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etlanpa
United Kingdom
560 Posts |
 Posted - 09/10/2008 : 13:25:56
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| Does anyone have any info on how these manifolds work (on a 6 cyl), and what kind of diameters/lengths of tubing is required?? |
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David Allison
United Kingdom
665 Posts |
Posted - 09/10/2008 : 16:13:12
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This is an interesting query.
Like a lot of tuning aids the inlet manifold on a blown engine has very little influence on engine tune.
Unlike an unblown engine the blown car has the mixture forced in and therefore does not really benfit from anything other than a common sense approach to the manifold layout.
The main thing is that the capacity of the inlet manifold should be no smaller than the swept volume of the engine across it complete firing stroke.
That is engine size on a four stroke - so it is nice and easy to work out.
The standard MG 6 cylinder cast alloy manifold holds around 2 litres of water.
Smaller manifold displacements can lead to overheated mixture - larger manifolds increase the potential intercooling effect.
Robin Jackson developed a number of products for blown MG racing cars pre-war.
The works also tried these and other options against the standard works options and found very little dfference.
That said they were running 1100 cc 6 cylinder engines not the larger sizes we tend to favour now.
In the 1980's F1 cars - all the manufacturers played with inlet manifold designs and most ended up with a design similar to the pre-war MG design.
This is a tube feeding one end of a progressively larger tube - with individual feed pipes to each cylinder of equal length.
The main tube on the F1 cars was increased to upto 12 times the swept volume for the BMW engine in an attempt to counter boost lag - but it turned out to be a blind alley and they returned to a main pipe of around twice that of the engine capacity.
My NA has its original inlet pipe.
The main pipe is around 1 3/4 inch bore at the input.
The outer casing is around 1/4 thick and has alloy fins along the outer line just inside the bonnet.
The feed tubes are around 1 1/8 - but the outer casing is much thicker here.
The works racing cars were the same.
My experience of the Jackson manifold is of mixed opinion.
There is no power advantage (we tried it back to back the same day with a standard one on a rolling road) and there was a significant increase in mixture temperature.
This required us to run a harder plug for the Jackson manifold.
The standard manifold is inclined to oil plugs 5 or 6 if you run race style plugs and try and run at low speed.
The Jackson manifold tends to oil plugs 3 & 4.
This is because of mixture concentration within the pipes.
The Jackson manifold also made changing plug numbers 2 and 5 tricky - plus 3 & 4 requires universal joints and endless patience with hot plugs in particular!
The tubular steel manifold does offer an advantage - it allows you to modify the thing easily - which Jackson did several times during his development of the engine for Horton.
It is however a less than ideal material for making a manifold pipe because of the strain on the welds.
There is in my opinion no reason to use a tubular steel manifold though - use thicker wall material than for an exhaust because of the expansion pulses.
The standard MG blown manifold does not suffer with the problems associated with the steel tube - it is cast alloy (no welds) - runs cooler (thicker material dissapates the heat better) and polishes up nicely - mmmmmmm shiny thing!
Hope this answers your questions.
Regards David |
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Bob Stringfield
United Kingdom
854 Posts |
Posted - 09/10/2008 : 16:40:05
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Thanks, David, for providing this answer.
I now have a replacement blower and drive ( Thanks to Richard Hardy ), twin pump - as original - plus Ki-Gass and some of the other bits needed to return PA 0603 / APP 186 to its build-sheet and early photo spec. ( Could this start a trend? )
Would you be kind enough to do the same for us poorer folk with only four cylinders?
What obtainable manifold would you recommend?
The wartime owner of the car took photos of the car in blown form with the bonnet closed and by the '50s the blower had gone. |
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kimber
United Kingdom
1529 Posts |
Posted - 09/10/2008 : 18:22:02
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When I was building my Q-replica about 15 years ago, Wilkie Wilkinson came to my house with my father and looked at the car. He (WW) made several interesting observations which in one case related to the Jackson-style inlet manifold produced by Colvin Gunn. The comment was something along these lines:-
"For testing purposes, the end of the pipe* was not fixed and when the length was altered you could see the power output rise and fall on the dyno" (*pointing to middle one, I think). Unquote. |
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David Allison
United Kingdom
665 Posts |
Posted - 10/10/2008 : 09:44:57
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Bob - the P types fitted with blowers at Abingdon would have all had a cast alloy manifold similar to the 6 cyl manifold.
