stock vs. modified cylinder head flow numbers

[650performance]

I've had several people email me asking about what they should expect in terms of added flow in a ported vs. stock, unported head. I don't claim the modified head tested here is the best ever, but it is a very good one and provides a useful comparison for anyone looking for real numbers to evaluate the success of their modifications.
Craig


COMPARATIVE FLOW NUMBERS
STOCK & MODIFIED XS650/750 CYLINDER HEADS

Test Conditions:

The stock head is not modified in any way and has been bead blasted to ensure the ports are clean and fitted with stock valves.

The modified head is the Harry Lillie modified "master"used for modeling the CNC porting program and has the modified, oversize XR valves. Both tests were conducted within a half hour of one another on the same flow bench with the same operator employing the same fixtures and procedures. Both were tested at 28"pressure.

Results:

The results are expressed as a percentage difference at each valve lift increment. I am not sending out the flow numbers because your flow numbers will be different on another bench, and I don't want to get into discussions comparing apples to oranges. However, you should see virtually identical improvement over stock (castings do differ) with your modified head. As you can see, on the IN port the mods really begin to kick in at .300"lift, while the EX porting becomes effective between .150"and .200". The numbers underscore that the EX port is the most obvious limiting feature of the head and was one of the primary reasons the OU-72 was developed.

It is important to note that the not-so-secret "trick" to make the EX flow better is to optimize its shape to both improve total flow AND to reduce turbulence within the port to the lowest possible number. Needless to say, this takes some sophisticated flow equipment to measure. By getting this balance right, the port can flow "more than its numbers," as it was described to me.

Valve lift Flow difference from stock head
INTAKE EXHAUST

.050" 0% +12%

.100" 0% 4%

.150" +2% 3%

.200" 6% 18%

.250" 6% 22%

.300" 9% 22%

.350" 19% 27%

.400" 37% 25%

.450" 46% 24%

.500" 56% 23%


Test conducted at Competition CNC on Super Flow 600 - Sept. 2008

 

[blackbetty]

Dude I'm glad you came around, your last few posts have been badass.

 

[arcticXS]

Thanks for sharing this info. Surely is food for thought.
I also read the other post about "too much power" or more accurately, overstressing the crankcase. There was some discussion around the suitability for the "Lillie Replica CNC Head" for street applications.

Now, I am building my XS for the street (and unpaved roads), but still would like more power than stock. So far I only have done the following carb, filter and exhaust upgrades:
Mikuni RS 34s from Topham in Germany, 1 1/2" single wall headers (35 mm ID) and modified Norton Commando peashooters (straight through glasspack, 35 mm ID baffle)

I plan on rephasing to 277, 750 kit, cam and porting (or the CNC head...)

So will the CNC head be a good choice for a tractable street 750 with the mentioned upgrades, running on 98 RON pump gas, or should I explore other options?
Is the CNC head only worthwhile with the recommended pistons, or will it work OK with Mike's 750 kit, Wiseco and other common piston choices?

Is 65 RWHP a realistic number, on pump gas with reasonable reliability and service demand?

Thanks for any input!

 

[650performance]

Hi arcticXS ...
A good basic engine setup for a XS street engine is a 750 kit, 34 or 36mm Mikuni flat slides, 9.5 or 10 - 1 c.r., a Shell #1 cam, 1 3/4" TT exhaust pipes or the Jemco side pipes, electronic ignition, and a free-flowing head. With this setup you will have no problem finding 64 or 65 RWHP. Tony Guest's tracker with a similar setup (but with Megacycle #30 cam) dyno'd over 68 RWHP.
Yes, the CNC head will work fine. It is a high velocity design and is very "streetable." However, it is expensive when compared to a mild port clean up and a race quality valve job performed by someone who is an experienced head work expert.
Rephasing doesn't make any more power, however it can reduce vibration, which in turn makes the bike easier to live with.
I've attached a couple of pictures of Tony's tracker which, if you look closely, has 19" wheels with flat track tires mounted. I suspect this bike would be real hoot on dirt roads.
Good luck on your project, arcticXS.
Craig

 

[dwyatt]

I'm running one of Craig's heads on my roadracer, but with a 700cc bore and I can attest it most definitely romps, especially out of corners. Get his modification guide and study it before starting on the motor, it's very educational. Also, Craig is a good one to pick his brain, I all but stalked him for a year........

 

[Jack]

Thanks for sharing this info. Surely is food for thought.
I also read the other post about "too much power" or more accurately, overstressing the crankcase. There was some discussion around the suitability for the "Lillie Replica CNC Head" for street applications.

Now, I am building my XS for the street (and unpaved roads), but still would like more power than stock. So far I only have done the following carb, filter and exhaust upgrades:
Mikuni RS 34s from Topham in Germany, 1 1/2" single wall headers (35 mm ID) and modified Norton Commando peashooters (straight through glasspack, 35 mm ID baffle)

I plan on rephasing to 277, 750 kit, cam and porting (or the CNC head...)

So will the CNC head be a good choice for a tractable street 750 with the mentioned upgrades, running on 98 RON pump gas, or should I explore other options?
Is the CNC head only worthwhile with the recommended pistons, or will it work OK with Mike's 750 kit, Wiseco and other common piston choices?

Is 65 RWHP a realistic number, on pump gas with reasonable reliability and service demand?

Thanks for any input!

Never! Never! Under estimate the true potential of the XS head when you place it in the right hands of a practitioner who has the knowledge to move the air in the right places and Jerry Heiden(Spelling ) who's in your back yard comes to mind. Not trying to discourage you or anything but Jerry's a well known XS enthusiast with credentials to back it up. What it lacks on the later models is a piston to utilize the quench area provided in the chamber ,that's it and thanks to Dave Rayner,such a piston exist through Ross pistons. Do some checking first before you dump a load of money on a head that might not be necessary.

 

[bt24]

I'm running one of Craig's heads on my roadracer, but with a 700cc bore and I can attest it most definitely romps, especially out of corners. Get his modification guide and study it before starting on the motor, it's very educational. Also, Craig is a good one to pick his brain, I all but stalked him for a year........

How can I get a hold of this modification guide??

Thanks,
bt24

 

[scabber]

http:/www.650performance.com/ got this from 650 central site

 

[1974jh5]

650 performance,that's a great paint scheme on the bike in your post! I also like the see through front brake caliper...

 

[650performance]

Those were the colors Shell used on his bikes when he sponsored racers in years past. The glass tank and seat as well as the frame are also authentic Shell bits.
Craig