Daves Compound Twin turbo XUD

[darrenjlobb]

No, the big turbo feeds the little turbo. Little turbo boosts up first, once that's going it effectively creates more exhaust gas - enough to spool the bigger turbo.

The best bit is when the bigger turbo comes on blow, it's not exactly what you'd expect. Two turbos boosting at 15psi makes 30psi right? Wrong. The little turbo is now compressing air that's already compressed, this creates super-awesome boost (technical term). With one blowing into the other they "compound" their pressure ratios and produce more boost than their individual capabilities added together. Say both turbos individually boost at ~2bar absolute pressure (absolute pressure = boost pressure + atmospheric pressure; ie what we know as 15psi of boost), expressed as a ratio to atmospheric pressure this is 2:1. So we have two turbos compounding their pressure ratios of 2:1 each, multiply the ratios together and we get a final pressure ratio of 4:1. Knock off a bar to account for atmospheric pressure and you're left with 3bar or 45psi of boost...

This is kinda incorrect, without going in pages trying to explain.

The only things to consider with compound setup are PR (pressure ratio) and flow in say cfm, these are the things you will find on any turbos compressor map.

Boost pressures (psi) are totally irrelevant here as it means nothing. The turbo wont make x psi, it will make x PR, when i say PR, i mean the ratio between the pressure entering the turbo (so normally atmopsheric) or in compound whatever the other turbo is making) vs the outlet pressure.

You can quickly calculate what total mani pressure you can make, and by using compressor maps you can work out what pressure ratio to run each turbo at to keep both turbos near peak efficiancy.

Flow is also critical, as its stupid having a big turbo that can flow more air than the small turbo can make at peak, as it will just surge (hence why i was changing my 2256v for a 2260vk. The whole point of compounding is to allow the small primary turbo to sit CONSTANTLY in its PEAK effiancy band (which will be at a lower PR than you need to make the power), thats where it can FLOW its most air, which is most important, but normally on there own, you cant do this as you need more boost / manifold pressure, so by using a compound, you can get the full PEAK FLOW from the primary, and still get your high manifold pressure.

Turbos are happy, engines happy everythings happy, im crap at explainging things but thats my attempt as I think there is some confusion here.