These manifolds hold around 1 litre meaning, this manifold is actually a little on the small side for a PB engine (especially with boost above 8 psi).
Andrew - you are quite right about changing the pipe lengths on the Jackson manifold - increasing the pipe length of the primary or secondary chamber will increase the capacity of the manifold and so influence power output and torque (but this only confirms my theory of larger manifold - better charge).
However make the manifold too large and you can end up with condensing problems (droplets of fuel forming around the inlet tract and falling into the cylinder un-atomised) - you would need to get your sums seriosly wrong though.
In my opinion most of the Jackson manifolds I have seen are a little on the small side for the engine capacity.
Also as a fabricator - I am always dubious of those using thin wall material which is mig welded and under pressure of expansion.
Phil Bayne Powell had one on the K3 in the 70's-80's - which was I think a genuine pre-war Jackson example.
This had quite thick pipework with a large capacity Primary and secondary chamber.
The sub connecting pipes (leading from primary to secondary) were around 3 times the diameter of the inlet valve feed pipes (common sense when you think about it).
However the K3 with a BTH mag and Jackson manifold made plug changes a major operation (it almost felt like you were dismantling the car) - there was very little room to work in.
I have seen later Jackson style manifolds using thinner pipe material and shorter lengths - but these were in my opinion a bad idea for the reasons already stated.
Wilkie did more work on blown racing engines than almost anyone in the UK both pre and post war.
The cars he prepared for the Evans family were always very well finished and went very well.
He told me though that he actually did very little actual tuning on the MG's until after the racing shop closed in 35 - because he did not need to.
After the closure of MG racing activity the pressure was on to keep cars competitive - without factory help that meant doing it yourself.
Wilkie used his experience combined with trial and error to prove and disprove a lot of tuning myth, fantasy and truth.
Most of the Bellvue cars were pretty much standard Abingdon specification - what he did find was small tweeks.
Opening up behind the inlet valves, matching ports and manifolds, minimising valve throats, maximising valve lift.
Wilkie was an example to all - use common sense and good engineering practice and you cant go wrong!
The BRM engine (probably one of the most highly forced induction racing engine used in road racing) used a simple single stage "log" style inlet manifold - mainly because there was little space for anything else granted.
Dragster engines (the most highly forced induction engines ever) use a simple "bucket" mounted between above the inlet tract, the feed pipes simply dropping to the inlet track.
All because - "if you force it in fast enough - it finds were to go its self" - this said to me at a meeting with a dragster engineer some years ago when I questioned his manifold design.
As with all things - plenty of common sense, reduces the amount of money required to put it right if it goes wrong and it probably wont!
Making manifolds (inlet or exhaust) is great fun and exasperating in equal measure.
Use thicker seamless metal for inlet than exhaust and use TIG welding where possible. TIG gives a more targeted heat and does not stress the metal as much as MIG welding.
Inlet manifolds on blown cars expand a great deal - the charge pipe on the big blower that used to be on the N special was 6 mm thick, but it stretched by + 3 mm across the plane in all four axis the first time we ran the engine at full power.
I would prefer to use aluminium for inlet manifolds however - it reacts better to the continual pressure and vacuum than mild steel.
It is also more resistant to the corrosion caused by condensation in the inlet tract.
Stainless steel just looks incorrect on a pre-war car but that is my opinion.
You can buy suitable pipe material for manifolds from most steel stock holders - although you will almost certainly need to order a minimum quantity.
Stainless steel is now very expensive - unless you have freinds in the food industry who can give you suitable off-cuts.
The dairy industry actually use suitable pipe for MMM engines and they also do elbows for bending around bonnet sides and chassis tubes!
Also this stainless steel is 316 grade which is a decorative steel and can be highly polished.
Aluminium is also pricey but again most of the pipe sizes required for a MMM engine are easily available.
