350 cylinder head air flow

[smotorboy]

I've pulled a cylinder head off due to the compression being a little weak on the front cylinder and it is always good to see just what is inside an unfamiliar power-plant.

So here are a few of my observations on these engines so far:

The area ratio between the Intake and Exhaust valves is outside what would be considered normal design parameters.

The Exhaust valve area is only about 50% of the Intake Valve area. This is a "good news" / "bad news" thing.

"Good News" is that a large intake valve helps fill the cylinder in the mid and high rpm operating ranges...

"Bad news" is that the smallish exhaust valve will limit the flow when flow levels are highest, such as full throttle in the higher rpm ranges. Supercharging or Nitrous Oxide would see it right away.

Net result is that this ratio, while not ideal, is better than having the reverse situation : smallish intake vs. large exhaust ratio.

The large 14mm diameter sparkplug hole is a shame,,, a smaller sparkplug could have made a little extra room for a larger exhaust valve. .

Happy New Year to All !
Robert

[mad muller]

hi,robert its a very technical assesment you have posted, being from the old school of bikers i wouldnt have give it much thought, if the compression is down i would check bore,rings,etc, looking at the picture it looks like a recut and grind the valve seats while the head is off,could improve things. paul.

[EVguru]

The area ratio between the Intake and Exhaust valves is outside what would be considered normal design parameters.

Actually, it shows just how far ahead of his time Lambertini was. The modern trend is towards larger intake to exhaust valve ratios. The Intake operates under less than 15psi difference, whilst the exhaust is several hundred psi. Ports also tend to flow better as exhausts than they do as intakes.

The large 14mm diameter sparkplug hole is a shame,,, a smaller sparkplug could have made a little extra room for a larger exhaust valve.

It's more to do with the bore size and leaving a generous land between the seats to prevent cracking. A full 400cc version (rather than 375 rebore) can take advantage of the 500 heads. The 501 heads with larger valves are prone to cracking between the seats. The 250 head uses the same casting as the 350 (including ports) but has a smaller inlet valve.

[smotorboy]

Paul, I quite agree that the seats need resurfacing,,,too fat a fuel mixture equaled too much carbon which held the Exhaust valve off the seat a bit it seems....the rings were sealing fine as per leak-down test......

EVguru, I think Lambertini's design is very very smart for day to day street performance as evidenced by my bopping about SoCal freeway traffic for a month or so on my K2... It cruised brilliantly at 75 mph or so (which is the torque peak in 6th) and returned excellent fuel mileage at the same time. Up high in the revs it's breathing seemed to drop off rather quickly. Low & mid-range engine response was just excellent.
High-velocity flowing ports and valves sized for this type of performance created this very nice performance curve.
However, now I'm looking for maximum HP output at red line engine speeds from this engines so things need to be tweaked a bit. The quickest way to deal with the exhaust port issue would be to install a camshaft that keeps the exhaust valve open longer.
You are so right about "cracking " being a problem, especially with air-cooled heads. I do think after looking at things , Lambertini could have scooted the exhaust port over just a bit, kept the "meat" needed, and fitted a slightly larger valve. It would have complicated things a bit for sure, and the pay-off would not have been all that much for the street.
I'm running the numbers for the flow improvement from a 4-valve cylinder head fitted to our 62mm bore and will post the results soon. It will be a total pain to design and machine out custom cylinder heads, but I might just do it if the the result would do what I need...

Thanks for the feedback Guys,
Robert in California

[EVguru]

The K2 cam is milder than a Sport and the exhaust makes a difference too. Martin Gelder's Sport

feels restricted at the top end compared to mine, which seems to wants to pull forever.
Mine is fitted with the very rare Riservato Competizione LaFranconis which I think are much less restrictive than the Sitos on Martin's bike. At Cadwell this year the rider of a race prepped Velocette (675cc no less!) was surprised not to be able to outdrag me on the straights.

[3potjohn]

Has anyone tried the silencers NLM sell?

[mgelder]

Ooh, that's my bike!

[waves]

I've been experimenting a little with different 'silencers'.



The short ones above have a 2" bore perforated tube which is supposed to packed round with sound-deadening material, but isn't. They're deafeningly loud, in an unpleasent way, at all speeds and throttle positions.

So I fitted some 'dB Killer' baffles.



These haven't done much to the infernal din, but do feel slightly more restrictive at high rpm. Without the baffles the short silencers felt somewhat less restrictive than the black Sitos at motorway cruising speeds, with them in the performance feels closer to the Sitos.

The next step will be to either find a way to repack them (the internals are seized solid) or bite the bullet and shell out for a new set. I suspect a lot of noise is coming from the walls of the silencers resonating and repacking should at least make them quieter on the overrun and on small throttle openings.

Once the noise issue is sorted, I might stick the bike on a dyno to a) check the fuelling and b) see just how pathetic the power output is. All that will be in the spring, however.

Happy new year all!

MG

[72degrees]

After having my 'Rick Racer' 2:1 fracture just before the "silencer" at Cadwell I had to repair it by using a short section of pipe as a sleeve and brazing it all up. I managed to get the removable cone off eventually and repack the megaphone well around the perforated tube. It's not only noticeably less offensively noisy but also seems to be going better. Not sure I can trust it for the proposed Dijon trip though so I may invets in a new 2:1 from NLM. The problem with that will be fitting it on to the 250 frame with 375 motor as it is all a bit under tension.

I had a crack develope between the valve seats on the original hill climber and NLM repiared the head and reclaimed the exhaust port thread for me at the same time. At least the seat inserts don't fall out when severely provoked like on my old AJS 350 single.

[3potjohn]

Martin-I'd seen your megaphone idea on the Realclassic site and was sort of interested particularly as i gather the black Sito pipes are now extinct. Mine are just beginning to go under the rear end,in spite of me filling them up with oil and burning it off on a run so will need to do something soon.
What will you use for packing material?

[mgelder]

You can buy various types of 'proper' packing material from shops that specialise in motocross bikes, and there's some that expands to fill the space available the first time it gets hot:

http://www.thermalvelocity.co.uk/Silenc ... p-117-425/

Haven't used it myself, but when I do I'll let you know how I get on...