Porting a type 4 head, Not much of a short side Options

[Shivers]

Because the port is almost straight, with little long side short side concerns, with what there is , do you find it necessary to slow the flow down on the “short side” to get good flow out of the valve. I was reading about something mepstein posted a picture of. Small divots, like a golf ball. So I’m going to do that on the long side, just wondering if there is any reason not to also do it on the short side. Thanks guy’s (IMG:style_emoticons/default/smile.gif)

[HAM Inc]

Hey all. It's been a while since I jumped into a discussion. I'm glad to see that folks are still passionate about their 914's.

I'm not going to get out in the weeds on esoteric theory but want to address a common misconception about T4 heads. A couple of decades ago I think we had it pretty well tamped down, but like bell-bottom jeans it seems to come in vogue every twenty or so years.

The misconception is that the exhaust port on these heads is the weak point restricting performance. In terms of flow nothing could be further from the truth.
We have spent hundreds of hours flowing every T4 head VW produced and they all, from 1.7 to 2.0 914, have a 70-72% exhaust port. Meaning the exhaust port flows 70-72% of the intake port. This is an excellent target for a N.A. engine.

I found many years ago that when working with 1.7 and 1.8 heads that if I spent ten minutes improving the exhaust port I would have to spend 20 minutes on the intake ports to maintain that same flow %. The exhaust ports on the 2.0 914 are pretty well maxed out by the factory. We make our big gains on them with seat work and changing the throat/valve O.D. ratio.

The shortcoming of the exhaust ports isn't flow related but rather the fact that it makes a very hard turn directly under the combustion chamber in a location that's about as far from the spark plug as you get. This area is more likely to see detonation. Because of the sharp turn the cross-section thickness between the exhaust port and the chamber is relatively thin and is therefore weak and subject to distortion when heat spikes, especially when detonation is present. The heat and pressure will push the chamber downward into the exhaust port. Of course you can't see this with the naked eye, but the area of distortion carries the exhaust seat with it, leading to serious leakage and eventually a burned valve if it gets bad enough and if it gets really bad the head will pull away from the cylinder creating a head leak.

My advice for the O.P. is to leave the ports alone. I have a hunch that there are plenty of other areas of his 914 that need attention.

[Shivers]

Hey all. It's been a while since I jumped into a discussion. I'm glad to see that folks are still passionate about their 914's.

I'm not going to get out in the weeds on esoteric theory but want to address a common misconception about T4 heads. A couple of decades ago I think we had it pretty well tamped down, but like bell-bottom jeans it seems to come in vogue every twenty or so years.

The misconception is that the exhaust port on these heads is the weak point restricting performance. In terms of flow nothing could be further from the truth.
We have spent hundreds of hours flowing every T4 head VW produced and they all, from 1.7 to 2.0 914, have a 70-72% exhaust port. Meaning the exhaust port flows 70-72% of the intake port. This is an excellent target for a N.A. engine.

I found many years ago that when working with 1.7 and 1.8 heads that if I spent ten minutes improving the exhaust port I would have to spend 20 minutes on the intake ports to maintain that same flow %. The exhaust ports on the 2.0 914 are pretty well maxed out by the factory. We make our big gains on them with seat work and changing the throat/valve O.D. ratio.

The shortcoming of the exhaust ports isn't flow related but rather the fact that it makes a very hard turn directly under the combustion chamber in a location that's about as far from the spark plug as you get. This area is more likely to see detonation. Because of the sharp turn the cross-section thickness between the exhaust port and the chamber is relatively thin and is therefore weak and subject to distortion when heat spikes, especially when detonation is present. The heat and pressure will push the chamber downward into the exhaust port. Of course you can't see this with the naked eye, but the area of distortion carries the exhaust seat with it, leading to serious leakage and eventually a burned valve if it gets bad enough and if it gets really bad the head will pull away from the cylinder creating a head leak.

My advice for the O.P. is to leave the ports alone. I have a hunch that there are plenty of other areas of his 914 that need attention.

Thank you for your time. And yes plenty to do. Summer around here is hellish, so I was going to do a temp work area in the living room. I won't be welding for a few months so polish the exhaust ports and divots in the manifolds. (IMG:style_emoticons/default/biggrin.gif)