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Old 07-21-2012, 08:30 PM   #1
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Default Injector Flow Variance

Just got my rx-7 550s back from DW and this time the spread has increased -- looking at 4% for low and 2.5% for high duty cycles. What's the rule of thumb here for what is considered acceptable?

By my math I'd be looking at .2 to .5 point difference AFR from the leanest to the richest cylinder at worst case under high load. Obviously under cruise conditions it isn't a big deal. It has been pointed out to me that the differences in air mass getting to each cylinder will likely exceed that of the injector flow rates. My gut feeling is that this spread isn't a big deal -- then again, running one cylinder at 12.0 AFR and the other three at 11.5 with a gauge readout of 11.6 seems like risky business.

What are your thoughts MT?

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Old 07-21-2012, 11:54 PM   #2
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Depends how close to the limit you want to tune it. If you're an average joe running a safe street tune thats more than fine. If you're running for a national championship and fractions of a hp matter then you need to get them flow matched.
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Old 07-21-2012, 11:56 PM   #3
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Any sort of flow info on the stock intake manifold and its variance per cylinder?
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Old 07-22-2012, 12:32 AM   #4
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I intend to get as close as my fuel system will take me to 300whp on a 2871R. The injectors will probably see a base rail pressure of 55-60psi, the effects of which I don't know for sure as far as the spread goes. To me it's looking like I will have to target a 11.2ish AFR in boost/under load to be safe across all cylinders, which means I will be giving up a bit of power even on a moderate tune.

+1 on aaronc7's question, could help with injector placement.
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Old 07-22-2012, 12:59 AM   #5
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Does MS allow you to fire injectors individually? If so run the car on one cylinder at a time and tune AFR for the lowest flowing cylinder. If you had enough time on your hands you could potentially figure out if there is any difference in runner flow and match the injectors accordingly.
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Old 07-22-2012, 01:12 AM   #6
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I assume it would make sense to put the "richest" one in #4 as it tends to run hotter unless you have a reroute?

Question for the masses: Is there any other rhyme or reason as to which cylinders typically run richer or leaner? I'd guess if you have a spread in the injectors you can at least use it to your advantage.
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