The '2 ton bugswatter' - back for a gentle stroke

[Audi_CQ]

If we now improve the flow on the exhaust more of the gasses can exit the combustion chamber, this in turn will allow the turbo to spin harder for more airflow. However as the combustion chamber has more space to fill this time round the turbo has to shift more air to achieve the same boost pressure level. So everything starts to work harder to achieve the same pressure levels. If there is then any form of restriction on the wastegate side of things at this increased level of flow when target boost is achieved this becomes a rapidly accelerating downward spiral in boost creep. Which will only get worse with increased rpm and flow up until the point the turbo exceeds its map and can no longer flow enough to fill the cylinders at which point it starts to drop of again unless you force it to overspool.

When everything has to work harder to hold boost up you also have to run more exhaust through the turbine to make it spin faster so shouldn't create that much more flow trough the WG should it?

[JP]

Des, yes written differently to try and simplify the explanation. Port design on a turbo head is different from a na head for this reason. Think of the exhaust port having to work like a two stroke exhaust where it is capable of scavenging due to pressure differential rather than blow through.

Putting that aside we can go round and round in circles if we had a way of having no wastegate with a sealed mani and once boost was achieved suddenly have a 44mm hole we could proof or disprove the theories outlined. Unfortunately our only tests we could carry out where on the dyno with g60rallye's car where a collective decision was made that the wastegate when fully open with no exhaust connected to it and flames shooting into the bay was not allowing enough gases to bypass to prevent excessive 1+bar creep.

Audi CQ we can only assume that with a good flowing tubular mani coupled with an improved flowing tial hotside and improved billet blade design that at the point of optimum ve around the midrange of the engine we set a snowball at the top of the hill into motion. As mentioned by Jeffer any issues had he's resolved with a larger 60mm wastegate. There is others out there that have gone the route of 2 x wastrgates to resolve similar issues.

One thing that has been noticed in the past is cars that achieve x amount of power when changing to a larger wastegate take of the same setup would create slightly more hp but more noticeably a lot lower egt's pre turbo. I know when we mapped my mates 750hp 1.8t at 2.3bar we never saw egt's exceeding 800celsius on the dyno measured 4" away from the back of the exhaust valves on all 4 ports.

[diesel des]

Jp, you will never get scavenging on the turbo engine unless the drive pressure is the same or less than boost. No amount of porting or fancy shape will overcome the differential pressure. But back on topic, can you remove your wastgate and test it on the bench for stroke. Eg put regulated air into it to force it open. Record the pressre that takes and its stroke.
I can't see how some guys have done over 700hp on bigger turbos than yours, or at least similar sized flow wise and can bring boost down to 0.75 bar with a stock wastgate, when your fancy one won't do the business.

Thats why I'm asking you to test it on the bench. Maybe it flows like crap or a tiny stroke?

[diesel des]

Where is it getting its impulse from? If the wastgate isn't seeing full inlet pressure for some reason it won't open fully. But test it on the bench first.

[JP]

There has only got to be a pressure differential between compression in combustion chamber and exhaust ports not intake to exhaust.

Anyway with reference to your on the bench test

G60Rallye's wastegate was taking a direct feed of the turbo with no actuator bleed valves or anything connected. We t'd into the feed with a boost gauge of the dyno to measure boost pressure going to wastegate. This figure 100% correlated with the boost figure measured by the ecu at the manifold all done by Dave Rowe and motech management.

At 3700-3800 rpm the logs show the boost holding momentarily before creep sets on and escalates rapidly. With the top breather open you could see the internal aluminium part of the diaphragm hit the inside of the wastegate cap indicating it was fully open and staying open. Which was further confirmed with an airline test of the wastegate on and of the car.

None of us there including the guys at dsa aylesbury could believe that the 44mm hole could not flow enough yet the evidence was there. Concerned I then went home and took my car for a spin logging on my zeitronix unit for personal piece of mind and unfortunately came back with the same results.

Subsequently speaking to jeffer hearing he had had the same issues and resolved it by going to a 60mm wastegate we could only draw the conclusion that flow being provided exceeded what the turbo needed and the wastegate could bleed of to hold lower boost. The only thing that all three cars have in common is the head design tial hotside and the 44mm wastegates.

[vcosmin]

Not really good at this , but how do you know that the wastegate is fully opened, when you tested it ? I mean, you only see the duty cycle of N75 and presume that the wastegate is opening as it should ? I have a 46mm Tial, ported head, etc. Car did 500hp with 1.1 bar boost, no problem with wastegate se dyno below, that was only spring pressure, and the boost holded fine.

[driftin jo]

Jp, you will never get scavenging on the turbo engine unless the drive pressure is the same or less than boost. No amount of porting or fancy shape will overcome the differential pressure. But back on topic, can you remove your wastgate and test it on the bench for stroke. Eg put regulated air into it to force it open. Record the pressre that takes and its stroke.
I can't see how some guys have done over 700hp on bigger turbos than yours, or at least similar sized flow wise and can bring boost down to 0.75 bar with a stock wastgate, when your fancy one won't do the business.

