knowing how to build...

OK so ive been wondering how motor builders know how to port a head. is it just R&D? or is there some real science that goes into it? and what makes one cam better than another? :busted:

There are basics that pretty much anyone willing to do the research can do that will get you about 60-70% of the possible gains you can from just porting. Finding the rest is where the professionals come in. There is a lot of testing different things to see what works and what doesn't. Of course there is science behind it, however using that science to a T is impossible. Mathematically predicting the results of different porting configurations is possible, but making it accurate is increadibly tedious and requires difficult math. It ends up being easier, cheaper, and less time consuming to experiment to see what works in the end. Some day if I get a ton of free time I would like to model a head on the computational fluid dynamoics software I have access to and see if that is helpful.

They probably use a flow bench like car engine builders.

They probably use a flow bench like car engine builders.

Indeed they do. However, flowing air through a head on a flowbench and flowing air through a head on an engine are two different things. The flowbench helps approximate, but a head that performs well on a flowbench doesn't always perform the best on an engine.

^^^true, and i believe they even use fluid rather than air because it is easier to meter:thumbsup: On that note,hey kj do you know anyone that is capable of doing extrude honing on a thumper head

In the mid 60's I built the data system for the first GOOD working flow test machine for a motorcycle.

My friend C.R. Axtell built the machine and I built instrumentation for it.

The suction pump was a Paxton supercharger driven by a 5 Horse Power electric motor. (Jerry Branch's machine did not pump near the airflow that Axtell's did. It takes a lot of suction to get something that simulates an engine. His did a good job. He had a cylinder for the head to mount to so that it would work line an engine. In its heyday, 3/4 of all the Harley Davidson flat trackers went through his shop.

Now back to basics.

A well set up flow test machine is as good as you can get. You can't hold a steady condition on a motorcycle, so any reading is meaningless.

Don't try to build one and expect magic results. It takes a lot of mistakes to make anything much better. The factories do a LOT of research.

Don

^^^true, and i believe they even use fluid rather than air because it is easier to meter:thumbsup: On that note,hey kj do you know anyone that is capable of doing extrude honing on a thumper head

There are a few companies out there that do it, but I wouldn't. It tends to leave an extremely smooth finish, almost polished. A port should never be polished.

Most people will say that a rough port helps fuel atomization, but this is incorrect. When a fluid travels through a pipe, the fluid touching the wall does not move, this is called the non-slip condition. As you travel down the pipe more and more fluid slows down due to viscous effects in the fluid (basically friction). If all of the fluid is flowing straight (called laminar flow) as it would in a polished pipe, then the fluid in the center of the pipe moves the fastest, and then the velocity of the fluid decreases as you move towards the wall (where it is zero). This is called a laminar boundary layer, and it will choke off the flow into an engine since only the very center of the flow can flow without restriction. A rough port creates turbulent flow right at the wall of the port, inducing a turbulent boundary layer. This is a thinner layer of slow moving fluid near the wall, allowing a larger cross-sectional area of the pipe to flow without restriction, and thus allowing more air to get into your engine.

Most the stuff you read on the internet forums about porting and port design is garbage reiterated from articles written by people who started the garbage.

This dude knows what he is talking about: Professor Gordan Blair. Anything written by him you really should look into.

He led the design of this software: http://www.profblairandassociates.com/GPB_Products_Main.html

If you search his home page, there is a ton of tech info that is WAY over most peoples heads, including mine.

umm... porting is really complicated... yeah...

but, cams are simple...

they're spun by the cam chain, lifting valves at the right time (also called timing)

some cams have a bigger lift, longer duration, etc.

umm... porting is really complicated... yeah...

but, cams are simple...

they're spun by the cam chain, lifting valves at the right time (also called timing)

some cams have a bigger lift, longer duration, etc.

Figuring out how much lift, how much duration, how much ramp, how much overlap, and when they should start to open relative to the cycle andignition timing is far from simple at all however.

Figuring out how much lift, how much duration, how much ramp, how much overlap, and when they should start to open relative to the cycle andignition timing is far from simple at all however.

well... yeah...

but sohc engines are fairly simple...

making the cam, however, is really hard

There are a few companies out there that do it, but I wouldn't. It tends to leave an extremely smooth finish, almost polished. A port should never be polished.

Most people will say that a rough port helps fuel atomization, but this is incorrect. When a fluid travels through a pipe, the fluid touching the wall does not move, this is called the non-slip condition. As you travel down the pipe more and more fluid slows down due to viscous effects in the fluid (basically friction). If all of the fluid is flowing straight (called laminar flow) as it would in a polished pipe, then the fluid in the center of the pipe moves the fastest, and then the velocity of the fluid decreases as you move towards the wall (where it is zero). This is called a laminar boundary layer, and it will choke off the flow into an engine since only the very center of the flow can flow without restriction. A rough port creates turbulent flow right at the wall of the port, inducing a turbulent boundary layer. This is a thinner layer of slow moving fluid near the wall, allowing a larger cross-sectional area of the pipe to flow without restriction, and thus allowing more air to get into your engine.

wouldn't this turbulance also have the added effect of better, or more evan fuel atomization as well as increased flow?

well... yeah...

but sohc engines are fairly simple...

making the cam, however, is really hard

Making the cam is easy! Specify a lift profile in tabular form, send to cam grinder, and CNC machine grinds cam lobes from a hardened and rough ground spool.

Designing the cams is extremely tough. Lift and duration are the two most basic factors you have to take into account. The important factors, and the ones that really make power, are the ones that control how quickly the valve can open/close, and keep it from bouncing off the seat or floating over the nose of the profile. You have a basic profile of lift over the duration of the cam rotation, the first derivative of that is valve velocity per degree, second derivative is valve acceleration, and the third very important derivative is valve jerk, or change in acceleration per degree.

Having control over jerk and acceleration while maintaining maximum valve open/close velocity as limited by the diameter of the bucket is key to making power. You can have two cams with identical duration/lift that act completely 100% different while running.

Without writing a small book this will be tough to explain here.

wouldn't this turbulance also have the added effect of better, or more evan fuel atomization as well as increased flow?

Only at the walls of the flow, which is a very thin layer since turbulent boundary layers are thin. The rest of the flow remains untouched. Also, since this layer is slow moving, very little of the turbulent air makes it into the cylinder on each stroke.

I dont think most know how much work really goes into building a cam. I for one am not going to explain it. I dont need a headache.:busted:

There are a few companies out there that do it, but I wouldn't. It tends to leave an extremely smooth finish, almost polished. A port should never be polished.

I was thinking of getting the exhaust ports honed for my yami. Do you think it would be worth it?

I was thinking of getting the exhaust ports honed for my yami. Do you think it would be worth it?

Since all it really does is erodes the walls of the port leaving a smooth finish, you would be better off just taking some sand paper and sanding down any burs or casting flaws. This is free and easy. If you want actual port work send it to someone like Jesse Williams who will actually resize and reshape the ports for optimal performance.

KJ790 are you an engineer / studying engineering by any chance?

KJ790 are you an engineer / studying engineering by any chance?

You caught me.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now