16-08-2012, 11:41 AM
(15-08-2012, 04:29 PM)Jarrus Wrote: i use 77% ve 0.42 BSFC and 17:1 a/f ratio
Just a quick FYI there - VE is higher than 77% - go for more like 80-82%... And go for 20:1 AFR - being an IDI you need a much higher air content as you'll never get the fuel burning completely...
DI Smoke threshold is so much higher than an IDI...
One of my next projects is to get a wideband on and measure some AFR to see what's going on...
(16-08-2012, 07:15 AM)CJ_Derv Wrote: i can cruise on the motorway and at just under 3k doing 75mph the turbo doesnt read on the guage More mpg)
Less boost doesn't equal less fuel - I assumed this also - you'll actually find that the little GT15s are more efficient on the motorway... My VNT I used to open the vanes up on the motorway assuming it'd reduce pumping losses etc... But infact having about 5-8psi continually helps massively, it increases thermal efficiency seemingly (like a raised Compression Ratio) - I used to achieve 55-60mpg on the motorway if grannying it... If I ran the vanes wide open I'd only get about 42-45mpg MAX....
Also yeah, with regards to compounding small turbos with big turbos... You can't have the two apart in sizing too far - not unless you're intending to only compound for a short period of time... I.e. compound until the big turbo comes in, but then you need to wastegate past the turbine small turbo and open a valve past the compressor of the small turbo too... But it intoduces all sorts of problems of when you lift off the throttle with surging and backfeeding through turbos... I.e. you CAN do it, but it can be a complete bastard...
I'd stick to either sequential where you completely bypass the small turbo and actually block it off as such... OR stick to only compounding, but then you need to be very careful about overspeeding the smaller turbo... There's a chap doing compounding of turbos at the moment on a ~2.0, using a GT3702 and a GT3282... Fairly similar in size (in numbers terms they're not THAT close, but when you think law of diminishing returns, they're pretty close...) but using like a TD04L 15g and a T2 or something, they're so far apart in terms of flow vs shaft speed vs what the engine demands you'd always be on the limit of overspeeding the small turbo and instantly popping it...
As I've been saying for a while now - MODERN turbos with correctly sized housings will do the job of big, old turbos dropping the boost threshold and lag to within stock T2/K14 levels... My GT20 was LESS laggy than a T2, but would do 180-185hp... If you can get hold of a modern GTB, you're talking so much flow down low, you'd have to modify the cylinder head to actually take advantage!
