Big valves or small valves

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acotrel said:
If the bike's weight distribution is correct for the power of the motor, the bike should be spinning it's back wheel before the front lifts. There is omly one corner on Winton Raceway which is slow enough to get my rear wheel slipping as I accelerate out of it. It is a hairpin. You don't get many of those on public roads.
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When you see a race bike lifting the front, it is wasting power which could be used to move the bike forward. Back in the old days when we first had two strokes, most lifted the front very easily. The guys who did that usually got beaten by the guys who didn't. I once rode a BSA Rocket 3, that lifted the front on the throttle easily, but it did not move forward very fast while it was doing it. The ideal would be to have the back wheel slipping while the front is just beginning to lift
 
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This guy is coming out of a turn spinning his rear wheel and lifting the front a little. I wonder if he knows how to ride the right way. lol

Big valves or small valves
 
What you find in corners, is that when you are on full lean and following the others, you usually pussy-foot around so you don't crash. But if your bike is more upright, you can put more power on the ground. The weight of the bike needs to be as far forward as possible or the handling can destroy your confidence. More power is always good, but you need to be cable to put it on the ground. Most race circuits have more tight corners than long straights.
 
worntorn said:
Sorry, very definite pogo stick wind up on that one. It's getting close though.
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You would have to be on the bike to know if you could give it more stick at that angle of lean. That guy is in the situation we all find ourselves in with most race bikes. Once you are in the corner, you are committed to behaving yourself.
 
acotrel said:
You would have to be on the bike to know if you could give it more stick at that angle of lean. That guy is in the situation we all find ourselves in with most race bikes. Once you are in the corner, you are committed to behaving yourself.
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Huh?
 
acotrel said:
You would have to be on the bike to know if you could give it more stick at that angle of lean. That guy is in the situation we all find ourselves in with most race bikes. Once you are in the corner, you are committed to behaving yourself.
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Chances are whoever "we" here is has never been on a MotoGP bike or anything resembling one.

I wish I had some pics of the Sounds of Thunder twins, but to be honest I found the old twins road racing to be incredibly boring as a photographer. Nobody was campaigning a Norton when I was out using a camera anyway, so it would just be more of my off topic thread content destruction.
 
In Austral;ia, the only modern racing for Norton twins has been Period 4 historic. Theoretically you could race a 1960s Bonneville on methanol in that class. But you can also race a CB750 on methanol, and there are no controls on increases in engine capacity. In effect Rex Wolfenden'a 1100cc methanol fuelled CB750s won almost every race for about 30 years. The only guy I ever saw who beat him convincingly had a 930cc methanol fuelled Laverda 750 SF. I was talking to Rex a while back about the large capacity engine - he said 'they are within the rules and we don't always beat the Nortons'.
There is a lot of psychology in road racing and some guys are beaten before they start. At one stage there were fourteen 1100cc CB750s. in Period 4.
With about 100 BHP on tap, you have to get the bike vertical before you gas it.
 
Here's a link to a picture of a well designed port with features to make it easy for me to describe how I think the intake works.

https://www.lotus-cortina.com/library/wsmanual/shengine.htm

It has a tapered length to accelerate the air, a straight length up to the thin spot at bottom of spring pocket to port, the valve pocket has a larger cross section area than the straight length before and the throat after at the valve seat.

The charge motion is pressure drops as air accelerates to end of taper. Pressure stabilizes somewhat in the short straight to better deal with the variable pressure pulsing of the flow. Entering the larger valve pocket area decelerates stalls pressure rises all in an effort to crowd the pocket before exiting. The throat is smaller than the valve so pressure rises to keep air attached to wall from pocket to combustion chamber.

All i've got are 850 heads and can't help seeing potential to make them a whole lot better like the respected lotus twincam. They share the same architecture of about 45 degree port to pocket. The problem is how to add material to fill the dead areas. Without seeing the 650 head I bet a 1.5" valve means more material to get things proportionally in the ballpark. What size carbs will it be running?
 
Maybe this one qualifies as a partial throttle to full throttle wheel lifter - skip to very end of video.

 
lcrken said:
Ouch! It hurts me just to look at all that exposed skin just waiting to be ground away. Cool wheelie, though.:)

Ken
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In 110+ deg Fresno summers its either suffer heat stroke or risk road rash.
 
