Dominator head to move the spark plugs to a central position. I know this has been done but it's a tight fit. I was considering dropping the size of the plugs to 12 or even 10 mm on the basis that it would make the machining easier and the spacing marginally less critical. I'm using standard sized valves. I was wondering if anyone else here had carried out such a mod and if they had encountered any issues they'd be willing to share?

Thanks,

Purity.

Fast Eddie · Sep 8, 2016

10mm is normal when doing this on Triumphs. It will cut the thread partially into the OD of the valve seat on some Triumph heads and it works fine.
But with the inherently better shape of the combustion chamber on a Norton, I'd question if this added any benefit.

Danno · Sep 12, 2016

Doesn't look like there would be room either inside the chamber or on the outside of the head to move it enough to make a difference.

Rohan · Sep 12, 2016

If there was mention that the valves were being re-angled there might be some gain

Its the twin plugging that always makes you wonder....
(being somewhat difficult in Norton twins).

texasSlick · Sep 12, 2016

+1 with Danno .... not likely to make a difference.

+1 with Rohan.... if dual plugs can somehow be shoehorned into the head, some improved combustion and increased power will result. BUT it requires dual ignition systems as well .... one cannot fire two plugs off the same circuit ... only one will fire, the other will do nothing.

Slick

lcrken · Sep 13, 2016

The only dual plug conversion I've seen reference to is the one the late George Mez used to do. I posted a picture of it in this thread

short-study-ignition-timing-and-combustion-t20321-150.html?hilit=dual%20plug%20head

It looks like a difficult machining operation, and has some less than optimal design features, so I don't know if it really had any advantages, but it's still interesting.

Ken

acotrel · Sep 13, 2016

If you do a twin plug conversion, unless you use a double-ended coil to fire each pair of plugs, how do you know they are all working ?

Rohan · Sep 13, 2016

Spark half the plugs in air, and see if you still have all cylinders ??

This is the problem they had in race vehicles, like you say...
One plug is good, 2 are better - IF they are all working.
Twin plugging has had brief bursts of popularity - then they return to one good one...

acotrel · Sep 15, 2016

Twin plugs in a Triumph motor is a good idea and I've seen it done. However in a Norton engine, I think it would be a waste of time. Why is it that for the same capacity and with no modification, Norton engines are always faster than Triumph engines ? When you pull the head off a well-used Triumph engine which as 12 to 1 comp. pistons, the side of the crowns which is away from the plugs are always coked up. You don't see that in Norton engines. My feeling is that the Norton cylinder head is a much better design.

acotrel · Sep 15, 2016

A way to get Triumph 650 and 750 engines going faster might be to change the cylinder head to the Norton design. However the Rickman 4 valve heads did not seem to make Triumph engines faster.

Rohan · Sep 15, 2016

acotrel said:
However the Rickman 4 valve heads did not seem to make Triumph engines faster.

We have discussed this before.
Folks who knew what they are doing have made them fly.
The current crop of Hinkley Turnips mit 4valve heads are no slouches either.

Danno · Sep 17, 2016

Rohan said:
Spark half the plugs in air, and see if you still have all cylinders ??

This is the problem they had in race vehicles, like you say...
One plug is good, 2 are better - IF they are all working.
Twin plugging has had brief bursts of popularity - then they return to one good one...

The new H-D Milwaukee 8 4-valve motor has twin plugs in each cylinder.

Danno · Sep 17, 2016

acotrel said:
Twin plugs in a Triumph motor is a good idea and I've seen it done. However in a Norton engine, I think it would be a waste of time. Why is it that for the same capacity and with no modification, Norton engines are always faster than Triumph engines ? When you pull the head off a well-used Triumph engine which as 12 to 1 comp. pistons, the side of the crowns which is away from the plugs are always coked up. You don't see that in Norton engines. My feeling is that the Norton cylinder head is a much better design.

Mostly the intake port position and angle make the Norton head capable of producing more power via better breathing than a Triumph head.

Rohan · Sep 17, 2016

Danno said:
The new H-D Milwaukee 8 4-valve motor has twin plugs in each cylinder.

Plenty of engines NEED twin plugging. !
Which says something about their combustion chamber design....
And it has had a resurgence in popularity lately too.

If they are designed like that from scratch, and with the reliability of plugs and ignition systems these days,
no problemo.
It has been suggested its mostly to benefit plug manufacturers though - or something to put on the brochure ?!

Fast Eddie · Sep 18, 2016

Danno said:
Rohan said:

Spark half the plugs in air, and see if you still have all cylinders ??

This is the problem they had in race vehicles, like you say...
One plug is good, 2 are better - IF they are all working.
Twin plugging has had brief bursts of popularity - then they return to one good one...

Click to expand...

The new H-D Milwaukee 8 4-valve motor has twin plugs in each cylinder.

Yeah, but its got a 3 foot bore...

acotrel · Sep 19, 2016

If you want to make a high -comp. 650 Triumph engine go faster, move the plug to the centre so the bit of the combustion chamber which is normally shrouded by the piston crown also does it's thing.

acotrel · Sep 19, 2016

Rohan said:
acotrel said:

However the Rickman 4 valve heads did not seem to make Triumph engines faster.

Click to expand...

We have discussed this before.
Folks who knew what they are doing have made them fly.
The current crop of Hinkley Turnips mit 4valve heads are no slouches either.

Rohan,
Are you personally one of the people who knew what he was doing and made one fly ?

acotrel · Sep 19, 2016

In the early 1960s in Victoria, most of the top A graders rode 500cc Manx Nortons on methanol. Rod Tingate raced a 650 Triumph and was competitive. He later fitted the 4 valve Rickman head and did not get much benefit from it. He was the guy who was Kim Newcomb's mechanic with the Konig in MotoGP. If there was more speed to be found by using the Rickman head on a 650 Triumph, he would have found it. The only value is that the motor might have copped a few more revs before crashing valves.

Rohan · Sep 19, 2016

acotrel said:
Are you personally one of the people who knew what he was doing and made one fly ?

I have seen one in action someplace, and waz impressed.
Clearly some folks know what they are doing.
This was discussed here, a while back.
I'm not totally familiar with what constitutes a Rickman or Weslake or Nourish 8 valve head though,
or how you distinguish the variants thereof.

The TSS was claimed to have 125 mph potential, out of the showroom, which was more than the 2 valve heads ?
And the top end was faster than the bottom end, too...

acotrel · Sep 23, 2016

Generally speaking replacing a two valve head with a four valve head gives about a 10% boost in horsepower. If I still had my short stroke Triton, I would love to have a Rickman head, mainly because it would allow more revs to be used. A better investment would be a 6 speed close gearbox. There is a problem with Australian Historic road racing - the old style capacity classes are no longer used. Against similar bikes, a well-tuned 650 Triumph is quite fast enough for most riders. In any racing involving old bikes, if you haven't got the gearbox, you are nowhere. The extra that the 4 valve head gives on a 650 Triumph does not count for much, if the other guy has the 6 speed box.