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#851103 06/08/21 10:00 pm
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Hey Folks,
In the process of rebuilding a '69 Bonneville.. thought I'd go a little warm on the motor this time... Normal I do bone stock... Anyway... I bought a head from a guy a long time ago that was a "race" head... Looked nice. new vavles, keepers.. and clean. I'm putting the motor together this week and noticed the port shape of the exhaust ports is really odd. Instead of having just a straight shot, gaping hole, for an exhaust port, this head has has a ramp built up with aluminum and has a ramp shape on the way out. Anyone ever seen this before? My guess it would help low-end but not sure on a race head why that would be beneficial.

Attached is a very dark picture of said port...

[img]https://drive.google.com/file/d/1KWthO-wazxJ5rTZeFnQXVkC5-kCq_jP2/view?usp=sharing[/img]

Thanks!

...Joe

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It's called "D" shaping the port. It is a common exhaust port tuning trick. Done right it works.
John

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Thank you, John! In my 50 years of brit bikes, I've not seen this.

Do you know what the purpose is? Ramped up flow?

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John, I have some BSA heads with big inlet valves but I also enlarged the exhausts. If I have new seats fitted I could use std 35.8mm or 37mm with the 44.5 inlets. I'm wondering if it makes much difference, most are 38 or 39mm. They go fine but I'm wondering if they are detrimental?

Joe do you have pictures of the intakes?


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Joe, thats the idea. Better flow. My race 650 has had this done and taken a step further with the pipe outlet (spigot) raised.
If this is done to your head, I too would be interested in seeing the intake ports.

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Larger radius on the inside of the curved duct means less gas speed difference between outside and inside of the curve.

That makes it tend more towards laminar flow.


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Hi Mike,

I can take a shot of the intake today. I'm pretty sure it's just flowed but not shape changes. I'll get it posted this afternoon!

Thanks for the reply's!!!!

...Joe

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Originally Posted by triton thrasher
Larger radius on the inside of the curved duct means less gas speed difference between outside and inside of the curve.

That makes it tend more towards laminar flow.
Yes, that's why the intake port floors on my 650 race Triumph are raised just before the turn into the valve area. Port velocity is increased and good things happen.


79 T140D, 96 900M Ducati ,61 A10 .On a bike you can out run the demons.."I don't know what the world may need
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Here's a pic of my intake port.. nothing special. It's enlarged a bit and smoothed with the manifold...

https://drive.google.com/file/d/1E4l8jhvkzDJ_C2CkCd-RPPTiIF5Sx96c/view?usp=sharing

...Joe

Last edited by JoeG; 06/09/21 6:36 pm.
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Hi Mark

The site says your PM is full

Some interesting stuff maybe.

https://www.youtube.com/channel/UCJv1SDdwEMhcl5n_dkvdKng/videos

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Originally Posted by Mark Parker
John, I have some BSA heads with big inlet valves but I also enlarged the exhausts. If I have new seats fitted I could use std 35.8mm or 37mm with the 44.5 inlets. I'm wondering if it makes much difference, most are 38 or 39mm. They go fine but I'm wondering if they are detrimental?

Joe do you have pictures of the intakes?

I welded “fillets” in my exhaust pipe on the A65, it worked well and gave a better top speed and more seat of the pants feel at midrange torque. However the fillet took up about 1/6 of the port area and it made carburettor tuning a right royal pain in the ass, so I got my die grinder and removed them again. I was having to use leaner slides #4, standard needles at lowest setting and 105 needle Jets. The difference was definitely noticeable Straight away after I removed thee fillet, even after I rejected the carbs to suit its new requirement. I’ve since modified my silencer again so it needs some more tweaking, but top end speed is still not quite as good.


Now let’s all have a beer beerchug

68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (undergoing restoration)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)

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Doesn't appear much has been done there. Raising the floors a little and blending/ streamlining the guides could show some gains.
I'll look to see if I can find a pic of mine.


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