Intake Manifold Design

mattho9 · Post by **mattho9** » Wed Aug 16, 2006 2:08 pm

FIntruder wrote:So is the shape of the reflective surface, the part of the plenum that the pulse reflects off of, important? Most plenums look to be curved in that section. Would a flat area reflect better? Then a square or rectangular plenum would work. .

No, The shape is one of the least important aspects. The pulse is reflected when it meets a change in gas composition, area, or temperature.

FIntruder wrote:My project is a low revvin' motorcycle engine without much room for runner length. The lower the rpm, the longer the runner needs to be for peak. I guess it comes down to what it always does, you go with what fits. I'll have to do some math but is there only one length at which the pulses are timed right? With a shorter runner the pulse with move back and forth more quickly and thusly may have two peaks.
It seems to me a shorter runner will have many more waves hitting the valve at the right time. My runner through the TB and into the plenum may be about 8". Also two cylinders are sharing one TB and plenum so there are two sets of waves in there. That could be either really good or they may cancel each other out and my motor will not even start. I guess I'll be the first to know.

This is much more complex than that. A short runner manifold at low rpm will definitely have many reflection cycles. They will reflect first emitting from the open intake valve and reflect back at the plenum towards the valve. Waves can undergo superposition when two waves meet. This totally alters the speed at which the wave travels. Plus the amplitude changes thus altering the tuning benefits. Very complicated subject.....

Matt

Bow · Post by **Bow** » Thu Aug 17, 2006 5:34 am

FIntruder wrote: My project is a low revvin' motorcycle engine without much room for runner length. The lower the rpm, the longer the runner needs to be for peak.

What is your max power RPM going to be?

What size Engine?

FIntruder · Post by **FIntruder** » Thu Aug 17, 2006 5:14 pm

1360cc Suzuki Intruder engine. 680cc per cylinder, makes 62 peak HP at 4500rpm and has a broad torque curve that peaks at 84 ft lbs with the small 36mm CV carbulators. It kept revvin' past 4500 but the power went away. I hope it will perform better with the 45mm throttle body I'm plannin' on using.

Bow · Post by **Bow** » Fri Aug 18, 2006 12:08 pm

Tuned Intake Runners

Peak Power Target @: 4500 RPM

Tuned Manifold Runner
Second Pulse Length: 24.00 In. 609.6 mm
Third Pulse Length: 21.56 In. 547.5 mm
Forth Pulse Length: 16.44 In. 417.7 mm
Fifth Pulse Length: 12.00 In. 304.8 mm

Minimum Port Cross-sectional Area Required

Peak Power @: 4500 RPM
Cylinder Bore: 3.701 In. 94 mm
Cylinder Stroke: 3.858 In. 98 mm

Minimium Area: 1.252 Sq. In. 8.074839607 Sq. cm
807.4839607 Sq. mm

Engine Air Capacity CFM

Bore: 3.701 In. 94 mm
Stroke: 3.858 In. 98 mm
Number of Cylinders: 2

Engine Displacement 83.00 ci 1360 cc
Volumetric Efficiency: 0.86

Total Air Capacity: 92.95 CFM
Single Cylinder Air Capacity: 26.098

That is what I get with your numbers.

FIntruder · Post by **FIntruder** » Fri Aug 18, 2006 4:11 pm

Thanks for the numbers. Did you get all of that just from what I gave you or do you have a data base that had the bore and stroke? Is it important to try to tune on an earlier pulse? A 1.5" diameter intake runner would be large enough from the info you gave, is going with a larger size, say 1.75" gonna have any drawbacks?
I'm not looking to get more RPM out of this motor, more interested in drive-ability, single throttle body and being able to tune each cylinder independently if necessary. The front carb is jetted differently than the rear but I am not sure if it is because the front has different needs due to heat or if it is because the front is a downdraft Mikuni and the rear a sidedraft Mikuni.

Bow · Post by **Bow** » Mon Aug 21, 2006 7:14 am

FIntruder wrote:Thanks for the numbers. Did you get all of that just from what I gave you or do you have a data base that had the bore and stroke?

I looked up the bore and stroke of your engine on the internet.

Is it important to try to tune on an earlier pulse? A 1.5" diameter intake runner would be large enough from the info you gave, is going with a larger size, say 1.75" gonna have any drawbacks?

The pulses you tune for are basicly for ease of installation. The higher pulse gives you a shorter runner. Running a larger intake runner than your intake port on the head will degrade your performance due to the decreased velocity of the air flow.

I'm not looking to get more RPM out of this motor, more interested in drive-ability, single throttle body and being able to tune each cylinder independently if necessary.

You sound like my kind of guy.

The front carb is jetted differently than the rear but I am not sure if it is because the front has different needs due to heat or if it is because the front is a downdraft Mikuni and the rear a sidedraft Mikuni.

I am not sure about the carb jetting, but if I was doing a project like this, I would match the throttle bodies to a size close to my carbs and attempt to reuse as many of the stock parts as I can. The factory did the design work, why try to reinvent the wheel?