Admittedly the 20b is not the motor for you if you want a fuel efficient vehicle but I would like to get an idea what kind of mileage could be reasonably accomplished. This more for me to determine how far I could get away from the house before I run into the point of no return fuel wise. I was considering an aggressive street port with semi peripheral ports. I know another person with full peripheral ports is getting around 8mpg. I know it's a compromise to either power potential or fuel economy. I'm hoping with the right information I can make a good decision about what port to run.

 

According to my sources, the 20B Cosmo gets ~14mpg in the 10-15 Mode test which is roughly half city/half highway driving if my understanding is correct. Assuming it is a Bone-stock 20B-REW in let's say a FC, I'd imagine it would get maybe 1-2mpg more thanks to having a manual transmission. Also, this is highly dependent on how it is tuned. Obviously running richer means more fuel usage. I'll see what I get once my Project OldTree is back up & running after re-sealing the oil pan & turbos.

 

mileage really really depends on your right foot, and the tune.

with my FC 20B, i got 19.9mpg on the 1000 mile sevenstock trip, mixed city highway was 14-16, and if you step on the gas it goes single digits.

my friend has a B2600 with a stock ECU 20B in it, and the stock ECU goes into open loop @3200ish, so on the freeway its in open loop getting like 12mpg (the sevenstock trip is like 5 gas stops each way).

if you wanted mileage, you'd want to use the 3 rotor's extra low RPM torque, and run a 3.9 diff with maybe bigger wheels, the FC runs ~2800rpm on the freeway, and the 20B would do better @like 2300-2500rpm

since the 20B is 50% more engine, the tune is more important, and any spots where its richer than it needs to be will basically have you at 8-12mpg

 

Yep! This is the reason I want a try to drop the highway cruising rpms. I'm gonna be running 26" diameter in the back so my rpms at 75 mph with the m12 version of the T56 will be around 2,300 rpms. Right now I still have the stock tranny and 4.10 rear. Next weekend I will be driving 350 miles to Dallas for my shake down run before my long journey to Vegas and Sevenstock. I have a very economical tune in the low load ranges and no overlap with my ports. So I hope to see some decent economy numbers.

OP, if you introduce a ton of overlap, you can kiss any notion of fuel economy out the window. At low rpms, you will simply loose to much unburned fuel out the exhaust.

 

I think the most appropriate measurement would be gallons per mile

 

Wow way less mpg than I assumed the 20b got.

 

It all depends on how you build it. Most people port the engine for power so economy goes out the window. When your building the engine to make 500+ rwhp, fuel economy isn't something most are considering. On the stock Cosmo, don't forget it's twin turbo charged. The primary turbo spools really early in the power band to give the larger Cosmo more bottom end torque. Anytime your spooling a turbo to make torque, your using more fuel hence the 14mpg mixed driving. Hell I could never get over 14mpg mixed in the stock Fd (and I drove it like a granny shifting at 3k around town) because the primary turbo spools so easily. On the highway I could get 25-26mpg easy. And un-ported single turbo 20b can get decent fuel economy if it's tuned well and driven normally. Bottom line is the engine has 6 fuel injection and 6 ignition events every two rotations of the e-shaft. So it injects fuel like a 6 cylinder 4 stroke piston engine. I think it's possible to get mid 20's highway fuel economy with proper gearing. Direct injection could raise that higher.

 

What I noticed on the highway drives I've taken with Project OldTree is that 70mph in 5th gear with a 3.909 diff (4.10 speedo gear) and FD Wheels (225/50/R16 tires) is that the tach is around 3200rpm. For my purposes, this is unacceptably high and just isn't suitable for the extra grunt of the 20B-REW. As you said, 70mph should be around 2300rpm, so a lower ratio for the highest gear is necessary. What I'm considering for my car is a Camaro/GTO T-56 with a Dodge Viper 6th gear (0.50:1)

T-von, what are the details of your intended T-56 conversion? I've got some ideas that you might find interesting, so I was thinking we could put our minds together and hammer out some solutions to all the issues that would come up, such as getting the FC's speedometer to read correctly. IIRC, there are some kits from Jegs for Camaros that give it a 4.10 diff ratio, so that may be a starting point.

 

2300rpm is really low, it might be a little soggy coming over mountains, but i guess you can always shift...

um so mine was non sequential stock turbos, so i tuned it like the factory, it was running 14:7 up until about 0 psi, and mileage was fine, the non sequential is so laggy that staying out of boost was really easy.

on an NA its a little harder, because you don't have the boost/no boost dividing line.

sounds like you're in good shape though

 

West Texas is flat flat flat so no worries there! Lol! Plus my primary runners are longer than the stock 20b uim/lim so I should have a little more bottom end torque to help out down there. We'll see! I really need to find a junk t56 to test fit since my engine is moved back.

 

Wow, learned a bit from this thread. Always wanted to go 20b and like you guys said - it wouldn't be to get better gas mileage

 

I totally missed this comment. For me I need to actually see if the tranny will fit 1st with my engine moved back. Then I will have to relocate the shifter forward. Fitting this tranny is gonna be one of my goals before DGRR and the Fd testing event in 2014.

 

i could see the needle move towards empty under WOT lol... but on the highway it wasnt so bad crusing on 6th gear(t56) that is..

 

What was your highway mileage? I'm still trying to get my engine running after the complete teardown of the entire car these past 2 months so I didn't make my Dallas trip. I keep running into all these little issues. He'll if I don't get things sorted soon, the car will miss Sevenstock.

 

i'm not sure what my MPG was but if i had to take a guess i would say about 200-220 miles to the tank.

 

in the FC with stock TII transmission I always got about 17mpg at 80mph on the highway, 14 or so mixed cruising, and ~6-8 in city or on spirited 2 lane driving. I am hoping for another 1 or 2 mpg in the FD with my 6-speed setup and more efficient intake/exhaust.

 

Hmmm these 20b mpg numbers are all over the place.

 

Which is understandable because everyone has a different setup, different driving characteristics, and different tunes. It's not like they're all on stock twins and stock ecu, so the fuel economy will largely be in the hands of the owner and what setup they decide to go with