Re: Porting Your Own Heads(comments Only)

[71XS1B]

Hello Jack,
I sent you a PM. Thanks for the info on your Mig setup. I sold my Hobart Handler a couple of years ago because I rarely used it, and I have been kicking myself ever since. I look forward to seeing the salvage operation. Cheers... Dan

[71XS1B]

Thanks Jack.. Since this appears to be a common problem with the 650 head, others may be interested in seeing the fix, even if its not easily accomplished. I'd say post them here, but otherwise you can send the pics to [e-mail].

BTW, I still toss around the idea of letting you do your magic on my spare head, problem is, I'm your basic tight a$$ and I'm certain you have your hands full anyway... cheers, Dan

[ahrma814]

I think it is great that people are focusing upon improving the flow of their XS heads, but here's a quote from a 2008 interview with Bud Askland (who built Kenny Robert's XS race engines in the '70s, so he really knows what he's doing) to keep in mind:

"Bud said that all things considered the ports in the XS engine are a bit too large to begin with, and lesser is better if you are going to do any porting. The lesson is that the tuner should not remove metal except to recontour. Do not “open ‘er up so she’ll really flow.” If you have the skill and experience, you do have the option on the intake to use epoxy to do a more comprehensive reshaping and obtain higher velocities, but that option doesn’t exist for the exhaust (although of course you can always add material by welding). So, the bottom line is that the tuner has to walk a very fine line between gas velocity and shaping the port walls and valve pocket areas. It is very easy to overdo it and actually make the ports flow worse than stock. "

If they are"a bit too large" for a full-blown, factory supported 750cc race engine, then you should be even more cautious about removing metal.

[jack]

I think I've stated this very clearly in the porting threads that you need to proceed with caution when porting and from what I've gather most are following the rule of removing the least amount of meat possible without enlarging port volumes and most feed back has been positive. When it comes to CFM vs velocity,it's arguably the most debated topic out there when it comes to porting any head and in the end it's all about the engine combination and it's intended usage not one head fits all.

[dwyatt]

My money went for the factory replica porting, hard to argue the credentials. Just depends on what you want, homemade would be fine for a street rod, but if you want the best it can be, that's another story.

[ahrma814]

Because porting is such an art, and the head is only one component in an engine's modification package, it can be tough to determine whether your efforts were really effective. Here's a tool that can help. The following link is to a comparison of a stock head to a modified head that is expressed in percentage of improvement. This gets us away from differences in flow bench readings. You can look at a given valve lift and see the improvement in flow for both the IN and EX ports. By the way, the EX port in this CNC-modified head has not been reshaped to the "D" pattern advocated by a number of XS tuners. For all I know, a well done "D" EX port would have better numbers. If someone has that data I'd love to see it. Here's the link: downloads.650performance.com/Flow_numbers_stock_vs_Lillie_908.pdf
Craig

[dwyatt]

Good stuff.

[jack]

Craig..............Thanks for the links ,at least I know now my work is up to par with an CNC porting.

Intakes Exhaust
100 31% 24 %
200 19% 20%
300 14% 18%

400 11% 33%
480 10% 34%

These #s are from a mildly ported head with a slightly exposed cap in the middle of the short turn that was just rolled back and still delivered a decent flow through the lift curve and with stock valves. The exhaust side in my opinion doesn't require a D Shape to push the #s,a well ported exhaust can flow in the neighborhood of 90 CFM and just a hair more push with a welded up chamber. Been laid off from work for a about two years and during that vacation time I've been working on a D Shaped intake side,looks very promising for delivery the goods down stairs for street use,will be sent off next week to get flowed and reworked if needed.

[dwyatt]

I smell a dyno throw-down here.................

