Racer intake/exhaust length &max RPM

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Allflatblack
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Racer intake/exhaust length &max RPM

Post by Allflatblack » Tue Apr 17, 2018 5:05 pm

You guys have really helped me out along the way and have certainly offered me your opinions over the last few years relating to the subject topic but I would just like to see what you might have to say one last time as I drop into the final fabrications for this road race bike. Intake and exhaust.

I have built a B50 with Std bore. 12.25:1 compression,approx .050" squish. Long Carillo rod, Je piston. I have a dual plugged Ted hubbard ported head with Ti Valves (approx. 47mm and 41 mm ex.) Beehive springs. Shortened intake rocker arm (.45" lift) Megacycle X4 cam. The intake port is 42mm ID, Ex. 1 7/8" ID. 1 3/4" slip fit exhaust ID.

David Rutherford has suggested that he has been most successful with a 9" intake runner and a 29" long exhaust. that is my fallback design.

I am using several calculators to verify the best size pipes and lengths to get the most out of this engine. A. Bell (Four stroke performance tuning) put together a series of calculations years ago that are considered the gold standard and many people have put together good calculators such as http://www.mezporting.com/exhaust_length.html

My questions:

RPM: The main feature needed to use these calculators is max RPM or peak torque RPM. Since the engine is not yet built I do not know what that is, any good guesses out there? I am assuming redline at 8,000?

Carb sizing: The inlet port is 42mm, the valve is bigger than that and the lift is .45". Carbs are available in 40mm and 44mm. To me 44mm seems right because of the venturi effect of 44 stepped to 42mm, but 44 seems wildly big.
Unfortunately an AMAL MK2 40mm is $350, and the old one that I have is junk. I have a brand new 44mm Mikuni ($99)
I have a lot of experience with the VM for racing. Any opinions? Will I be wasting my time and money trying to set up a 44mm, or have you heard of a hot bike road racing doing well on something like this?

Thank you once again for any help.
AHRMA #647
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Re: Racer intake/exhaust length &max RPM

Post by Allflatblack » Mon Apr 23, 2018 3:50 pm

I have the info that I need from Rob Hall
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Re: Racer intake/exhaust length &max RPM

Post by stew79 » Mon Apr 23, 2018 7:36 pm

i thought you would have had many replies to this post, but you never can tell. the inlet port size in the head sounds a bit big, ok if it reved to 10000 rpm. i cant see much use it having a vm carb over 38mm, any bigger will give terrible air flow at less than 5000 rpm and over half throttle (a pumper carb would help a bit but will make set up very hard) 50 thou squish is ok (a bit big perhaps) but over how much area of the piston ? is it wedge shaped towards the center ? 2 plugs can make no difference to performance, one plug (the right type, heat range, etc) fired from cdi at the right time would be better than 2 being fired from a std electronic ignition unit (possibly at the wrong time ?) with so many variables going on, the exact lengths of in and out "pipes" seams a bit trivial ?
stew

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Re: Racer intake/exhaust length &max RPM

Post by Allflatblack » Thu May 17, 2018 4:54 pm

Yep!
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Re: Racer intake/exhaust length &max RPM

Post by koncretekid » Sat May 19, 2018 6:04 pm

Sorry, I have been out of touch here in Colorado. Your set-up is very similar to my road race bike (not surprising since we both used Ted Hubbard and E & V parts), except my intake port is nowhere near 42mm. In fact, I didn't think that was possible without cutting and welding as I have on my land speed motor. I assume you're using the Megacycle X4 cam set-up at 108º centers or slightly retarded.

That said, I don't think my intake port is over 34mm and I used a 36mm Amal Mark II carb with good results. I also tried a 40mm Amal Mark II at Bonneville on that motor with 108 mph top speed in otherwise road race trim, but that was at 4400 feet elevation. 8,000 rpm may be your redline, but I'm guessing your maximum hp occurs closer to 7,000, so gear accordingly. My intake runner is 6" long plus the carb which is 3" long, so 9" total from the cylinder head to mouth of carb, and my exhaust is about 34" long outside measure plus a megaphone.

Good luck and keep us posted.

Tom
life's uncertain - go fast now

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