I have a guy wanting to gasflow the head on his DS650. The rest of the bike is still stock, except for a slip-on exhaust pipe and CDI. Is it worth while gasflowing the head with stock carb, cam, and piston?

 

Explain the term gassflow.
As in putting the head on a flow bench? If so, that is the last thing I would do.

 

No, not putting the head on a flow bench, and trying to get the highest flow figures possible. That can seriously mess up the performance.

I have a lot of information that I consider, and use to flow the head. I look at the cam duration, the lift, intake and exhaust valve diameters, and based on that info, I use some rules to what port size will be best for both flow and velocity. The math can also help me predict where the peak HP will be made, and I can port and build the motor specifically to suite the owners power requirements. There is no use, porting a motor for 9000 RPM, and it is running a stage 1 cam, and the owner want to use it for technical trail riding. I have collected a lot of information over the years on this subject, and the math I use is a good guide to get you started. I have been using this info for a while now, and the other heads I have done came out great, and produced very good HP output, where I wanted it.

I have not opened the DS head yet, and the reason I am asking if gasflowing the DS is worth it, is to try and save myself the cost and effort to remove the head, only to find that I will be wasting my time, and could have spent the money better elsewhere on the bike. Especially since the owner of the DS will be running stock cams, piston, and carb.

For example, on the raptor heads I have done, I only clean the intakes, and reshape a few areas on the right hand intake port. I don’t make the ports any larger, since the math I use, indicate that they where fine for the amount of flow I needed. They might actually do better with me making the port diameters a bit smaller. On the raptor's I also don’t touch the exhaust ports, because they are to large according to the math. Some flow benches have proven this fact on the raptor exhaust ports, and only need to be enlarged when you make 70HP plus. I might have to do more work on the intake and exhaust ports, if I have a request to build a 720 or larger capacity motor, but for the 686cc’s running stage 1 or stage 2 cams, a massive amount of head work is not required.

The KFX700 heads I did, needed a massive amount of work, with reshaping the combustion chambers, as well as intake and exhaust port work.

The LTZ400 head on the other hand needs very little work, with only a clean up on the intakes, and a bit of work on the exhaust side. Most of the work is purely at the exhaust. The pro's that race these LTZ's, even with the Yoshi 450 kits, hardly touch the heads, and some even racing it in stock from, so again, it confirms the math I have, that little work is needed on these heads.

Just to give you another example, on how accurate this math is. I took the LTZ head, cam info and worked out the size intake and exhaust valve diameters I needed to produce the correct flow and velocity for the stock cam. The intake and exhaust ports on the stock head was within 0.5mm of the valve diameters indicated by the math, so was the intake port and exhaust port sizes. Except for the one oval shaped exhaust port, the LTZ is a near perfect match to the math. Coincidence… don’t think so.

If anyone know the intake and exhaust port diameters of the DS, I can work out if it is worth the effort ot open the head. Like I said, with stock cams, piston and carb, there might be very little to gain by gasflowing the DS head, and not worth the money and effort for the gains.

 

By gassflow are you referring to doing a leakdown tests?


How old is the motor - light use/heavy, is crankcase vent tube showing oil?

 

No, we call it gasflowing, otherwise known as porting. Cleaning and reshaping the ports.