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I don't think I have ever wheelied a bike while racing, even a two stroke. It is a waste of time. In racing the speeds are relative, it all happens in slow motion unless somebody stops. If you reach the end of a corner at the same time as another rider, but you are going 10 MPH faster, they need a lot of power to pass you before the end of the next straight. A lot depends on torque and gearing. I do not believe the ability to wheelie a bike is a good measure of the effectiveness of it's motor
 
edgefinder said:
Here's a link to a picture of a well designed port with features to make it easy for me to describe how I think the intake works.

https://www.lotus-cortina.com/library/wsmanual/shengine.htm

It has a tapered length to accelerate the air, a straight length up to the thin spot at bottom of spring pocket to port, the valve pocket has a larger cross section area than the straight length before and the throat after at the valve seat.

The charge motion is pressure drops as air accelerates to end of taper. Pressure stabilizes somewhat in the short straight to better deal with the variable pressure pulsing of the flow. Entering the larger valve pocket area decelerates stalls pressure rises all in an effort to crowd the pocket before exiting. The throat is smaller than the valve so pressure rises to keep air attached to wall from pocket to combustion chamber.

All i've got are 850 heads and can't help seeing potential to make them a whole lot better like the respected lotus twincam. They share the same architecture of about 45 degree port to pocket. The problem is how to add material to fill the dead areas. Without seeing the 650 head I bet a 1.5" valve means more material to get things proportionally in the ballpark. What size carbs will it be running?
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I do not think of inlet ports and exhaust systems in terms of normal gas flow. I suspect the speeds are sonic. What we have are sound waves. What works at low gas speeds, does not necessarily behave the same at ultra high gas speeds. When you use a flow bench to develop port shape, you are making an assumption that what you see on the flow bench actually represents what happens when an engine is revving. With two-strokes, much of the noise you hear when they are running comes from the inlet port. You can theorise about the optimum port shape, but it is probably unpredictable. It is what happens when you run the motor on the dyno which is more important. I think most dyno runs are made at full throttle looking for top end power. With ports, what suits high revs does not necessarily suit low revs. You can boost power at the top end of the usable rev range, but lose at the bottom. Then the way you gear the bike comes into play. The early 1950s Gilera fours had seven gears.
 
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One of my 750 valve jobs with reangled 1.5mm oversize intake valves (larger hard seats for unleaded gas).
Big valves or small valves




Maney stage 2 reangled oversize valves (+3mm intake and exhaust).
Big valves or small valves




Maney stage 3 reangled oversize valves (+5mm intake and +3mm exhaust).
Big valves or small valves
 
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If there is detectable benefit, at which parts of the usable rev range does it occur ? Or is the motor faster at all revs - more torque ? I would have thought the cam profile and timings were more relevant. But if you have faster or higher lift, the extra weight of the valve might cause float to occur at lower revs than normal.
 
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In my Triumph motor, I used the hollow tappet adjusters which take an Allen key, and I cut the lock-nuts in half. The cams had large base circles, less lift and very slow lift and closing rates. I regularly revved the motor to 10,500 RPM, but the crank was short stroke and much lighter. If you did that to a Commando engine, you would have lost the plot. You can go in either of two directions - not both. And there is no happy medium. Whatever you end up with, the gearbox must cope. So start with the gearbox and work back. If my 500cc Triumph had a 6 speed TTI gearbox, it would have been a world-beater in it's day. As it was I could choose how I wanted to lose a race, by gearing high or low, If I geared low, I could beat them everywhere except at the ends of the straights. If I geared high, they were quicker in the slow bits, but I was faster at the ends of the straights. Which ever way I geared, I still got beaten. I sometimes wonder what gearbox Percy Tait had in his Daytona 500 when he beat Agostini. Back in those days I always rode in the Open All Powers class against much bigger bikes. The Seeley 850 is a soda - there is little effort involved in doing well. I just let the bike do the work- it has no problem keeping up
I had a really bad childhood.
 
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jseng1 said:
One of my 750 valve jobs with reangled 1.5mm oversize intake valves (larger hard seats for unleaded gas).
Big valves or small valves
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When you say "one of my 750 valve jobs" are you talking about work you do for yourself, or are you actually doing valve jobs and port work on Norton heads as a service?
 
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