Injector size

[Tristansgn]

So I currently have a set of seimens 55# injectors with a modded ECM. I'm running a 66mm turbo with a razors alky, mostly just bolt-ons but nothing done to the actual engine. Looking to do a 218/218 roller cam kit, and install GN1 ported aluminum heads that i just bought. I don't know what the duty cycle is on my injectors right now, and I won't know til my power logger is shipped from full throttle. Heres my question, if I buy injectors that are bigger than what I might need, 72#, 83# etc. does it make much difference as long as my chip is burned to match? I want to make sure I pick ones that are big enough, but I don't want to go too big. There's a lot for sale on TB so I want to take advantage of a good deal. Thoughts? I live in slc Utah so I'm at a higher elevation if that helps...

 

[87-WE2]

I had this question last year as well; here's my experience with it.

Check out my mods - there aren't many. I'm mostly stock, but when I bought my car it had an old ATR Pit Bull chip with 32's (I think) & pulled some nasty KR. I was told on here that in order to be safe, I needed to retire the chip & get something else. I knew I would eventually be building up the car for more power, so I decided to get a chip/injector combo. I talked to Eric @ TurboTweak and told him I was considering 42's and a chip. He asked what my goals were, and I told him high 11's but that the car is mostly street driven. He recommended going with 60# as they would cover me the whole way to my goal as well as have fine street manners. He explained further that as I pick up new pieces to go with my combo, I can get different chips along the way if I have to (which are relatively inexpensive), but I know I won't need new injectors until my goals change. I went ahead with the 60's based on his recommendation.

With the new chip & the 60's, my slightly modded car runs quite a bit better. I have no comparison times to back it up, but it runs much smoother than before and I also lost a stumble that I was having before. Zero KR with the new chip also. I don't have powerlogger, but I would imagine my injector duty cycle is below 50%!! My point is, the 60's I have are WAY overkill for my motor, but they work just fine.

I don't mena to tell you who to buy from, but I greatly appreciated Eric's advice. He easily could've sold me smaller injectors today, just so he could sell me bigger ones tomorrow. His honesty and integrity (as well as quality and service after the sale) scored a repeat customer

I'm no expert & you might wait for other more knowledgeable guys to chime in, but it's my understanding that unless is just GROSSLY more than what you need, it's OK to have a little too much injector. It will also depend on your goals for the car and where you are now.

 

[mgmshar]

There is really only two downsides to running injectors that are bigger than you need...

1. At low engine loads like idle or low speed cruising, a big injector may not atomize the fuel quite as well as a small injector. This might hurt your fuel economy, engine smoothness, and emissions under these conditions.

2. At lower engine loads, big injectors tend to operate at very low pulse widths. At these low pulse widths, the injectors tend to be very non-linear in their response. What I mean is that at low pulse widths, changing the pulse width by a certain percentage does not result in the same percentage change in fuel delivery. Example: increasing the pulse width from 20 to 25 ms (25% increase) will result in an almost exactly 25% increase in fuel flow. However, increasing pulse width from 2.0 to 2.5 ms (25% increase) will almost certainly not result in an exactly 25% increase in fuel flow. The bigger the injector, the lower the pulse widths get, and the bigger this non-linearity problem becomes. Also, the lower the pulse width, the more sensitive the fuel flow response becomes to small changes in applied voltage (ie the system voltage). The explanation for this gets a bit hard to explain while typing on my phone.

Regarding Item 2, well, it's up to the chip maker to put things in the programming and tables to account for this. Having burned a few chips in my time, I have some ideas on how this is done, but the experts know a lot more.

Honestly, the injector technology and skill of the chip burners have gotten so good that running injectors that are way too big for your combo is now generally OK, IMO. I would buy injectors that will cover whatever your possible plans are (within reason, I wouldn't go right to 160-lbers) get a good chip to match, and be happy with it.

Mike
Sent from my HTC Droid Incredible using Turbo Buick

 

[Tristansgn]

This all makes great sense, I thank you both for chiming in, I probably will only look into maybe 72-83# injectors but didn't really know what the downsides were. Not really driven much so fuel economy I'm not too worried about and my car is in a county that doesn't require emissions. Does anyone have touts on low vs high impedance injectors? My ECM is modded, so it will run both but I don't really know the benefits of either....

 

[mgmshar]

Theoretically, high impedance injectors are more prone to the stuff I mentioned in Item 2 above than low impedance injectors. Because of this, about 10 to 15 years ago, the largest high impedance injectors you could get were about 50 lbers. I still remember thinking how awesome it was when they came out with 57lb high impedence injectors - bought them in about 2003 or so, still have them on the car, have gone 119 mph in the quarter with them. But I digress....

I think the improvement in technology now allows for much bigger injectors that still have high impedance, yet drive and idle just fine. Honestly, if I were you, I would work with one of the good chip makers on this board and ask them to recommend an injector that's around the size that you want. They're the ones who have to tune for each new injector that comes out, so they will have good knowledge of which injectors are the most linear, have caused the least tuning problems, etc.

Mike
Sent from my HTC Droid Incredible using Turbo Buick