Valve Float scope of limits?

Valve Float scope of limits [repeated in text are so forum search function can find again]

What is the consensus of valve float rpms in the various valve trains from factory to out of $ite? My ponder is how cheap can ya go before valve train is not the rpm limiter compared to the crank fling, nullifying stroke friction or piston parting. Generally only hi lift aggressive ramp cams are used to allow higher rpm power so cam should be factored in too. My 1st question to public forums, [after asking where ignition system was] was what limits rpm 1st, 2nd, 3rd and 4th.

I've caused a couple of engines to float their valves a big block Ford in a jet boat and my 1st factory Combat. Big block blew up, Combat almost blew me up in early learning curve of its rpm power limits.

see Peter Williams on http://www.peterwilliamsracing.com/pw3cam.pdf stating that JPS norton with 2S or 4S camshat start floating at 6600 rpm with peak power at 7500 !
with PW3 valves floats at 7500 with max rev at 8000.

I'm pretty sure some will be very surprised by these quite low figures but I 'd not pretend to know better than PW about Commando

Well, ain't this a coincidence. Guess whe I'm about to do.



I have had JS's stage 1 cam and BSA lifter setup for a couple of seasons now so I thought it was time to compliment this with his Beehive springs and valve kit.
I also just got the Powerarc from Old Brits and have it set at 7000 rpm redline. I cannot say I have had issues with valve float and this should gaurentee that I won't. Along with the new 32mm Premiers, I just may take some laps at Grattan this year.

JRD thanks for the PW3 lesions and sense of olden days valve clash range. I don't understand the part about valve float occurring over last 1000 rpm's and still making more power yet not clash or blow off peak power increase. Interesting engineering coincidence that moderately upgraded valve trains and upgraded bottom ends both have similar rpm limits.

pvisseriii with an over kill top end like you have bought and wise Powerarc rev limiter @ 7000 valve train should be bullet proof and hardly stressed wear wise, unless maybe you insist on dry sump start ups : )

about power increasing despite valve float , I guess it's because efficiency ( read torque) decreases less than rpms increase. so still power increment.

hobot said:

pvisseriii with an over kill top end like you have bought and wise Powerarc rev limiter @ 7000 valve train should be bullet proof and hardly stressed wear wise, unless maybe you insist on dry sump start ups : )

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No, actually I am a proponent of have at least 6 to 8 oz of oil in the sump all the time. This is of course its normal state. If draining or just a routine oil change, I believe in adding a cup off oil to the sump through the exhaust cover. Never start a dry sump system dry. I guess that makes sense. I does to me anyways.

Hobot, you started this so don't blame me for getting off topic.

I use the standard 850 cam, but advance it 15 degrees with a two into one exhaust pipe. I always try to change below 7000 rpm, but it always sees 7,500 on the tacho because it spins up so quickly when your overall gearing is low enough to permit a respectable start in a clutch start race. My crank is balanced to 72%, and I haven't had a problem yet with either the bottom end or valve float. I intend to use the combat cam and advance it 6 degrees to compensate for the exhaust pipe. I now have a six speed TTI box, so I will be able to increase the overall gearing, and still get decent starts using the lower first gear.
My current cam timings are :
IO 65 btdc
IC 59 abdc
EO 97 bbdc
EC 27 atdc
With the combat cam they will be:
IO 65 btdc
IC 83 abdc
EO 94 bbdc
EC 56 atdc

My carbs are 34mm and the inlet ports are tapered back to standard 32mm within 25mm of the manifolds. I also use methanol fuel with the ignition advance at 34 deg. before TDC. The motor is set up to pull, not rev with the power up high. I haven't used the six speed box yet, but I should be able to work up to a good speed at the end of the straights and still get decent starts in races .
As far as cam timing is concerned, have a look at the table in Tuning for Speed, and in particular at the timings for the 1958 model 7R AJS. It was the fastest 350 single around at that time.
If your motor doesn't blow up in the first five minutes, it will last forever.

I still have to change the selector drum in the TTI gearbox. When I've done that I will start the bike and get my wife to film it, and I will put it up on Youtube. - 'A look is worth a thousand words'. When I start it up down at the circuit, it gives all the guys a thrill. I've had an awful lot of hot Triumph twins in my life, the Seeley commando 850 is much more exciting. It is a real hoot, it really means business.

JRD » Mon Feb 04, 2013 12:48 pm
about power increasing despite valve float , I guess it's because efficiency ( read torque) decreases less than rpms increase. so still power increment.

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Well yeah but that's what's hard for me to understand as one would think valve float would bleed sealing so more bleed power than rpms gave back. Likely even more power if the valve train stayed in control in JPN engine.

Alan, I love your intensity and go getting spirit so letting it rub off on me as I age with a power lust. I'm paying attention to you exhaust and cam degree relationships and will apply to Peel's eventual dialing in. I don't like shifting nor sluggish launches so Peel's TTI will have lowest first and widest spread 3 more gears. On THE Gravel play i have to be careful w/o rev limiter as can over rev in every gear, so water skiing background comes in handy crossing the choppy surface. I've got a few more chapters to add to your self steering post so may renew that again. You would shit your pants learning The Ways of THE Gravel like I did so would have to lend you crash cage'd Peel as definitely going to crash a lot before getting the hang of it but after that you will see tarmac as childs play. The most spun up into redline I get on THE Gravel is approaching apexes after starting the spun up power about where everyone else is braking like crazy. If power runs out ie: torque curve increase falls off so rear hooks sideways, bye bye Hi Side City.

JMS has as good as it gets and great advantages he lists. Peel may give most expensive ever Commando a real run for the money so spending on other stuff as her valve train tested beyond belief so will do what Peel's bottom end can stand. Peel's already dropped big cash at JMS and glad of it.