I was just talking with Bigbaja and we were discussing his situation. Now after serious contemplation I have came to the conclusion that bigger is not always better. Since Scott put his bigger valves in he lost 5 ft.lbs. of Torque and 3 H.P. plus he said the top end fell off quite a bit. Now to me this would dictate that with the bigger valves and bigger cams the only logical course to take to get back that top end and power would be to add more air to the head, correct? It seems to me that you now have the capability to flow a ton more air but the carb you are using is now not adequate enough to compensate for the extra cfm. So my theory is that in order to see increased power, torque and top end one would need to add Dual carbureators and more compression. So my new theory is for the power to money spent ratio for the bigger valves, then the bigger valve are indeed a waste of money. If you stay with the stock valves and add a bigger cam duration camshaft and up your compression you will gain more torque and h.p. Actually more torque. For example: I take 2 720 engines, both with 12:5:1 compression pistons. One engine has the bigger valves with heavy porting, the other with stock valves and heavy porting. Now we run a bigger cam on the stock valve motor, let's say a Ron Woods race 2 cams and then we take some smaller cams lets say some Ron Wood sport cams or WeB cams 110 grind and put those in the engine with the bigger valves. In theory both of these engine should run nearly if not Identically on a dyno. When you boil it down the smaller valve with a higer duration and lift will put just about the same amount of air into the cylinder as will the bigger valved head will with the smaller cams. So basically what I am trying to say unless you got lot's of money to burn you can get the same performance out of your engine with the stock valves, so don't waste your money unless all you are going to do is drag race your bike in the 1/8th or 1/4 mile flat drags. If you want a nice dunner that wil kick butt on the dunes and the flats then you don't need those bigger valves. I feel that you will loose more torque with bigger vavles and cams than you do with stock valves and mild cams. And let's face it torque wins hill shoots not H.P. on a four stroke. I would love some input on this from some of you engine builders on here. I think my theory is pretty sound jsut from my personal quad and my other Samcoe compatriots.

 

I think your correct, if your using big valves and big cams you need big carbs to go with them, especialy if your big bore/stroke too

 

I think the biggest waste of potential power comes from porting done incorrectly. This is truly a case where bigger IS NOT better. Good cylinder fill is accomplished by keeeping the velocity in the intake tracts as high as possible at all rpms. I believe that larger valves are a good thing as long as the intake tracts are NOT enlarged. Polished ports and enlarged ports will compromise cylinder filling and will cost HP.
These are the very same features that show great flow numbers on a flow bench but in reality cost you power. For further explanation
read here
high velocity porting
My intake tracts are only 65% the size of the stock tracts- velocity is the key! It'll give you better atomization and cylinder fill!
Certainly the dual carbs help the top end , but they are all about velocity too [img]i/expressions/face-icon-small-happy.gif[/img]

 

Punisher,why do I have smaller cams and more torque.

 

Porting a cylinder head is not about hogging large amounts of material from the intake and exhaust tracts. It about straightening, reshaping, fixing what you have to work with. There is no perfect port but they all can be improved. Moving air around corners is simple, moving air and fuel in a suspended state around corners takes years of experience. Throwing cams and valves at an already well designed engine may or may not make power. You have to find someone that has proven combinations and dyno sheets to back them up then you know for sure what you are getting.



MrHorsepower

 

I've always wondered how to calculate the max flow the cylinder could suck through those valves -- and still get good fuel atomization.

There has to be a theoretical max valve to carburator to stroke/bore relationship where the you reach the reality of dimishing return.

jbt

 

Dear Mario[img]i/expressions/face-icon-small-happy.gif[/img] I beleive you have more torque with your smaller cams and bigger valves because is because your air fuel ratio is actually better suited for you carbueration. You have the bigger valves which allows more air into the motor but with the smaller cams you let in just the right amount of air/fuel to compliment your headwork and compression. I think the old man was on to something there.[img]i/expressions/face-icon-small-wink.gif[/img] My machinist said the thing he didn't like the most is the seats that Ron Wood uses. They are to large and it takes away from the velocity that you need for correct atomization. He wants me to send that head back to him so he can put in different seats and really make that head the Shiznit! Now if I just had some darn money.[img]i/expressions/face-icon-small-mad.gif[/img]

 

After years of learning about v8 hotrod motors, Now I'm looking at modding these 4stroke singles. I had bikes before but most people don't mod CR500s much. I bought my DS to be a nice mellow play bike but you know how that goes.
I don't think anyone would consider these heads restrictive stock. Almost any hotrodder would kill to have a four valve head. Majorecho, are you saying that your intake tracts were filled in somewhat and reshaped? I think it's great people are tweaking these motors. Look how far banshees have come over the years and they have to replace most of the components for reliability.
Has anyone looked at the setups on say the road race hondas.A v-twin 1000cc around 55-60 horsepower per 500cc stock and then the race team builds them. What kind of dimensions are they running?

 

Yes, the floor of the intake track is raised, since that is the area that flows the slowest. It did wonders to the torque curve, it was so flat that my dyno guy thought it had already been doctored.

 

Good timing to see this topic,
All very good info and very intellegent responses. My mods are listed below, and hopefully someone can lean me in the right direction to make the next step work properly. I am sending my head out for oversizing the valves and completing the porting. I have what I call mild porting so far. I very much agree about the mild cam vs. valve size, vs. carburation issue, so my question is this. Will I need to move up to dual carbs to complete my next round of mods? The individual that will be doing the work( who shall remain nameless untill its done) will be doing a complete bench flow check to verify and modify properly. This is ONLY a question of money vs. time here. Do I get the valves done without the carbs, or do I save my bubblegum wrappers and get both at the same time? I will be heading out to LS Oklahoma on October 9th - 12th and also attending a rather large event here in the midwest called The Fudd Run put on by the famous Monkey Butt Atv Club. The Fudd Run is on Sept. 7th which gives me a month to complete the mods. (this can be done due to my terrible habbit of building machines the night before a ride) . If I infact need to get the dual carbs now, should I go with the FCR 39's or (unless Im mistaken) does lectron make dual 41's? Is there any pro's or con's to either of these? Once again, thanks to all ahead of time for the great responses, and I look forward to reading them.

DStroyer

Mods: Tm45carb with ported and extra polished intake (homemade), direct mount k&n, rwr cdi, 60% rwr springs, dual titanium valve springs, 11.5:1 piston, sam coe porting, .395 lift web cams, alba rull race exhaust. I think thats it. I have a fever now, and MUST feed it.