Thats why I'm asking you to test it on the bench. Maybe it flows like crap or a tiny stroke?

I don't think it will flow well enough for a few reasons, the gate is at 90 deg to the gas flow and with a good design exhaust manifold as he has the gas velocity is nice and high because this a is big turbo and does not create much resistance to flow through the turbine and thus does not decrease the gas velocity much and so does not increase the pressure pre turbine, without the increase in pressure the gas will just carry on going through the turbine as it will just follow the path of least resistance. The gate is only going to start bleeding pressure off when there is enough pressure to make it happen, if this, on this set up is after the turbo has started to provide positive pressure to the engine and thus more power, more gas flow, you then get the snow ball affect that he is talking about as now the gate is flowing but is is too late to overcome boost creep.

A good few people have found this happen with big turbos.
Lots of people simply think a bigger turbo needs more flow to spin, but what they forget is that a smaller turbo may make 600hp at 2.5 bar boost were as a bigger turbo will make 600hp with much less boost, say 1.8 bar because of the less restrictive turbine.
If you then look at how this is going to affect the flow through a waste gate you will see that the smaller turbo can be controlled with a much smaller gate as the pressure is higher pre turbine so when the gate opens the flow is much higher as the gas again follows the path of least resistance.

[Thuppu]

When I measured my MVR the "hole" is actually 40mm in the valve seat area. Only the inlet is 44mm and then reduces to 40mm.

[diesel des]

There has only got to be a pressure differential between compression in combustion chamber and exhaust ports not intake to exhaust.

That's 100% correct when not considering overlap. With overlap you need more boost than drive, or you get the opposite of what you want happening eg exhaust end gas staying in the dead none swept part of the chamber.

But it sounds like you have tested this wastgate well on the g60 dyno, so it appears that it flows poorly when bolted to that type of exhaust manifold. Maybe the stock gate flows better than these upgraded valves. Or the manifold is not well setup for getting wastgate flow. Do you guys use the same or similar designs of manifold?

[diesel des]

What's the stock wastgate size and stroke. If the upgraded one is 40mm what's its lift or stroke?

[Jonnybravo]

Interesting point raised about the manifold design, this is not the first time I've heard of a similar issue with a Nortech manifold, that said I'm quite interested in the results using the bigger gate to see if that does cure it.

[JP]

I would've questioned the manifold design if it wasn't that its not only the nortech manifold that has had the issue.

In all honesty as far as manifolds go perfect cold welding no warping on v band flanges. Internal diameters spot on. Runner lengths pretty much equal within mm's, cnc'd collector....

When taking the car for a spin the one thing I noted was despite hitting 1.8bar and the car being ridiculously quick in 3rd it didn't feel like I was driving a turbo car the transition in power delivery was too smooth, it was more like the 6.3ltr stroker I used to run in my old capri. I normally get this on smaller frame turbo's with headwork but the only time I've achieved this on a large frame turbo is when we did our own ported equal length tubular mani for the mk2 good with the gt40 my mate runs, which had to also have a 60mm wastegate.

Going to start on it later in the week after they finished stripping car for paint allready got an idea in mind for a second take of blending into first to achieve the 60mm as I've got a suspicion the collector is too short to chuck the required 2.5" pipe on at the right angle.

[driftin jo]

I would've questioned the manifold design if it wasn't that its not only the nortech manifold that has had the issue.

In all honesty as far as manifolds go perfect cold welding no warping on v band flanges. Internal diameters spot on. Runner lengths pretty much equal within mm's, cnc'd collector....

When taking the car for a spin the one thing I noted was despite hitting 1.8bar and the car being ridiculously quick in 3rd it didn't feel like I was driving a turbo car the transition in power delivery was too smooth, it was more like the 6.3ltr stroker I used to run in my old capri. I normally get this on smaller frame turbo's with headwork but the only time I've achieved this on a large frame turbo is when we did our own ported equal length tubular mani for the mk2 good with the gt40 my mate runs, which had to also have a 60mm wastegate.

Going to start on it later in the week after they finished stripping car for paint allready got an idea in mind for a second take of blending into first to achieve the 60mm as I've got a suspicion the collector is too short to chuck the required 2.5" pipe on at the right angle.

My point was that it is because the manifold is a good design that that gate does not flow well, this and the fact you have a large turbo well ported head, stroker displacement etc.
You build a good manifold to flow well through collector and into the turbo, this will
have the affect of not flowing well through the gate, thus a larger one being needed in this situation to get the required flow to reduce spool.

[cooperquattro]

Interesting point raised about the manifold design, this is not the first time I've heard of a similar issue with a Nortech manifold, that said I'm quite interested in the results using the bigger gate to see if that does cure it.

My mate had same issue with nortech,on a 650bhp tigra,ended up using 2 wastegates

[Jonnybravo]

My mate had same issue with nortech,on a 650bhp tigra,ended up using 2 wastegates

Danny by any chance ? I know quite a lot of the Vauxhall lads, not met Danny yet ( too far North ) but have heard of issues similar to what JP has had. It is nice to read some facts on the what it is though, or could be, you won't find that on any Vauxhall websites, far too secret squirrel about everything !