Plenum design for high boost

[Arttu]

Slightly related to discussion in "Main jet" thread...

Here's some food for thought if you are designing a system for high boost pressure (relative term, I know). It would be a good idea to think a bit about forces that pressure applies on the plenum. So take a largest surface of your plenum, or any other pressurized part of intake system, and calculate how much force planned boost will make on it. You may get surprized if you haven't thought about this before  

To get some fun for this thread I'll add an anecdotal example. Here's what slight mishap with boost control can do.

 

So whole lid of the plenum just popped of.

And after a quick fix.

 

Original construction wasn't looking that bad. Thick enough material and proper welds. But the lid had just too large area without any support from the middle. After quick calculation I found out that even planned 2 bar boost would make about 1200kg force on the lid. And in this case there was about 3 bars... 

Edited December 16, 2018 by Arttu
Typos

[Leblowski]

That Dyno operator is very very very lucky

[Arttu]

1 hour ago, Leblowski said:

That Dyno operator is very very very lucky

Yep, good that I wasn't sittin on the bike as usual. Luckily bike had an exhaust that tried to burn my leg so decided to stand on the side So I got just some bruises on my arm.

[no class]

yikes ! that was a close call !  

[clivegto]

I had one split on a weld at the back @Paulmblew one to bits as well.

[Paulm]

8 hours ago, clivegto said:

I had one split on a weld at the back @Paulmblew one to bits as well.

Split mine twice, and blew it off the back of the carbs more than once as well. All good fun.

[mick-ne]

you wouldn't believe some air could do that, best step up my welding practice lol.

Lucky break been off the bike at the time

[no class]

1 hour ago, mick-ne said:

you wouldn't believe some air could do that, best step up my welding practice lol.

Lucky break been off the bike at the time

….lol....it has me second guessing the welds on my plenum now !

[Goo'Roo]

[nitro]

But look at the quality of the weld. The weld was not gone through the material. It was only on the edge.

[Gixer1460]

35 minutes ago, nitro said:

But look at the quality of the weld. The weld was not gone through the material. It was only on the edge.

No arguments please but he does have a point - look at the tacked joints inside - there is no penetration from the outside finish welds, likewise the top closing weld on the far edge - maybe 0.5mm deep (visual guess) - I'd say not if, but when, for weld fracture.

[Arttu]

3 hours ago, nitro said:

But look at the quality of the weld. The weld was not gone through the material. It was only on the edge.

Sure, the welds could have been better. But that's not the key point here in my opinion. The point is that if you have large enough surface without any support in the middle and you put in enough boost it will break or bend somewhere unless you use stupidly thick material. But if you have some additional support here and there or if you think about the plenum shape more carefully you can get away with far more lighter structure.

[nitro]

Yes, there is a lot of stress to the material. My drag bike plenum has 2mm wall thickness without problems so far. Reached 2+ bar boost a few time. But want go back to 3mm anyway on the new bike. It will have only one throttle flap.  So plenum gets the whole pressure if there is a backfire.

Edited December 20, 2018 by nitro

[Gixer1460]

That's an interesting comment re: backfires. If anyone has had a mooch around a Top Fuel bike or car they tend to have sacrificial burst panels built into the manifold to protect the blower essentially. The one on Ian King's bike was a 2" / 50mm dia disc of what looked like tin foil that held 40 - 50psi boost but a backfire would rip it to shreds and release boost pressure safely.

[Arttu]

As far as I know back fires are a real problem only on supercharged engines. And to be more specific ones with roots or screw chargers. Those won't release pressure backwards no matter how high it gets so that's the reason for burst panels. Centrifugal chargers will let air flow backwards when the pressure gets higher than their maximum boost capability.

[Gixer1460]

Well i've blown the manifold off a few engines whilst using dodgy ignition kill  with an air shifter on turbo motors, and seeing as a lot of people bolt the manifold to the head, you lose that potential pressure release. Nitrous assist is always good for a spectacular backfire. Anyway it's all info for the masses.

[nitro]

Yes, the roots chargers are closed, so no chance to get away with the pressure whithout rated breaking point when backfiring. Turbo chargers are open, but you also have a pressure peak. And its enough to bend the butterflys.