My V-max WITH PICS.....

[tyler440]

well i cant figure out where i saw the stock 38's were recommended for stock bore and rpm..... and actually i have quite a bit of stuff to do today at work so i cant afford to be on here all day like usual hahaha maybe someone else can help me out hahaha

[tyler440]

ps you maybe be right, that with major porting and pipes you will want the 44+'s im not sure tho and honestly if i were you i would start with the 38's... buts its up to you

[Justin]

I got some perfect Delaughter mod jugs /pistons and head for sale $200

[tyler440]

what size justin?

[Bob Vehring]

Maybe I can clear up some carb questions.
A 34 mm hole in a carb will flow X, a 38=X, 44's=X and so on. each change in the dia. of the hole lets more air through, that's easy to see. Their might be some very minor differences from 38 to 38, but if the hole is 38, it flows X.
Now we go past the carbs to ports
Statically ( nothing moving), a port is again just a hole, it flows X, change its shape or make it bigger, it flows more. Now dynamically, which means running with the piston going up and down. The port is really a completely open hole for a very short time span. At some point during each rotation, a port starts out closed, the piston as it moves uncovers it 1/2 way, them continues on to full open. As the piston reverses direction the same thing happens closing. For the techno geeks that measure this stuff, and they do, its called time/area values. They measure how much of the port is uncovered at a given deg, and the chordal width of the port opening to do their math and figure out flow.
Transfer ports are usually square because they are small. Square sides on a port give max flow as soon as the port is uncovered. A big sq. ex. port is instant death to an engine. The oval shaped sides are what guide the rings back into their groove as they expand. Too sq. of an ex. port will break rings very quickly. This is a big issue to home porters who don't understand this and get carried away .
Getting back to flow, also remember at least at some point things are under pressure. the transfers under a little pressure from the piston coming down and compressing the mixture in the crankcase. The ex. is under a great deal of pressure as the plug fires. extreme pressure. Most all of this high pressure leaves the cyl under what is called Blowdown, the beginning opening of the ex. port. That pressure sets off a strong pulse wave which travels threw your pipe. Now IF the pipe is designed right, by the time the piston has changed direction and is now closing the port, that pressure wave is trying to cram some of the fresh charge that has been pulled out the port and into the pipe, back into the cyl. That is exactly what every curve in your pipe is about, making that happen at exactly the right time.
So now taking this back to what works well on a sled. Cars, bikes, sleds whatever, people always make the same mistake. They look at racers to see what their doing, them copy it. Take a drag sled, it is meant to go 500 ft at a time, it is geared and clutched to hit max RPM right away and the pipes are made to make power at 9000 rpm or more. It can use big carbs because it is always wound up. At that RPM you have max velocity flowing threw the engine, and it can use it.
Velocity is something we haven't talked about but is every bit as important as flow, on the Flow bench, velocity numbers are what we look for.
Velocity is very easy to understand. Go outside, turn on your garden hose all the way. The water just falls out the end. Now take you fingers and close down the end of the hose, the water sprays 20 ft or more. Its the same volume of water, you just increased the velocity.
An engine can be built to do whatever you ask of it, the hard part is being realistic and asking it to do what you need, without going overboard. We never take trail sleds to races anymore and today it takes over 1000 cc's and a turbo to be king of the lake.
I'm looking to beat the guy that pulls up next to me on the trail, or when we hit an open field, maybe through the woods. Either from a stop or from30 mph, when I hit it, I want it to pick up the ski's and go. If the 120 hp guy has to wait for his engine to clean out and start building HP, hopefully I have 6 or 7 sled lengths by then, and by the time hes making power and has it wound up, races is over, were outta room, I win.
Right now, both my Yamaha's are very much over ported for what I want, Wendys mostly stock '81 ElTigre 6000, 85 hp has no problem being every bit as fast on anything but a long lake. I'm going to fix that

[Justin]

Standard bore.

[tyler440]

good read as always bob!!!! tho i think it needs a bit of a closing statement.... are you saying big carbs and big ports will take a bit to "clean up" when you get out of the woods and into a field? which would be worse for coming out of the woods and into a field... too small of carbs or two big of carbs? i have no problem running 38's with a big bore setup or even justins newly announced delaughter topend... i just want to match the carbs to the motor so i get the most bang for my buck..... im sure there is no straight up answer, but since i have figured out about everything else on these sleds i am now on to the more advanced stuff (trying to learn all that i can)

[Bob Vehring]

