Over sizing injectors

[Chassis 33]

Is there any advantage to be gained in significantly over sizing injectors? For example you calculate that you need say a set of 250cc/min operating at 80% duty cycle to get the power you want. Is there any advantage, or indeed disadvantage, using 500cc/min injectors and running them at 40%? Pressure pulses in the fuel rain becoming a problem maybe???

Regards
Iain

[trackcar]

I see no reason why you would want to deliberately run an injector which was massively over-specced. most ECUs can rescale the injector map numbers so you dont lose resolution at low airflows/horsepowers/fuel pulses, but why not just run the right sized injectors in the first place (especially as your car already has some just perfect for the job!) ?

[stevieturbo]

I would never size injectors based on the power you hope to achieve now...as if you end up wanting more, you will then have to buy again.

So I say always buy injectors, with a fair bit in reserve.

If the ecu you are using is any way decent at all, it will not pose any problems whatsoever.

[CNHSS1]

some of the guys into jap engines have a bit more work mapping huge injectors 750cc-1000cc as at low outputs they tend to atomise the fuel less well and the fuel becomes a stream rather than spray which can cause emissions issues for road cars or CATs, but as said, it can be done.

[900T-R]

stevieturbo said:

I would never size injectors based on the power you hope to achieve now...as if you end up wanting more, you will then have to buy again.

So I say always buy injectors, with a fair bit in reserve.

If the ecu you are using is any way decent at all, it will not pose any problems whatsoever.

No - the higher you can keep the fuel pressure for a given output, the better the injectors will be able to atomise the fuel at the lower end of the load/rpm scale. As with turbos, for driveability you should always use the smallest that supports your hp target.

[900T-R]

CNHSS1 said:

some of the guys into jap engines have a bit more work mapping huge injectors 750cc-1000cc as at low outputs they tend to atomise the fuel less well and the fuel becomes a stream rather than spray which can cause emissions issues for road cars or CATs, but as said, it can be done.

I (or rather the very capable engine management specialist at Beek Auto Racing) already have massive problems mapping the 346 cc injectors on my car using non-lambda controlled Bosch LH 2.2 for low rpm/loads. Standard inj'size is 214 cc. In the long run I'll have to look at either aftermarket management or a Trionic 5.2 retrofit.

[GreenV8S]

The only reasons I can think of for doing that would be to get very short injector times to allow phased sequential injection at full power, or to support very very high power levels.

The main disadvantage I can see is that the injection duration at tickover and lower rpm/off load will be very short compared to the opening/closing times and it might not be easy to meter the fuel accurately. Perhaps with low impedance injectors that might not be such a problem, but you might need to run two injection stages, or play games with the fuel pressure, to get the metering accurate enough for a road car.

[K13 WJD]

ive been here before....and had nothing but bother....it was on a turbocharged 1600 vtec....4 big injectors ( we were looking for 330BHP)...and it ran like a whore. i spent 4 days mapping it, to no avail.....so, std injectors back in, custom made inlet manifold with 5th injector, and trust ecu running it......and hey presto. a 3 hour session on the rollers, and 310BHP at the flywheel !

[rev-erend]

Surely if you buy the right sized injector you get the benefit of good atomisation and low speed running and if you do decide to have more bhp you always have the option of uping the fuel pressure a bit to cope.

[stevieturbo]

900T-R said:

for driveability you should always use the smallest that supports your hp target.

But often, those targets change. More than once too.

Much easier to buy bigger, with a good safety margin built in. Then you have room to grow, instead of changing injectors all the time.

FWIW, Ive used 1000cc injectors on 2.3 litre engines, and never had any issues tuning them. Buy good quality modern injectors, and if you cant tune anything say below 800cc on a 2.0 engine, then either the ecu is crap, or you need a new tuner.

the only issue I have ever had, is when you throw in a big camshaft with too much overlap into the bargain, as this really will affect things down low. But on a turbo'd engine, thats a bit daft in the first place.
On a n/a engine, it could become an issue with massively oversize injectors, but I can never see any reason for doing that, as n/a never make that much power anyway.

Build in a good 30-40% safety margin, and all should be well.

