Firing order

[OzzyMike]

An interesting question regarding the Jowett Flat Four Engine spark-plug firing order.

I have the firing order listed as 1-4-2-3

An internet search regarding firing orders for flat four engines lists 1-3-2-4 for the Subaru and 1-4-3-2 for the Volkswagen.

Is !-4-2-3 correct for a Javelin or Jupiter?

Does anyone know of another make of flat four engine that used the 1-4-2-3 pattern?

Regards from Ozzy Mike in Australia.

P.S. This unusual firing order probably gives the Jowett flat four engine its distinct exhaust note

[Forumadmin]

Yes it is correct and unusual. One problem is that it causes uneven load on the carbs requiring a large balance pipe.

[OzzyMike]

I remember, as a schoolboy in the 1970s, seeing a Javelin drive past the school gates.
I instantly knew what I was looking at as I recognized the unusual exhaust note!

Regards from Ozzy Mike

[David Kemp]

reading a bit about flat fours in wikipedia, it says conventional flat fours firing order causes poor exhaust scavenging. Is this why Jowett chose their firing order? To improve exhaust scavenging?

[Forumadmin]

It is worth comparing the exhaust and inlet manifolds of those three cars with different firing orders. Note only if the cars are not super or turbo charged.

The principle of the manifolds' designs are to provide either scavenge for exhaust or ram for inlet at the maximum power revs. Ideally a four branch manifold allows tuning of each individual pipe (similar to an organ pipe) where the distance from the valve to the branch and then to the silencer (where the gas is essentially vented or damped) governs the standing wave set up by the opening and closing of the valve.

When a shared or siamese port is used the situation is not ideal, but with careful design the reflection (from the branch or silencer) of the pressure wave after one port opens is out of phase just before the closing of the port causing a sucking out of the remaining hot gas and the sucking in of the petrol mixture.

You might think that the gas in the exhaust is always at high pressure, since the engine is revolving so fast. But waves do occur and create low pressure, as well as high pressure, areas. It is use of these low pressure areas that travel up and down the pipe that can be tuned to get more power.

To explain this accurately read this.

[Jack]

So instead of the original design you could fit a manifold with separate pipes from each cylinder but you would need to carefully calculate the lengths of each pipe and where they join? Has nobody done this for racing etc to test it? I remember a discussion about twin exhausts, but from memory the feedback was that it didn't gain much.

Jack.

[Forumadmin]

Yes a few people have split the manifold. The offside cylinders require a bit of work to get the pipes pointing backwards though. I even tried back in the 1970's having learnt the maths how to calculate it all at Uni.
Could ask Frank Wooley whether it made any difference as he did the mod and raced . The only real way is to get it on the dyno.
Before you start all that, the engine needs to be good and the exhaust needs to be 'straight thru' so as to reduce back pressure. You then have to choose your rev peak where you want the improvement. I found inlet manifold tuning was much more important as that is where the problemm is on a Jowett.

[ian Howell]

It probably won't help much, but many years ago I worked for a firm that has recently taken over an outfit called Nerus Engineeriing that specialised in tuning - mostly Minis and Ford Escorts in those days.

I clearly remember having demonstrated to me the difference to the engine performance that a small 'trumpet' arrangement made when fitted to the inlet of a carb. These trumpets were aluminium castings, polished inside and - most importantly apparently - having a thick rim of apparently circular cross-section.

Unknown to me, the engineer we inherited from Nerus (who maintained our company's Escorts)- fitted one of these trumpets to my car during a routine service. The difference was immediately obvious as I burnt rubber just leaving the car park in my otherwise usual manner!

On another occasion he changed the back axle, forgetting the difference it would make to the indicated speed. This was pointed out to me by a policeman with a speed gun. Strangely he smiled and took no further action when I explained about the Nerus connection. How times change!

[David Kemp]

Thanks for the explanation, in theory I could mathematically design a perfect exhaust for driving in my most common drive revs. I know nothing is perfect. On intake design, my understanding of the Jowett intake was that Jowetts calculated the required volume of air needed to silence the engine, without killing performance that is why the long intake tubes & silencer in the hood(Javelin).
On the Jowett web there are pictures of Jupiters with headers, one lot goes over the engine, very dramatic!

[Keith Clements]

Trumpets for the inlet pipes are indeed beneficial although offer no silencing or air filtration. The shape is ideally paraballoid to provide a laminar non-turbulent flow. I made some and raced with them for many years. Since they were easy to remove I made the change on the dyno where the difference was obvious. Also of note was the loss of power when the standard AC air cleaners were fitted. As a result of this test, I used full flow racing foam filters for rallying and they are still on the car. These were fitted with Hoover vacuum cleaner bags in the Sahara desert!

On the Javelin the inlet system is not really designed for noise suppression, although having the substantial oiled air box where it is does helps damp the staccato beat. I changed the carbs to Delortos and found that they were upset being connected together via this air box and replaced it by fitting a pair of cylindrical paper air filters. The carbs starved each other due to the tuned pipe connection at about 65mph. This probably also occurs with the standard Zeniths and is yet another design flaw. The solution was found with the bonnet popped open a little and the car accelerated through wher e the flat spot was. Further tests with and without the oil bath filter proved the point.

Early Jupiters had the air cleaner behind the rad, so I made this set up out of a Renault 16 air box in preparation for the Saharan rally. After tests, I decided to stick with the foam filters, although for quiteness it was much better.

[Jack]

To get around some of the filtration issue, couldn't you just fit trumpets with filters on top? Or does that affect the tubulence (I would expect it to) of the air?

Something like the filter on the left of this pic would have similar amounts of filtration to the standard Jupiter metal filters:



Jack.

[Keith Clements]

For all but track use, I would recommend a good air filter. Even small particles of dust can seriously wear the piston rings and bores. A paper or foam filter inside and AC 'dome' might be the answer to maintain 'original' look. Alternatively go back to the early system where there is scope for using a larger paper filter.

The wire mesh is useless at filtering air but it might stop a stone going in and smashing the cylinder head.

[ian Howell]

7hp twins Firing Order

I have never checked the firing order in my 7hp but I was inspired to do so by this post which I came across whilst rambling through Jowett.Net and it rang a serious bell with me.

I took the cap off the distributor and sure enough the rotor arm was pointing to No.2 cylinder and the ignition leads were crossed over.

I decided to re-set everything, so I removed the distributor and carefully refitted it with the rotor arm pointing to the No.1 cylinder contact and making due allowance for the movement of the rotor arm due to the skew gears on the dynamo.

Next I refitted the ignition leads so that the plug lead outlet on the nearside of the distributor cap fed the nearside cylinder and vice versa for the offside cylinder.

Then, with the ignition ‘OFF’ and the advance/retard control in the ‘full retard’ position (I got that wrong once on my Bradford, and I still have a lump on my right hand!) I carefully reset the contact breaker gap and set the timing to TDC.

After finally checking with the ignition ‘off’’, I confirmed that all seemed to be correct. Choke out, ignition ‘on’ and away we went! The engine now starts easier and runs much more smoothly than before.

In an old newspaper cutting from April 1960 I also found a related article by Olaf Proli who apparently had similar problems in Norway at that time.