"it Blowed Up, Sir"

Stan- I was geared too short(1, maybe 2 teeth) and don't run a fairing yet, but I think the Manx is putting out a bit more than 50 hp. On the front straight, he pulled away from me when I was fully in his draft (10 in off of his back tyre) and had a better drive. It's really a moot point as Roper is so fast of a rider. Great riders make slow bikes look fast.
In regards to wieseco pistons, I HAVE shattered one once. IRP in '96. Way too much time on the piston.
Scott- Pay attention in class! The rz has a bridged intake, rd doesn't.

 

Stan, I understand the composition of the forged pistons and that it makes them more susceptible to deformation. The very thing that makes the aluminum forgeable and less likely to crack, low silicon content is the culprit. This also makes them swell more hence the need for the additional clearance. I wanted to try a set, having done so just confirms my preference for the OEM type cast pistons, RD or RZ even with the cracking issue

As for the exhaust issue you mentioned in your comment to Lyn, I believe on motors with pipes (and even a motor with stock pipes), there is a building negative pressure at the piston face for the full period that the lower front window uncovers the crankcase to the exhaust. I am not positive of this but based on wave/pulse timing the negative pressure has to have started to build at this point in the cycle.

 

I'll buy into the idea that there's low pressure outside the skirt, but what about the pressure under the piston as it's traveling upward? Isn't that low as well, pulling mix into the case thru the carb?

 

I'm not sure how much HP the new Manx make,but I did pass and beat Dave at Summit this year,on Kurt Liebman's Drixton.I had an experimental head on the Drixton,and it made no more than 50HP right after the race.(I'm bolting up a proper head right now)We seemed to have about the same power,the lighter Manx having an edge at the end of the straight.I'm talking modern rear-wheel HP,not an old water brake at the crank,Fred.I thought the best G50s didn't make 60,and the Manx,I thought,was slower.Mark,don't believe me,prove me wrong and get the bike on a dyno!I know how much Rich's bike makes,your comments seem to put it in the same ballpark.
Bob,I'm completely in agreement with you about the piston issue,I tried Weiscos myself in Tinas RD350.Oh,yes,I forgot!what happens when a piston has worn to too much clearance?it breaks!Boy,do they make a racket when the clearance is that big!I think the unknown issue is how much piston clearance?Tighter clearance will slap the piston less,making it last longer,too tight and it may seize.Bob,about the exhaust port thing,I'm only listing my opinion,I'm looking for input from others to set me straight.Your negative pulse theory may be OK in a narrow RPM range,but I shut down in corners(sometimes)and let the RPMs drop.

 

I agree with you Stan and I am not sure what is happening at lower RPMs, Just offering my take/hypothesizing on what may be happening.. I would think that even at lower RPMs the wave and negative pressure exists, in fact it may be just as strong as when the motor is on the pipe at the point the skirt opens the crankcase. With the lower RPM the uncovering would take place later in the wave cycle. While at lower RPM's the overall strength of the negative pressure would be reduced, the uncovering would seem to occur closer to the peak of the negative pressure cycle, whereas with the higher RPM, the uncovering takes place early in negative pressure cycle prior to maximum effect.

 

The crankcase opening takes place during the last 2mm of stroke, the piston has virtually decelerated to a stop, I think with the inertia of the intake charge at this point the crank case is at least neutral or even starting to build positive pressure. Even if there is negative pressure in the crankcase I would think it to be very slight, nowhere near the negative pressure level at the piston face. The exhaust gasses exited the last cycle at 1700 fps, that creates alot of vacume in its wake.

 

Stan,

It's really tough to tell what a particular Manx puts out HP-wise. I only know that the Manxman repro street-bike with lights, etc. that was built in South Africa (stopped production in 99) was guaranteed and dynoed to put out at least 45 HP. This motor had the SOHC, 2 valve head and used the AMC 4-speed, hence it couldn't be tuned for max power or you wouldn't get it off the line.

Some Manx's have DOHC heads and 4 valves. I didn't ask Dave what his had. I do know that the Austrian-made 6-speed transmission goes for about $ 2,500. I wouldn't think that one would spend $2,500 for a gearbox unless the bike was built to need it, hence I figured that Dave's motor was one of the real peaky ones making something more than 50HP. But I am only speculating.

 

I thought the best G50s didn't make 60,and the Manx,I thought,was slower

There's been a lot more development work done on Manx engines in the past few years than on G50's. I don't know exact figures on horsepower but it probably is close to 60. Over in Britain Andy Molnar and the Summerfield brothers are making brand new Manx engines and allied with 6 speed boxes they are very fast, the guys riding G50's & Seeley's, even the new 'replica' ones, have a problem staying with them. Admittedly, the front 3 or 4 guys riding these are in the Roper class but I wouldn't be surprised if the bike that was at VIR on Saturday was producing close to 60BHP.

 

Stan- Lighter weight should help acceleration off of the corner, but doesn't have any affect on top speed. I've been racing long enough to be able to tell the difference between getting out motored and out ridden. W/ Roper, I'd have to say it was both! Rich told me after the 500 race that I had an advantage over him on all of the straights, so I don't think our motors are that close either. I'll try to get it strapped to a dyno sometime, but it wont be anytime soon. Still, shame on me for showing up at the racetrack w/out any sprokets and running tires that I don't really care for. I'll make sure to have both of these problems taken care of next time I'm at vir. Hopefully Dave will be there and I won't have any excuses if I get beat. Honestly, I don't think Dave was riding as hard as he could. I rarely do when I'm leading. The first pass I made on him was on the brakes going into t7. The follwing lap, after Dave went by on the front straight, I tried again into t7 and he moved his braking marker back 40-50 ft. Needless to say, I didn't make the pass that time!

