Combustion chamber polishing

[ICman]

I'm getting ready to get a valve job done on my V-twin motorcycle. I'm wondering about polishing the combustion chamber (before/after the valves are ground?). Some people polish the whole chamber, while others do just around the exhaust or intake ports.

Polishing smoothes the air flow, but I thought air movement should be turbulent prior to and during the combustion cycle to improve the burn. If so, polishing the intake port and much of the combustion chamber might be detrimental. Yes?

Polishing, or smoothing the flow thru the exhaust port makes sense, though. This would allow for the quick exit of burnt gases.

Overall, does polishing/porting really help in a street engine? Does it reduce the carbon buildup? Increase power noticeably?

Just thought some of the experts might know. Thanks,

. . . Steve

 

[patprimmer]

Improving air flow through the ports is the main source to increased power.

The difference between a satin finish and a polished finish in the ports is negligible if all other factors are identical.

Polishing the entire combustion chamber and the chamber side of the valves and the piston tops, improves thermal efficiency by reducing surface area, and consequently heat losses. Whether or not it's worth the cost or trouble for a daily driver is another matter, as polishing alone will give only a barely measurable increase.

Polishing does reduce carbon build up.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[ICman]

Now I'm confused... By reducing the surface area, the heat transfer thru the head would be lower, thus increasing temperature. That increases thermal efficiency, but isn't that a bad thing, especially for an air-cooled engine?

Can enough material be removed, via grinding/polishing, from around the ports to significantly improve flow? Or, is this part of the science of porting a combustion chamber?

I guess I could rig up a flow bench and test, but I don't think it's worth all that effort for a daily driver. Don't engine designers do this when they design the engine? I would think they would want to design the combustion chamber to allow a complete burn as well as maximum intake and exhaust volumes. The valve sizes and locations are pretty well fixed, so the shape of the combustion chamber and/or piston top are about all that can be changed, right?

I realize the extra step of polishing the combustion chamber is too expensive for the manufacturer to reap any benefits. The basic shape is part of the casting, though. It appears it is not worth the effort to polish the combustion chamber, unless you're planning to race.

Thanks for the reply.

 

[MaxRaceSoftware]

Improving air flow through the ports is the main source to increased power.

The difference between a satin finish and a polished finish in the ports is negligible if all other factors are identical. ---pat
---------------------------------------------

Yes, i don't see any flow gains from just polishing the combustion chambers...if you don't do the polishing, the customer will think you cheated him or the Porting job is not that good ?

The only way you would see a Flow gain, is if you happened to blend-in a top angle into the chamber or you heavily polished the areas around the valves to the point you signifcantly "unshrouded" the valves by your polishing

its almost impossible not to accidently blend-in the top angle into the chamber while polishing combustion chambers
and that might lead you to believe the polishing (finish) alone is increasing the Flow

as in this picture for example

http://www.maxracesoftware.com/492_SBC_Chamber_3_800x600.jpg

Larry Meaux (maxracesoftware@yahoo.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers
Support Israel - Genesis 12:3

 

[evelrod]

Control of boundry layer flow through intake ports is much better in a 'non polished' finish. Indeed a stock cast finish will flow better all other conditions being equal. I have my doubts about exhaust ports as well but there are engineers here that have MUCH more experience in port flow than me. I will restate the old example I was shown so many years ago---take a drop of water and put it on a piece of glass and try to blow it off, then do the same on a piece of fine sandpaper. I prefer a nice, smooth satin finish, myself. The shape is much more critical than finish.

Pat, I have seen a couple of polished combustion chambered heads after extended street use and although the carbon buildup was less overall, they had areas where buildup was heavy AND had possibly detonation generating sharp edges. It would seem to me that most highperformance 'tweaks' should remain on the racetrack.

Rod

 

[bazeleur]

Wouldn't a polished port give you less intake charge heating (due to lower surface area) which increases power and reduces the tendency to detonate? I would imagine this would (theoretically) show up as a high-rpm power gain, as port wetting isn't as much of a problem at higher rpms.. or is it?

 

[patprimmer]

Rod and Larry

I absolutely agree with everything you say.

I always blend valve seats anyway, so polishing in that area would see no change then.

ICman

Unshrouding the valves on the chamber side can have a significant increase over a production head, of course depending on the original head design and casting quality.

Sand casting is not a precise process. Metal will be left in the chamber so that even with a bit of core shift, the chamber will not extent over the outside edge of the bore and expose the gasket, so hand finishing accurately to the bore can often improve the situation.

Cylinder head castings are often used on more than one engine configuration for production cost and inventory item reduction reasons, therefore it might be compromised for a smaller bore version than you have.

Cylinder head work is expensive, and to be very effective tends to move the power band upward.

Modern 4 valve per cylinder heads are normally very good straight from production, but older design 2 valve heads can often be improved with some expert hand work. If you do it wrong it can produce a negative effect. Intuition is not a very good guide, and few people can look at two ports and pick the best, without prior experience with that particular head.

A multi angle valve job, narrowed seats, some detailed work to assist flow around the valve guide boss, blending out any edges in the valve pocket and valve seat areas, and matching the shrouded side of the chamber to the bore generally help with little chance of a negative result. Changing port shape and size without a flow bench and a spare head is fraught with danger.

Polishing the ports will reduce charge temp marginally, but this will have nowhere near as much effect as a gain in airflow.

Polishing the chamber and piston tops can only help, even if for a limited time as Rod suggests, but is it really worth the cost and time for other than a race engine.

Significant gains in airflow of the port requires changes to many other engine components to optimise it, especially induction and exhaust, but possibly cam and / or displacement.

Unshrouding valves reduces compression, and if this is not corrected, then a net reduction in power might result.

Remember, the newer and better the original design, the less the opportunity to improve.




Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[mburgess]

ICman,
What motor is it & whats the applications. Larry, Rod & Pat prob have a lot more experience than I do, but for road use I would focus on,
- blending the carb rubber into the inlet port.
- cleaning up casting lines & dags.
- check the short side radius, in some older engines when the head is machined for valve seat inserts the port is often machined to match the seat throat. However this can sometimes leave the short side radius as a sharp edge.
- when cutting valve seats make sure the crown cut is blended into the chamber roof (shown in Larry's link).
- For port finnish I use a split rod or a coarse cross buff. Standard Abrasives have a good range. You want a honed appearance on the ports. Helps break fuel pooling on port walls.
- Remember alloy is easy to remove & hard to replace. Remove as little material as possible.
- For road use a good 3 angle valve job should is adequate.
- While the engine is out spend some time dialling in cams & shims.
- Lastly once all done spend some time on the dyno & tuning. Geetting the fuel & timing curves spot on can often make more gains than the mods mentioned above.
Sorry guy's if I have repeated some info above.
Regard's MB