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rx7 460s = bad gas mileage?

 
Old 06-02-2008, 12:07 AM
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Default rx7 460s = bad gas mileage?

I've noticed a drop in gas mileage since installation of my rx7 460s yet I've got it tuned for mid 15 afrs while cruising, just like with the oem 1.6 injectors.

Anyone else notice a difference? Could it be because you don't have as much resolution with the bigger injectors?
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Old 06-02-2008, 12:13 AM
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with my 460's I get about 28mpg highway, haven't figured out my average combined mileage though. I got my highway calculation on an extended higway trip back from Deals Gap to my home in MD, near constant 80mph cruise (not on cruise control), split between 2 drivers, with one highway pull against a 350z. Seems like a decent representation to me.

I'm sure I'd get better mileage at 60mph or so, but I say 28mpg.

As for around town driving, it seems to get rather bad mileage. I think that it's a function of my heavy foot though!
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Old 06-02-2008, 12:17 AM
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run HiRes.
Tune better.
Try using a lower required fuel number, and then raising your VE cell values by the inverse of that.(like JoePerez did, or was it AbeFM?) That will give you more res.

I've been too busy to tune mine well since turbo and I've been changing setups regularly but I still get 26-27mpg on my 460s during regular combined highway/around town driving. i have a heavy foot also
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Old 06-02-2008, 12:23 AM
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I'm getting about 22mpg so its not bad, but before the injectors I was getting about 30. This is freeway only averaging about 75.

Ill probably try what you suggested paul.
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Old 06-02-2008, 12:38 AM
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Oh, but what do you mean by tune better? What should I do better about the tune for gas mileage/hwy cruising?
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Old 06-02-2008, 12:50 AM
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I got 26 on my old bad tune with 550s. Run high res and tune better.

Might want to try messing with timing during cruise to see if that affects your mileage any.

Is your odometer reading correctly due to a wheel/tire size change?
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Old 06-02-2008, 01:34 AM
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Or you just beat on it more because you aren't at 95% DCs with 1.6 injectors at 5500rpm. :P
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Old 06-02-2008, 03:55 AM
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Originally Posted by crashnscar View Post
Oh, but what do you mean by tune better? What should I do better about the tune for gas mileage/hwy cruising?
I have my cruise tuned for 17.0 afr not 15's???? and I get good gas mileage
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Old 06-02-2008, 06:41 AM
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Running overly rich in boost (11:1) will also drain some serious fuel quicker then 12:1.
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Old 06-02-2008, 07:32 AM
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I got 28 or so MPG in my old car with 460's
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Old 06-02-2008, 08:12 AM
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I've heard conflicting opinons about this:
If you tune your cruise very lean (18:1) will you damage your exhaust valves/pistons?
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Old 06-02-2008, 08:32 AM
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I got upper 28-30 mpg average with my 460's tuned to 14.6 cruizing and 12.0 in boost.
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Old 06-02-2008, 09:08 AM
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Before turbo with 460's I got like 22 mpg. Now that I got a turbo it seems to have gone up, yesterday the fuel needle(I know not very accurate) was at the middle mark and the car did 124 miles.

Ill know the exact improvement after I fill up.

I tune for 12:1 in boost, and 14.7 in cruise.

P.S. When I first installed the turbo, it was like 10:1 in boost if not even worse, and I went through like 6 gallons of gas in less then 70 miles.
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Old 06-02-2008, 09:37 AM
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i typically see ~160-170 when i hit the middle mark. after that if falls more rapidly.

Accel/decel enrichments can play a huge role in fuel consumption. Have you messed with those?

I cruise around 16afr and in decel it goes off the gauge. No reason to be throwing fuel in there when you are dragging.

Tune your idle manually. Set VE Analyzer(if you use it) to a min rpm of like 1800 so it doesn't screw with idle fueling. If you let your car warm up before taking her out(as you should) you could be wasting a lot of fuel during that time if your warmup enrichments are too high.

I love using VE Analyzer but after I've gotten a decent tune with it I manually adjust my cruise cells to bring it to around 16 afr.
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Old 06-02-2008, 10:00 AM
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Did you consider that with twice as much injector you can make twice as much power? There is a very direct relationship between horsepower and fuel use.
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Old 06-02-2008, 10:20 AM
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wait?! are you trying to say that if I figure you have (4) 460cc injectors, so that's 175 lbs/hr. If I multiple that by .80 (duty cycle) and divide by .55 (BFSC), I can see how much B.H.P. the pump is capable of flowing for?!?!?! so like, 254BHP at 47.5psi of fuel?! Insane!
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Old 06-02-2008, 10:46 AM
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Originally Posted by paul View Post
run HiRes.
Tune better.
Try using a lower required fuel number, and then raising your VE cell values by the inverse of that.(like JoePerez did, or was it AbeFM?) That will give you more res.
Nah, Abe has MS-2, so he's too busy whining about how bad his idle is to tweak the fuel.

crashnscar, the mod Paul refers to is that I set my ReqFuel constant to 3.5ms, since according to [url=http://www.msextra.com/manuals/MS_Extra_Software_Manual.htm#hrcode]The Documentation[/u], this is exactly 100 times the native resolution of the HR code (35μs) and thus, each single digit increment in the VE table directly correlates to one whole step of the smallest possible change in fuel.

If your ReqFuel was, say 7ms, then the fuel would only actually change once every two VE digits. Ie: from 50 to 51 = no change, then from 51 to 52 = a change...

It results in a bizarre looking VE table, but for injectors in the 440-460 range at normal street-ish levels of boost, it means that we are able to use very nearly the entire 8 bit resolution of the VE variable. Here's an example:




But I hear you on fuel economy. After I switched to MS (the 465s had been in for a while previously) my economy went to ****. We're talking 19 MPG in town. Over time as I refined my VE table, enabled overrun cut, stopped wasting so much fuel cranking and ASE, leaned my idle back out to ~14.5, and so on, it's improved. Currently, I'm back to 22-24 MPG city (which is typical for my driving) and on a recent trip to LA and back I got over 29.
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Old 06-02-2008, 01:18 PM
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Originally Posted by Joe Perez View Post
the mod Paul refers to is that I set my ReqFuel constant to 3.5ms,
I did the same thing after reading Joe's previous thread on the subject. Works well for me running 550cc RX7 injectors.
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