Javelin exhaust

[richard turner]

Was there a reason Jowett ran the exhaust system for the Javelin in the way that they did? Could it be modified to a twin pipe system from each head? I seem to remember this caused exhaust flow problems but have forgotten the details.

[Jack]

Hi Richard,

Another thread I asked the same question, some answers there but worth asking on that thread if you are thinking differently.

http://jowett.net/forum/viewtopic.php?f ... it=exhaust

The summary seems to be that the gases going through the exhaust pull gas through the relevant other valve as they pass the join. It isn't clear quite how much effect this has, but that is the engineering reason why it is done.

Jack.

[paul wilks]

Now I am NO engineer or even mechanic
BUT my understanding is that over the many years I have been a member of the club different members have asked the same question!
Some have even tried the 'twin exhaust' "solution" only to find it didn't work!
No sense in trying to re-invent the wheel and go down a path that others have tried- without success!
Perhaps someone who has tried it would like to comment?

[Chris Gibson]

To get any improvement over the Javelin system look at the Jupiter where the pipe from the RHS cylinders joins that from the LHS after the LHS manifold so the RHS gasses do not go through the LHS manifold.

The Motor Sport review of Javelin engine development by Bill Boddy states that routing the exhaust system around the front gave a 1.5% power increase compared with two separate pipes leading from the original siamesed exhaust ports inside each cylinder head.

Looking at two later horizontally opposed engines, the systems used on both the Lancia Fulvia and the Alfasud have separate pipes leaving each cylinder which are then siamesed in pairs near the gearbox and the join up to a single pipe behind it. Both were front wheel drive cars so there was no problem with prop shaft interference.

Photos on the attached doc.

Lancia & Alfasud exhaust.docx

It would be difficult to emulate the above systems on a Javelin because the cylinder heads are not handed, therefore the exhaust ports in the right hand head are angled forward (and those in the LH head rearwards).

The Citroen GS engine had a rather weird system where the individual pipes from each side joined together towards the front of the heads then turned back under the block to eventually meet up with a single pipe between the front seats, BUT there was a spherical damping chamber on the left hand 'manifold' only
Try putting 'citroen gs exhaust system' into Google and click on 'Images' to see what I mean.

I don't know what the Ferguson R5 used as it is not visible on any of the info I have. Nip into Coventry Motor Museum and have a butchers!
If anyone is interested I have scanned a copy of the March 1965 'Automobile Engineer' analysis of this engine [11 pages with drawings etc]

[BobCulver]

Older cars seemed to get by with very basic exhausts. The Consul had a simple gallery and even the Zephyr used the same despite the special disadvantge of a common manifold with a 6. The normal 4 cyl principles as in many text books apply. Ideally successive firing cyls should be well separated, as moderns. And the stub attachment point for these cyls and length is important. The normal 4 cyl configuration very difficult to adopt for a Javelin.
There are two flows in a pipe; the moving gas and pressure pulses moving at the speed of sound. it is the latter which fancy pipes harness. The pulse from an exh well opened should not arrive at another just as it closes. As the pulses turn corners easily the Jupiter system probably does not make much difference, but it does keep the heat off the LHS valves. Some books claim that the notion of moving gas having a suck effect is largely fiction.
I suspect LHS pressure pulses reflecting delayed from the blind RHS end contribute to the curious Javelin exhaust.

The VW Beetle kept the four pipes separate into the silencer, where the pressure pulses would be attenuated.

[Keith Clements]

Sorry Bob you do get scavenging through negative pressure. Please read this.


We've all heard the magazines talk about 'back pressure' (and most of what they say is utter cac) so how do we ensure we have a good, efficient, exhaust system (bearing in mind that 'back pressure, any back pressure, is a bad thing)?

Well, some history:

Back in WW2 the RAF Spitfires ran, what were essentially, open headers (they called them 'ejector stacks'). After the Battle of Britain, when they were looking to use Night-fighters, in order to cut down the exhaust glare, they ran aircraft with extended exhausts and found that engine power was increased.

