Turbo systems, over-boost & plumbing for dummies!

[Enright]

I started to formulate the following analogy while trying to explain over-boost to someone at Malvern.
I’ve since refined it a little and thought it might be useful (and a bit of fun) to share with the forum.

Think of your Elan’s turbo system as being like a bathroom washbasin. Bear with me – it’s not quite as mad as it sounds, and you can even have some fun in the bathroom verifying it!

The plughole is the turbo itself, the overflow is the wastegate, and the taps regulate the amount of flow of the air/fuel mixture. Oh and the water level is the exhaust gases and power produced. For the sake of this demonstration, the plug is never in, but don’t complain to Severn-Trent Water about me wasting their precious resources. Right, let’s take our washbasin out for a spin…

As we potter out of town behind other washbasins with the taps only slightly open (low boost), the flow does not trouble the plughole’s capacity to discharge it. The water level never rises high enough to need an extra helping hand from the overflow.

We’ve now reached some 40-50mph roads, so the taps can be opened more and more. The water flows faster and the level rises because the plughole can’t get rid of it quite quickly enough. But up to a point the plughole can still cope without the level rising up to the overflow.

Hooray, we’ve reached the national speed limit roads, they’re clear apart from the toilet in front, and there’s a big enough gap to overtake it, so let’s open the taps fully! The level rises above the overflow, but in a nicely matched system like our Elan’s, the overflow is (just) sufficient to prevent the level from continuing to rise and wetting our feet. They’re already quite wet enough from the leaky roof anyway.

OK, so that’s roughly how everything works under normal circumstances, but where do wastegate wiggles, Pecos and Pipers fit into all this?

Well, think of a stuck wastegate as being like a blocked overflow – with Elans, nasties gunk it up over time if it’s not regularly used (thrashed). In this instance, as the taps are opened and the level rises above the overflow, there’s nowhere for the water to go. It just keeps on rising until the bowl overflows. Fortunately as soon as water hits the floor, the ECU backs the stop-cock off to only half open, allowing the level to drop again before it will open it again to allow any more fun. That is over-boost. Hope you were wearing your seat-belt when the ECU took control, and didn’t meet any on-coming bidets while trying to overtake that bathtub!

The purists will already have spotted the slight flaw in my description so far, in that the overflow is not always fixed open. There’s a clever little level monitor between the overflow and the top of the basin that opens and closes a flap over the overflow so that the level maintained is nice and high. This is where the MAP Sensor and Boost Solenoid Valve fit into the system. One of them is an input into the ECU, and the other is the corresponding output. Those same purists might also point out that the overflow is mounted a lot lower down the side of the basin, near the plughole, rather than up near the basin rim, but hey, this is my analogy.

Now let’s say that you install a K&N filter and fit a Piper exhaust. This is just like fitting bigger taps. The problem is that when you open them wide, the increased flow can often exceed the capacity of the overflow. And that’s why we port the wastegate by a few mm – it’s usually just enough to raise the total discharge capacity to safely beyond what the taps can provide.

Can anyone think of any glaringly obvious flaws in this or omissions that I might have made?

[Steve A]

I'm off to the bathroom with a file

[Dave Eds]

Is it the same in Australia?

[GeoffSmith]

My analogy contains batteries, a zener diode and some resistors but when mine goes wrong, the sparks fly!

[maz_r]

I'm keeping up with the analogy so far, but (well, you did say this was for dummies!):

Is the amount of water in the basin equivalent to the increase in power?

where does a blow-off valve fit into your analogy?

I guess upgrading the turbo is equivalent to getting a bigger sink!?

More dummy questions pending...!

[GeoffSmith]

Can anyone think of any glaringly obvious flaws in this or omissions that I might have made?

The only thing I can think of is the positive feedback from the water flowing down the plughole to the height of the header tank in the loft.

[GeoffSmith]

I guess upgrading the turbo is equivalent to getting a bigger sink!?

Larger taps or lifting the header tank is probably closer.

[Enright]

I'm off to the bathroom with a file

I deny all responsibility - have been doing successfully for years!

Is it the same in Australia?

Of course not silly - it's all upside down so the water ends up on the ceiling.

My analogy contains batteries, a zener diode and some resistors but when mine goes wrong, the sparks fly!

Ah but far more people are familiar with their bathrooms, and I didn't fancy electrocuting myself to prove a point!

Is the amount of water in the basin equivalent to the increase in power?

Yes, exactly.

where does a blow-off valve fit into your analogy?

Ah, very good question - this one took a bit of thinking about:
A blow-off valve operates during gear changes, and allows for a faster spool-up time.
So it's like when you turn the taps off and the water level starts to drop, there's a bit of lag in the system as you turn them back on again before it stops dropping and begins to rise.
A blow-off valve helps to minimise that lag.

I guess upgrading the turbo is equivalent to getting a bigger sink!?

Hmm, I'd say bigger plughole, bigger sink and bigger taps all in one go (more power and flow).

Does that nail your questions Maz?

Can anyone think of any glaringly obvious flaws in this or omissions that I might have made?

