Why does the exhaust go round the front?

[Jack]

A bit of idle thinking about engine rebuilds, Javelins and Jupiters, life, the universe and everything else this morning while driving around the M25 lead me to a fairly simple question, the answer to which was not 47.

Why is the exhaust from the off side routed around the front of the engine, providing a lovely combination of hand-burning temperatures in a convenient location for anyone attempting to work on anything while the engine is hot (we've all done it), and also increasing the temperatures in the engine bay of both the Javelin and Jupiter (I assume that a big hot tube at the front of the engine bay is likely to result in increased temperatures, but welcome any actual evidence around this)?

Thoughts please - I haven't looked at the layout of the exhaust ports yet, but it can't be a huge amount of work to use a near side exhaust manifold on the offside or fabricate one to fit can it? Other than purist concours reasons, why doesn't everyone make this change? The only obstruction from memory is the fuel lines coming up the offside underneath the chassis on the Jupiter, and perhaps the lack of hole through the chassis on the Javelin, but these should be fairly easy to resolve wouldn't they?

It would also facilitate running twin exhausts, which might have a limited benefit for competition, probably lost in the additional weight, but even if they were rejoined behind the engine it would avoid the burning hot exhaust tube round the front.

Jack.

[Chris Spencer]

Someone has probably already done it - when building the manifolds for my race cars many years ago (V8 engine) - all pipes were made the same length in order to balance the exhaust flue exit - it would make sense if you could likewise with the Javelin / Jupiter engines - or just leave the exhaust in it's normal route and have it ceramic coated - it reduced the under bonnet temp of my V8 Rover by over 30% by having the manifolds coated.

[paul wilks]

I could be wrong(!) but many years ago I did read of a member coming to the same conclusion and fabricating separate exhaust manifolds for twin exhausts pipes. I think it was a Jupiter but can't be sure. The results were (from memory) disappointing and the owner reverted to the original 'set up'.
Does anyone else remember the details or have I dreamt it?

[Keith Clements]

Frank Wooley.
Note that the basic principle of exhaust tuning is that the pressure wave caused when the exhaust valve opens regularly, creates a wave with positive and negative pressure points between the valve opening and the expansion box or end of the pipe. Although I did some theory at University on how to design exhaust systems there are so many variables that theoretical design is virtually impossible.
The challenge is to get the negative pressure part of the wave to hit the valve just as it opens to aid the exhaust of gas from the cylinder and to do this at the chosen revs that suits the cam profile you are running.
The design of the Jowett Engine and its firing order does this reasonably well probably at about 3000rpm.
Race cams and higher rev peaks would benefit from a redesigned exhaust system that would require separating the exhaust manifold and 4 branches tuned to the new flow characteristics and frequencies.
Is it worth the trouble? It would be almost last on my list after balancing, porting, carbs, tyres, cam, aerodynamics and gearing. The most you might get would be 5 bhp.

I think Frank will say that he did not think it made much difference, but it was worth a try.

[Keith Andrews]

Being a V8 (American) enthuist, I have often wondered about the jowett set ups..
And further to what Keith above talks about..
With V8s getting up in the 350/ 450 cube inches....dual exhausts are the way to go...
For low end rpms/ torque one goes for factory cast truck rams horns for small block chevy, down into approx 2 1/2" diam pipes with a H or X cross overs..
For higher rpms / HP one goes to headers...(extractors) These are designed with primaries as closr to the same lengths going down into a collector, the min 2 1/2" diam dual pipises, again with H or X crossover pipes.
The length of the primaries depends on the rpm range / power band one want peak HP at ... outside that range the headers perform against premuim performance.
the postion of the X / H crossovers is determined by the pressue wave of the exhaust... this is a constant...and found by opening up the ehaust pipe and examining the rings / despostes formed in the pipe, and fitting the crossmembers to suit.
The advantage of fitting in the right postion is marginal and olny used in top competion where that extra 1/100 of a second is required....general racing / street ...just blow a hole in the side of each pipe with the gas torch and weld the cross pipe in... equal distant from the collectors.
Pipe size.. dual 2 1/2" pipes are more than enough for a 350 to 400 cube inch engine reving out to 6000/6500 rpms.

Is it worth the trouble? It would be almost last on my list after balancing, porting, carbs, tyres, cam, aerodynamics and gearing. The most you might get would be 5 bhp.
I think Frank will say that he did not think it made much difference, but it was worth a try.

I would think a dual semi tuned system , if the old one is in need of replacing, would be worth the effort... about the same effort as aerdynamics....which unless have a flat front car... does realy come into play till over the 90 / 120 mph range.
Something well documented with drags cars and hot rods....a flat nosed T bucket running mid 13s at 90 odd mph with a given engine doent get significat advantage of aero dynamiocs....but if wants to get down into 11s secs or lower some very expensive serious hP is required from there..

[Jack]

So what we are saying is that it wouldn't have a detrimental effect on the running of the engine, there is no physical reason why it isn't possible on the Jupiter as it has been done, it would just require a bit of thinking and engineering perhaps to do the same to a Javelin?

I might just give it a little go then, and see if we can route it round the back to connect up with a single through pipe. It isn't really for performance reasons, just that if there were obvious performance reasons for running the pipe round the front that we wouldn't want to lose those. Really it is to reduce engine bay temps and avoid burnt hands (particularly on the Jupiter) but would be at the expense of concours originality I suppose.