Well buddy, there might be an answer, but like so many things I haven't had the time to really R&D it yet.
The problem I see many racers have is they pick a sport and really get into it, but many haven't done anything else. My kids have many National Championships in Kart racing One of the things that really help us, we just didn't do one type of kart racing,we did Asphalt and dirt ovals, sprint, Big tracks and enduros and Indoor karting. All those thing are really quite different, and teach you many different things. A kart racer that simply stays in, say, Sprint racing never learns all those other tricks. As far as engine building I go back even further having raced cars, bikes and sleds. All teach you new things.
Years back, pre EFI bikes commonly used what is called a CV carb. CV stands for constant Velocity. These carbs had a butterfly just like the SRV or SRX carbs. This was controlled by a cable and trigger just like normal. Now the difference is behind the butterfly was a slide pretty much like a normal round slide carb. The opening and closing of this slide is controlled only by the demand of the engine vacuum. So if you slam it open at a stop or very low speed, the butterfly opens all the way but the slide opens only say 1/4 of the way, as the engine picks up the vacuum pulls the slide open further. the engine never gets more air then it can use. Basicly its always sized right for the condition.
This is not a new idea, back in the 70 a man named Ron?????? sold a carb for car racing. It affectionately became known as the " Flying Toilet". It never like most things new or out of the box got very popular, but those that understood it, made it fly.
Now days all higher HP 2 strokes have variable ex. port timing, this is kind of the same idea but on the other end.
Holley offers somewhat the same choice on their 4 barrel carbs. Racers use mechanical secondary carbs but for street use they make vac. operated secondary carbs. Like most other things the home hot rodders often pick a big mechanical sec. carb, then wonder why it bogs off the light..
My friend that used to race VM's back in the 80's then went on to building other sled engines for racers had good luck with CV carbs in cross country racing years ago. Today he only builds engines for the oval racers which is a different ball game He keeps telling me I have to try it, its on my list. until then, make a choice and live with the draw backs. Its not like its not going to run,your just leaving something on the table. I hate that

[scooter141]

Very good read....

I knew some of that, and didnt know some.
My deal is, it was built by a company, all together, at one time. There is a reason it has those pipes, carbs, and porting.
Granted I am changing the bore, but I think ill be allright, it might have a bog, or a sputter, but if it runs good enough for me, ill ride the CRAP out of it. I have jetted bike, cars all my life this is a normal deal.

I was a Kawasaki Service tech (1 yr) then the Service Manager (2yrs). So I have some experence with tuning, and Diag of engines. As I said I have been riding all my life. I had 3 CR 250's ( 1999-2000-2001 yea the old 2-smokes. went to 4-strokes in 03) built by Reynard Modifications, man they were crazy fast, but he really didnt port the crap out of em, it was a combo of porting, pipe, timing, head mods, and carb mods. Like you said its all about airflow, in and out......

Anyhoo, I'll give it a whirll, if it sucks, ill change it.

Todd S.

[Bob Vehring]

Todd, none of my rambling is really aimed any one particular post. I just stumble through here and try to add things I think might help someone, or, really anyone.

[Joe]

My neighbor runs an engine porting business out of his garage. He has lots of good results. He tells me that to make power, you need lots of air and the right amount of fuel. Efficiency comes from high compression. Like a garden hose, there are lots of places to restrict flow and reduce power. From the factory, most engines are pretty well matched in all the air flow paths. Air goes in the airbox, through the carb (or throttle body), through the manifold, into the cylinder, out the exhaust manifold, through the exhaust, through the muffler and out of the vehicle. ANY place, there is a restriction, you lose airflow and power. If you lose compression due to leaks, low static compression, excessive valve overlap, etc, you will lose efficiency. To make serious power, you need lots of air and high compression pressures. High pressure also produces high temperatures which can melt things inside the engine. You can get high compression pressures with a supercharger, turbocharger or manifold resonance. Two stroke engines use manifold resonance to bring the cylinder pressure up (exactly as Bob described). Resonance is a finicky thing to exploit. To get a high pressure, the temperature and RPM need to be exactly right. This results in a very narrow power band. Wider power bands have a lower resonance energy and a lower peak pressure. The snowmobile clutch CAN be set up to exploit this very well. Much better than a fixed ratio multispeed gear box. Getting it all to work together is a fine art. I stick to stock pieces because I know they were designed to work together with each other at the same RPM. I can really appreciate the skill and patience of those of you who can get an engine to operate above the design RPM and above the design cylinder pressure without compromising reliability. I have enough trouble getting all the pieces to work together reliably at the design RPM and pressure. Keep up the good work, and keep us informed of how your experiments work out (or not).

[scooter141]

Todd, none of my rambling is really aimed any one particular post. I just stumble through here and try to add things I think might help someone, or, really anyone.

Oh I know you werent smashing me at all. I LOVE to learn about the mechanics of horsepower. It was a very informative set of posts.

I am a gearhead by birth, a mechanic by choice. If its got a key or a kickstarter, I love it.

I try to learn all I can at everthing. My problem is I learn from doing something, I can read about it all day long, but till I do it or see it with my own eyes I have trouble "grasping it".

Thanks Again. Todd S.