The face plate against the cylinder head should also be as thick as possible - this prevents deformation and makes the fabrication easier.
I would use 8-10 mm plate.
Machine the holes in the plate to match the cylinder head, I would also dowel the face plate to the head and blend the holes to match the head as accurately as possible.
Once the manifold is all welded and normalised - have the face plate machined across the face to ensure it gives a flat surface for the register and gasket joint.
If you do make your own manifolds make sure the entire assembly is thoroughly pressure tested - a small leak can cause a big under bonnet fire.
Remember - common sense in everything.
Hope this helps
Regards David
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etlanpa
United Kingdom
560 Posts |
Posted - 10/10/2008 : 10:10:01
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I shall be using aluminium - 10swg or thicker
Thanks for all the advice!
Would 2 blow-off valves be adequate? |
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David Allison
United Kingdom
665 Posts |
Posted - 10/10/2008 : 12:14:44
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Blow Off Valves:
1 should always be enough to be honest!
Seriously though the positioning of the valve is more important than anything else.
The most common cause of a blow back is a backfire - usually from the plugs getting too hot (either too low a heat range or retarded ignition).
The standard MG set up is pretty neat although crude and effective.
A modified exhaust valve mounted over a steel plate with a spring to hold the valve closed.
The valve is machined the opposite way around - so the seat of the valve is on the base and the valve is pushed open by a pressure rise - letting the explosion out of what is often mistaken for a tiny carburettor.
The spring is normally set at a seat pressure just above maximum boost (the NA is set at 16 psi and it has around 10-12 psi boost).
A lot of people make the mistake of setting the seat pressure to high - to prevent the backfiring.
In practice this makes the backfire more sudden and potentially damaging.
It is better to have the thing bleeding off too easily than firing off and breaking the blower drive shaft with the shock loading.
I have seen a number of blown engines with multiple blow off valves, but in practice these work no more or less effectively than a single valve placed centrally along the feed pipe.
The potential for backfires increases with boost - the more boost - the higher the charge temperature and the higher the potential for overheating the charge.
MG engines tend to use harder plugs than almost any other pre-war racing engine I know of - anything you can do to reduce the charge temerature is good and gives a better choice of plug to suit road or race conditions.
My standard Abingdon set up running on 99 octane fuel (running a little over rich) runs a BP6HS NGK plug for road use and a 7 for racing (sometimes 8 for a really hot day).
The harder plugs wet up too easily for road use and the 6's are too soft for sustained wide throttle openings.
The rule of thumb is the more boost the more likely the blow off.
Also the smaller the manifold the more likely the blow off.
On a Jackson manifold the blow off was placed centrally along the secondary - but I have seen others with 2 - 3 and in one case 4 valves.
I am a simple soul (lazy too) the more bits you introduce the more complication you potentially cause - my advice stick to one!
Have a look at some cars to for inspiration - but most of all try to make the manifold look right.
Hope this helps
Regards David
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Gordon
United Kingdom
692 Posts |
Posted - 10/10/2008 : 14:19:53
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For what it is worth here are some thoughts based on manufacturing a 'Jackson'type inlet manifold on my PB.
It is running at ~14psi max boost and the manifold temperature close to the engine after full power running is about 30 to 40 C. I use the standard twin SU inlet manifold matched to inlet ports and then a fabricated stainless steel plenum chamber with 2 branches to the original inlet manifold.
The feed to the plenum chamber is central. The plenum chamber is approximately 2.5" diameter. The feed to the plenum chamber from the blower passes under the radiator but above the axle - yes there is enough room with suspension fully compressed! It is all made out of food industry stainless steel, professionally tig welded, pressure tested and stress relieved.
When building this manifold and blower installation a major concern was to avoid the damage from a backfire and I so wanted maximum blow off area and rapid response. I therefore followed dragster practice and have a ~2" port sealed by aluminium sheet (cut from lithographic plate scraps). There is a ring clamping nut with a viton 'O' ring seal. I must admit that I didn't do the nut up quite tight enough to start with and blew the disc out at full boost! The noise gave us quite a fright!
