Bathtub combustion chambers

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Fast Eddie

Fast Eddie

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I guess this is mainly @ Sir Comstock, but others may have input too.

My question is (as kinda started in a other thread, but separated here so as not to hijack):

Do bathtub shaped combustion chambers lead to significant be merits on big Norton engines?

I have used them to great effect on big Triumph twins, taken out to 840cc by using Commando style bore and stroke measurements. So, my assumption is that it would also benefit Nortons.

Do any of our resident experts have experience with this they could share?
 
Bernhard said:
God knows how you are going to manage to get a Bathtub combustion chamber in a Norton cylinder head, the engine designers take a great amount of time and trouble to cast and machine a cylinder head and along comes someone else who likes to feck it all up :!: :(

http://www.bing.com/images/search?q=Bat ... tedIndex=1
Click to expand...

This is how its done Bernhard: http://www.mez.co.uk/mezporting/bathtub2.jpg

Its quite common these days and has been done on Nortons Triumphs BSAs even Vincents.
 
I have a head modified by the late George Mez, but I haven't used it.
The idea behind it is to increase compression without skimming the head or using taller pistons.
 
Pros of bathtub chambers:
Increases squish area and speeds the burn.
Raises compression ratio without the problems caused by domed pistons.

Cons:
It limits airflow because of valve shrouding
It reduces swirl in the chamber

My experience on a Norton has been a gain in power in the lower 2/3rds of the rpm range and a limit to power in the
upper 1/3rd.
It seemed to be of more advantage in a short stroke / large bore engine where there was still room for big valves and less shrouding. Jim
 
Ok that what my research showed too comnoz but take another look at the site for the 2nd spark plug modification that may help best at top rpm.
 
Further to Jim's post, speeding burn rate translates to less ignition advance, always a good thing. The combustion chamber is more compact with a bath tub configuration, most always a good thing for earlier, more complete combustion.

A bath tub head translates to less piston dome which is good for the following reasons:
Less piston dome should translate to less piston (reciprocating) mass.
Less piston dome translates to less piston crown surface area to take on heat.
Possibly more efficient scavenging during valve overlap (speculation on my part).

As Jim alluded to above, bath tub heads also tend to be a necessity on short stroke motors and often also need to rely on domed pistons to get adequate compression ratio.
 
The biggest effect would have to come from the reduction in piston mass and the lack of shrouding. In my Triumph engine, the side of the piston crown away from the plug was always coked up while under the plug it was clean. A friend does the bath tub thing to Harley engines, and claims it is a good move. I've thought about doing it to the commando engine, however there is no easy way back if it is of no advantage. Also softening the head while welding can be a problem.
 
Anyone got a view of the top side of these?

Bathtub combustion chambers


Bathtub combustion chambers
 
I chatted with Mez years ago regarding his dual plug Commando head but he was not willing to say much about it. From the pictures one can surmise how he did it and it entails whacking out cooling fins beneath the rocker box. Furthermore, the plugs enter the combustion chamber at a wonky tangential angle rather than normal (or perpendicular) to the hemisphere. This creates what I believe is an undesirable pocket in the combustion chamber.

I kicked around an alternative better approach with Herb Becker which is marginally doable but the challenges would be plug electrode interference (even with using a very small plug) and having to place and remove plug wires with a hemostat. All machining would need to be conducted from the combustion chamber side. As much as the dual plug concept is great it would be (in my opinion) too much of a moon shot.
 
Interesting this should come up just now. A few days ago I was looking for some info in "Advanced Engine Technology" by Heinz Heisler, first published in 1995 in Britain, and later reproduced by SAE, ISBN 1-56091-734-2. Anyhow, in the process, I ran across a discussion of experiments Alfa Romeo did with dual plug heads, which has some interesting conclusions. They found that combustion was considerably faster with the twin plugs, and they could use 4 degrees less advance. They also found the twin plug engine had smoother running and more torque in the lower speed range, but the benefit "became marginal with increasing engine speed". I recall reading another report on a similar study where they got the same improvement in low end performance, but stated that they saw no improvement in peak horsepower. I'm trying to recall where I saw that data, and if I find it I'll post the reference. There's been a lot of study of the different fast burn techniques, which includes multiple spark plugs, mostly for interest in clean emissions and improved mileage. Somewhere there's a study that looked at various numbers of spark plugs in one combustion chamber, all the way up to 12 plugs!

Ken
 
Hi Ken,

I recall reading similar. I think it has to do with increased combustion chamher turbulence at higher engine speeds reduces the added benefit of a second plug.
 
Dual plugging by itself with no other changes adds 2 to 3 percent to power levels according to the Porsche factory. When high performance Porsches go back to Porsche to be upgraded for even higher performance, the heads are first converted to dual plug. The pullback in ignition advance due to dual plugs allows for a big boost in compression without detonation. With boosted compression there are also fueling and cam changes that will now work .
So it isn't so much the effect of dual plugging on it's own that is so beneficial to performance as is it the other changes that dual plugging allows which really add to output.
A large close fitting squish band also helps prevent detonation from high compression, hence the drag racer's name for squish heads-quench heads. Of course the squish also adds power on it's own by creating more swirl.

Glen

Bathtub combustion chambers
 
IIRC it was Ricardo that 1st tried close metting surfaces with the concept the metal would suck the heat out of the feul in detonation prone rim areas so called it quench bands. This is separate phenomina for the slapping together to jet turbulance out into stalled areas at TDC when mixture is compressed to honey viscous.
 
On a cracked 8 valve Triumph head (TSS) I once had bronze skulls fitted. These are single pieces of bronze (one piece per cylinder that is) that form the combustion chamber shape and valve seats and plug thread all in one pre-machined piece, the head is then machined to accept it as a press fit. The skull also sits on the barrel, sandwiched by the head so it cannot move.

The point of saying all this? Well, IF (hence the original question) bathtub combustion chambers are good on Nortons, couldn't the same process be applied? Meaning, machine up accurate skulls of the required shape and design and then fit them into the Norton head? This means there would be no requirements for welding, so no damaging of the heat treatment etc and it would be a very repeatable process, for someone with the pre-requisite skills and equipment of course.

Waddya think Jim...?
 
Eddie, fitting bronze skulls is not a 5 minute job...more like 10 minutes :lol: But a nice project for someone with a well equiped workshop and plenty of time. ..like jim as!




Fast Eddie said:
On a cracked 8 valve Triumph head (TSS) I once had bronze skulls fitted. These are single pieces of bronze (one piece per cylinder that is) that form the combustion chamber shape and valve seats and plug thread all in one pre-machined piece, the head is then machined to accept it as a press fit. The skull also sits on the barrel, sandwiched by the head so it cannot move.

The point of saying all this? Well, IF (hence the original question) bathtub combustion chambers are good on Nortons, couldn't the same process be applied? Meaning, machine up accurate skulls of the required shape and design and then fit them into the Norton head? This means there would be no requirements for welding, so no damaging of the heat treatment etc and it would be a very repeatable process, for someone with the pre-requisite skills and equipment of course.

Waddya think Jim...?
Click to expand...
 
If you were going to use dual plugs, it might be wise to use double ended coils to fire both plugs together on each cylinder, otherwise you'd never know if one plug had stopped working. Even then if you fouled a plug you'd probably have to change two.
 
I suspect if bathtubs worked reasonably well on Nortons there would be more examples about by now. Mez was doing his work some years ago.

IIRC Seeley 920 has a bathtub head, and if I understood correctly 'could never really get it working properly'. Which I though meant clean carburation as much as anything else. I assume you would end up with very high compression and need specialist fuels.
 
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