Landracing Forum
Tech Information => Technical Discussion => Topic started by: konon on March 08, 2014, 10:34:55 AM
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Would adding a 45 degree elbow in a intake tract , to turn the carbs out , and allow longer intake length, be a restriction ? This is a BSA A65 (650) motorcycle.
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I would try to make it an ellipse if possible
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In a word, YES.
The real point of fact is: How much of a restriction? The ONLY way to know for sure is to flow bench test the combo, both ways, back to back. Manifolds ALWAYS add SOME restriction. Manifolds with turns, add more. A gentle/gradual turn will add less restriction than a tight/sharp turn. How much might be tolerable would need to based on the "flow demand" that the engine requires for the bhp desired. AND, more importantly, ANY turn messes with the "wet flow" of the fuel you are using. Since fuel is heavier than air, it will turn "poorly".
Not trying to discourage you, just want you to put some thought into what you are going to do. Take a look a some of the airflow numbers I've posted on Milwaukee Midget's Build Diary, pages 230-245 approx.
Also read:
A/ any book on Fluid Dynamics
2/ Engine Airflow by Harold Bettes
d/ How to Port & Flow Test Cylinder Heads by David Vizard
AND, Most Importantly: Generate and believe your own data. Don't listen blindly to some "expert" or "guru", regardless of how much "experience" they have. Part of this journey is developing your own cerebrum.
:cheers:
Fordboy
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The real point of fact is: How much of a restriction? The ONLY way to know for sure is to flow bench test the combo, both ways, back to back.
That is what you have to do!
I have flowed hundreds of carbs, manifolds and heads. Even when they have the same casting number and have never been touched and look exactly alike they MIGHT or MIGHT NOT flow different numbers.
Doug Odom in Big Ditch
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The real point of fact is: How much of a restriction? The ONLY way to know for sure is to flow bench test the combo, both ways, back to back.
That is what you have to do!
I have flowed hundreds of carbs, manifolds and heads. Even when they have the same casting number and have never been touched and look exactly alike they MIGHT or MIGHT NOT flow different numbers.
Doug Odom in Big Ditch
Doug,
That is my experience as well.
What I know for certain is: You can not tell anything from looking at the parts . . . . . because, It's Complicated . . . . . . . .
Check out some of the flow bench testing numbers for stock & "modified" Weber Carbs on MM's Build Diary. Don't know the page #'s, sorry. Just go to MM's Build Diary and do a search for "weber carb" and all the relevant postings will pop up. (I love technology!!! Welllll, when it works . . . . . . :roll: )
:cheers:
Fordboy
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A question that should be addressed before any plans are made: is the intake tract intended to be resonance tuned for length?
If not, the curved section should be not only an ellipse but have slightly greater cross-sectional area than the tube ID before & after it to compensate for the radius' flow reduction. Merely squashing a tube to get an ellipse retains only the circumference but reduces the X-area.
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The distance between the bell mouth inlet and the inlet valve face is important to get the max HP out of those A-65's. It is a pretty short distance. The carbs pretty much bolt onto the back of the head with a phenolic spacer about 1/8 to 1/4 inch thick between the head and the carb, is my experience. No room for a manifold.
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I've had this bike since 1969 so I don't want to do modifications to frame. With the cam I'm running I need 14.inches from valve to bell mouth. I was told it didn't matter were the carb was, I could make stacks to get the length. So I'm using 1/2 inch spacer with manifold to keep carbs close to head for a strong vacuum signal and try different length stacks on the dyno. I was hoping for 2nd harmonic wave ,but I don't think I can get enough length for that. Does this sound like it might work better than the 45 degree elbow ? Not sure the carbs clear my legs anyway.
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If the 2 options interchange easily try them at the drags and compare the speeds . Same for back to back acceleration runs between reference points on a road reading the speedometer .
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IIRC Jennings goes into some detail about the difference in effect between length to the carb and length to the stack end.
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Thanks for the help.
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The third harmonic was what we were tuning the intake tract for in those days. It is shorter and you might be able to get it without a lot of work.
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What classs are you going to run? I like the A65s and have though about building one for the salt. One thing to think about is the fact that from the factory they are 654cc...which means you have to run in the 750 class unless you destroke or put in liners...and still can trun in production classes.
I could be wrong though, but thats my take.
Keep you intake straight. mod your frame.
Hans
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There was a production A-75 BSA. A batch of them were made and sold here so the BSA 750 flat tracker could race in the US. Some of the tricks used to set up those engines might be useful for running in the 750 class.
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I was going to sleeve the cylinders and use 74.5 mm pistons. to get it under 650. M/PG 750 is 160 mph at Bonneville and 130 at Ohio. I'm still making a plan.
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Konon, some experience I had. The intake tracts with the carbs just where BSA put them can be made to provide more mixture than those engines can use. The dellorto carb website has a handy graph for carb size. It gives realistic estimates for carbs on those engines.
Looking at my build diary you will notice how my engines have moderate compression and cam timing. I pay a lot of attention to intake harmonics, exhaust tuning, and getting the proper spark advance curve. These are habits I developed from tuning A-65's. The engines are not durable with high compression or radical cams. I was forced to look hard at other means of getting performance.