just got my ls/v built with about 8.8 comp wiseco pistons and had it tuned, cruising afr's are 14.7 to 14.9. but I seem to be getting 23.5 mpg consistently. my previous turbo b16 would get no worse then 27 and that was railing on it non stop. Iv even driving a full tank with getting on it maybe once. I am wondering if the compression is giving me bad mpg, or if its the rc750cc injectors like they take longer to warm up or something?

specs on car:
94 civic ex 2250lbs
16x7 volk racing te37s
gsr tranny

please enlighten me

 

You are boosting, hence, the increased air into the cylinder from the turbo needs more fuel.

First, those injectors are designed to drop lots of fuel, fast. For this reason, it is difficult to get them to spray small amounts of fuel at idle especially.

Second, with the same amount of air and fuel, higher compression ratios can make more torque than lower compression ratios. In order for a lower compression piston to make the equivalent power, it would need more air to create similar cylinder pressures, which needs more fuel to burn.

 

Talk to Tony1 about the new Bosch injectors, as he brings up Precision & RC injectors a lot when he talks about them. In short, most big injectors have crappy atomization. That brings down gas milage, as you waste gas. Also bigger cams tend to waste more gas when cruising, as more overlap can let raw gas flow right though the head.

One of the biggest reasons you're probably loosing milage is because of the rotating mass. There is quite a bit more with the longer stroke. If you dug up the thread with the math done on flywheels, an 8lb flywheel vs stock is similar to removing ~250lbs from the car in 1st gear due to it being *rotating* mass.

If you bypassed the coolant line to the TB or use a hondata gasket, that could be another issue. Warm gas/air atomizes better, and that increases gas milage. (While cruising, you simple add more throttle to make up for the less dense hot air.) Ambient temp & humidity also plays a roll, as do tires and their rubber compound & air pressure. LOTS of factors.

 

You should lean out the -10 in/Hg to 0 psi fuel maps. On the Vitara D16 setups (7.5:1 to 7.8:1 CR) with small spool-happy turbos (IHI, T25, Greddy SOHC kit, etc) I typically run the entire NA range in the 14.x:1 AFR range. The idea behind this is that you take such a hit in efficiency by lowering the CR that there isn't enough airmass in the engine at these points to require the cooling of a rich mixture. To restate this from a different perspective for clarity: a stock a engine makes 10 whp at 2000 rpms and -8 in/HG. After dropping a couple points CR and retuning it doesn't make that power at 2000 rpms until 0-2 psi. Why run the engine richer in these circumstances? It makes no sense to do so.

I've a couple Vitara setups that hit 32-33 mpg, two of these were engines I tuned early on and set the AFRs "by the book" which resulted in 22-25 mpg. Taking the car off the dyno and experiencing it on the street was the clincher, if the car isn't making any power and won't pull itself why set the AFRs at 12.5:1?

 

I hit send once, get double posts. Vb is teh sux.

 

Thanks for that explanation Joseph Davis! ! !

 

I bet that is a big part of it along with the injectors.
I went from a b16 with a lw flywheel to a 89mm stroke and heavy fw. and Im pretty sure the aftermarket rods weigh in a bit more too.
those bosch injectors sound nice, my old 450's were tuned really well (on the street) but wouldnt support the power I hope to make.

also I beleive I might have a boost leak at the throttle body, one of my buddys who is into FC's said that boost leaks kill your mpg... is this true?

 

I think you have to give a lot of the blame to the temperatures as HiProfile said if you live in an area that gets cold. I can't believe how bad my MPGs have gotten this frigid winter with my intercooler ensuring that my IATs are often in the single digits. . . .

 

ya, didnt even think of that. its high 20's low 30's around here.

when I was railin the b16 it was 50s at worst.

 

Going from B16 to b18 make difference in gas mileage too. Low compression pistons make difference too. More throttle opening.

 

Yup, winter vs summer up here in Canada, I would see a difference of +/- 100 km each tank of gas. I get 19 mpg in deep cold winter, but I get 25 mpg in the summer of mixed driving (mostly city, and ripping it once or twice each trip).

Although I don't have precise logged data, when I dropped from 11.5:1 CR to 10.5:1 CR on my old GT2871R setup, everything was identical to the setup (same turbo, manifold, exhaust, cams, etc..). I lost about 10% in fuel economy, and the car was fine tuned, with AFR's only in the 11's under maximum power. I would guess that 10% drop in every 1.0pt drop in compression, in my case at least.