 

Hey Lynn, save your money. You dont need Guido. My luck at Road Atl. hasnt been good. Besides, Josh might be the one to look out for at the GNF.

I agree with the reply that said something about proper put together being the key to reliable two-strokes. In eight years of racing RDs, I have four non-finishes. One broken piston, One crash in the rain at summit point and a DNF and a DNS at the GNF in 99 both for chasis problems. I had a tire rubbing from making hasty changes at the end of the year.

So Evil take your time with preperation and everything will be fine.

R.Lucas #345 Rd 400

 

BEWARE...Josh has a new Spec II top end

Rich, thanks for the encouragement. Will you be at Nelson's next weekend? Josh has a real racing top end now, so good luck with him in F-500 and F-RD. I know you guys had a couple good races last time at Nelson's. I stayed pretty close (well kinda close) to him at Putnam in F-500 before I blew my motor. (I know that opens the door for comments from the 4 stroke guys). I'm hoping to be able to follow you guys around for a few laps this time. Hopefully no blowing motors and no stripping spark plug holes this time for me.

 

Richard:
I'm still looking for Guido. Or maybe since we are down south now I need Mad Dog or Tiny. I know your luck at Road A has been kind of spotty, but you are awfully fast there and Mark Morrow says you are getting faster. Well, maybe Gene L and I can fight it out for sixth or seventh.

Met Josh and his dad at Talladega. Good folks. We had 10 bikes the F RD and F500 grids! Best turnout I've seen since last year's GNF. This year's race should be good.

Besides Josh we have to look out for Chris Stevenson. He's on an RD 350 and rides it very well. He's down on motor (using DG pipes) but he has done very well.

 

Sorry Evil, Josh is all yours. Unfortunatly I have to be somewhere else on labor day.

Lynn, your right, there is lots of talent in form rd this year. As fun as it was chasing Mark in 500 at VIR, Gene gave me an even closer run in the rd race. He could have won it, but knowing I need the points, he showed his hand early and gave me a chance to repass for the lead. As you know he is not easy to pass. He was going for it and it was all I could do to get by. BTW,RRR might be in the cards so if Josh and Chris can make it,we could have a pre-GNF showdown. And yes Gene will be there.

Hey Stan, FYI Gene does all his own work. We consult all the time as do you and I but, we each make the final calls on how we like it. BTW How did you make out up ther in the great white north?

R.Lucas #345

 

Richard:
I need to mend some fences here. When we were at CMP earlier this year I fell in the F 500 race when it began to rain. Gene and his friend Bruce worked like dogs to help me straighten the front end so I could race F RD. Then I went out and beat Gene in that race. Well, he kind of beat himself because he jumped the start and then the rain began again and he couldn't make up the time he lost on the stop-and-go. Anyway, I don't think I acknowledged the help those two guys gave me at trophy presentation. I want to use this forum to tell everyone what they did. I've helped guys too, in fact, I think most of us have helped our competitors - that's part of vintage racing. I just didn't publically thank them. Plz pass that on to Gene and Bruce. Also, send me your e-mail address please. I lost it when my hard drive crashed. [email protected]
Lyn

 

What? You mean your computer is a two-stroke as well?

Lyn, I should have the time to try the seals in the caliper tonight. Let you know how it turns out.

 

Hmm does this mean 4-stroke riders own Apples?

Robert "F" Macaulay

 

Sort of.

Still chugging along with OS 9.1...

 

Guess I will stick with my 2 stroke

Bob - thanks again for the help getting my shifter straight!

 

Some of us are closet PC users and run AIX4.

 

In an effort to push this thread back to technical issues <G> I'll report on conversations I just had with my nameless Yamaha Tech Rep and Kevin Cameron about the notch in the exhaust skirt of RD 400 pistons (all models except the "F" (2V) pistons) and the electrodes of spark plugs.

My Yamaha friend said that he does not remember the reason for the 2mm notch in the regular RD 400 pistons. It uncovers the exhaust port around TDC, exposing the crankcase to the exhaust system (See Stan Lipert's earlier post). My friend stated substantially the following: Yamaha may have been trying to make the engine more enviromentally friendly by mixing a little exhaust with the fresh charge in the crankcase. They might have been trying to reduce the surging that occurs in two-strokes at mid-throttle and mid-RPM. Or, they might have been trying to use a small rise in crankcase pressure to force the reeds shut quickly. (He is a two-stroke expert, former RD racer, and long time Yamaha employee.)

Kevin said: It was probably Yamahas attempt at exhaust gas recirculation to reduce emissions. At the time the exhaust port is exposed, crankcase pressure is at a minimum; this would permit a small amount of exhaust to mix with the fresh charge as it enters the crankcase. Using the RZ pistons is a good idea because there is no exhaust skirt notch and the intake skirt windows are larger than on the RD pistons. The reduced life of the RZ pistons, 400 to 450 miles, when compared to RD pistons shouldn't be a consideration.

Spark Plugs: When Champion came out with the thin electrode plugs many years ago Bobby Strahman said that they thought that the plugs would help engines accelerate after a period of closed throttle. Kevin said he never proved that one way or another, but that the smaller electrode might require less voltage to create a spark and that the spark should be more intense because of the reduced radius. He suggested a test using a thick electrode plug in one cylinder and a thin one in the other cylinder and comparing them after a practice session. He did restate his previous recommendation on using a colder plug as the ambient temperature increased. (I use a NGK B9EV when it's cool and a B10EV when it gets hot (over 90 degrees)>