Msv?

L

Lazyeiger

Member

May 5, 2003

23

0

• Jan 20, 2004

• #1

I have read that a higher MSV tends to favor low/mid range torque whereas higher MSV favors top end/overrev.

I have also read that higher compression favors low/mid rnage torque.

Now I have a 7cc combustion chamber on a CR125 with a .030 squish clearance and a 40% squish band comes out to about 25M/S at 11Krpm if I increase the squish to 50% it comes out to about 35M/S

Now if I reduce the compression The MSV increases. So which senario favors top or bottom end torque?

Does this also imply that increasing compression calls for an increase in squish area to keep the MSV the same??

Also how does the blending radius factor into all of this, I assume that a sharp raduis causes more turbulance, but how does this effect the MSV?

Thanks JBB

 

J

Jman271

~SPONSOR~

Oct 18, 2001

317

0

• Jan 21, 2004

• #2

The range for central bowl should be 15-30 m/s. Things that lower MSV, low rpm, smaller sq area ratio, high port, larger bore, shorter stroke,more sq clearance, higher comp ratio. Things that raise MSV, higher rpm, larger sq area ratio, lower port, smaller bore, longer stroke, less sq clearance, lower comp ratio. It would help to know how far ATDC in mm your ex opens, connecting rod length, as well as current CR to determine what you are trying to achieve. If your MSV get too high for 11k, you will get some problems like piston damage and detonation, MSV too low, it will run rough at low speed. Is this a flat top, or dome? What yr is this CR? If you give specs, I can maybe pump it in the PC to see what happens. What are the goals of this head design, are you looking for more low torque?

 

L

Lazyeiger

Member

May 5, 2003

23

0

• Jan 21, 2004

• Thread starter
• #3

It's a 1999 6 speed CR125 used in a shifter kart. We have just changed class from a class with a 10.5cc head volume minimum ( actual head volume 8.5cc) with a spec ignition with a 12.5K rev limiter. the class is basically 125cc unlimited.

exhaust opens at 27.5mm ATDC = 83 degrees
Conrod 104mm
I am looking to increase bottom end ( 8.5K rpm) to peak and keep a good overrev to at least 13K
Domed RS HRC "A Kit " piston (0.8mm flat ring)
Wolf programable ignition (I have the programer)

Go here to get some idea of the torque in the lower class configuration. http://www.angelfire.com/nm2/bettsracing/images/Dyno03212003.JPG

Thanks JBB

 

marcusgunby

LIFETIME SPONSOR

Jan 9, 2000

6,450

2

• Jan 21, 2004

• #4

that is one peaky torque curve-you sure the cr engine makes a good base to work from?

 

L

Lazyeiger

Member

May 5, 2003

23

0

• Jan 21, 2004

• Thread starter
• #5

We have no choice the sanctioning body defines the engine, either CR, YZ, KX or TM 125. purpose built european kart engines (ICC) are making a entrance into the US karting scene and a fully built CR125 is competitive with these. Top built pro engines are putting out somewhere in the 19 ftlbs area at about 12K so around 43HP.

Most of the peaky narrow powerband is caused by the 12.5K rev limiter and crappy ignition curve on the stock box. Also don't forget on a kart the RPM very rarely drops below 9K rpm so who needs power down there anyway!!!

JBB

 

J

Jman271

~SPONSOR~

Oct 18, 2001

317

0

• Jan 21, 2004

• #6

I don't know if you have PV or not, but the 8.5k mark may be around the time it opens, which if that is the case, maybe hard to retain low end torque when PV is WO. But if it has plugs or no PV, you may just have to shoot for max hp @ desired rpm. I know this sounds like jibber jabber, but let me look at the #'s and post back when I can. If I have this correct, you want more power from about 8.5k up to 11k, w/ overrev to about 13k? When, (mm) do trans open? Alot of other things will calculate for more power beside head design. If I know that, I might be able to hone in on it. Also, how much deck clearance do you have? Sorry for all the questions, trying to help----the CR's as of late seem to favor adv port timing meaning that they have upper mid power, but lack low end torque, so it may be like trying to get blood from a stone if trans are too high or ex too wide. I will post back when I get some info,,,,,

 

L

Lazyeiger

Member

May 5, 2003

23

0

• Jan 21, 2004

• Thread starter
• #7

PV is plugged. go here for a picture of the port map.

http://www.angelfire.com/nm2/bettsracing/map.html

I am looking for a overall increase in torque from 8.5K to 11K - 11.5K with good over rev to 13K - 13.5K. I am not worried about piston/ ring/ crank wear as I change ring every 2 hours piston every 5 and crank every 10.

I don't understand your comment about the exhaust being too wide? I was under the impression that the exhaust can never be too wide? Bell states that widening the exhaust will always increase power over the whole band?

Thanks JBB

 

J

Jman271

~SPONSOR~

Oct 18, 2001

317

0

• Jan 22, 2004

• #8

Once you get to over 70% of bore width on ex, you run into excesive ring wear and piston life, this is even more so w/ main ex, no bridge. Also, if you go too wide you will encroach on the pin boss and the jug will be useless. I think 2mm was a safe buffer distance to pin bosses.

 

Eric Gorr

Engine Builder

Jun 29, 1999

384

12

• Jan 22, 2004

• #9

I'm reading this and having a chuckle because shifter kart guys are trying to talk to dirt bikers. Low End. Low end on s shifter is wide open for most dirt bikers.
When you build a shifter its basically a time-release-bomb. My suggestion, try the flat-top Wiseco 762MO5400 and the stock 99 head with .8mm squish clearance, then program in more advance at 8-9K. Joe Mossbarger is an engineer at Wiseco and a shifter kart racer. His dyno testing efforts have shown that the forged pistons can sustain more timing advance.
Regarding exhaust port width on a shifter, I get better luck with 98% of the bore width with a lower height, say 28.7mm effective stroke. I think that will give you what you're looking for.

 

J

Jman271

~SPONSOR~

Oct 18, 2001

317

0

• Jan 22, 2004

• #10

Yeah, what EG said!!!! LOL!!!!

 

L

Lazyeiger

Member

May 5, 2003

23

0

• Jan 22, 2004

• Thread starter
• #11

Thanks for the info Eric!

You say 98% of bore for exhaust, you mean 176 degrees around the bore? 53mm from one side of the port across the bore to the other?

So this does have the wrist pin crossing the exhaust I assume that with such a wide exhaust most of the pressure is gone before the exhaust shorts to the primary?

I have heard that Wiseco re "engineered" the 762 piston at the end of last year and took a lot of the meat out of the top, now it can't take the abuse of the high compression and advance in shifter karts?

Can you comment on the flat top V's domed piston differences? Do you have any experience with the Wiseco 793?

How about I lower the cylinder 1mm to make the exhaust open at 28.5mm and recut the head?

Thanks JBB

 

Eric Gorr

Engine Builder

Jun 29, 1999

384

12

• Jan 25, 2004

• #12

Yes 53mm wide and yes the exhaust and transfers overlap when using a piston like the RS with cutaways on the outside.
I prefer to use the 762 and 793 because they're full surface pistons (no cutaways) EN coated, and pinned on center. Wiseco has shifted towards designs with thicker crowns no away. I haven't melted or indented a crown on a 762 yet. Using the 762 with a flat top head allows you to get away from using a spacer plate and that may get your exhaust port height where you want it. However you could always turndown the base.