Question on porting the cylinder head

[Ivan]

My cylinder head has a rather large lip around the intake valve. The valve sits about 1/8 below the surface of the combustion chamber. Would it be beneficial to work on that lip to help unshroud the valve, especially at lower lift, or would the loss in compression undo the benefit?

Here is a pic of what I am referring to:

[Ken Talbot]

Ivan, try using the word 'porting' in the search tool here at the new forum. A few guys have "been there done that" already. You'll find quite a few threads on this.

[trbig]

I think with a porting search, you'll just bring up things about working on the intake and exhaust ports. What you are refering to isn't really considered porting. Although I really have no knowledge to lend here about your question, I do know that the little guys in white coats put a lot of thought into the things that were designed for this bike. Not to say that things can't be improved... I would just be pretty hesitant about blindly changing anything.

The only person that comes to mind that may know that I can think of right off hand is Dan Hodges. I see it's his birthday today. Maybe wish him well in a PM and ask him?

Tod

[Ivan]

My search for porting was vague if anything. Nothing on working the combustion chamber.

I did see Dan Hodges pic of his head, but it was hard to tell if the combustion chamber was worked. It was definitely polished, but not sure about the ridge.

Although I spent lots of years wrenching on cages, I have never really gotten into porting and polishing. Usually it was just bolting on a set of aftermarket heads that had all this done to it.

Good thing I am using my spare head as practice. It has been run with gas in the oil and the cam journals started to gall, not too bad yet, but not a big loss if I screw up.

Good idea on talking to Dan. I will PM him tomorrow if he doesn't catch this thread.

[TADracer]

My opinion is....

Generally I would say that unshrouding the valves is a good thing and will increase flow but, be careful what you do inside of the 11's combustion chamber (and the ports too). It is not a true hemispherical chamber but is a set of three overlapping hemi cuts which in my opinion makes its characteristics a bit more complicated unless one knows all the ins and outs of the chamber design. In porting and polishing alot of times less is more. One of the arrows on your photo is the little ridge between the intake and exhaust. Remove this area wrong and it may affect the scavenging durning the valve overlap event. As in any porting operation, once too much material is removed and you are past the optimum shape the head is never going to perform the way you want it. Same is true for the chamber.

If I were going to fool around inside the chamber with the idea of gaining optimal performance the first things I would do is get a good baseline of the stock head's performance and another head for when you mess up the first one. Then, measure each chamber volume with spark plug installed along with the thickness of the head gasket, the bore volume, allow for the piston dome and do the math to get your true compression ratio. Then make a SLIGHT adjustment, polish it up, re-measure the chamber volume, calculate the new compression ratio, bolt it on and try it. Compare the results to your baseline. Then, if not where you want it to be, make another slight change. It is not going to be just a 'stick it on the bench and grind' type of thing. It is going to be alot of trial and error to get it exactly where you want it to be.

Then there are other things that can be done like installing taller valve seats, trimming the valves, etc.

Also, consider the other things that may need to be changed or tuned, such as possible milling the head to get compression ratio back up (depends on the amount of material removed inside the chamber), change in plug type or heat range, Possible re-jetting carbs, ignition timing and on and on.

I would try to pick Dan Hodges mind a bit. He does alot of motor mods on 11s and probably would have some good advice for you. But one thing I can tell you, I did a slight bit of porting on mine a long time ago and it was a bit time consuming for sure but it was definately a fun project to do.

[Guy_b_g]

I had my heads ported.

Here is my 2 cents.

Unshrouding the valves is a good thing, if you kmow what you're doing.

I had the head ported by a professiomal, but I specified that that ridge be relieved...here's why.

I have a 1978 XS11, and had a spare 78 and a 1981 head. I wanted to go with the bigger valves in the 81 head, vs the smaller 78 valves.

As I disassembled and looked at both heads I noted that the valve stems are shorter on the big valves, and thus the valves are set farther into the head. I think this was done solely to avoid valve interference between the larger exhaust and intake valves and that no other design changes were made to the heads. They simply moved the valves a little deeper into the head. They did this primarily to regain some of the performance lost when they went to a milder cam because of EPA considerations. I don't think they felt the need to go to the extra expense of unshrouding the (larger) valves after insetting them.

I base this only on my inspection of the differences between the 78 and 81 heads I have.

I can't tell you what you would gain (if anything) by unshrouding the valves, but I have a machinist buddy that's done a lot of work on racing heads, and he also recomended that I unshroud them, after looking at the 78 and 81 heads (I took them apart at his shop to get his feedback on what improvements could be made).

