RossD 44 Posted September 16, 2007 Right, this is something I'm puzzling over.... My TU build is coming along nicely, but one thing that has been at the back of my mind is my injectors and the fuel pressure. I'm aiming for 150bhp from my TU5 (1.6). The injectors I have are Bosch 744's, which (I think) are 240cc/min @ 3bar. Using some dodgy maths, at 80% duty cycle these injectors can supply enough fuel for about 145bhp. Now after trawling my local scrap yard, I came up trumps and found a 4 bar pressure reg in an Audi V6. At 4 bar, again using some dodgy maths, these injectors should flow 277cc/min. Which is enough for lots more than 150bhp. Now comes the dilemma. Will running the injectors at the higher presure cause any problems at low power? (Using injectors that are too big for an engine can cause dodgy results when they are opening for a very short amount of time... Is the same true when you run a smaller injector at higher pressure??) RIght.. now to the question Is it better to run the system at 4bar and have lots of fuel in reserve if its needed when it goes to be mapped, or run it at 3 bar and maybe run out of capacity... (Bearing in mind its not all about peak power.) Cheers Share this post Link to post Share on other sites
Sandy 191 Posted September 16, 2007 The closer the injectors are to 85% duty at full power, the better, I find as a rule. Smaller is risky on transisent and bigger means poorer resolution and sometimes poor mixing at low load. Share this post Link to post Share on other sites
RossD 44 Posted September 16, 2007 The closer the injectors are to 85% duty at full power, the better, I find as a rule. Smaller is risky on transisent and bigger means poorer resolution and sometimes poor mixing at low load. Cheers Sandy. I wasn't sure if 80% or 85% duty cycle was the magic number, I chose 80%! I suppose they are much of a muchness. If, as you say 85% is the number to aim for, then running at 3bar won't be a problem. In which case I'll just keep my 4 bar pressure reg as a spare Share this post Link to post Share on other sites
Sandy 191 Posted September 16, 2007 I've used 200cc injectors in TU8v's around this power before and they're fine at the top end of duty, but idle and cruise thay can be a bit short on duration for ideal fuelling. Share this post Link to post Share on other sites
Guest hassan Posted November 5, 2007 Right, this is something I'm puzzling over.... My TU build is coming along nicely, but one thing that has been at the back of my mind is my injectors and the fuel pressure. I'm aiming for 150bhp from my TU5 (1.6). The injectors I have are Bosch 744's, which (I think) are 240cc/min @ 3bar. Using some dodgy maths, at 80% duty cycle these injectors can supply enough fuel for about 145bhp. Now after trawling my local scrap yard, I came up trumps and found a 4 bar pressure reg in an Audi V6. At 4 bar, again using some dodgy maths, these injectors should flow 277cc/min. Which is enough for lots more than 150bhp. Now comes the dilemma. Will running the injectors at the higher presure cause any problems at low power? (Using injectors that are too big for an engine can cause dodgy results when they are opening for a very short amount of time... Is the same true when you run a smaller injector at higher pressure??) RIght.. now to the question Is it better to run the system at 4bar and have lots of fuel in reserve if its needed when it goes to be mapped, or run it at 3 bar and maybe run out of capacity... (Bearing in mind its not all about peak power.) Cheers dont forget all that the 214 cc/min injectors of the mi16 can deliver fuel enough for the 158hp at 3 bar fuel pressure..........hwich means that the 240cc injectors at 4 bar can deliver enough fuel for 200hp at least. Share this post Link to post Share on other sites
Ryan 99 Posted November 5, 2007 A good rule of thumb is 5cc per 1bhp @ whatever the test pressure is. So 240cc * 4 injectors = 960cc/min 960 / 5cc per bhp = 192bhp @ 100% duty Then scale that according to whatever duty cycle you want. 85% would give 163bhp. Share this post Link to post Share on other sites
Guest hassan Posted November 7, 2007 A good rule of thumb is 5cc per 1bhp @ whatever the test pressure is. So 240cc * 4 injectors = 960cc/min 960 / 5cc per bhp = 192bhp @ 100% duty Then scale that according to whatever duty cycle you want. 85% would give 163bhp. this rule my be nearer to be true in forced induction engines where the mixures is more reach up to 1:11 A/F ratio ..... but in naturally aspirated engines if we go with this rule then the MI16 engine cant get output more than 145 bhp at 85% duty cycle which is not logic. the scientific rule is: InjectorSize = (HorsePower * BSFC) / (#Injectors * DutyCycle) unites: injector size lb/hr 1 lb/hr = 10.5 cc/min. BSFC lb/(hphr) 0.45 naturally aspirated to 0.55 in forced induction. take 85% duty cycle. notice: we may say an engine to deliver its output, its BSFC must be less than 0.45 say 0.42 as in MI16 engine at its maximum output .... sure it reflects how much this engine is efficient in fuel consumption compared to its output figure. http://www.mi16.nl/main.htm (Tech specs link). 1 g/(kw hr) = 0.001621029 lb/(hp hr). Share this post Link to post Share on other sites
petert 607 Posted November 7, 2007 dont forget all that the 214 cc/min injectors of the mi16 can deliver fuel enough for the 158hp at 3 bar fuel pressure......... They can actually deliver 180-185hp at 85% @ 3 Bar. The flow rate of injectors is very much only a guide. Bosch won't tell you what a particular injector delivers. All those figures on the net is data gathered by various people over time. The only way to find out is bench test them yourself into measuring cylinders. Some people even use injectors in a throttle body that were designed to flow into a port, then wonder why performance is less than expected. Share this post Link to post Share on other sites