My intention is to replace this with a standard blow-off valve in the future when I am confident in not having any backfires. So far I have had no backfire at all.
Sorry about being vague on pipe diameters but the car is in England and I am in S.Africa for the moment.
I can provide some pictures via e-mail that show the assembly if this is of any help to anyone.
Part of the reason for going this route was that it was much cheaper than buying repro manifolds and I preferred to have the pipe from the blower concealed - ie not going around the side of the radiator.
Whilst this is my own design and not be to everyones taste at least it requires no modification to any of the original bits of the car and can be easily removed by a future owner if he wished.
Hope this was of interet and may be some help,
Regards
Gordon
Gordon |
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David Allison
United Kingdom
665 Posts |
Posted - 13/10/2008 : 09:42:02
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Gordon
I have seen pictures of your installation and confirm it is very neat and tidy.
There is a lot of really useful information available on supercharging - most of it based on turbos it has to be said but the rules are pretty much the same.
The important thing is to maintain as low an induction temperature as possible - the reason MG directed the inlet tube around the radiator was because they found improved power in doing so.
More by accident than anything else but what they did was produce a very crude air to air intercooler.
The inlet pipe also runs very close to the bonnet side - this also provided a degree of intercooling.
However at the time engineers actually believed that heating the inlet gases was beneficial and went to lengths to avoid cooling.
Reg Jackson and Cecil Cousins noticed the inverse of this knowlege during development of EX 120 and the C type.
When they went to the AB cylinder head, the inlet pipe originally went under the radiator and vertically up behind it.
On EX 120 with progressively larger blowers they noticed the number of blow backs increased - the cowling on the radiator to improve streamlining was also preventing enough cooling air passing through the radiator leading to heat build up inside the car.
Eventually the blown version C type was found to suffer the same problem, leading the works to remove the cowling.
On the J4 the problem returned (bigger blower) but the engine was not overheating.
Jackson reasoned that diverting the inlet pipe might work - and it did.
Backfires are always caused by an overheated inlet tract.
This can be internally (plugs to soft) or externally (hot weather) both amount to the same thing - change the plugs for a harder grade.
This is my last post on this site - I hope my jottings have been worthwhile.
I had my last drive of NA0307 on Saturday - it is sad when you have to give up something you love - but for the good of the car - it has now passed into my sisters ownership.
Regards David
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Gordon
United Kingdom
692 Posts |
Posted - 13/10/2008 : 12:13:49
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David
Many thanks for your post and all the other ones you have made - always to the point and filled with knowledge and good advice.
I will greatly miss your postings and you will leave a void.
All the best
Regards
Gordon
Gordon |
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McEvoy
United Kingdom
252 Posts |
Posted - 13/10/2008 : 12:56:01
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McEvoy & Pom developed an " air " intercooler for the R type that comprised a well finned interconnecting tube between blower and manifold, I've no records of how effective it was.
David I echo Gordon's comments entirely, I 've enjoyed your posts always informative and thought provoking, they will be missed
all the best - Bob |
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Bob Stringfield
United Kingdom
854 Posts |
Posted - 13/10/2008 : 20:18:37
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| Seconded. Many thanks, David. |
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JMH
United Kingdom
911 Posts |
Posted - 14/10/2008 : 20:42:57
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A late comment, but I've been away from home.
The original Jackson 6 cylinder manifold from K3007 ended up on K3003. When PBP rebuilt 003 he used it on that car (as far as I know it's still fitted)& 007 had a replica made. I can honestly say that it makes plug changing an absolute b***er to do! It's even worse when they're hot. The increased total volume of the inlet tract gave us issues which we believed (rightly or wrongly) centered on the methanol "puddling" & wetting plugs, which ultimately resulted in myself limping of the startline at Silverstone on 1 1/2 cylinders (the M types must have been dead chuffed). We changed to the std 1934 pattern & found it much better. Ultimately the Jackson design offers better mixture distribution, but is best suited to higher flow rates. The std 1934 pattern has (for us at least) offered a more drivable set-up. I guess the theory is pretty similar to the split vs std Weber set-up people used to use on the BMC A series.
Regards
JH |
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