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Author Topic: The ideal head job  (Read 3400 times)

Fried Ape

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The ideal head job
« on: November 18, 2014, 10:31:20 am »
The cylinder head on the NF is a typical old-fashioned design: it is large and hemispherical, with two big valves set at a wide angle. The valves are big and heavy with thick stems, so need powerful springs to haul them closed.
As a result it is not unusual to find that the valve heads have 'tuliped' - they can can start to distort under the spring pressure. The valve seats also get a hard time, and so do the guides.
The easiest way to get more torque (power) out of the engine is to raise the compression ratio. However, the only way to do this is to raise the crown of the piston (trying to avoid the valves). This results in a combustion space shaped like the skin of half an orange and will need an even more extravagent ignition advance than thee engine has as standard.

So I stole an idea from Dave Degens that he used on his 24-hour racing Triumphs.
This means getting the sides of the combustion chamber welded-up and milled to make the combustion space more like an oval bathtub.

It leaves space for the valves but fills-in the two sides to reduce the combustion space and to provide a bit of squish from the piston.
While you are having this done (I used the Cylinder Head Shop), get the head drilled and threaded for a second spark plug. This can be fitted so that it is vertical with respect to the bore, and comes out 'inside' the U of the rocker box.


The two plugs are easily fired together using a double-ended Japanese ignition coil.

The last step, couresty of Len at the Cylinder Head Shop again, is to replace the valves and springs with something more athletic.

This shows a valve with an 8mm stem diameter and a pair of lighter springs.

The end result is that the engine pulls even more like a train, but is able to hold a decent speed on motorways and up hills.
It now pulls a higher gearing, with the rear sprocket reduced by two teeth.
The carb will probably need a bit of rejetting, but it's not that difficult.

Cheers

« Last Edit: November 20, 2014, 04:38:18 pm by Fried Ape »

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banquo

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Re: The ideal head job
« Reply #1 on: November 18, 2014, 10:47:56 am »
Looks good Paul. Might be better to link to the pix on Photobucket, as I'm pretty sure we have very limited upload space on this free forum.
Was the head still like that when you sold the bike?

Edit: Pix now linked to Photobucket
« Last Edit: November 18, 2014, 01:40:29 pm by banquo »

nick949

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Re: The ideal head job
« Reply #2 on: November 18, 2014, 12:56:59 pm »
Excellent stuff Paul - thanks.

Nick

Bruno

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Re: The ideal head job
« Reply #3 on: November 18, 2014, 03:55:55 pm »
Very nice.That combustion chamber shape looks similar to the work Gerry Branch did (and still does) on Harley EVO engines.Had the same work done on a soft tail I had.

Fried Ape

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Re: The ideal head job
« Reply #4 on: November 19, 2014, 10:40:08 am »
"Was the head still like that when you sold the bike?"

Yes - new owner delighted. He rides out with someone else who has a New Falcon. The modified engine and taller gearing means that he can out-accelerate them and hold a decent 70-75mph speed on motorways.
Pre modification I found that I often had to slipstream lorries on motorways.

KenJ

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Re: The ideal head job
« Reply #5 on: November 19, 2014, 11:40:42 am »
Good to see Paul on the new Forum. What that man doesn't know about the NF.....?

Having bought Paul's bike, I can confirm that once I'd sorted out the jetting, fitted a K&N filter and a free flowing exhaust (it's certainly not a silencer) it goes like the clappers. Running on the original 34 tooth rear sprocket, I was clocked at 90 mph by a friend following on a modern Triumph. It blew a head gasket shortly after!

By doing plug chops, I ended up with a 140 main jet, which runs slightly lean. One higher may be better, but haven't got one to try. I had a few blown head gaskets, but now it's torqued to 40 lbs, no problems. While I had the head off on first inspection, I took 2mm off the overall diameter of the push rods (drumsticks) and matched in the tapers, also grinding down the steel cups to match, while narrowing the lock nuts on the rockers. With the lighter valves, the reciprocating weight of the valve train has been greatly reduced, allowing a safer upper rev limit. I've no idea what the engine was revving to, when I was clocked at 90, but it was high.

With twin plugs, the timing can be retarded due to the shorter time needed to burn the fuel. I read that Porsche engines were retarded by 10 degrees, when switched to twin plugs, so aimed towards that. On the NF, this equates to zero static timing. At this, my NF really barks, being extremely loud. Advanced to 4 degrees static, seems to give a softer engine at tick over. There is a definite power band, when the engine changes rapidly from plodder to racer. To harness the extra power (??) I fitted the shiny new 31 tooth rear sprocket in the spares kit, that Paul included with the bike. The sprocket transformed the bike, giving a usable first gear and motorway cruising speed of over 70 mph, with an overtaking speed of 85 mph readily on tap. I have no desire to test the flat out speed on this gearing.

My early intention was to skim 10lb off the flywheel, but I like the easy one swing start and slow tick over pace of the bike, feeling that I have the best of both worlds at the moment. Once the weight is taken off, it can't be put back on.

Did you get a measurement of the compression ratio Paul?