The only thing I can think of is the positive feedback from the water flowing down the plughole to the height of the header tank in the loft.

Huh?!?

[GeoffSmith]

Can anyone think of any glaringly obvious flaws in this or omissions that I might have made?

The only thing I can think of is the positive feedback from the water flowing down the plughole to the height of the header tank in the loft.

Huh?!?

The higher the exhaust pressure, the higher the air pressure.
The higher the air pressure, the higher the exhaust pressure.
The higher the exhaust pressure, the higher the air pressure.
etc. etc.
Positive feedback which must be regulated (by the wastegate valve).

[Pasco]

Hi yet another newbie to ask you regulars daft questions

The above analogy makes perfect sense to me and is a good one well thought out Op.

I,well my partner now owns a Medina green 1994 S2 number 165 with the following spec Everest mountain chip K&N air filter main cat removed and a sports back box.

I am experiencing over boost on a regular basis would you expect this with the cars current spec or could it be down to the waste gate actuator sticking?

Regarding porting the waste gate am i right that the optimum diameter is 21mm? if not what is considered to be the best and how can you accurately grind it out to such a small tolerance, is a special tool needed ?

I live in Burton on Trent Staffs.... are there any other owners close to me who have done this mod i could have a chat with and are there any Meets close to this area i could visit ?

Many Thanks to anyone who cares to reply.

Best wishes

Paul

[Enright]

Hi Paul, and firstly welcome.

That sounds like classic sticky wastegate territory to me, and would be my first bet. Usually it takes a full exhaust system upgrade to a larger diameter to introduce overboost into the system, rather than just a few components.

You should find a helpful article on doing the wastegate wiggle in the WikiLEC (link near top of page).

If you're unlucky and it comes to wastegate porting, then you can pick up a set of Bosch grindstones from B&Q or Homebase for about a tenner. The conical one is 20mm dia and does the job perfectly. However don't be under any illusions that this is a quick and easy job just because the materials are cheap. You'll be attacking the part of the car that's been through the most violent heat cycles for getting on for 20 years. It's a nightmare of seized and snapped fasteners and scuffed knuckles. Doing the job in anything under 2 weeks is a bonus. It's basically just about as bad as doing a head-gasket and de-coke.
So I hope a wiggle sorts you out!

Can I recommend putting B-o-T in your profile? It will save others having to ask in future. As it happens I'm not far away, living in Nottingham and working in Derby. PM me your number - having done the job a couple of times (and talked about it endlessly!) I'm in a reasonable position to explain the details.

[Pasco]

Hi Neil thanks for the prompt reply most kind.

Right... Releasing a sticky waste gate it is then will try to have a go this weekend unless gets in the way

[rip]

Can anyone think of any glaringly obvious flaws in this or omissions that I might have made?

Yes, you've sort of glossed over boost creep.

This is when the overflow cannot cope so the water level rises. It has not yet got so high that water is pouring onto the floor.
At this point, you are getting more boost than you should be, but not enough for the ECU to back off the stop-cock.

[nicowalker1]

Can anyone think of any glaringly obvious flaws in this or omissions that I might have made?

Yes, you've sort of glossed over boost creep.

This is when the overflow cannot cope so the water level rises. It has not yet got so high that water is pouring onto the floor.
At this point, you are getting more boost than you should be, but not enough for the ECU to back off the stop-cock.

ME and Rip bost enjoy the boost creep who needs and everest just need to learn when to let off the pedal a little! My dad took mine out after being warned, as Dads do he ignored me and floored it in second. Followed by 's**t, what was that!' as thought hed blown it! I sat with a smug smile

[chris.t]

can turbo lag be incorporated in your analogy

[Enright]

Yes, you've sort of glossed over boost creep.

This is when the overflow cannot cope so the water level rises. It has not yet got so high that water is pouring onto the floor.
At this point, you are getting more boost than you should be, but not enough for the ECU to back off the stop-cock.

Well if there was a gap, then that fills it nicely, thanks.

can turbo lag be incorporated in your analogy

Yes Chris, quite easily - stiff taps!
No seriously, that's the answer.
If the taps can be opened instantly then the water level changes instantly.
But the more the resistance the slower the flow rate changes, and the slower the water level is affected.

[bratboy_y2k]

Is it the same in Australia?

yeah, but everything turns in the opposite direction.

[HJ2]

Depends of the altitude

[Cpnetwork]

Hi Paul,

My name is Christian and I am now the owner of the exact car you had - S2 number 165. I still have the overboost problem and wondered if you had any luck with the 'wiggle' or whether you actually went ahead and increased the bore size. If not then I know it's something I will need to have done...

I guess you must have sold it pretty soon after the post as the previous owner who had it before me had the car for about 2.5 years.

Thanks for you help,

Christian

p.s. as I am not mechanical, any ideas from anyone on this forum where i can get this professionally fixed?

[Jeemy]

CR Turbos will rebuild and bore your turbo for you, I consider myself fairly adaptable and technical and this was not something I felt confident to do and put back into a car. so I had them to it and it came back gleaming. speak to craig there.