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Author Topic: Project Return of the MAC - Vintage 350cc Velocette build  (Read 8952 times)

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Offline thefrenchowl

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Re: Project Return of the MAC - Vintage 350cc Velocette build
« Reply #45 on: June 26, 2020, 04:58:59 AM »
Hahaha!!!

I gave the Jonghi a well deserved retirement from Bonneville... I just about finished reconstructing it after the salt did attack absolutely everything on it... 2019 was the worst salt I've seen in my few visits there...

Anyway, back to the ports. Since a flow bench is not sommat everyone can have in his garage (and it will only give you basic static figures... not real life engine breathing), yes, it's worth looking at what's been done on similar engines as the ones we try to tune.

As far as OHV and OHC engines go, they always seem way over-carbed (and over-valved, but that's another topic!!!) from the word go at any factory... Donno why, buyers seem to think bigger is always better... While peak power will usually be synonymous with biggest carb available, that leaves the torque curve way too high in the rpm range... And you'll need low down torque as well as high up power with only std street 4 speeds to play with.

In all the iron head Sportsters I've had, quite a few, the best ones were always the earliest ones fitted with the oh so simple Linkert DC carb, 34mm venturi, so none of this S&S 1" 7/8 (47mm for you metric boys) nonsense here...

As far as the port goes, a small decreasing taper (ie getting smaller) towards the valve speeds up the mixture and help it getting and staying laminar. Last "venturi" about 2" from the valve, just before the bend and then it gets bigger to join the valve OD.

Once you get the basic engine going, look at the air trumpet to see how much mixture gets back out due to inadequate inlet track length (could be either too long or too short). Factories did use telescopic inlet tracks to tune the length at the revs of the peak torque, that's easy enough to do on a loaded engine... Look at the Thruxton inlet track compared to a std MSS/Venom one... They had to lower the oil tank to clear the GP carb!!!

Bye for now,

Patrick
« Last Edit: June 26, 2020, 05:20:18 AM by thefrenchowl »
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Offline jungblut13

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Re: Project Return of the MAC - Vintage 350cc Velocette build
« Reply #46 on: June 26, 2020, 05:11:28 AM »
I apologise for one too many "F"'s Mr Frenchowl :-D

P.S. Those were my last words to my ex-wife too :-D
« Last Edit: June 26, 2020, 05:13:15 AM by jungblut13 »

Offline wobblywalrus

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Re: Project Return of the MAC - Vintage 350cc Velocette build
« Reply #47 on: June 26, 2020, 09:23:32 PM »
What is the engine displacement and the rpm that you expect peak power to occur at after full development of the engine?

Offline wobblywalrus

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Re: Project Return of the MAC - Vintage 350cc Velocette build
« Reply #48 on: July 01, 2020, 10:57:17 AM »
A big problem with engine building is figuring the valve and carb size, as mentioned in earlier posts.  Here is the method I use.  First, using the actual rather than nominal displacement, and the realistic peak power rpm limit my wallet can afford, this is entered into the equation on Page 83 of Harold Bette's "Engine Airflow" book ISBN 978-1-55788-537-1.  Let's assume 21.1 cubic inches (346cc) displacement, 7,000 rpm at peak power, and the rpm coefficient from the table in the book is 1196 for 28 inches water.

peak power cfm = (rpm at peak power x displacement per cylinder) / coefficient at peak power = (7,000 x 21.1) / 1196 = 123 cfm air flow at peak power.

Now, using this airflow and the horsepower equation on the same page and the horsepower coefficient for 28 inches of water from Page 82, which is extrapolated to be 0.26 for a one cylinder engine:

peak power hp = coefficient at peak power x displacement per cylinder = 0.26 x 123 = 32.0 hp at peak power

These are what I always called the Dynojet equations and they are based on measured wheel chassis power and measured intake air flow for the full system including manifold and carb.  This is based on my foggy, aged and pickled memory and there is a chance that I am wrong.

Crank dyno power is used in most automotive based analysis programs and equations.  Assuming a primitive style chain driven primary like on Harleys and old BSAs and Triumphs, crank dyno power = wheel dyno power / .90 = 32 / .90 = 35.5 crank hp.  Assuming a more efficient drive system like a geared primary, crank dyno power = chassis dyno power / .92 = 32 / .92 = 34.8 crank hp.     

These power estimates, based on my experience, are pretty good high estimates of what to expect.  The PipeMax program is easy to use and it gives port and valve flow recommendations.  It relies heavily on an accurate volumetric efficiency estimate.  What I do is to use the crank horsepower calculated by the preceding estimate with an engine in good condition to get the expected VE.  Then, I use the Pipemax program and others like Dynomation combined with advice from experinced tuners to get an idea of the best valve and port size.  The smallest intake port and valve size with adequate sonics is what I use. 

This is what works for me and I am an average redneck backyard tuner.  Do not consider it to be gospel.   

 

         

Offline thefrenchowl

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Re: Project Return of the MAC - Vintage 350cc Velocette build
« Reply #49 on: July 03, 2020, 07:00:20 AM »
Hi all,

The all conquering Guzzis 350cc won 5 consecutives World Championships between 1953 and 1957, a period in keeping with the MAC design and lifetime...

Giulio Carcano, their designer/enginer/race head honcho, said that, most of the time, they had way less than 33 HP to compete with the others guys, HP obtained at great cost with a SOHC design of impeccable racing heritage, having started in 1926 as a 250 cc with an horizontal cylinder .

The period 350cc DOHC single Norton Manx claimed more than 35 HP, but, too high and too heavy, being a reduced size 500 ie wrong approach, never got near the Guzzis.

I would think to extract anything near 30 HP from a MAC with street gasoline would be a miracle unless umpteen hundreds of hard earned cash are spent on this project.

The Velocette flagship, the 500cc Venom Thruxton, highly tuned with special head, valves and carburettor was rated at 42 HP @ 6200rpm by the factory and capable in Veeline version of 115mph on a good day... Lets not quibble and agree they found a few more, say 45 HP on their winning Production TT racer, hit 120mph, that's it...

To get speed out of his MAC, I would suggest the HP approach will not be a success and ROTM will have to explore other avenues to get the speed he wants...

ROTM should look at Carcano's work to get speed w/o HP, ie lightness, friction reducing measures, fairings and low frontal set up, small section tyres, efficiency, weight distribution etc...

All things that don't need that many dollars to implement, but mostly brain power.

Patrick

« Last Edit: July 03, 2020, 07:17:21 AM by thefrenchowl »
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Offline wobblywalrus

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Re: Project Return of the MAC - Vintage 350cc Velocette build
« Reply #50 on: July 03, 2020, 10:49:56 AM »
The Bonneville cylinder displacement is 30.3 cubic inches with a 236 cfm intake airflow at 28 inches.  This is flow for the complete system with carb and filter.  Peak power rpm based on the Dynojet equation is (1196 x 236) / 30.3 = 9,300 rpm.  Peak power is 0.26 x 236 = 61.3 hp per cylinder.  The cylinder volume is 498cc.

The engine makes between 51 and 53 hp at 9,000 rpm, per cylinder, depending on the dyno.  The goal is to get 55 hp per cyl this year.  There has been over a decade of work on this motor with hundreds of dyno pulls.  Patrick is right, the Dynojet equationa are a bit high, and especially for a vintage engine.  They might result in oversizing if you use them.