[eliot]

On my 5.7 I had 26lb injectors - When i fitted the turbos, I needed bigger injectors. I could get hold of low impedance 60lb injectors for a good price, so I put the injector size into my software (megasquirt) and checked the idle pulsewidth - which was about 1.2ms,which was pretty low - but acceptable, so I went ahead and got them.
They mapped in great and my final map was actually much flatter and smoother than when I had the smaller injectors.

So in my case, I more than doubled the injector size without issue - so it can be done if you understand what youre doing. Max duty cycle is about 55-60%, so Ive got plenty of head room. (60's should take me to 550-600 bhp if i can ever get a gearbox to hold)

eliot.

[stevieturbo]

The 60lb Siemens Deka high impedance injectors doing the rounds, are bloody superb pieces.

if they fit the bill...you will look hard to find a better injector, and the are very good value.

[eliot]

stevieturbo said:

The 60lb Siemens Deka high impedance injectors doing the rounds, are bloody superb pieces.

if they fit the bill...you will look hard to find a better injector, and the are very good value.

They were mototrons's from New Era Performance, but are in fact Siemens Deka's - cost $439 a couple of years ago.


I did have to change my fueling from 4 squirts to 2 squirts per cycle - so if oversizing, I assume you have a mappable ECU.

Edited by eliot on Saturday 24th November 09:12

[stevieturbo]

Where you running it in 2 stroke mode before ?

Yes, those are the same injectors.

[900T-R]

stevieturbo said:

900T-R said:

for driveability you should always use the smallest that supports your hp target.

But often, those targets change. More than once too.

Only if you're into drag racing, or dyno queens. In all other cases, the car the engine's in pretty much determines how much power you you want/need before the overall dynamic package starts to worsen instead of improving on what you have, i.e. in my FWD Saab (ca. 1250 kg) it'd be plain silly to go for much more than 300 bhp so I'm not sizing my turbo, injectors, fuel pump etc. for 500. If it runs OK in the lower part of the rpm/load map with oversized injectors I can almost guarantee it will run better with properly sized ones.

Getting the power is the easy part, OE-level driveability is something entirely different.

[eliot]

So to summarise; yes you can oversize the injectors - whether you should depends on your particular requirements and circumstances.
If your car/ecu has issues with larger injectors - and you intend to keep increasing the output of your engine (i.e. more boost) then sooner or later - you will need to address your injector control problem.

[GreenV8S]

How short can you practically go on the injector duration? Say for high impedance injectors with a 1ms nominal opening time?

[dnb]

If your injector opening model is good, then you can get to silly small numbers. I think my VEMS setup will tolerate 700us or so before the injector model doesn't sufficiently match reality.

Of course, this assumes you have the injector parameters accurately calculated! There's much more to it than just the flow rate.

I'm another fan of the same Siemens 600cc injectors everyone else is liking They gave a good flat VE map on the Griff.

Edited by dnb on Sunday 25th November 00:10

[stevieturbo]

Different ecu's seem to handle it differently.

Im running high impedance 73lb injectors, which are circa 750cc at 45psi.

Although Im running them at 66psi base pressure. I guess that would make them maybe 850-900cc ? give or take. The numbers on my map seem to indicate circa 1.7-1.9ms at idle, although that seems unusually high.

Mine did idle fine at that though. it did idle better with the Siemens injectors, but they were too small. ( same fuel pressure )

IMO The RC Eng 73lb's I am using arent very good at the low end. If I could find a better replacement, I would use them. Fired by my DTA, albeit batch fire, not sequential. I need to sort those triggers so I can run sequential.
Ive been told the 60lb Siemens can be drilled out to flow closer to 90lb...but I emailed the guy who had this done about getting my set done, and he hasnt got back to me yet.

That said, the 1000cc Low impedance injectors from RC Eng, controlled by a Motec, fired sequentially seem to work perfectly on a 2.3l 4cyl engine

[ringram]

GM ECM's seem to have problems with very low pulse widths and large injectors, end result is rich idle and low MAP conditions.
Sizing to 80% duty seems the go. But if your ECM can drive a very low pulse width to eliminate this issue, then there shouldnt be any reason not to go large.