porting question

[Guest]

Ok, so I have my engine all apart and waiting for parts and money for more parts, and I have done some work on the head by unshrouding the intake valves.
I am wondering if I should open up the exhaust ports to match the headers. As you can see from the pic there is quite a difference in the port size and header inlet. The header is the size of the gasket.

[Mark M]

I say leave them alone. My 1100E has a large flat bottom area that has a huge step into the header and it is there for anti-reversion effects. I have read that removing this step will hurt performance through the midrange, but I have no first hand confirmation of that.

At least on the 16 valve 1100 engines, I believe the limiting flow factor is the size of the exhaust valve. You can port all you want with minimal gains until that valve is upsized. Not sure on your 750.

BTW, have you read this site for porting info? It is very good info, if presented in a wacky manner...

http://mototuneusa.com/thanx.htm

Mark

[gearhead13]

Yes, I have read about anti-reversion, but I thought it was only for turbo engines. My exhaust valves are only 20 mm, quite small. Why would V&H make the header so much bigger then the exhaust opening HMMM were they thinking of anti-reversion I wonder?

[willie]

I'm curious to see how others respond but IMHO I think you should, at the very least, radius the step between the head and the header. I believe the turbulence caused by flow over that step is detrimental.
Willie in TN

[bucketsofgs1000]

From a physics point of view, that sudden increase in pipe diameter would cause the fluid flow speed to drop at that location, but it would also cause a pressure drop. Thinking about those changes from a mechanical point of view, the flow speed drop would seem detrimental as you are trying to get rid of those gases quickly, however the pressure drop would generate a minor scavenging effect since you have the higher pressure in the small port being drawn by the lower pressure in the header pipe. Perhaps V&H did some research and determined that the pressure drop was preferable. Or, more likely, since V&H had engineers sourcing parts for the system, they selected a pipe that would have sufficient clearance on the OD to easily mount in the bore of the exhaust port while having a thin enough wall that when one edge of the pipe is against the bore's wall the other edge of the pipe is not blocking the exhaust port at all. Just my thoughts....

[Mark M]

I'm curious to see how others respond but IMHO I think you should, at the very least, radius the step between the head and the header. I believe the turbulence caused by flow over that step is detrimental.
Willie in TN

The sharp step is critical to the anti-reversion effect. Any smoothing or radiusing will reduce the effect.

For anybody thinking it was V&H research, you are looking too far ahead. It was Suzuki's research and V&H simply built an exhaust system to fit what Suzuki built. This detail work is some of what gave Suzuki the best inline air-cooled fours ever.

Mark

[Nessism]

The stock pipes are double wall so the ID of the pipe is a lot closer to the ID of the port. Guess the aftermarket guys just go for max size regardless of how that effects flow.

[Old Colt]

There is nothing wrong with opening the port up. What is generally considered right is to leave the floor alone and open the port to leave a 0.050-0.060 step out to the pipe ID. Done this way the port will be a D shape with the flat on the bottom.

[renobruce]

I had my head professionally ported, intake and exhaust. I'll shoot some pics and post them for you. I'll throw an exhaust gasket in there to see if there is much difference.

[rapidray]

Trust me when I tell you this as I have been doing this for a LONG time. If you port the exhaust to the sixe of the gasket that motor will be a TURD! On 16 valve motors, that I do all the time, even on 1500+ cc, we don't open the port near that much! There is a ratio of exhaust flow to intake
flow that you want to achieve for maximum power & efficiency. Too big on the exhaust & the bike won't get out of it's own way. The flow bench & dyno don't lie! Well, the flow bench does sometimes but that's another story but the dyno doesn't lie. Ray.

[49er]

Keeping the correct inlet/exhaust valve size ratio is really important for maximum torque and it's duration. That's why many tuners opt for experimenting with bigger carbs and playing with diferent cams to increase lift, overlap and duration of opening, instead of enlarging one or both of the stock valves. If you ever get the chance to experiment with radical variations of these factors, you will appreciate just how pronounced an affect they can have on a stock performance power curve. Mr Zuki, on most of the GS models, did really git it right, especially with the pre-emission models.
Don't touch the size or shape of your exhaust port, unless you are planning to build a drag only machine. IMO, blue printing to the header is more important and keeping the step size to a minimum.

[trippivot]

hell no

exhaust only needs to flow 60% of intake side!!! no porting there!!!!!
unshrouding the intake???? you have already done un-repairable DAMAGE!!!!

going slower is the easiest thing to do to your bike.

Ok, so I have my engine all apart and waiting for parts and money for more parts, and I have done some work on the head by unshrouding the intake valves.
I am wondering if I should open up the exhaust ports to match the headers. As you can see from the pic there is quite a difference in the port size and header inlet. The header is the size of the gasket.

[trippivot]

I want to offer a positive tip.

a deep-deep 60 degree cut on the valve seat into the aluminum casting will benefit improved flow numbers with the least amount of risk/work!!!

there are many many more improvements for volumetric effy. than cutting on the combustion chamber.

PM me and I can give more specific "secrets" to get some real bang for your buck and effort!!

remember!!! you want to flow 100bbs' not 1 big bowling ball!!

PORT VELOCITY not port volume !!! (if that makes sense) maintaining 300 feet per second is optimal

[gearhead13]

WOW, so many different opinions.
I guess the only opinion that really counts is my own.

[Mark M]

Trust me when I tell you this as I have been doing this for a LONG time.

Ray, is it correct to say that the 16 valve 1100 head is limited by the exhaust valve size? I have heard that a 1mm bigger exhaust valve really helps wake them up when combined with bigger cams. When you are porting the exhaust, do you remove the flat bottom and step at the end of the port or leave them?

Mark