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Bernard
United Kingdom
69 Posts |
 Posted - 22/05/2008 : 13:10:26
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I am newish to MG's and in the throws of rebuilding my J2 engine. The new bronze guides obtained are some 12mm longer than the steel ones removed. This increased projection into the port will reduce the available port area by some 7-9% and as I am not supercharging this seems to be significant. I am considering reducing the inlet guides by some 10mm to assist gas flow and leaving the exhaust longer to improve heat transfer from the valve. This may of course increase wear on the inlet guides but how seriously?. Has anyone done this or have comments. I vaguely recall in my youth that reducing guides was a standard performance enhancer. Appreciate your comments.
Great forum thanks for whoever set it up. |
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David Allison
United Kingdom
665 Posts |
Posted - 22/05/2008 : 16:09:37
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Bernard
The same is true of all engines really - the length of the valve guide does impede the porting - however the difference is so little on a MMM road car engine as to be unimportant in my personal view.
If you are planning to use the car for competition use - then this is a different story!
Every little advantage you can gain is worth it.
You can always shorten the guides to match the iron ones - the bronze guides are easy to machine at least.
The head boffins all have different views on the influence of the guide on gas flow and in truth it is probably only on ultra high revving engines with high compression ratios that gain maximum benefit of such small advantages.
I know from my own experience with the A series BMC engines, that Les Ryder (ex-Weslake and Coventry Climax) always left the guide protruding into the port by quite a way. I simply copied what he taught me.
My engine never seemed at any disavantage and later I tried another head with the guides blended into the port and found no advantage at all (in fact that engine was always a badun for a variety of other reasons).
The biggest influence you can have on a MMM engine when setting up the cylinder head is to take your time and make sure the valve lengths are all correct and the rocker fingers and cam timing are correct. The number of people who have tred and failed to find a simpler and faster way of doing the work are legend - if they find an advantage it takes longer (and MG had already tried it), some persuade themselves they have an advantage - NAH!
Read the manual - Blower is correct - it is the bible according to St Jackson and St Cousins (they tried every trick in the book in the 30's why re-invent the wheel) - it works and the cars always run better set up this way.
The newly available rockers without the adjuster are worth a try on a competition engine - but the adjuster does give a small margin of adjustment for a road car which is worth having.
Andrew Smith did a lot of tuning work on his PB - he took the compression up and used very small land accross the valve lap face - he does read the posts and might be able to give you some extra tips.
Hope this helps - sorry it is so long.
Regards David
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Bernard
United Kingdom
69 Posts |
Posted - 23/05/2008 : 13:03:47
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Hi David,
Thanks for your reply and aditional information. It does seem strange that with all the references to polishing ports and improving gas flows it makes no difference to then leave a block of flats in the middle of the port but I am certainly no Harry Weslake.
Thanks again
bernard |
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David Allison
United Kingdom
665 Posts |
Posted - 23/05/2008 : 14:04:07
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Bernard
It is more to do with the amount of work you want to put in and what you get back in return - very high revving - high compression engines re-pay much easier on small gains than low revving ones.
The head design is quite intelligent - the ports are straight and have a steep entry and exit ramp to speed up the gas flow.
The area around the valve guide and stem are quite well thought out too (although you can improve things here) and the swirl effect caused around the guide and stem of the valve could be considered beneficial.
There is a need to clean up the area around the valve seat, both inside the port and also around the valve head inside the compression chamber - the engine also benefits from a higher compression ratio (although you have to be carefull not to over do things) and larger carburettors (again dont over do it).
The AB head came about because of the introduction of the supercharger on the C type - the cylinder head is very much designed with superchargers in mind - hence the port sizes are a bit too large and the compression ratio too low for the standard J2.
Gas flow on blown engines is nowhere near as important as on un-blown ones - again there are benefits but probably not worth chasing.
Because with the gas being forced in - it tends to force its way back out again quite readily.
Gas flow work is not a new science - Fred Dixon used gas flow work on single cylider motor cycle engines in the late 20's to great effect on Rileys in the 30's - he was of the opinion that blowers were cheating and tried in vain to gain parity without forced induction. But I dont think he was the originator of any type of new science - it was the application of common sense.
This work was also used by others like Wilkie Wilkinson, Walter Hassan and Leo Kuzmicki to very great effect.
On MMM engines the most successful un-blown engine tuners have all been exponents of the common sense approach, combined with lots of reading and long hours spent in the workshop with a grindstone.
Enough ranting for now - if you consider the work worth doing for a non competition engine - then by all means go ahead.
I personally doubt it is worth the aggro - but I am a lazy git (my NA is blown)!
Regards David |
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ags
United Kingdom
275 Posts |
Posted - 24/05/2008 : 02:24:36
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Hi Bernard and David,
Thanks for the introduction David, but I would not claim to have done much "creative" tuning on my PB, I merely tried to apply the advice of the current experts in the 70s and 80s - particularly David Vizard who seemed to write especially sensibly. However, back to the point on valve guides. Even with standard iron guides it did seem to me that a lot of port area might be wasted by eddies forming from the blunt ends of the guides so that I tapered my inlet guides from the edge of the port casting to a rounded edge about a sixteenth thick at the standard length. I have no evidence that this alone made any provable difference, gas flowing was always going to be the step taken "when I could afford it". However the operation took very little time once I had measured things up. I did not do anything complicated, just gripped the guides in a drill chuck and used a fine file manually with calipers to keep all the guides reasonably similar.
As David says I am convinced, admittedly on little or no evidence, that sharp edges to valve seats and wide seat faces are bad for air flow when the valves are just opening or on the point of closing. In this area I really took my time to get a radius as big as possible. About a quarter of an inch my memory says, checked with a simple alloy gauge. I found it surprising how narrow a seat would still give a good seal for the inlet valves. My mentors said that 30 thou was all that was needed as long as the circle of the seat was unbroken when checked with engineer's blue. I used to leave them at about 40 thou (say a mm) in the belief that the valve and seat would soon batter each other to about this extent once in use. Exhaust valves were also radiused, but the seats made much wider, about 125 thou. The major problem here is keeping the valve cool so the wider the seats the better from this aspect so compromise is necessary with the flow properties. The heat problem is the reason why I did not taper the exhaust valve guides.
However as David has said the port sizes on the large cam engines were decided with blowing in mind and are very oversized by modern standards. Therefore there is not much need to worry about anything except preventing aerodynamic restriction from unwanted turbulence at critical places.
Well, that is what I did and it seemed to work and be reasonably reliable.
More technical ramblings from
Andrew Smith MMM571 |
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