Why the T3 Cross-Over So Long?

[Dimples]

Is there a design function to the cross-over to extending so deeply into the mufflers?
What are the consequences of cutting it shorter?

[Tom H]

From what I have read, true/not true??. The deeper into the muffler, the less prone to heat discoloration of the muffler.

Tom

[guzzisteve]

I always thought that the closer you can get the x-over to the quarter size outlet holes the better. That way the exhaust forces the exhaust out.

[moto]

I believe that a longer exhaust pipe at a constant diameter creates a column of exhaust with higher inertia than would happen with an exhaust that increased its diameter away from the port. This means higher inertia of the gas column as it moves away from the head, leading to better extraction of the spent charge during the overlap period of the cam, when the piston tries to draw gas in from both the intake and the exhaust valves. That in turn draws more fresh mixture in through the intake valve, and means more power. I believe this effect is more important at lower and medium RPM. The crossover pipe between the two longer, constant-diameter exhaust pipes is a secondary issue that doesn't figure into the benefit of long exhaust pipes.

Just my understanding. I'm no expert.

[ray bear]

my convert is the same

[turboguzzi]

I believe that a longer exhaust pipe at a constant diameter creates a column of exhaust with higher inertia than would happen with an exhaust that increased its diameter away from the port. This means higher inertia of the gas column as it moves away from the head, leading to better extraction of the spent charge during the overlap period of the cam, when the piston tries to draw gas in from both the intake and the exhaust valves. That in turn draws more fresh mixture in through the intake valve, and means more power. I believe this effect is more important at lower and medium RPM. The crossover pipe between the two longer, constant-diameter exhaust pipes is a secondary issue that doesn't figure into the benefit of long exhaust pipes.

Just my understanding. I'm no expert.

Exactly,
In guzzi racing exhaust system for higher rpm,  the pipes are usually much sorter.

[chuck peterson]

Yes

A friend did track days by taking off the muffler leaving the crossover which is designed to scavenge the spent exhaust. He said each header isn’t big (wide) enough alone, but the crossover, being a split that widens the available exhaust space, acts like the exit side of the venturi. Guzzi has two venturi, one in front of the carb and the other formed by the exhaust and crossover

Taking off the crossover takes away the venturi effect.

I’ll be back with the right info from wikipedia…

https://en.wikipedia.org/wiki/Venturi_effect

In fluid dynamics it’s a Venturi. Same as air. My old brain forgets the term for that little piece in front of the carb that first constricts the airflow (same as fluid) and then expands to create faster flow

 (my understanding is the exhaust pipes on guzzi are a little too small)

and then there is an expansion (the crossover that almost doubles the area for exhaust to both pipes) that make a scavenging effect for faster airflow, pulling the spent gases away faster then the piston can push into the exhaust header

So he suggests…i never went to track days. Ymmv





This also explains Guzzi moving the crossover closer to the exhaust valve to above the rotor/stator on the higher powered LeMan type bikes. With the header being stuffed by higher rpm usage, creating a need to split in the exhaust gas sooner, and also a second crossover below the transmission

Looking at the unmuffled exhaust of the La Franconi, here in the race bike foto, you can see the slowly expanding exhaust pipe as it goes away from the second crossover below the transmission This too has a scavenging venturi effect. Notice if you straighten this race exhaust you’ll have approximately the same length as your stock T3 headers, but without mufflers

The stock guzzi bits are pretty well thought out. Yes you can just remove the crossover but that removes the venturi effect.

Start your bike with just the headers and it makes a funny pop pop pop sound, not the explosive roar you might expect