Author Topic: Porting Intake  (Read 299 times)

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Offline 604f_1

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Re: Porting Intake
« Reply #10 on: November 30, 2015, 02:14:42 pm »
The only reason to make the bottom of the intake port close/open progressively is to take it easy on the piston skirt . Make the intake 70% wide and the bottom of the port totally flat , run it 5 minutes and look at the piston skirts , it sure won't look good !

Offline oldpro

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Re: Porting Intake
« Reply #11 on: November 07, 2016, 06:49:04 pm »
You are right,sme manufacturers curved the bottom edge of the intake port so the port didn't open suddenly with a huge "POP",with really affected idling and slow speed running,that sudden change of vacuum was hard to compensate the carb for. But todays carbs meter much better than the old Tillotson HL,the curved port edge is just paranoia left from the old days...
I always try to get maximim area from the the port in it's given duration,race bike engines never had a port floor like that.
Considering that an intake with 170 degrees of dur. at 13,000 rpm is only open for .00024 seconds(thats 24 thousanths of a second!) you must obtain all the area you can get.
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Offline oldpro

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Re: Porting Intake
« Reply #12 on: November 09, 2016, 11:03:43 pm »
OOps I made a math error,intake port open time in duration is .00021 seconds. Increasing intake duration to 185 degrees =.00023 of a second actually increases the port open time by 9%,-a possible 9% power increase. I will take that anytime,in fact I will do the mods needed even for a 1 or 2 percent increase,it all adds up quickly.

Offline Al Smith

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Re: Porting Intake
« Reply #13 on: November 14, 2016, 06:22:15 am »
You might find a less than mirror finish on the intake will work better .Too smooth it has a tendency to cause  the fuel mixture to liquify/condense on the walls of the port .

Going from atmospheric pressure across the carb to a partial vacuum in the crankcase is a pressure drop and in effect has a tendency to cool slightly .

Some manufactures have given the intakes a dimpled finish which has tendency to break up the air flow to prevent or lessen this effect .I've found a finish about like using Craytex is about as smooth as it needs to be .Hours of polishing with jewlers rouge and felt fobs doesn't help much if any .
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Offline oldpro

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Re: Porting Intake
« Reply #14 on: November 14, 2016, 03:08:08 pm »
Also more intake open time(Duration) will help to increase "ram effect" which will occur in a race motor with enough rpm,intake duration,and large enough carb to allow it to happen. Ram effect actually supercharges the crankcase to a certain degree.
I have actually documented ram effect on a race motor by conducting vacuum and pressure tests on a running engine from idle to full throttle. My tests showed that there was constant pressure in the crankcase during ram effect,and there was no vacuum even on the piston upstroke... The fuel/air column reaches enough velocity and energy to overcome blowback from the desending piston,and rams into the crankcase.
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Offline 3000 FPS

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Re: Porting Intake
« Reply #15 on: November 14, 2016, 03:38:30 pm »
Wow I have never heard of ram effect on a 2 stroke like what your describing before.  Interesting.
PP 505, 475, 445.

Offline Chris-PA

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Re: Porting Intake
« Reply #16 on: November 14, 2016, 04:20:00 pm »
OOps I made a math error,intake port open time in duration is .00021 seconds. Increasing intake duration to 185 degrees =.00023 of a second actually increases the port open time by 9%,-a possible 9% power increase. I will take that anytime,in fact I will do the mods needed even for a 1 or 2 percent increase,it all adds up quickly.
Off by 10 - at 13k a 185deg duration is 0.0024sec, or 2.4ms.  It's still a damn short amount of time and it's amazing they run up there at all. 

I'm sort of skeptical the carb actually responds to each revolution/intake pulse at all at those rpms, rather I'd bet it responds to the average.  The outlet check valve is probably fast enough, but I'm not sure the metering diaphragm/lever is.  What I was thinking about is that these carbs to not give a constant fuel/air mixture as air velocity varies (because they have no air corrector jets), instead the volume delivered goes up as close to the square of the air velocity.  This is why a saw 4-strokes - with only a small increase in rpm (=air velocity) it gets so rich it misfires.

Given that, I was wondering what those very fast air pulses from the flat lower port edge do in regard to metering.  The short intake can't help in that regard.  Still, I guess if you can tune it then it must all work out.

Also, the carb venturi shape is optimized for flow in one direction, but it is still going to add fuel in the opposite - so those strong pulses may contribute considerably to spitback.  Whether that is a problem might depend on the air filter, etc.

Offline 3000 FPS

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Re: Porting Intake
« Reply #17 on: November 14, 2016, 07:48:31 pm »
Also more intake open time(Duration) will help to increase "ram effect" which will occur in a race motor with enough rpm,intake duration,and large enough carb to allow it to happen. Ram effect actually supercharges the crankcase to a certain degree.
I have actually documented ram effect on a race motor by conducting vacuum and pressure tests on a running engine from idle to full throttle. My tests showed that there was constant pressure in the crankcase during ram effect,and there was no vacuum even on the piston upstroke... The fuel/air column reaches enough velocity and energy to overcome blowback from the desending piston,and rams into the crankcase.

Do these engines still require an impulse for the carb and if so how does that work out.
PP 505, 475, 445.

Offline oldpro

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Re: Porting Intake
« Reply #18 on: November 23, 2016, 11:04:05 pm »
Do not go crazy and "square" the port out. Of course you are right,you must,round the bottom edge to help gently guide the piston skirt back into the bore. I.ve seen so many blown up motors(mostly pistons shattering).this is always the cause,guys who think they can "square out" the port for more size,but it's fatal,put a gentle curve on the bottom edge of the intake,then chamfer it very well! Your engine will last forever while the other guys try to figure out their piston shattering problems...



 

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