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Ironhead Sportster Motorcycle Talk (1957-1985) For all those that wanna talk about Ironhead Sportster Motorcycles

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  #31  
Old 6th August 2017
Slojoe55 Slojoe55 is offline
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What's stock valve size on a set of 74 heads

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  #32  
Old 6th August 2017
needspeed needspeed is offline
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Quote:
Originally Posted by Slojoe55 View Post
What's stock valve size on a set of 74 heads
This post from Dr. Dick covers it all.

http://xlforum.net/forums/s...08&postcount=7
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  #33  
Old 6th August 2017
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A couple of tidbits I learned from an old engine builder;

First; If you look into the intake port you'll notice a slight rise in one section of the passage. I think it's the valve pocket side but when you look into the intake port, it's pretty evident.
DO NOT grind that down flush with the rest of the passage. It's there to help with air flow and fuel/air mixing. Smoothing the passage there can make performance plummet.

Second; Bigger isn't better. You can get intake valves that are so big that air flow/velocity in the area around them drops off and intake mixes around them just kinda fall into the hole when the valve opens instead of being carried in as with higher flows.
One study showed that performance increased when the intake valve size was reduced and the ports closed up slightly.
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  #34  
Old 6th August 2017
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I've looked down the intake ports of some amazing heads. Ducati's done by Syd Tunstall, Norton's done by Jeff Clatworthy and my own Baisley heads.

What that told me was I can't see the magic. I know it's there because I see the results.

I know the theory, the port velocities, nozzle effects, tapered ports, valve to intake port sizes, 5 angle valve jobs narrowed and shaped guide bosses, shorter guides, on and on.

Cast iron is really hard metal to shape. Well, compared to alloy at any rate. Even so, I suspect I could ruin a good set of heads in under an hour.

But I decided it was smarter to just leave it to the experts.

I pay my Doctor. I pay my Dentist. In fact, any expert who has spent thousands of hours to acquire knowledge and skills that I don't have - I pay them for their expertise.
Would I LIKE to be able to port my own heads as well as those guys ? Sure, but I'm not willing to invest the time and money they have to get there.

Spend your time and effort on building a reliable engine that will get you home (or to the finish line) first.

An extra 10 HP will not improve your chances against any 600 CC Sport bike out there.
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  #35  
Old 8th August 2017
11B40 11B40 is offline
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The average Joe smokem has no flow bench and what a good intention modification might do is in doubt.
"An extra 10 HP will not improve your chances against any 600 CC Sport bike out there"
Amen. I remember the first time I grudge raced a Kawa triple. It was at Fremont Drag Strip and this $900 out the door chain saw dead stock planted a 12 second pass on me I wasn't laughing at it anymore. Now the Honda/Suzuki/Kawas really go good. It takes a stroker to play that game and regardless, you still have no chance really if the guy can ride.
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  #36  
Old 8th August 2017
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Default 900 bike

Quote:
Originally Posted by BrokenIron View Post
Yea that's kinda what I gathered, oh well.
I don't need to have a race bike, but it just seemed so gutless that I want to give it a boost. Now granted, I think a lot of my power loss was from other things. My Indy described some issues that I can't for the life of me recite. But it made sense.
In your opinion, what are things that could rob power from an old 900, other than the fact it's an old 900?
Basic stuff.. any ideas?
--- so Image that same Motorcycle weighing 75-90 lbs Less.. Can it be done ? Yes ...
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  #37  
Old 15th August 2017
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Quote:
Originally Posted by mrmom9r View Post
1970 cylinder heads do not need ( or want ) porting for a stock displacement motor.
ta da!
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