DragBikeMike
Serious Thumper
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SuzukiSavage.com Rocks!
Posts: 4166
Honolulu
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Modifications to the exhaust port included:
A general clean-up of the port by removing rough cast areas, blending the valve seats into the aluminum port wall, and attempting to blend and increase the short side radius.
Back-cutting the valves 30°.
Cutting back the valve guides until they are flush with the roof of the port.
Filling in the step in the roof of the port to provide a smoother flow path. I refer to the step as the “Dipsy-Doodle” (DD).
Enlarging the port opening (location “D”, see sketch) from 1.3” to 1.5” diameter and blending the adjacent wall to provide as straight a flow path as possible. Material removed from top and sides of port, floor of port left as-is. This is a “D” shaped port.
Note: I used a 1.5” ID test adapter to connect the test bench to the exhaust port. When I tested with adapters smaller than 1.5” ID, flow dropped dramatically.
Let’s look at the various modifications to the exhaust and discuss the pros & cons.
The general clean-up of the port provides about a 1.5% increase in flow. It is straight-forward and relatively low risk. Just like the intake, pretty much anyone with a Dremel tool and steady hand can pull it off. However, the opening is a lot smaller and there are lots of nooks & crannies that are hard to get at. Be careful around those valve seats so you don’t end up with any rat bites in the seating surface.
Back-cutting the exhaust valves 30° provides about 1.0% increase in flow, with gains mostly at lifts up to about .250”. If you have the tools, it’s easy to do and seems low risk. Like the intake valves, you will need a lathe or Neway Gizmatic to do it. If you don’t have the tools, a trip to your local engine rebuilder and a few bucks should get it done.
Cutting back the valve guides until they are flush with the port didn’t seem to improve flow very much. I found that odd. However, with various combinations the flush guides appeared to have a positive effect. Removing the guides, cutting them, and reinstalling is not an option as the interference fit is reduced dramatically when the guides are pushed out. You must install oversize guides. It is possible to grind or machine the guides in-place, but it just doesn’t seem to be worth the effort. I’m gonna leave the exhaust guides alone.
Filling in the Dipsy-Doodle improved flow by 4.5%. It worked well. It should be inexpensive and relatively low risk. In the event of a failure, flow should carry the filler material away from the valves. Fishing chunks of filler material out of the muffler shouldn’t be too hard. The trick will be coming up with a product that can live in the extreme operating environment.
A good anchor profile will have to be cut in the surfaces of the DD. That should be easy with a Dremel along with number 113 & 199 high speed cutters.
JB Weld “Extreme Heat” paste is rated to 2300°F, but it’s single-component water-based. That means it shrinks as it sets up. I currently have a test specimen filled with the product. Stay tuned for the results.
As expected, enlarging the port opening from 1.3” to 1.5” was a grand slam. It resulted in a 25.3% improvement. Except for some minor cleanup, all material was removed from the sides and top of the port. There’s not much flow across the bottom so leave it alone. You end up with a “D” shaped port. It should be noted that this modification’s effectiveness can only be realized if a proper 1.5” or larger exhaust pipe is used. Using the stock header kills the effectiveness of this modification. More on this later in the report.
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