I haven't installed the modified head yet.

Just my 2 cents.

[trbig]

I hope you can tell something from these nasty heads... but here's a '78 head...








And here's an '82 head that uses the bigger valves.






I don't know if you can tell.. but it doesn't appear that there's a whole lot of difference on that lip between the heads. Yes, the larger valves do sit up higher in a larger compression area, but that lip is pronounced on both.

As I said, I know Dan Hodges has done work to this area on some heads and is a walking mechanical encyclopedia.

Tod

[Guy_b_g]

Here is a pic of the head I had ported, as you can see. nothing radical, no large changes, just a little goes a long way.

[Dan Hodges]

Flow Grooves Around Valve Seats

The grooves or depressions that the valve sits in were called flow grooves by Yamaha. These grooves were employed to unshroud the valves at very low lift and did. If you simply grind/remove the grooves you will increase the volum of the chamber and ruin the head as 1 cc of volum equals .23 of a point of compression and every quarter (.25) point of compression is worth 1% more power. You can remove the valve seats and weld all of this up and then put new seats back in but the heat of the welding weakens the structual intergity of the parent aluminum of the head unless it is re-heat treated which is very costly. On the intake side of the 80-82 head, you can machine the head for a larger seat and use 39 mm intake valves which gets rid of most of the groove. Any larger valve than this and you will have valve interference problems and these 39 mm valves will only work on a 72.5 mm or larger bore. The head of the Eleven is partially hemispherical in shape as it has a north-south valve arrangement but is actually polyspheric in shape and employs 6 individual cuts. They cut spherical-section pockets for the valves and an overall spherical-section cut over the bore area (squinch-quench bands), flow grooves around the valve seats and machined a final relief where the spark plug nose enters the chamber. You will also notice that the intake ports curve as they enter the head and not straight on as in a true hemi head and they did this to create swirl. If you have access to a flow bench, you can clay up some of these grooves/crevices and the results will surprise you. If you close up the grooves in a stock head you lose flow and if you machine away the grooves you gain nothing but you lose compression. I could go on and on about this subject but basically I recommend you leave the grooves around the seat as they came from Yamaha. I am attaching a picture of a state of the art two valve motorcycle race head and if you look closely you will not see any flow grooves but that's another story.

[Dan Hodges]

Head Games

Here is a pic of the head I had ported, as you can see. nothing radical, no large changes, just a little goes a long way.

Here are two pictures of a well developed head, mine.

[Guy_b_g]

I stand corrected.

Two questions though.. Why doesn't the 78 head have these grooves?

And based on the pic of the chamber, do you feel I have damaged the performance potential of my head?

(by the way, it looks much rougher in finish than it actually is due to the lighting)

Thanks for weighing in.

[Ivan]

Wow, Dan. Thanks.

I haven't gotten nearly as agressive on the intake runners as I have seen others do, mostly just smoothing the surface blemishes, and working on the valve guide protrusion a bit.

Looks like de-carbon and polish is all I will do for the combustion chamber. Or maybe I'll just polish the carbon. could help with compression.

Thanks again.

[trbig]

Don't smooth too much of the roughness on the intake ports. That casting flash helps with the atomization of the fuel.... so I've heard. Dan's looked pretty smooth though, so is this another falicy that we've all been told so long that we believe it for fact?

Tod

[Ivan]

From what I have been told, and seen (my dad's spray plane) a satin type surface is the best for flow. The small pockets in the matte type surface actually hold onto a very small section of air, and the rest of the air slips by with more ease. Almost in the same way oil lubricates a cylinder by hiding out in the cross hatch. My dad paid high dollars for a new fangled super paint that was to control the boundary layer along the wings and fuselage. It was like a bad looking rattle can paint job that orange peeled, but was very thick. He said it helped the cruise speed abut 5 kts on the same power setting. That was back in the early 90's though, so I don't know if they still do that or not.

Anyhow, same stuff goes for intakes.

As far as atomization, I thought that was what swirl polished valves were about? Could be wrong. Dan?

[Guy_b_g]

My understanding is that the intake ports are to be left a bit rough to increase flow, but the exhaust and chamber polished to help shed carbon.

The intake ports are considerably rougher than the exhaust ports on my head. Though the exhaust ports aren't polished to the degree that 2 stroke engines I've personally ported in the past were polished.

I am also planning to coat the chamber and valve faces with KG Gear-Kote as a heat barrier and to help shed carbon, as well as the top of the pistons. The skirts will be coated as well to reduce friction.