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Exhaust Bridges

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The only part of that I agree with is the "no risk of bridge seizure".

 

The only part of the increased port area that will actually benefit you is the part above the transfers.

 

I doubt the exhaust bridge has a significant heat transfer to the piston.

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The only part of that I agree with is the "no risk of bridge seizure".

The only part of the increased port area that will actually benefit you is the part above the transfers.

I doubt the exhaust bridge has a significant heat transfer to the piston.

Either way, you want that heat transfer. The cylinder is water cooled, and without thst heat transfer you'll melt the piston. I doubt it's significant on an engine that big, but important all the same. Wonder what happened to that guy arguing about taking the bridge out of his 125 a few months back.

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Either way, you want that heat transfer. The cylinder is water cooled, and without thst heat transfer you'll melt the piston. 

 

 

The exhaust bridge certainly isn't water cooled.

 

 

 less heat transfer to the piston

 

And if I'm understanding DirtRider correctly, he seems to be stating that the exhaust bridge transfers heat TO the piston, not from it, and by removing the bridge he has eliminated that transfer of heat TO the piston. This is of course wrong, and I may be misunderstanding him, but I don't think I am.

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The only part of that I agree with is the "no risk of bridge seizure".

 

The only part of the increased port area that will actually benefit you is the part above the transfers.

 

I doubt the exhaust bridge has a significant heat transfer to the piston.

 

Bridged ports only flow 80% as well as non-bridged ports. The bridge transfers more heat than what you would think. There's very minimal coolant where the bridge is on the cylinder. 

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Bridged ports only flow 80% as well as non-bridged ports. The bridge transfers more heat than what you would think. There's very minimal coolant where the bridge is on the cylinder. 

But the heat transfer through a single body, cylinder, will be pretty constant. 

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Says who?

 

A. Graham Bell Two-Stroke Performance Tuning  :) Hillclimbers and draggers use this port layout on 500s, in addition to me using it for MX. Power gains everywhere due to the increased flow.

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I've read Bell's book, and nowhere in it does it say "A bridged exhaust port will only flow 80% as well as non-bridged ports".

 

Port flow is all about time/area.

 

The primary flow benefit you achieved from removing the bridge is from the area of the bridge located above the transfers. During the blow-down phase the increased port area allows faster evacuation, provided there isn't so much time-area that it allows blow-back into the crankcase within the intended rpm operating range of the engine. And this also needs to be balanced with the wave tuning of the pipe, if evacuation is too rapid low-end torque suffers greatly. 

 

Once past the blow-down phase, the increased port area below the transfers only serves to increase flow volume at higher rpms, and decrease flow velocity at lower rpms, effectively moving the power higher in the rpm range.

 

You could have accomplished the same thing by simply opening up the sides of the port by the same amount as the width of the bridge. Granted, you've eliminated the chance of a bridge seizure, but you've greatly increased stress and wear to the rings.

 

And the bridge does NOT transfer heat to the piston, it's the other way around.

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I've read Bell's book, and nowhere in it does it say "A bridged exhaust port will only flow 80% as well as non-bridged ports".

 

Port flow is all about time/area.

 

The primary flow benefit you achieved from removing the bridge is from the area of the bridge located above the transfers. During the blow-down phase the increased port area allows faster evacuation, provided there isn't so much time-area that it allows blow-back into the crankcase within the intended rpm operating range of the engine. And this also needs to be balanced with the wave tuning of the pipe, if evacuation is too rapid low-end torque suffers greatly. 

 

Once past the blow-down phase, the increased port area below the transfers only serves to increase flow volume at higher rpms, and decrease flow velocity at lower rpms, effectively moving the power higher in the rpm range.

 

You could have accomplished the same thing by simply opening up the sides of the port by the same amount as the width of the bridge. Granted, you've eliminated the chance of a bridge seizure, but you've greatly increased stress and wear to the rings.

 

And the bridge does NOT transfer heat to the piston, it's the other way around.

 

Could've sworn that was in the book. Must've heard it from one of the 500 gurus that I am friends with.  :excuseme:  I suppose it would be better to say that the gases flow more freely because there is no obstruction (i.e exhaust bridge).  :thinking:

 

I did this as a bit of an experiment as to how well it may work for Motocross. Yes, ring life suffers, but I go through the top-end every season. (Bottom end every two seasons) I have two other cylinders that I can work with if this one doesn't turn out well. Next cylinder I'm going to run will be 189 exhaust, 125 transfers, 123 boost port, with winged exhaust ports.

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Fired her up yesterday. 5 primer kicks, then one good kick. (Started up the first kick). Easily the most pissed off 500 I have ever heard in person. I took a quick video, but it seriously does it very little justice. Still have some fine-tuning to do.

 

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I've read Bell's book, and nowhere in it does it say "A bridged exhaust port will only flow 80% as well as non-bridged ports".

 

Port flow is all about time/area.

 

 

 

 

its gordon jennings book that says it if i recall. and i believe he is right.  but since a cr500 doesnt have aux exh ports your better off leaving the bridge in. the port window can be made much wider and flatter.

 

time*area is nothing more than x amount of window area, thats open for x amount of time. it tells us nothing about volume of mixture flow into the cylinder or exh flow out of the cylinder.  you can have 2 different cylinders both with eqaul time*area but one could be superior simply because its duct geometry is better

Edited by harryhandshake

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