I got thinking about it because we have the louvred front valance on Project B, and if they were trying to increase airflow and reduce temperatures, re-routing the exhaust makes sense to me. Once we've had a play with it (several months away) we'll post some photos.

Jack.

[robert lintott]

I am not sure that separating the exhausts would not have a detrimental effect . I recall investigating this many years ago and the conclusion was much the same as Keith Clements ie the firing order meant there was an advantage in tuning the system to operate more efficiently as a result of the offside gases alternating with the nearside in the single pipe .

If heat is a problem it would be possible to make a metal shield around the danger areas as is done with motorcycles and some racers with external pipes . Most of the surrounding hot air goes under the engine I would think, if not a baffle could be made up to direct it downwards . Bob

[Forumadmin]

It is one hell of a lot of work, and you could, and probably will, have a detrimental effect. There would need to be a lot of experimentation, instrumentation and rolling road trials to prove it was any better than the original.

It is very difficult to get the offside bank of cylinders to point their exhaust ports backwards. You have to modify the heads and manifolds as there is no room to bend pipe with sufficient radius through 180deg, .You will probably loose ground clearance if you want not to have a siamesed manifold. You have to bring No 2 exhaust next to the rocker cover and through to join the other tubes just beside the gearbox. It was OK on Frank's racing Jup as he was not worried about ground clearance , but it was no good for a rally set up.

Heat from the front exhaust is not a problem, indeed in damp conditions it is a positive benefit to reduce icing of the carbs. See Mini and the like that put their air inlet over the exhaust in winter.

Note my Subaru Outback had a similar set up to Jowetts with the exhaust going across the front. Sure the GTB had twin Turbos and thus somehow managed to accomodate all the pipework, but Turbos do not use exhaust tuning like normally aspirated cars. If it is good enough for Subaru in the 21st century it should be good enough for all but the most tuned racing Jowett. Note the whole engine needs to be tuned to take benefit of any modified exhaust as it is all about gas flow from inlet through the cylinder head and out again..

[george garside]

I remember Horace Grimley explaining in some detail about the amount of work involved in arriving at the best exhaust layout for both Jav & Jup - unfortunaltely I can't remember the detail but running the pipe round the front had I think something to do with getting an extractor effect and this was superior to 2 separate pipes. Interestingly the highest tuned factory jupiter, the R4, retained the ''round the front'' exhaust and Jowetts would ssurely have used 2 separate pipes on the R4.. if a few bhp could have been gained by so doing.

I can't see the 'round the front pipe contributing greatly to underbonnet temperature as it was directly in the wind at the front. The substantial inlet manifold on inline engines, being in the middle of the engine compartment and behind the radiator in a relatively enclosed space probably contributes far more heat to the engine compartment.

george

[Tony Fearn]

A bit of idle thinking.

Seems to me Jack, that having read all these informed comments, you might just leave well alone and wear welding gloves occasionally.

Tony.

P.S. Like the pun!

[Keith Andrews]

Firing order would be an issue as mentioned above...and rather than 2 exhausts , 2 into 2 , 2 into 2 with 1 exhaust....with such a small engine and low rpms flow would not be an issue.
The other effec of headers is cam timing, with extraction, there would be a tendecy to pull that concentated poll pf mixture that hangs over the inlet valve, right thru and out the exhaust...
Runs the engine lean with very rich exhust
One would be very suprised the number of engines that died a slow and at the end spectacular death because if this...

[Jack]

The drawing board beckons.

Some interesting ideas above, and useful references for others who will no doubt have the same idea. I think I would have attempted it by fabricating a new manifold - the design is fairly straightforward and shouldn't be hard to do this, but it doesn't sound like the benefits are there.

One thing that does surprise me a little is the lack of any kind of shield or guard on the Jupiter particularly - just a thin plate from the bumper support would prevent hands (or anything else!) from coming to rest in just the wrong place. Everybody seems to have made the same mistake and touched it at some point.

Chris, does the special fancy coating on your exhaust manifold make it touchable at normal temps? I'd have expected it to still be too hot to touch, but if not it seems a sensible, albeit expensive, precaution for the front pipe. I've seen a couple of Jupiters with exhaust tape wrapped around this pipe, presumably for the same reason, but I don't really like the potential for tape to hide corrosion and hold water in the weave, and it looks a bit naff in such a visible location.

Jack.

[Keith Andrews]

the universe and everything else this morning while driving around the M25 lead me to a fairly simple question, the answer to which was not 47.

No, it is 42...

[Forumadmin]

My SA of course does not have the exhaust cover and bumper fitted, hence another Health and Safety issue that Jack is worried about. But Jack have you ignored the fan that cuts your fingers off, the fan belt and pulleys exposed to catch your tie in (not that you wear one), the rear bumpers that catch your shins, the bonnet that can gobble you up (or the boot on the Javelin), the hood frame that gets you when you get in or out, the top of the windscreen ideally placed to hit you head on in a crash......?

[Chris Spencer]

And my Jupiter chassis that you walk in to to with your shins when it is stood on it's side on the workshop floor - of course now we have suspended it to the rafters you just have to wait for it fall on your head !