Nothing then really happened until the late 50's / early 60's when the Japanese started producing 2-stroke M/C engines. - These rely upon exhaust efficiency to work.

Shortly after this people like Colin Chapman and the men at Coventry Climax started producing the 'bunch of bananas' exhausts.

So what was / is going on?

Aside from restrictions in the Silencer (which cause back pressure! and which we don't want in an efficient system!), the lengths of the Primaries / Secondaries / Collector affect the 'pulse tuning' of any exhaust.

Consider a church organ - different pipe lengths and bores make different notes. The sound you hear is due to the resonance or 'standing wave' that is set up as the air passes through the pipe. This standing wave has both a negative pressure component and a positive pressure component, the wavelength being directly related to the sound you hear because wavelength is directly proportional to the inverse of the frequency.

Now it's the same with an exhaust since it is effectively a pipe flowing gasses. Firstly we want the gas pressure in the exhaust to be lower than that at the cylinder head to assist scavenging through gas inertia. Secondly we don't want the exhaust gas of one cylinder to pressurise another cylinder.

Here comes the interesting bit. By altering the length and bore of the primaries and secondaries we can ensure that the negative pressure component of each exhaust pulse reaches the cylinder head when the exhaust valve is open, thereby further assisting cylinder scavenging. This will depend upon engine rpm and the valve opening time, i.e. exhaust valve duration. So, for example, on a 4-cylinder Jowett engine, we can use the negative pressure pulse from no 1 cylinder to assist the exhaust scavenging of no 4 cylinder.

What we are doing is pairing cylinders that are 180 degrees apart. In Jowett Javelin case that is 1 will help 4, 4 will help 2 , 2 will help 3 and 3 will help 1 to scavenge the exhaust and suck in the charge.

Where it gets really 'tricky' is if we use a wide valve overlap, i.e. both exhaust and inlet valves are open at the same time (hence they 'overlap'), we can use (in the example above) the negative pulse from no 1 cylinder not only to assist the scavenging of no 4 cylinder, but, because of the negative pressure and the fact that no 4 cylinder's exhaust and inlet valves are both open, this negative pulse will actually assist in sucking the new inlet charge into the cylinder. Hence gains in power and torque.

The downside is that this will only work perfectly at a given rpm. If you tune for max power you will inevitably reduce the torque lower down and 'close up' the engine's 'power band'. This is why race engines idle badly with associated popping and farting and lumpy idle rpm.

Similarly engines with a wide torque spread produce less peak bhp.

As with everything there is a series of compromises being made

Exhaust tuning theory is actually fairly simple; it's all about getting the negative (and, hence, scavenging) pressure pulse to arrive at the exhaust valve as it is opening. To do this we have to set the pipe lengths and diameters correctly.

The formula for Primary pipe length is:

P = [(850 x ED) / RPM] - 3

Where:
RPM is the engine speed to which the exhaust is being tuned. Say 5000 rpm on a reasonably balanced Jav with standard cam. 7500rpm if you are brave and have done a lot of work.
ED = 180deg plus the number of degrees the exhaust valve opens before BDC which is 50 deg in the normal Javelin camshaft.
P = Primary pipe length (on a 4-1 manifold), or Primary pipe length plus Secondary pipe length (on a 4-2-1 manifold), in inches.
That works out at 36.1 inches.

Generally road engines will require the manifold to be tuned to the max torque rpm whereas race engines will be tuned to work either at max bhp rpm or a speed midway between the max bhp rpm and max torque rpm.

4 -1 manifolds restrict the power band, whereas 4-2-1 manifolds give better mid-range power but reduce top end power by as much as 5-7%.

Generally speaking with a 4-2-1 manifold the starting point for Primary pipe length is 15 inches, thus Secondary pipe length is P - 15 inches. Changing the length of the Primary pipe tends to rock the power curve around the point of max torque. Shorter Primaries gives more top end power but less mid-range, and vice-versa. There is, however, little change in the peak torque or the rpm where this occurs.

