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Background
This isn't an RS6 question - but I know a few of you guys are pretty sharp.
I have a 9.3 litre marine V8 with a somewhat lumpy cam - it idles nice and smooth, but I get a fair bit of reversion going on. So what?

My issue is noise. On a marine engine, you usually manage noise by dumping the cooling water into the exhaust which muffles it nicely. However - I can't do that on my motor because of the exhaust reversion going on - it ends up getting wet all the way back up the exhaust. (Or at least, it would....)

I also don't have space to fit automotive style mufflers.

The Idea:
If I have an SAI pump into the exhaust that can mostly cancel out the reversion pulse with it's flow at idle, this means I could run a wet exhaust and get everything nice and quiet, but without getting the water travelling all the way back the exhaust. It'd be switched so once we hit 2000 rpm the SAI valve closes and the pump shutsdown.

So - the questions:
a. Is it possible to estimate the size of a reversion pulse at idle?
b. How much flow does an SAI pump push through?

So you'd splice into the exhaust and pump air through it to encourage gas flow in the right direction through venturi effect? Effectively lowering the pressure in the exhaust manifold?

9.3 is a lot of cubes. I'd be worried that a puny car SAI pump wouldn't be able to displace enough air to have an effect. Hard to quantify, though. How hard would it be to knock something up to try it? Maybe you'll need one pump per bank.

Is there any scope to modify the exhaust geometry? Like deviate it into a smaller diameter pipe at idle to increase gas speed? Maybe with an actuator?

Raising the pressure - ie the "vacuum" pulses from reversion are offset by the pump.
Yup, was thinking it'd be a pump per bank.
Opportunities to get creative are almost nil - distance from the end of the 36" equal length manifold to the end of the exhaust system is about 14" inches, including passing through 3" of transom.
I remember I had an old landrover with a valve on the exhaust so at idle it was sucking air in through. I don't think that would work here due to the short exhaust length, and therefore the mass of air changing direction (and therefore able to flap a valve) probably isn't high enough. Hence the positive pumping idea.
I've also considered just having a butterfly in the exhaust so that at idle, we shut the butterfly to maintain a positive pressure in the manifold - which opens as you open the throttle.
That means I couldn't inject the water upstream of the butterfly though - but maybe I could get away with that.
Did actually make the butterfly and the shaft - but lots of options and no idea which ones are <beep>, vs worth trying.

Can you not use something as per anti-lag devices like lots of Evos have? Not exactly how they work either but assume its a similar air pump method rather than fuel injected WRC style.

I thought they were running mega <beep> on the spark so they ran wider throttle opening and were burning the mixture in the exhaust?
I could probably get away with a bit of <beep>, but probably not that much....

Hmm. Apparent R3T4RD is a banned word? This is a car forum FFS.

I don't think an Evo style ALS with all the pops and bangs is the result Matt is aiming for.

Having said that, I presumed he was attempting to make his vessel quieter to help his smuggling operation, but I could be wrong.

MattV8 wrote: ↑

Wed Jul 24, 2019 2:46 pm

Hmm. Apparent R3T4RD is a banned word? This is a car forum FFS.

Just replace it with spaz.

MattV8 wrote: ↑

Wed Jul 24, 2019 2:46 pm

I could probably get away with a bit of spaz, but probably not that much....

Works perfectly.

Anyway, pops and bangs won't be ideal when I'm trying to sneak past customs in the dead of night.

No chance they'd catch you though with your 9.3 litres.....

what exactly is 'reversion' ? I'm assuming you mean waters coming back up the exhaust on tick over ? in turn does that mean the exhaust(s) are under water ?

On all motors, there's a moment where both the inlet and exhaust valves are open. When running under load, this increases the volumetric efficiency since it lets the exhaust help suck in the new inlet charge.

However, at idle, there's not enough mass flow, so what actually happens is a bit of exhaust gets sucked back in. The more radical the cam, the more pronounced it is.

MattV8 wrote: ↑

Wed Jul 24, 2019 8:36 pm

On all motors, there's a moment where both the inlet and exhaust valves are open. When running under load, this increases the volumetric efficiency since it let's the exhaust help suck in the new inlet charge.

However, at idle, there's not enough mass flow, so what actually happens is a bit of exhaust gets sucked back in. The more radical the cam, the more pronounced it is.

Thanks, got the jist of it now.

ahoooga wrote: ↑

Wed Jul 24, 2019 7:22 pm

what exactly is 'reversion' ? I'm assuming you mean waters coming back up the exhaust on tick over ? in turn does that mean the exhaust(s) are under water ?

No its coolant water. Instead of running your own coolant its picks up water from the river/lake etc.. and dumps it out the other end - sometimes into the exhaust system.