[ahrma814]

Yeah, this is fun stuff. Harry's flow set up included some instrument(s) he used to measure turbulence, and I have notes recalling his comments that he felt that minimizing turbulence in the EX port was an important consideration when trying to get the right balance between gas speed/quantity/turbulence. I don't pretend to understand the nuances of how he weighted those values.
Jack, I'm impressed with your IN improvement at low valve lifts. Do you add epoxy so the modified port has a small cross section?
Harry's porting was specifically designed to work with the Megacycle 40 cam, although we discovered that it works equally well with Shell's #1 grind, which also happens to be a nice "hot" street cam. (With that said, we have all become aware of the problems users have had with trying to dial in newly-purchased #1 cams ... there seems to be a quality control problem.)
Dwyatt, is your bike together? Have you run it? Put it on the dyno?
Craig

[dwyatt]

Craig,

I had a slow down during the holidays, but am almost finished. I forgot to order the cam when I was nearly ready to begin final assembly and had to go back to work before it came in. I should have it finished in a couple more weeks, then off to the dyno.

It's sitting patiently on my bench with newly bored cylinders and JE pistons in place and the ported head assempled with everything but a cam, resting nicely on top of it all. I hope to get it to the track in June.

[ahrma814]

Sounds like it is coming together nicely, Dennis. A couple of the guys in Australia who bought the head are running and from what I've heard are very pleased with the results.
I've had some health issues and haven't been able to go forward on a couple of developments I have in mind for the XS engine. Once I get to them, and if they work, I'll let you know so you can incorporate them into your engine.
Jack's comments perfectly illustrate why comparing flow numbers doesn't make any sense. There seems to be no doubt he has an EX shape that flows well and on his bench when he sees 90 CFM he knows that's a good number. On Harry's bench, his EX port reads approx. 127 - 128 at .350" valve lift. Seems like a big difference, but if you flowed them back to back on the same bench they would in all likelihood be very close. On the IN side, it's apples and oranges because Harry shaped his port for mid to high flow and Jack designed his for low and mid range improvement. Once of the reasons TT pipes work so well on a street bike with Harry's porting is that they fill in the off-idle to mid-range part of the powerband (at the cost of high end power - it's always a compromise.). You can see that in the dyno curves in your engine modification manual.

[dwyatt]

Get well soon, looking forward to any additional tips.

As to the performance differences between the heads, Lillie was porting for all out racing, so priorities and power curve needs were different than most of those that follow Jack's path, both seem to fill their niche. Your head fit my needs perfectly - proven power and super coolness factor, being a copy of the factory racing specs. Only thing I'd change is now that I've gone through the exercise of doing my own valve and guide work, should have let your guys handle it, much better deal than I thought, but you knew that ..........

[plattey]

Hi, new to porting and polishing and dealing with gas velocities. I have two questions.
1. If the exhaust port volume is too large for the exhaust valve size, could a larger exhaust valve be installed? Would the gas velocity suffer from the larger exhaust valve and hurt performance?
2. For epoxy use on the intake, what brand would be recommended? jbweld? Or is there an epoxy that is specific for intake ports?

Thank you

[jack]

Nov 18, 2010 23:15:41 GMT -5 plattey said:

Hi, new to porting and polishing and dealing with gas velocities. I have two questions.
1. If the exhaust port volume is too large for the exhaust valve size, could a larger exhaust valve be installed? Would the gas velocity suffer from the larger exhaust valve and hurt performance?
2. For epoxy use on the intake, what brand would be recommended? jbweld? Or is there an epoxy that is specific for intake ports?

Thank you

If you think XS exhaust port volumes are greater than normal which in some cases they are,then installing a larger valve won't increase the gas velocity,the port will just become lazier on the bottom end. One small fix to reduce port volume would to cut down a Virago 750 exhaust valve to XS stock exhaust valve dimensions to reduce the port volume(minimal amount) but the tulip shape head along with the thicker valve margin will assist in speeding the flow up. Another fix would be to install Mikes XS or 650 Centrals exhaust Ar inserts in combination with a 1 3/4 header with a 6 or 8 inch chucked down outlet to 1 1/2.

As far as applying epoxy goes JB Weld is a good product and as been used successfully in reshaping intake ports and when run through a few engine cycles it becomes rock solid and will need chiseling to be removed the only draw back is when applying a barrier finish you need to remove small amounts at low air speeds as it become very powdery,leaving a smooth finish. Devcon(Spelling) Aluminum plastic epoxy is another good product as is Splash Zone A788,expensive but the best from reviews of some of the top dogs in head porting. But regardless of which epoxy you choose to use preparation of the port or floor is the key for solid adhesion