Ideally the Primaries should come off the cylinder head in a straight line for around 4 inches before any turns occur.

Inside diameter of the pipe can be gained from:

ID = sq root [cc / {25 x (P + 3)}] x 2.1

Where:
cc = cylinder volume in cc. which is 1498/4 cc for standard Jav.
P = Primary length in inches.

That gives a required Internal Diameter of 0.9 inches.

In some engines it can be useful to have a 'step' between the exhaust port and the Primary (ie the Primary bore is greater than that of the exhaust port). This tends to be the case in engines with rectilinear exhaust ports.

For a 4-2-1 system then, Primary pipe diameter is calculated as above. Secondary pipe diameter is given by:

IDS = sq root (ID x ID x 2) x 0.93

Where:
ID = calculated inside diameter of the primary pipes.
For the Jav as above this would be 1.17 inches diameter for the secondary pipe.

The pipe diameter can be used to change the peak torque rpm, a reduction in diameter of 0.125 inches will drop the peak torque rpm by 500-600 rpm in engines over 2 litres and by 650-800 rpm in smaller engines. Increasing the pipe diameter by 0.125 rpm has approximately the opposite effect.

The total length of the Collector and Tailpipe (to the front of the silencer) should be equal to P + 3 inches (or any full multiple of P + 3 for a road car).

Tailpipe internal diameter is given by:
IDT = sq root [(cc x 2) /( (P + 3) x 25)] x 2

Where P is calculated as above.

For our case that is 1.75 inches.

The 4 primaries or 2 secondaries go into a collector which is a tapered tube.
Collector length is given by:

CL = [(ID2 + ID3) / 2] x CotA

Where:
ID2 = diameter of Collector inlet
ID3 = diameter of Collector outlet.
CotA = Cotangent of angle of Collector taper (which ideally should be around 7-8deg (certainly less than 10deg).

The design of the collector should be such that the inlet pipes terminate abruptly otherwise the tuned exhaust pressure wave will carry on into the tailpipe and the calculations done to get the negative scavenging wave back to the exhaust valve on time will all be wrong.

Some work needed to design this bit with a 4 inch inlet and 2 inch outlet , you need a 24 inch collector.

THE standard textbook on exhaust design is PH Smith's "Scientific Design of Exhaust & Intake Systems". Still in print after more than 40 years, there aren't that many books that last that long.

Taken from here

Also see here for a system for a VW Beetle

More research in this paper that also looks at some of the extensions to the theory used in competition such as in F1.

The reflection of the pressure wave back to the same cylinder can help with scavenging. When the refective wave returns from where it expanded it returns as a negative pressure reflection . This is why the exhaust valve opening time is so important in the calculation. The refection should arrive just before the exhaust valve closes. In the Jowett engine (as in most) the inlet valve is already open so the negaive pressure sucks more charge into the cylinder. A later reflection can also be designed to arrive on another cylinder helping to extract more of the spent gas during the exhaust stroke .

Of course, the same can be done on the inlet manifold but this is complicated on a Jowett with the balance pipe and siamesed ports in each head, plus the uneven loads on each carb where cylinders 1 and 2 are competing for charge with 3 and 4 respectively. I demonstrated that the air box on the Jav upsets the carbs above 70mph by the standing wave set up across it. The bonnet flying open a notch increased power dramatically.

[Andrew Henshall]

The way to get a significant increase in power from a Javelin or Jupiter engine is to fit a set of extractors where the primary and secondary lengths are tuned to develop specific engine characteristics (top end power, or mid-range torque, etc). As others have pointed out, this is not easy to do on our engine because of the packaging constraints.

Joe Caudo has developed a very effective set of extractors for his rally Jupiter, but it is a real "plumber's nightmare"! see below:

DSC04447_s.jpg

DSC04449_r.jpg

The driver's side primary pipes run forward, then up & over the engine between the distributor & carburettor, and down the back, where they join to form a secondary pipe. The primaries from the other side pass behind the passenger side carby, and eventually join, then meet the secondary from the other side above the gearbox! This extractor system gives Joe 8 additional horsepower on its own vs the standard exhaust system.

