Bought a swapped 99 318ti, tons of questions

[rockthemullet]

Alright, so I picked up a 99 318ti M-tech with somewhat of a bastard engine in it.

M50b25 block (e36 325)
M52b28 crank/pistons (e46 328)
S52 head (including cams)
1996 OBD1 intake manifold
325 transmission
325 manifolds, midsection and a hacked catback, but it looks like an OEM BMW muffler

Car has been converted to OBD1 and is running an "Auto Authority" tune on a 413 red label DME

I'm getting pretty bad gas mileage... 23 if I baby the crap out of it. I honestly expected more like 30. Is it the tune? Can I get it dyno tuned on the Auto Authority chip or do I need to go new? Are there better mail order options for an OBD1 setup like I have?

Also, it seems like I have good torque but the top end is very lacking. I assume the four catalytic converters don't help. Aside from rebuilding the midsection, what else can I do to find some more power in the higher rpms?

Finally, the open diff in the rear simply will not due. I've looked around a bit but haven't found a definitive answer to what I can and cannot install in it's place.

[3.2TI]

Is this a rebuilt title or is it registered as an OBD2?

[rockthemullet]

Originally Posted by 3.2TI

Is this a rebuilt title or is it registered as an OBD2?

I'm sure it's registered as stock.

Not sure how that's relevant to any of the questions I have though.

[3.2TI]

For starters if you have OBD1 manifold you should expect more breathing which in essence generates more horse power, more horse power is more gas wasting. If your car is registered as stock or not is not relevant, I only ask because in your description you state obd1 manifold and in the bottom you claim obd1 conversion, they are 2 different things.

[rockthemullet]

Originally Posted by 3.2TI

For starters if you have OBD1 manifold you should expect more breathing which in essence generates more horse power, more horse power is more gas wasting. If your car is registered as stock or not is not relevant, I only ask because in your description you state obd1 manifold and in the bottom you claim obd1 conversion, they are 2 different things.

Surely, the OBD1 M50 manifold was not stock on a 99 318TI. When I mentioned the OBD1 conversion, I was talking in terms of wiring and DME. More horsepower = more gas wasting... to the tune of 23mpg? I think not. If it had a v8 and weighed 3500lbs maybe

[3.2TI]

I can't vow for the tune you have but still confused of your goal. You are not happy with your MPG but want more HP, this you need to figure out on your own. As far as more power on top end, do you have headers , high performance cats, and cams? This would surely increase your HP and MPG and TI

[rockthemullet]

Originally Posted by 3.2TI

I can't vow for the tune you have but still confused of your goal. You are not happy with your MPG but want more HP, this you need to figure out on your own. As far as more power on top end, do you have headers , high performance cats, and cams? This would surely increase your HP and MPG and TI

The car gets low 20s for mileage while I have other cars that get the same with near 500hp. If anything, with the increase in breathing efficiency, mileage should not decrease with a better exhaust or subtle cams. The car should be in the 30mpg range and I should still be able to modify it for more power and maintain that rating. Or are these engines THAT inefficient...

[3.2TI]

A good thing to test is how clogged up your cats are, these get old and become a carbon box with little filtering. A new cat back can improve MPG at about 80%.

[cjm1219]

Originally Posted by rockthemullet

The car gets low 20s for mileage while I have other cars that get the same with near 500hp. If anything, with the increase in breathing efficiency, mileage should not decrease with a better exhaust or subtle cams. The car should be in the 30mpg range and I should still be able to modify it for more power and maintain that rating. Or are these engines THAT inefficient...

I can get 31 mpg if I baby it and get 28 driving the hell out of it and this is on 138hp 4 cyl.

Not sure what a swapped TI should get but can't imagine it would could do better than 30 mpg in the best case scenario.

