Crux of the Biscuit

[hombre]

WT

I chose 7250 RPM as a practical limit because of piston speed. There is no discernable valve float at 7250 RPM. I'd like to be able to carry my max torque through to that RPM, but that may not be possible. The 300HP was just a theoretical calculation based on that goal. I expect the longer duration SC exhaust cam will flatten the torque curve even more, and help achieve this goal. The headers were designed to place the TQ in this higher RPM range. I'm hoping for a 3% increase in max TQ (from 212 to 218 Ft Lbs) with the SC exhaust cam and headers. With rear-rear sets and lay down launch position, my tire hook-up is actually improved and the front end stays down much better. So with the wishful 3% increase in torque, I still expect the bike will be quite tractable.

What do you think of Nev's suggestion to advance this cam, and possibly even retard the intake cam (wider lobe separation angle)?

 

[W. Tripp]

Hombre,
Hard to say without knowing the cam specs - I can only make an educated guess as to what they might be based on your results. But generally wider lobe seperation is a good idea with a centrifugal supercharger running moderate pressures at moderate rpm. But I have seen narrower centers work in some cases as well.

One thing I can definitely say is that more races have been lost due to racers not listening to their crew chief or engine builder than most will believe.

From what I see so far, Nev seems to know what he is talking about - there are lots of smart Ozzies, especially when they are not at the pub. He has not steered you wrong so far.

 

[hombre]

Given that Nev designed the SC cams, his advice carries a lot of weight. He was also right on the money about the broadened power band with the collector-muffler... from just looking at the photo.

 

[hombre]

Gleaned from research on supercharger cams and lobe seperation:

"The only other change to the engine was replacing the camshaft with a grind more suited to a blown application. Superchargers are recommended to be used with a wider than-normal lobe separation angle to optimize the engine for boost. This is a fairly general recommendation that follows established practice with blown engines, serving to open the exhaust valve sooner, and reduce overlap. In theory, opening the exhaust valve earlier helps deal with the greater volume of gasses produced with supercharging. The reduced overlap is beneficial since overlap scavenging is not critical for cylinder filling with a blower providing the fill. The wider lobe separation is well accepted... "

It appears that Nev is once again right on the money. I can personally attest to the tremendous amount of exhaust and heat produced by a SC R3. The proposed benefits of advancing the exhaust cam and retarding the intake cam dovetail with the above recommendation.

 

[hombre]

These are not R3 cam profiles, but you can see what happens by increasing cam lobe seperation. By advancing the exhaust cam (red curve moves left) and retarding the intake cam (blue curve moves right), lobe separation angle increases, and the overlap area (green) will be reduced. This should produce a torque increase.

 

[Brembo]

Richard reckons he got his (now MINE) to do 10 flat with a terminal (I think) of about 140mph. That is with ToRs, standard clutch apart from heavier springs and standard cams, but he's about 7 stones (98lbs) lighter than me! He also said that at that speed the bars are rotating in their mounting as you hang on for dear life. Can't wait for August and the Bulldog Bash (run by the HAs and the only place in this country you can get on a strip without full leathers) so I can give it a try.

If 10 flat is true, it will outrun amost all sport bikes in the 1/4 except a busa, gxsr1000 and a zx14. They are all sub 10's in the 1/4 mile. Most of the other big sport bikes are 10.1 -10.47. The zx10 also clames a 10.0. I'm with regulator, putting the power on the drag strip will tell what they will do and how much abuse they will take.

 

[hombre]

Brembo

I've seen your torque chart... and with the right rear tire and correct launch, you'll turn sub 10s no problem.

 

[goodfellow]

If 10 flat is true, it will outrun amost all sport bikes in the 1/4 except a busa, gxsr1000 and a zx14. They are all sub 10's in the 1/4 mile. Most of the other big sport bikes are 10.1 -10.47. The zx10 also clames a 10.0. I'm with regulator, putting the power on the drag strip will tell what they will do and how much abuse they will take.

brembo,
you may want to look at a car tire.bridgestone potenza reo1 or dunlop durreza star spec. go 205 to 225. you'll hook like no tomorrow. you got nev's clutch? if not, you'll need it with these tires. sounds like you're hazing a little bit on the top after you feather the bottom.

greg

 

[hombre]

More info on valve timing:

"Changing the timing of a cam's opening and closing events can significantly change the engine's power curve. Of the four opening and closing valve events, the intake valve closing (IVC) has the greatest effect on engine-operating characteristics. The IVC begins the point where all valves are closed and starts the change from the engine's intake cycle to the compression cycle. All engine power is made when the valves are closed and cylinder pressure can build on the piston. All other things being the same, the earlier IVC occurs, the greater the cranking compression and low-rpm torque. As rpm increases, intake charge inertia increases and generates a "ram effect." Closing the intake valve later takes advantage of the ram effect at high rpm and increases cylinder filling for greater power. The higher the engine's operating rpm, the later the intake valve should be closed to maximize cylinder filling at high speed. However, the tradeoff is that low rpm cylinder filling will be reduced, thereby reducing low speed performance. Note the IVC when comparing cams with similar specifications. All else being equal, the later the closing, the higher the rpm peak torque and horsepower will occur."

I'm checking cranking compression for the longer duration exhaust cam at the different valve timing positions suggested by Nev. Ideally, the engine should be dyno tested with the different cam positions. But it's to much work to open and close the engine just for testing valve timing... and we leave for Mallorca this weekend. I'll try to make an intelligent decision based on the cranking compression.

 

[W. Tripp]

Most of that is good info, but really pertains to symmetrical lobe designs. Generally speaking, widening the lobe centers widens the power band, but reduces the peak. The opposite happens when you narrow the lobe centers. Advancing both cams helps low end, and retarding them helps higher rpm. All generally speaking - of course. The lift and opening rate can alter the results nearly as much as the duration - just in a different way.

Duration determines the place where peak torque will occur. Adding 10 degrees can move the peak torque point 500 pm higher. But as you go higher or lower in duration the effects are less or more respectively.

How well the intake and exhaust is tuned to match the cam and heads can alter the results too.

When you start using asymmetrical lobe shapes (opening ramp differs from the closing ramp), things change.

There are a lot of small cam design issues that you rarely see in print (or the internet) because they are tricks that most want to keep quiet, and separates the really good cam grinders from the average ones.

-Do some research into what happens when you open the two intake valves on 4-valve heads at different rates - one more, or more quickly, than the other.
-Look into what happens if you open the exhaust valves too quickly, or too far.
-Look at what occurs with cross-flow in the chamber at low lifts, and how to reduce this.
-Look at the rate the piston is dropping away from the valves, and compare this to the rate the valves open.

And all of this is related to head, port, and chamber design, as well as valve seat angles.

It's kind of hard to make sense of all of this, but I hope it helps.