Port and Polish

[George inMinneapolis]

Next winter I may do the top end of my 1978 R80/7.

As long as the heads are off, would it be good/worth it to have a mild port and polish done? I read a thread on ADV riders and the suggestion was for a mild port and polish. Whatever that is. http://www.advrider.com/forums/showthre ... 894&page=4

They said the midrange roll on was improved nicely.

Your thoughts?

George

[Garnet]

Most R80s in North America are 8.2:1 compresion and rated at 50 HP. If I was rebuilding one I would use 9.2:1 Euro pistons, as those bikes where rated at 55 HP a nice little gain.

Motobins has those pistons for 75 BPs which is about $125 each.
http://www.motobins.co.uk/displayfinal. ... =&bike=991 part number 50111 at the bottom of the page.

[vanzen]

"Porting" will be beneficial or harmful
commensurate with the skill and knowledge of the one grinding the ports.
Choose someone who has an established track record of good results with the BMW airhead !
And fully understand the power delivery consequences of the re-design before letting a grinder touch a port !

IMO, the best effect of "Polishing" will be to please the customer's eye.

[mattcfish]

How much hp can be gained with a proper port and polish on these machines?

[George Ryals]

You should get 8% to 10% more air flow at the same differential pressure on a flow bench. How much torque/horsepower gain you can get with that depends on all the other stuff involved with the intake tract, cam, exhaust system , etc.

[Chuey]

Recalling older discussions of this topic it seemed that Funholiday had quite a bit of exposure to racing head modifications. He said, if I'm remembering correctly, that for an effective job of port modification, the "floor" (?) needs to be built up. It was something like that. In other words, not only did material need to be removed but some needed to be added, which would require welding.

With what the factory knew about head design, and considering that these bikes were designed with all around high performance in mind, my guess would be that they already have pretty efficient port designs and executions.

I would be disappointed if I compromised the lower end response of my BMWs. After having driven some cars that were very nice except for a flat spot just off of idle, I appreciate the juicy feel of my bikes. Further, I was just reading something that was saying the two valve design and our bigger pistons do not lend themselves to really high RPMs That's not that they can't go higher than stock but longevity seems to be important to most of us.

Disclaimer: I barely know how to do maintenance on these bikes. The above is just recollections of previous posts and personal preferences.

Chuey

[vanzen]

Recalling older discussions of this topic it seemed that Funholiday had quite a bit of exposure to racing head modifications. He said, if I'm remembering correctly, that for an effective job of port modification, the "floor" (?) needs to be built up. It was something like that. In other words, not only did material need to be removed but some needed to be added, which would require welding.

With what the factory knew about head design, and considering that these bikes were designed with all around high performance in mind, my guess would be that they already have pretty efficient port designs and executions.

I would be disappointed if I compromised the lower end response of my BMWs. After having driven some cars that were very nice except for a flat spot just off of idle, I appreciate the juicy feel of my bikes. Further, I was just reading something that was saying the two valve design and our bigger pistons do not lend themselves to really high RPMs That's not that they can't go higher than stock but longevity seems to be important to most of us.

BMW intake ports are large to the point that intake velocity is quite slow especially at lower RPMs.
Back in the late 70's, a popular performance modification involved building up the floor of the port,
a "raised port floor" and "revised port radius" that shaped the port into a kind of a leftside down "D".
The modification used venturi effect to increase intake velocity
improving power delivery and extending the limits of a performance built engine to suck
fuel.

Later on, after the demise of the large valve airheads, BMW incorporated a similar design.

[George inMinneapolis]

I asked Oak, and here is his reply.
G

George:

Sorry late with response. Mail to the ceiling....

I normally do not recommend porting and polishing. There are two chains of thought. A bit roughed up port on the intake is beneficial. It tumbles and swirls the air fuel mixture for more homogeneous preparation prior to combustion chamber use. Porting and polishing works against that benefit.

If it were mine to deal with I would omit the porting and polishing. I have yet to see where it has done any good. More than not, it is likely detrimental and a hype thing from the old days when engine dynamics was not fully understood.

Hope that answers your question..........Oak

[mattcfish]

I asked Oak, and here is his reply.
G

George:

Sorry late with response. Mail to the ceiling....

I normally do not recommend porting and polishing. There are two chains of thought. A bit roughed up port on the intake is beneficial. It tumbles and swirls the air fuel mixture for more homogeneous preparation prior to combustion chamber use. Porting and polishing works against that benefit.

If it were mine to deal with I would omit the porting and polishing. I have yet to see where it has done any good. More than not, it is likely detrimental and a hype thing from the old days when engine dynamics was not fully understood.

Hope that answers your question..........Oak

That's pretty much what I've heard. But.....
How about polishing the chambers and piston domes to reduce carbon build up? What about the exhaust port?
How acurately cast are BMW ports? At least getting them balanced by porting has got to help. Removing material from around the valve guides and installing shorter ones is another mod I've read about for improving flow.

[Garnet]

But.....
How about polishing the chambers and piston domes to reduce carbon build up? What about the exhaust port?
How acurately cast are BMW ports? At least getting them balanced by porting has got to help. Removing material from around the valve guides and installing shorter ones is another mod I've read about for improving flow.

Those things apply to your build much more than George's IMO.

You are using more cam and compresion and buliding the valve train to takethe resulting higher revs, as a result the improved flow may be noticed. Balancing the flow is a very imortant step and WAY beyond my meager knowledge.