The Bullet Board

Ace.cafe wrote: ↑Sat Jun 11, 2022 2:00 am Too early inlet valve opening is causing reversion up the intake. Big overlaps are like that. It's better at the higher rpms.

Speaking of this.

I've been reading one of Phil Irvings books and it mentions reversion and anti reversion cones in headers and exhaust systems to mitigate it.

The exhaust system on this is straight through but with stainless perforated core and fibreglass packing.

Wondering if it would beneficial with these cams with big overlap to design a anti reversion cone in the header?

Something like this?

At the moment the fitment of the header is not great. Even the port machining is misaligned to the casting. It's a terrible fit.
Knowledge from fluid dynamics and basic exhaust knowledge. I've seen the "Torque Cones" or anti-reversion cones used on Harley's and Triumphs. But these "cones" seem to reduce exhaust port diameter by a lot. My basic understanding they are mainly a band-aid for drag pipes.

Here's a rough idea so far I can fabricate up.
The port matched section of the header will help with (Colored Red) exhaust gases velocity speed and scavenging.
The stepped angled section (Colored Grey) will act like a check valve which will help prevent Reversion (Colored Purple)

With the Standard header there's only about 45mm of straight section coming out from the exhaust port on the head. So the Port matched insert will be 20mm long with the small inside beveled edge.

Have I got the right Idea?

At the moment its straight out the exhaust port to a poorly fitting header into 41mm ID pipe.

Well, something like that, but not so short. Having the step about 16" down the pipe is more like it for our stuff. This would mean a joint to a 1/8" larger inside diameter pipe. A stepped header.
The idea is to keep the exhaust speed high(negative pressure)for as long as the overlap period is in play. If the exhaust system keeps a lower pressure than the inlet at all times during overlap, then the exhaust will go out the exhaust pipe until the inlet valve closes, and won't push back up the inlet port as reversion. It won't matter that the inlet valve is open while the piston is still ascending, even if there is no positive pressure in the inlet tract, if there is strong negative pressure in the exhaust tract.
This also helps the whole overlap process of starting the pull of the inlet charge into the cylinder which is very important for a good induction cycle.
So, IMO, a stepped header at the 16" length down the pipe is a good spot. The step also gives a ridge inside to perform like an anti-reversion cone, but at a more advantageous location.
The other reason to step up to a larger size pipe is to begin the process of trading velocity for pressure as the exhaust gas enters the muffler chamber and trades off more velocity for pressure to help it transition into the 14.7psi outside air pressure as it exits.

I understand! Thanks for the details.

So the same principle, just 16" further down?

Straight out the head at 1-3/4" and add the reversion chamber 16" down the header?

Or

Port matched diameter 1-5/8" header 16" down to a reversion chamber out to 1-3/4"?

I would be more inclined toward the latter, if the port can be matched at the head. But you must have a tight fit of the pipe O.D. into the head, so see what you can work out.

Last night I popped up the barrel and flicked out the base gasket. It wasn't actually that simple. It was stuck on there like it was going to outlast religion. I won't use Permatex Aviation again! It's rock hard in winter.

Anyway I let the RTV copper gold silicone set overnight and run a compression test this morning.
It bumped it up to 130psi/9 Bar from 120-125ish psi previously



Adjustable crank pinion is still in transit. There's been a delay with postage but should be here early next week. I'm hoping with the 3dg cam advance will clean up the idle vacuum, and a little bump in compression it will restore a little torque and bottom end by shifting the curve a little earlier.

Should do.

"I let the RTV copper gold silicone set overnight..."

I'll add this to my catalog of gorilla snot utility catalog.
On working with BSA headers, I used to use a hi temp type to help keep the pipes in the head. The 'copper gold' sounds even better.

Guaire wrote:"I let the RTV copper gold silicone set overnight..."

I'll add this to my catalog of gorilla snot utility catalog.
On working with BSA headers, I used to use a hi temp type to help keep the pipes in the head. The 'copper gold' sounds even better.

Stand corrected. It's called Ultra Copper, but theres a few ither brands called similar things. But it's the best gear I've used for metal to metal seals in my trade. There's "black" but it doesn't have the heat resistance. Just oil resistance.

https://www.permatex.com/products/oreil ... rtv-3-5oz/

Thats the Fitter and turner coming out in me. Haha

Same with Loctite. I tend to use 243 (blue) on most things on the bike, 222 on M6/1/4" or smaller. 263 if I really never want it to come lose. 680 if you it to stay tight for eternity! Hahs

Hi-temp anti-seize where it's needed on the engine and normal anti-seize everywhere else if it's got a spring washer.

Silicone spray on the raw aluminium parts you don't want to corrode. And keep polished. Also fork tubes, shock shafts and cables. It won't run like InOx/WD40. Magic stuff!

Another small gain. The Hitchcocks 3-way timing pinion installed on the Advanced setting gained another 12psi of compression.

So its now sitting at 142psi.

I ran out of time this morning to start it. I haven't tried since its original configuration with 120psi.
At present the pilot circuit is surprisingly a very rich 45 pilot jet, which the mixture screw was best set at 1.5-2 turns out. I'm presuming due to the weak vacuum at idle it needed a richer pilot to compensate.

Anyway I'll re-jet the idle/pilot circuit tomorrow morning then hit the road with a pocket full of plugs and start changing the brass in the carb. Needle and main jet is pretty close. But I wasn't going to get carried away until I have the compression bumped up.