I was curious if anyone has given it any thought about running a pair of FIXED geometry compounds.. as in no VGT turbo in the mix.. I ask this because i have been seeing a lot of failures on the trucks side from VGT sticking or not able to work properly causing failures due to EMP spikes.. and there are a good amount of folks running small enough HP turbos to spool a larger LP fixed geometry turbo well.. I know there is someone on here having alot of luck running a fixed geometry single turbo.. with similar to stock spool..
Fixed Geometry Compounds?
- Thread starter storx
- Start date
numbnuts
Veteran Member
^^ is that in the jetta?
v8 coupe
Veteran Member
I have been exploring this for my setup because the intended use requires sustained high rpm demands I fear a vnt will have issues keeping stability when pr's are above 5bar.
well, i ask this question because.. with a small enough fixed turbo.. you could get good bottom-end spool and once the bigger one lights up.. you can just bypass around the smaller one.. but on the trucks.. i see them use internal gated turbos for street setups and then use the internal gate turbo along with a external for bypass on larger compound setups and it spools nearly the same as using the stock vgt turbo.. with a bigger turbo
v8 coupe
Veteran Member
That is more of a sequential setup then a compound setup. A properly sized pair for compounds will split the load and run both turbos at around the same PR. Utilization of a internal WG is only for turbine bypass to maintain proper boost pressure on each turbo.
The biggest issue with compounds using a VNT LP is tuning. For street driving you can tune it fairly well and get great manners if you size your turbos correctly. Look at Mike's and Matt's cars for ideas on sizing, or Jon's car which is using a cr170 turbo as the HP turbo. The issue becomes when you are going to try and maintain high boost pressures for long periods of time. For instance racing, where WOT is going to be maintained for 2-3 minutes almost continuously. You can start to encounter major surge and emp spikes on the VNT as it struggles to deal with the excessive emps. With a external W/G to regulate the EMP's from the VNT you can enter a sort of feed back loop. Where the w/g and VNT start to fight each other and boost levels can fluctuate wildly.
For me I looked at several options I am going with a more traditional setup utilizing to well paired internally wg'd turbos. i know this will reduce initial spool which I am ok with. Low end torque isn't something that is that important when the ultimate goal is power above 2k. optimization is dependant on power level goals and desired powerband. Bigger turbos will take longer to spool pushing power higher into the Rev range. the CR has the ability once fueling is dealt with to produce power higher into the RPM range then any of the older cars. We will see how power above 5k looks once there is adequate fueling.