It sounds like everyone I've heard of that has done this either uses a VERY small turbo pulling through a VERY large one. Wow, thanks for the takeoff in responses! That's awesome. I guess the trick would to be getting a WG big enough to bypass the small turbo, and feed the big one propperly. Once the big turbo is up and spooling, the smaller turbo can basically freewheel. I have had an idea about using the small turbo's WG to feed the big turbo. Remeber that sizing the exhaust housings will depend on how you setup the exhaust. If you size the small one to be running out of puff after the big one is up and running, you should be sweet. The smaller turbo can draw thru the big turbo, so you dont have to worry so much about it not being able to feed the air needed to make the HP - it will have enough air shoved into it, so that it can flow. So, for the feeder turbo, red the map as normal - as this turbo is going to be feeding ALL the air into the motor - so make sure it can cope with the airflow demands (IE, the big one) The maps work on pressure ratio and Lb's of airflow.Ĭhanging the inlet pressure simply doesnt have any affect on the pressure ratio, but it will alter the Lb's per min of airflow. Yes, in theory you just move the compressor map along the X axis. Here are a couple examples of some compound turbocharger setups: Neither turbo has to be that large, I'm mostly looking for the compound effect more than top-end power- especially considering I wouldn't have any rev range to use it. I'd love to have a turbo small enough to be able to have those same boost characteristics but still have some OOMF up top. The stock setup which makes about 170BHP and about 240lb/ft of torque gets its itty bitty Garrett GT2056V VNT turbo spooled by about 2000RPM, the same place it makes max torque. I'd really like to make a logical decision on the turbos for this if I follow this path. Now, if there was only a way to figure out what the compressor maps would look like against each other. It apparently also reduces EGT as well! The other clever part about using a diesel (especially one with a sequential gearbox) is that once both those turbos spool up, there's never a reason for them to release any of that pressure- great for road racing, which this project is aimed at. Wastegates before both turbines would essentially "fix" the boost at each turbo, and assuming that the small turbo's map moves right on the x-axis of its compressor map as the big turbo's boost increases, the dinky one should never be a restriction- right? It all sounds like an incredible solution to low-end boost, compressor surge, and overall boost pressure, possibly even getting that max boost down to a manageable RPM. Could the smaller turbo pulling through the larger one help with this by reducing the pressure in the compressor outlet?
One of the big problems I'm having with sizing a turbo to this engine is keeping out of compressor surge. Would this essentially move the whole compressor map for the small turbo along the X axis? In this way, you compound the boost going through the smaller turbo by increasing the pressure at its inlet. I'd like to make a better one.ĭoes anyone know how the compressor maps will work with a compound setup? I know the whole idea is to pull through a large turbo with a tiny one, while spooling the large turbo's turbine with the little guy as well. Everyone seems to base their designs off either what they had laying around, or a good guess at what will work.
I've been reading up a bit on compound turbocharging and quite a few people have pulled it off on small displacement gasoline engines, and even more people have pulled it off on large modern diesels. With a limited rev band (5000rpm redline) it is quite difficult to find a turbo with enough pressure at a low enough flow rate to get the boost above about 25PSI. I have a 2.5L 4-cylinder common rail diesel engine that is in need of some extra power. Working on a project, thinking of options.
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