There I was at work, staring out of the window (as you do), when I started thinking about injectors. I've got 550cc's because I was told by Pete@SE that's what I'd need for my engine spec, which ultimately is supposed to make 550-somink bhp and run a couple of bar of boost. So blindly I handed over the cash - duuuh, OK - without thinking about it any further. That's until Macca mentioned fitting 870cc's to his. 870cc's!?! :eek:

 

I thought: can't be /that/ hard to work out what size injectors anyone would need based on how much boost they intend to run. *This* much air needs *this* much fuel at *these* engine revs, no? Well, I dunno WTF I then went on to work out, but it's so _totally_ wrong it's laughable!

 

 

Take your standard 3.0 litre TT Z32 running 1 bar of boost...

 

I say 1 bar because obviously atmospheric pressure is 1 bar, so jamming compressed air in at 1 bar will mean the air is twice as dense. Or have I made some horribly wrong assumption about that for starters?!

 

Then I said: the engine's 3 litres (exactly, in my example) so each cylinder is half a litre. BUT, because the cylinder only sucks once every two revolutions, it's sucking half that amount of air. But that's cancelled out by the fact that the air is double-density... so lets just say it's half a litre per revolution.

 

Bit of maffs later... at 7500 rpm, one cylinder's sucking 3750 litres of air per minute, or 62.5 litres per second. Hmm. Sounds like rather a lot.

 

Trying to work out how much fuel's needed for that much air... I said, lamda=1 is 14.7:1 air:fuel ratio. Bad assumption? To get how much fuel, I do (1 / 15.7) * 62.5, which comes to (wait for it...) 3.98 litres of fuel per second!?! Eh?! And that's only on one fickin cylinder! Fire all of those b*stards and the car would run out a full tank in around 2 seconds! :eek: :confused: LOL!

 

 

Anyone here got a kid doing GCSE physics that can help sort this mess out?

on boost you want to be running 12:1.

 

4 720's should deliver enough for 500-550bhp, so 6 should be good for 750ish.

 

here's a useful link...

http://www.rceng.com/technical.htm

 

you've worked the fuel usage out wrong.

 

it's cc per minute, richies 600bhp S13 uses 1 gallon every 75 seconds or 3 miles on full chat.

Ta Jezz ;) Z32 fuel rail pressure, anyone?

think it's about 2.5 bar.

fuel pressure is 43 psi

http://www.300zx.co.uk/cgi-bin/manual.cgi?list=efec&dir=&config=&refresh=&direction=forward&scale=0&cycle=off&slide=91&design=default&total=190

not that I am bieng picky old mate, but that does say at idle, and it was discussed above that the maffs was being worked out at 1 bar of boost, but whats 7psi amongst friends??

No I know, I couldn't find the Zed pressure when i initially looked so gave the S13 pressure which is 2.5bar ( 1.98 @ idle). :)

It's why I said I think it's about 2.5bar, because I wasn't absolutely sure.

 

injectors are tested at 43psi, pretty that's the pressure they use to classify them, 3 bar.

 

that's probably how you lot manage to get 450 out of the 6 std injectors, while the S13 only gets 270/280 out of it's 4. They are the same sized injectors.

well, I did the maths on your links, as I was thinking of dropping my 555's back down to standard, but by their reckoning 400bhp comes out at 525.

 

but then i did the actual brake specific maths and turns out mine is .72 ffs

Z32 fuel pressure is 2.95 bar + boost.

you don't add the boost John for calculating how much fuel is actually being injected into the engine, you are right that the fuel rail pressure is that, but the actual fuel pressure being injected into the charge is ~3bar.

 

The +boost bit is just there to overcome the positive manifold pressure, otherwise if you ran 3 bar boost no fuel would come out of the injectors.

Sorry, not too clear. Yeah, DP between the rail and manifold is maintained at 2.95, fuel rail pressure as read with a gauge is 2.95 + boost.

Used to seeing the idexed figure as I normally do a fuel pressure check on boost if I'm chipping cars.

 

you don't add the boost John, you are right that the fuel rail pressure is that, but the actual fuel pressure being injected into the charge is ~3bar.

 

The +boost bit is just there to overcome the positive manifold pressure, otherwise if you ran 3 bar boost no fuel would come out of the injectors.

well, I did the maths on your links, as I was thinking of dropping my 555's back down to standard, but by their reckoning 400bhp comes out at 525.

 

but then i did the actual brake specific maths and turns out mine is .72 ffs

 

I'll swap you mine :hyper:

 

+ cash of course ;)

Si, problem is you've assumed AFR is volumetric base, it's mass:

 

Assume Cyl vol = 0.0005m³, RPM=7500 VE=90% this gives air requirement1.6875m³/min

Air density at 1bar, 300K is 1.177 so assuming compressibility=1 then at 2 bar, 300K it's 2.354 (maybe this is a bit low temp for charge but it'll do!)

Mass air flow = 3.972kg/min

Assuming 12:1 AFR then mass petrol = 0.331kg/min

Assuming 721kg/m³ density then Vol petrol = 0.000459m³/min or 459cc/min

 

Looks pretty close, but that's assuming complete combustion + all the assumptions above. Be interesting to get the figures for humid air at realistic charge temp.

All I'm saying to this is:

 

Si, your maths is shite! :D;)

john, tell me your copying that out of a book :)

Have a look here:

 

http://www.ztechz.net/turboupgradeforvg30dett/id5.html

john, tell me your copying that out of a book :)

 

No, don't tell anyone but I'm a chemical engineer :x:

john, tell me your copying that out of a book :)

 

 

Nah, he's got a little bloke sat on his lap typing away :D

No, don't tell anyone but I'm a chemical engineer :x:

 

 

Thank fook for that, thought it was just me being dense .lol

so reading that link of andys I dont need that fooking aftermarket fuel pump?? looking for the spanners

No you won't need an aftermarket pump - actually, if you raise the fuel pump voltage to ~16v, you can get a Z pump to flow masses ;) Looking into this at the moment - don't worry about stressing the pump either - they actually run better at these kind of voltages.... next link time:

 

http://www.splparts.com/Parts/_Z32/Engine/Fuel.asp

 

bottom of the page ;) GB anyone? Oh no you're all running poxy power :tongue: :D lol

Oh yeah, this thread belongs in the advanced tuning forum surely?????

Oh yeah, this thread belongs in the advanced tuning forum surely?????

 

 

Would suspect so.

 

I actually have an aftermarket pumpo and afr fitted, but dont think they are set properly (afr) and so removed the afr, and was looking at going back to the standard pump

Should of done it all in moles. :-)

 

I want to pick holes in an assumption though...

 

1 bar atmospheric + 1 bar boost does not equal twice as much oxygen, because the intercoolers do not cool perfectly... but does anyone know what sort of charge temp you actually get on a stocker or with say Greddy FMIC? Bet it's not 20 deg C in either case!

I know, but I've assumed air entering the cyl is at 2bar(abs) and 300K. It's possible on a very cold day I suppose if it went in to the intake at 280.

TBH it's a very rough calc!