Valve Tech - What's a 30 degree backcut worth?

Flow tested from .100" - .400"

Valves are M.I.J. Aftermarket OEM replacement 37mm J Intakes, from Z1.

The modified valve

Don't know about bikes. We did it to all our racing big block Mopars and saw as much as 10, or more CFM at lifts up to maybe .4" As I recall, cam choice and porting had a bit to do with it. Of course, those were much bigger valves.

Thanks for sharing the info. Roy

i have often wondered if back cutting the valve was actually worth any thing now i know i'm redoing a couple of heads for my drag bikes i'll be doing that before they go back together. 4 cfm is quite a bit especcally if it is free. thanks for doing the leg work, larry.

I always back cut the valves on racing applications but never had the chance to see what difference it really made, good info.
in my mind I figured it prob helped clean up the flow around the valve also, not sure if there is any way to teast for that.

I've looked at this a couple times and saved it.

Valve Angles - Make More Horsepower

They went as far as doing a double back cut, but it yielded no percievable results.
Not exactly bike tech, but still interesting :laugh:

Somtimes a backcut on the valves pays off and sometimes it doesn't. It depends on the port shape.

It's one of those things where you sneak up on the amount you cut as well.

Here's some numbers for you.

Stock 1983 GPz11 with much larger than KZ port and 38mm stock valves [the're back cut from the factory]

Flowed at 10" of test pressure.

.100 - 28CFM
.200 - 58CFM
.300 - 77CFM
.400 - 82CFM
.500 - 82CFM

Here's the KZ street cut I did with J Intakes and throats at 86% of valve diameter. Bowls are only 33.5mm Runners are still smaller than a J. Flowed without a radiused intake boot that generally adds 3 - 5 CFM to the numbers.

.100 - 30.8CFM
.200 - 55.4CFM
.300 - 78.0CFM
.400 - 84.2CFM
.500 - 86.0CFM

Head will get decked tomorrow and I'll post up some pics of the completed work.

It's getting married to some Cam Motion Z1 Grind cams [.400 lift with 246@.050 dur]and topped off a rack Keihin CRS29 Smoothbores and a 1075 Piston Kit. Should make a really nice street ride...

I could take this head to an easy 90+CFM no problem with 90% throats.

Great work and info Larry, cant wait to see the pics.
For that matter i'm even keener to get it down here to Aussie and fitted to the "old girl"!!!

Kt

Anyone here speak Japanese for I have no clue what the hell Larry just said.... :laugh: :laugh: :laugh:

I'm just glad I sent Larry my head to do his magic... Give me the spaghetti electrical work any day over all this CFM spanglish... :laugh:

OMR

Old Man Rock wrote:

Anyone here speak Japanese for I have no clue what the hell Larry just said.... :laugh: :laugh: :laugh:

I'm just glad I sent Larry my head to do his magic... Give me the spaghetti electrical work any day over all this CFM spanglish... :laugh:

OMR

Im thinkin OMR has had too many shorts in his pants:woohoo:

Old Man Rock wrote:

Anyone here speak Japanese for I have no clue what the hell Larry just said.... :laugh: :laugh: :laugh:

I'm just glad I sent Larry my head to do his magic... Give me the spaghetti electrical work any day over all this CFM spanglish... :laugh:

OMR

:laugh: :laugh: :laugh: :laugh:

Anyone notice the dead guy "Patton" in the image.... :ohmy: :huh: :dry: :laugh: :laugh: