maximum size for bridgeless exhaust port

dannygraves · Post by **dannygraves** » June 18th, 2008, 6:34 pm

I remember seeing a figure somewhere to figure out the maximum width and exhaust port can be without the bridge... anybody remember?

redrocket190 · Post by **redrocket190** » June 18th, 2008, 8:23 pm

From what I have read you can get to 62% and maybe 72% of bore, but the shape of the exhaust needs to be very symmetrical and elliptical so the ring can be eased back into its groove as it passes the port....

Please discuss.

Slomo · Post by **Slomo** » June 18th, 2008, 8:57 pm

What you have going that you need to worry about it?
Got a pipe that will support it?

...just curious

100hp honda · Post by **100hp honda** » June 18th, 2008, 9:44 pm

just put a chainsaw in there

. now i think of it brit has a poulan he would probly loan out

ou812 · Post by **ou812** » June 18th, 2008, 10:26 pm

Check out the shape on a 84 motor, that would be a good guide.

dannygraves · Post by **dannygraves** » June 19th, 2008, 8:12 am

slomas wrote:What you have going that you need to worry about it?
Got a pipe that will support it?

...just curious

I'm running an fmf gnarly modified by dewayne to rev... shortened head pipe, larger chamber, etc, etc, he didn't even tell me about all the things he did to it, but its a totally different shape.
I was just tossing the idea around, since I plan on making some changes to my existing porting and every time I look at that bridge it seems to go "neener neener!!"
The bike is a dunes only bike so I really want to get as much reving out of it as possible, its simply too hard to control my launches with the way it is now, although, it would make a sick hillclimb setup.

redrocket190 · Post by **redrocket190** » June 19th, 2008, 8:30 am

It is the port area/timing that is affecting the power characteristics of the engine. Obviously the bridge is introducing an element of inefficiency, but is probably an acceptable price for extended ring life. You can go the experience route and/or get some modeling software like TSR and play around with port area/timing variables.

dannygraves · Post by **dannygraves** » June 19th, 2008, 8:51 am

yeah, when I did all my porting work before I got some great results, but never touched the timing. I thinned down the bridge, widened the exhaust port, opened the boost ports a bunch and knife edged everything, etc. and it hauled ass, but it took the already hard to control '86 motor characteristics and amplified that. So now I am ready to try to move the power up in the rpm range to make it more controlable and faster in the sand.
I've already got the pipe, head, carb, ign setup for it, so I just want to get my porting to work WITH everything else, not against.
I figure raising the exhaust port some is a good starting point and dewayne gave me some really good pointers on transfer port angles and piston modifications. He was telling me to do a hole like glen does, but make it oval and the width on the upper boost port. I printed out a degree wheel last night, and tonight I plan to get to work!

redrocket190 · Post by **redrocket190** » June 19th, 2008, 9:00 am

Good luck. My understanding is that in isolation widening the exhaust port without changing the timing moves peak power higher in the rev range. Of course if remove the bridge you are in effect widening the exhaust port by increasing its area, but leaving the timing un-changed.

dannygraves · Post by **dannygraves** » June 19th, 2008, 9:19 am

what really got me thinking about the bridge was this
http://cgi.ebay.com/ebaymotors/ws/eBayI ... :IT&ih=010
look at the second pic, that port looks like a damn storm drain!

Uzi · Post by **Uzi** » June 19th, 2008, 11:26 am

Did you see this one? Looks like they started removing the bridge and then stopped about half-way

http://cgi.ebay.com/ebaymotors/CR-500-C ... dZViewItem

dannygraves · Post by **dannygraves** » June 19th, 2008, 11:31 am

WTF

I guess if they made it REALLY wide at the bottom, they narrowed it at the top, that would make sense...

Post by **AlisoBob** » June 19th, 2008, 11:59 am

You can see the "Stop Sign" marks from here.....

100hp honda · Post by **100hp honda** » June 19th, 2008, 4:26 pm

these guys have some of the fastest sand machines in the land. one of them could surely give you some pointers

http://atvdragracers.com/

dannygraves · Post by **dannygraves** » June 20th, 2008, 8:29 am

ok, so I kept the bridge... my exhaust port was already 70mm which makes it %78, so thats not an option anymore.
I busted out the degree wheel, I was at 175* ex 115*in now I'm at 190*ex 130*in

I also opened up a few more areas, shaped the exhaust runner better and opened it up and knife edged everything even more (this thing is dangerous in more ways than one

)
DeWayne gave me some advise about piston mods he suggested making a hole where glen drills his, but he said to make it oval shaped and the width of the upper boost port. He kept sayign 30 degrees, and I didn't really get what he was talking about until I noticed that part of the piston lines up with the bottom of the upper boost port at 30* past bdc and at 30*in height its about 1/2" tall.
I busted out the drill and really taught myself a few lessons...
1, BUY A DAMN DRILL PRESS!
2, holding a shiney new and very sharp piston with your hand while drilling it with a 18v chorless is gonna end badly
3,blood cleans easily off a shiney new piston with the use of some carb cleaner

so I drilled 2 1/2" holes right next to eachother, then busted out the sanding rolls to clean it up and make a nice oval.

I also figured out why my motor only seized on the one side... last time I had it apart I did a lot of porting, and I copied Glens extra boost parts on each side of the intake port. I made 2 mistakes though. '86 motors have a wider intake, so there was a lot less room to drill and again BUY A DAMN DRILL PRESS!
the two holes exited at slightly different spots. The side the seized had that port exiting where the bottom on the piston skirt didn't cover it much at all, the other (which wasn't that much different) I noticed that a good chunk of it was covered by the piston shirt. so it was getting more flow on one side. I reshaped them both to make them match as closely as possible.
while shaping some runners I busted out my high tech flow bench... the garden hose! it works great for seeing how things are gonna flow and turbulent spots. Just have to make sure to dry it off and wd40 the sleave right away.
anyway, I should be giving you guys a ride report soon

teemtrubble · Post by **teemtrubble** » June 20th, 2008, 8:45 am

You could put the piston in the vise...hahahahaha

dannygraves · Post by **dannygraves** » June 20th, 2008, 8:57 am

100hp honda · Post by **100hp honda** » June 20th, 2008, 4:44 pm

right on man

. what you do with the transfers and head ?

dannygraves · Post by **dannygraves** » June 20th, 2008, 5:43 pm

the head was reshaped by dewayne to rev. the transfers, I kept the basic design just opened everything up and reshaped the 2 closest to the intake side to blow the charge more rear facing than at eachother. the upper boost port got opened a LOT and and the lower ones even more. that was there from before. yesterday I only cleaned things up a bit and raised the ports.
When I did all the porting at first it was a powerful MOFO, but was all still just a down low uncontrolable hit. The hit was so crazy that against brits GSS bike on the bottle I gain on him in 4th on the hills, but have to puss out on the launch because its uncontrolable. raising the ports, the different head and pipe are all just for shifting the power higher in the rpm range to get more use out of it.

100hp honda · Post by **100hp honda** » June 21st, 2008, 7:09 am

cool. take some photos of your work if you get the chance

90cr500guy · Post by **90cr500guy** » June 21st, 2008, 7:24 am

The exhaust port on the '84 is approx. 35mm high x 58mm wide with an outside diameter of 46mm

dannygraves · Post by **dannygraves** » June 29th, 2008, 4:17 pm

took her for a spin today and it looks like I fully got what I wanted. the uncontrolable rip right off the line mellowed out, while making some very impressive higher reving power

Can't wait to hit dumont!

ISBB · Post by **ISBB** » June 29th, 2008, 5:31 pm

I cant wait to give it a spin.. its exactly what i want out of my motor.. A mid to top hit like no other and all the power in that neigborhood since thats where i like to ride

teemtrubble · Post by **teemtrubble** » June 29th, 2008, 6:34 pm

Cool did you take pictures before you put it back together?

dannygraves · Post by **dannygraves** » June 29th, 2008, 7:06 pm

ya, only a couple, I'll post 'em up later

I took some pics in the middle of it while I was working the degree wheel, but then got all excited and put it together. After putting it together I realized I needed to replace the wrist pin bearing, so it came back apart, only I left the head and reeds on.

BannedCR500Riders.com

maximum size for bridgeless exhaust port

maximum size for bridgeless exhaust port