Before anyone comments on the large diameter of the primary pipes, you need to know that this engine runs liners made from VW barrels, with way oversize pistons, a high compression ratio, a thick strengthening plate between the crankcase & the sump, thick strengthening plates replacing the tappet covers, big carbies, Carillo con-rods, and it loves to rev all day to 6500 rpm!

Andrew

[Keith Clements]

Good photos Andrew. I would love to see the view above the gearbox and from under the car. Seeing how the four pipes come together and how the collector and silencer are fitted would be very instructive.
My GTB Subaru acheived 300BHP from a flat four, OK it had a couple of turbos to help and its exhaust system was bigger than a drain pipe.
Our normally aspirated Subaru Outback had quite a complex exhaust with a pair of pipes coming around the front like the Jowett to join the other bank of cylinders. The system was all covered in fibreglass with an outer casing so difficult to see how it all fitted together.

[Andrew Henshall]

Thx Keith,

The Caudo Jupiter was standing on the floor in the workshop at the time so I was not able to photograph the undersides. It's a 9 hour drive from my home (each way) to get the photos you want, so you can understand if I don't head there today! These two photos might help people understand just how complex an installation it is (sorry about the blurred one).

DSC04445_s.jpg

DSC04446_s.jpg

Cheers,

Andrew

[richard turner]

Thanks to all who have contributed to a most comprehensive answer to my question. I will leave the exhaust as standard!
Rich

[Forumadmin]

I thought I would take some rough measurements as I was under the Jup today. The distance from No4 valve to Y join is 55 inches the distance from Y join to first expansion box is 55 inches. Distance from Y to No1 valve is 12 inches and from No 3 valve is 7 inches . So the resonant length is between 62 and 67 inches.

Does that give B flat, or C sharp?

There will also be a secondary resonance at 105 to 110 inches for No 2 and No 4 respectively.
The expansion box is 16 inches long, the intermediate pipe 60" , the silencer 12" and tailpipe 11 inches. The latter was about two inches longer as we cut a bit off to cure the midship bearing rubber rattle on the Baltic trip in Hamburg.

So now plug that into the equations and see for what revs they tuned the system.

The Javelin front pipe is about 36" between No2 and No1.

[Keith Clements]

A 62 inch resonant length is 3000rpm and a 67" is 2800 rpm.

The Javelin length between No1 and No 4 is about 43" which is 4200rpm. But the pressure trace is going to be very complex as each cylinder will interact, and there will be reflections back from the expansion box. Some of these pressure fluctuations could be benefical (negative) but I suspect many will be positive and harmful sending exhaust gas back into the cylinder on cam overlap.

[BobCulver]

The point I was trying to make is that it is not the moving gas which imparts extractor effects but reflected (reversed) pulses. As these go around corners the Jav and Jup systems effectively much the same. The stock tuned layouts harness the pulses but hard to fit to a javelin. Seems positive pulses reflected from the blind rhs unlikley to assist lhs.

[Keith Clements]

Yes Bob, the moving gas would have a sucking effect like in a Pitot tube used for air speed measurement or in a spray gun but this may be negligible compared to the reflected pressure waves caused by the valve opening. I say 'may' because my knowledge and the data available on the Jowett configuration is incapable of resolving.

Perhaps we need to put some pressure tranducers close to each exhaust port and connect to an oscilloscope or more modern display device. If only I was back at the lab in University!

[Keith Clements]

This is the 4 branch exhaust from a Jav I found lying on the floor at the Auckland shed when I visited. Here is the full view of the system.

There are over 1000 pictures in this album which is why I did not put into the Gallery. I should really sort out into those particularly relevant to Jowetts! The danger is that not being in the Gallery. they could become unlinked in the future and lost to the Jowett world.