Fuel injector size, duty cycle and the meaning of life...

[plohl]

Hi guys,

I have just picked up a set of itb's for my 1.8L NA mx5/miata/roadster. It is supposedly making 130rwhp with tighe 805C cams and I am using a haltech e6x and batch injection.

As far as I know I am using the stock injectors for the BP engine which are 254cc/min (ref: http://miataturbo.wikidot.com/fuel-injectors). Using the reference I calculated that the injectors would be good for about 155 crank shaft HP with a BSFC = 0.5 and 80% duty cycle.

Code: Select all

Math
cshp = (254cc/min x 4 x 0.8)/(10.5 x 0.5)
     = 154.81 cshp

So this tells me I am almost at the limit of the injectors.

I copied the injection pulse width (IPW) values from the fuel map and used excel to determine the duty cycles across the map. I used this the method outlined here to calculate the duty cycle http://www.hywell.org/plhelp/injdutycycle.html. My max IPW at 100% TPS, 7500RPM was 7.131, so the duty cycle was calculated to be 47.5%.

I had to think about this at first as i was pretty confused as to why it is so small, but then it dawned on me that the batch firing influences the IPW. I have had a bit of a search, but haven't found anything concrete. Is is safe to assume that the injection duty cycle is double? So the ducty cycle would be 95%!!!!!! As the injector is fired twice per engine cycle (720* crank rotation).

Or is it better to assume that only a percentage of the fuel injected during the other cycle is used?

Either way, i am not expecting huge gains with the ITB's, but is it safe to say I'll need bigger injectors? Just confirming this before I lash out and buy some.

[gslender]

Nah, all fuel is used, and is why batch fire with large injectors won't idle... You can't get the pw small enough as they are all adding fuel. At WOT, batch and sequential makes little difference, obviously due to the limited time between squirts.

As for the calcs, doesn't the ecu tell you the duty cycle?

G

[plohl]

Nah, all fuel is used, and is why batch fire with large injectors won't idle... You can't get the pw small enough as they are all adding fuel. At WOT, batch and sequential makes little difference, obviously due to the limited time between squirts.

As for the calcs, doesn't the ecu tell you the duty cycle?

G

Cool. It makes sense.

Stupid batch fire! Means i cant get too big an Injector. Hmmm...

I think it does but i don't have the car at the moment.

Before you say it, i know, i should get a new Ecu :p

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[sailaholic]

E6x is when haltec first moved from dos to windows. So no very basic and no duty cycle info we have found in the menus

[Steampunk]

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

[plohl]

seen that one dude - same equations i used.

I could get an adjustable fpr but I can see that adding up quickly.

[NitroDann]

Its fine, look what are your AFRs?

Dann

[Steampunk]

So the haltech is configured correctly for the 254cc? I would just use them until you definitely feel you've maxed them out because if you can't enter the new injectors' latency/lag/dwell time into the software then your fuel map will be out of wack.

[plohl]

Its fine, look what are your AFRs?

Dann

Will have to look at some logs, but from memory they're about 13.5-12 wot.

I thought about just seeing how they go but i essentially have to have everything done one weekend, so don't want to find out they're too small

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[NitroDann]

13.5 is too lean, and 12-13.5 isnt a very steady afr across WOT.

If the injectors can hold 12:1 or lower then their duty is fine.

Dann

[plohl]

looked at some of my logs - WOT afr ~12.5-12.

How can you be sure/justify they'll be fine when everything else is point towards them being just too small?

[NitroDann]

because they flow enough to make over 160hp at 12.5:1 and you are making 130.

[NitroDann]

They are getting close to the limits however, if it stresses you contact works engineering for a FPR, they are very well priced and small and sexy as hell. Ive had personal experience with the 500hp one and it was great. Anodised black too so pretty low key.

Dann

[plohl]

160 rwhp or at the fly? How do you getting that?

I'll look into some options, was hoping not to touch the fpr as it'll be a catalyst for me to spend more money.

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[NitroDann]

Just get some injectors from soemthing bigger.

flywheel HP, standard calcs.

Dann