Fried Ape

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Re: The ideal head job
« Reply #6 on: November 19, 2014, 11:47:14 am »
Ken,

great to hear from you and very pleased thatt he bike is working well.
I never did get around to measuring the compression ratio.
However, next time you blow a head gasket(... !) fill it with some oil or water to the joint line and tell me how much it takes.

Cheers

banquo

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Re: The ideal head job
« Reply #7 on: November 19, 2014, 12:53:16 pm »
You've been busy on here Mr Ape...  ;)
Thanks for all your efforts in uploading all the stuff. It's much appreciated.
Don't you feel tempted to go out and get another one..???  8)

Fried Ape

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Re: The ideal head job
« Reply #8 on: November 19, 2014, 04:20:12 pm »
Compression ratio....

So, the original engine is 6.85:1
If we assume that the additional welding added 15cc of metal, that would make the new CR around 8.1:1

The amount of weld added is a guess - here are the numbers for various amounts:

Weld  CR:1
5cc     7.2
10cc   7.6
15cc   8.1
20cc   8.6


Ooh, and an important point - the second spark plug is a 10mm. There isn't much room for anything bigger.
« Last Edit: November 19, 2014, 04:21:47 pm by Fried Ape »

banquo

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Re: The ideal head job
« Reply #9 on: November 19, 2014, 04:46:12 pm »
Just as a matter of interest, I wonder how much the combustion efficiency is affected by plug position, rather than by the double spark?

Did you ever try running it with only the 10 mm plug, and disconnecting the original?

I wonder if it would be worth sleeving down the original plug thread to 10 mm, so the 2 plugs are the same?

Fried Ape

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Re: The ideal head job
« Reply #10 on: November 20, 2014, 12:17:05 pm »
The plug position has a big effect on the burn time of combustion.
The standard head has the plug offset to one side to clear the valves. This means the burn starts at one side and has to cross the head volume. This is one reason why the maximum advance is so large - to give the mixture time to burn.
Raising the compresion ratio reduces the space in the head and means that the burn will complete more quickly. However, if this can only be done by raising the dome of the piston to fill the hemispherical head, the burn will be compromised because it has to find its way across a thin space with large amounts of wall drawing heat away.
Look at a modern 4-valve head - the valves are small and each pair is set parallel with a narrow angle between the exhaust and inlet set. The plug is central. The piston crown is wedge shaped to fill the head space but confine the burn volume to the space around the plug. The maximum advance is a lot less than for the New Falcon.
(And for comparison, take a look at the 2-valve head used in the Honda Jazz 1.4 engine. This has two plugs, as again the use of two valves prevents the plug being placed in the centre of the head. Honda also time the ignition to the two plugs separately so that the engine burns better at low throttle. Clever.)

This modified head design has three benefits: it reduces the head volume and provides some squish, and it starts the burn at both sides using two plugs.
The increased compression ratio helps reduce burn time and gets more torque out.
The (modest) squish helps reduce burn time.
Having two plugs decreases burn time.
Plus the combustion space has a better shape, being more compact.

The net efffect is that the engine should produce more torque (=more power), need less ignition advance and (thrashing excepted) be more economical.
What's not to like?

:-)

Fried Ape

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Re: The ideal head job
« Reply #11 on: November 20, 2014, 12:18:38 pm »
Oh, and providing both plugs are of the same heat range, it shouldn't make any difference that they are different sizes. Except for carrying spares.

banquo

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Re: The ideal head job
« Reply #12 on: November 20, 2014, 01:16:37 pm »
Oh, and providing both plugs are of the same heat range, it shouldn't make any difference that they are different sizes. Except for carrying spares.

Interesting stuff; the carrying of spares was what I was thinking about, but since having had plug problems 10 years ago, related to a now replaced coil, I always carry two, thus ensuring that I never need them...  8)

Glorious Span

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Re: The ideal head job
« Reply #13 on: March 10, 2015, 09:13:40 pm »

My own quest for NF engine perfection went as follows. First I bought a new advance/retard mechanism from an ebay seller. Then I fitted a Mikuni pumper flat slide carburettor, slightly larger than the original, and had it mated carefully to the inlet manifold. I had to cut off and re-weld the throttle pull upside down due to lack of clearance under the tank. Everything on the Mikuni is tuneable and I spent a good deal of time setting up all of the jets properly. A twin-plug head mod preceded that with some minor gas -flowing in the form of cleaning up and mild polishing. The valves stayed as they were, with new springs. My other Falcone came with an original but very different cam, and the second one's cam was re-ground to match. I have no idea of the origin of this cam, and cannot find any information about it, but the cam timing is radically different to the original. With the static ignition retarded, the engine now pulls well at very low speeds, from 30kph in top, but will also rev smoothly and willingly - 80kph in 2nd is no problem with more to come. This is a bad video, but listen to the engine pulling in 2nd gear from 4:20. This bike easily out-accelerates any other NF I have ever met.



banquo

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Re: The ideal head job
« Reply #14 on: March 11, 2015, 01:27:07 pm »
Not a bad video at all! it doesn't sound anything like mine, that's for sure!  8)