[islandtime]

HP has absolutely nothing to do with mileage. My last turbo Miata race car when driven on the road got better mileage than my stock miata street car and the race car had a fully builty 405whp motor. The motor when tuned properly will only use the amount of fuel necessary for the amount of air coming in which is determined by the throttle body, not the horsepower. It does sound like you're getting poor mileage because you're getting less mileage than my friends 328. As for the V8 comment, you'd probably get close to 30mpg with an LSx motor in there and still have 400+ at the wheels.

[M-technik-3]

So it has a Getrag 250 gearbox which is the same as the stock Ti, any idea if the differential ratio. Or better yet at 60 mph how many rpms is the engine turning?

I get 24 mpg average with my M52 OBD I conversion with the same gearbox as well but I have boat anchor 18"s on the car. When I swapped to my lighter K1 in 16's with a smaller width tire, it gained 4 mpg back.

328i has a 2.93 ratio in the back vs my 325/8 which has a 3.15. Maybe he has a different differential. Who built the car, previous owner here?

[rockthemullet]

Originally Posted by M-technik-3

So it has a Getrag 250 gearbox which is the same as the stock Ti, any idea if the differential ratio. Or better yet at 60 mph how many rpms is the engine turning?

I get 24 mpg average with my M52 OBD I conversion with the same gearbox as well but I have boat anchor 18"s on the car. When I swapped to my lighter K1 in 16's with a smaller width tire, it gained 4 mpg back.

328i has a 2.93 ratio in the back vs my 325/8 which has a 3.15. Maybe he has a different differential. Who built the car, previous owner here?

I have M6 rep 18s on the car, they didn't seem extremely heavy though. I know at 75mph, I'm around 3500rpm... I can check on the 60mph tomorrow. I'd love to find an LSD rear with a lower rear gear to take that down to around 2500rpm if possible.

Blubimma (if he's even on here, I know he's on other forums) assisted in the first swap on the car but I doubt he would know everything about it. I don't think the PO is on here, it was unmodded when he bought it though.

What kind of tuning are you running? Perhaps I should be looking for some 15s with snows at least for the next few months.

[BimmerBum]

Originally Posted by rockthemullet

Alright, so I picked up a 99 318ti M-tech with somewhat of a bastard engine in it.

M50b25 block (e36 325)
M52b28 crank/pistons (e46 328)
S52 head (including cams)
1996 OBD1 intake manifold
325 transmission
325 manifolds, midsection and a hacked catback, but it looks like an OEM BMW muffler

Car has been converted to OBD1 and is running an "Auto Authority" tune on a 413 red label DME

I'm getting pretty bad gas mileage... 23 if I baby the crap out of it. I honestly expected more like 30. Is it the tune? Can I get it dyno tuned on the Auto Authority chip or do I need to go new? Are there better mail order options for an OBD1 setup like I have?

Also, it seems like I have good torque but the top end is very lacking. I assume the four catalytic converters don't help. Aside from rebuilding the midsection, what else can I do to find some more power in the higher rpms?

Finally, the open diff in the rear simply will not due. I've looked around a bit but haven't found a definitive answer to what I can and cannot install in it's place.

Don't expect 30 mpg. The only way I can crack 30mpg in my 2.8 Coupe (not a ti but so close it is relevant) is if I drive 60-65 on the highway... I average around 24 mpg with mixed driving and can get up to 28-29 strictly highway. The car has a 3:15 diff.

What gear ratio do you have?

When you say you baby the car what does that mean? If you are babying it in town 23 mpg might be spot on, if you are driving 55 on the highway with the AC off 23 mpg might be a little low.

[rockthemullet]

Starting in second and shifting under 2k most of the time, 70 or so on the freeway.

I guess I'm a bit disappointed... I expected a bit more out of this combo. I guess I'm used to seeing low 20s out of cars with a lot more power, here's what's in the garage:

99 Cobra coupe - vortech supercharger on 10psi - 502/433 - 22mpg beating on it
99 Cobra convertible - eaton supercharger on 8psi - 456/402 - 23mpg beating on it
02 Regal - holset turbocharged on 10psi - 412/423 - 20mpg on E85

[rockthemullet]

60mph is about 2750-2800