Nibbles wrote:
First is more of a question.
1) Looking down the ports from the manifold (direction of gas flow), it's quite a sharp turn into the wastegate port. I'm sure there is some merit in putting a bigger radius on the inside of the turn to aid flow out through the wastegate, the question is how much would it reduce flow down to the turbo at the lower end where turbo efficiency is most needed.
2) Second one is probably more controversial. Being a dual port turbo, the gasflow in each port isn't steady. Alternate cylinders send pulses into alternate ports. If a substantial part of the metal in between the 2 ports were removed, this would create a single, much bigger, port. Each pulse of gas would then have a much larger exit path as it can spill across into the other port which would be in it's 'off' phase. Removing this metal shouldn't lead to cracking, unlike increasing port diameter and reducing wall thickness.
ANy thoughts ?
1) Logic has suggested exactly the same thing to me Chris, it was interesting to see that this approach was adopted on the ATS racing CT27 (which still suffered boost creep IIRC all be it to a lesser extent). I doubt if any noticeable impact on turbine efficiency would result from this "low down" as the WG will be closed and the WG port already full of exhaust gas (although not at full pre turbo pressure all the time). I also suspect that a downpipe designed with the inclusion of a WG guide vane within the downpipe would assist in evacuating the wastegated exhaust into the downpipe without causing a whole heap of turbulence like I suspect the stock arrangement does. Interestingly it seems that soem Garretts have this.
The thing that gets me though is that a lot of the turbo/wastegate positioning on aftermarket turbo/manifolds seems much worse and relies on exhaust manifold pressure to divert exhaust gases to the external WG and yet seem to function!
2) The above IMHO is therefore directly related to part 2. By machining out the dividing plate you are effectively removing almost all of the effect of the twin scroll (as per Turbo Technics but still retaining the smaller A/R). The exhaust gases are likely to backflow through the WG port and into the non-active scroll of the turbine housing/exhaust ports. I think it would assist in reducing boost creep but have a far greater impact on boost threshold and mapping.
One thing that has been bothering me for a while is the exhaust manifold design, with it being unequal length (by a large margin) I wonder if there is an RPM range where you effectively have 2 pulses simultaneously reaching the same scroll and overwhelming the WG with flow (choked flow?). I wonder if an equal length manifold would serve to again reduce boost creep by spacing the pulses better or would lead to slower spool (depending on RPM range where 2 pulses happen together)? When I get some time I want to do a few tests on the stock manifold to see if the stock design promotoes flow reversion in some ports (as I have seen in other cast designs) or if Toyota actually did a good job of equalising flow between cylinders despite the nasty angles and unequal lengths.
I'm not sure how much you have looked into it but I found using the Borg Warner EFR Matchbot very interesting as it calculates the % wastegating & pre-turbo exhaust manifold pressure for each RPM band at your desired boost level/engine.