Engine and Transmission Tech

Hey guys , I have a 3.5 rover low compression with a Hotwire fuel injection system throwing no codes and in good tune. I have an lt77 trans with .78 5th gear if I remember correctly and 3.27:1 rear end gearing. What kind of mileage are you guys getting? Mine is pretty bad and I can't figure it out, some people tell me they get 30 or so mpg with similar setups, so people are exaggerating or something is amiss with mine. My o2 sensors are new and I do have the correct 3900ohm tune resistor. Had the Hotwire meter at 1.6 volts and just turned it down to about 1.2v

What's the engine in Randy?
That setup in a sd1 only got 25-27 mpg per imperial gallon (advertized)
So 20 or so mpg per US gallon in something like an mgb would be realistic to me.
Cheers
Fred

I get 25-26 USmpg on the Interstate with a .63 5th & a stock 3.909 R&P ratio. Eingine is '63 Buick 215 factory 200hp.

That's an interesting way to think of it, Fred. However, 20mpg sounds unambitious to me so I quickly pulled a couple numbers. Not enough to draw a conclusion, but maybe enough to aid the discussion.

The conversion factor for gallons is 0.83267418, so 25 imperial = 20.8 U.S. and 27 imperial = 22.5 U.S.

I expect MGB-GT probably has a significant aerodynamic advantage due to frontal area alone but that's harder to pin down. According to [www.mayfco.com], MGB-GT's coefficient of drag = 0.40 and frontal area = 17.30 square feet. According to [www.britishv8.org], SD1's coefficient of drag = 0.39. Frontal area isn't given but width is quoted as 5' 9". Now, my MG is less than 5' wide at its belt line, so I expect it's fair to guess that MGB frontal area must be at least 13% less than SD1. (Surely MGB-GT can't be taller, right?) My understanding of aerodynamics is poor. If an MGB is roughly 10 percent more slippery, does that mean it'll be ~10 percent more efficient at high speed, everything else being equal?

Tires might or might not be equal. "High MPG" tires are offered these days...

MGB-GT has a weight advantage, which surely matters in town (accelerating) if not at steady state (highway): ~2300# vs ~3200#.

Oh... and I read a little deeper into that Rover article (mentioned above). Here's its paragraph on efficiency:

Quote:

But gearing and aerodynamics mustn't be allowed to overshadow the engine's own contribution to uncanny economy. Leyland claim 24-26 m.p.g., touring and tell me the average over the duration of the Press test days was 27-28 m.p.g., with a worst of under 22 m.p.g. and a best of no less than 32 m.p.g. On one non-stop motorway/dual carriageway journey (abroad, of course, Your Honour) my road-test car averaged over 80 m.p.h., in spite of roadworks delays, to the tune of an incredible 25.6 m.p.g. On the other hand, this commodious 26.9 cwt 3.5-litre saloon also proved much cheaper for commuting into the City of London than my TR6, with which regular transport I must inevitably compare it. While the 2.5-litre two-seater, and slower sports-car averages less than 18 m.p.g. for this journey, this Rover recorded 21.3 m.p.g. when gearing and aerodynamics would be of little benefit. And to think I praised the petrol-miserly Porsche 911 2.7 for recording 21.58 m.p.g. over that journey, albeit on 2-star. The 9.35 to 1 compression ratio Rover needs 4-star in its 14.8 gallon tank.

Quote:

I get 25-26 USmpg on the Interstate with a .63 5th & a stock 3.909 R&P ratio. Eingine is '63 Buick 215 factory 200hp.

I also get 25-26 US mpg on the interstate with a 0.73 5th and a 3.25 R&P. Engine is a Ford 302.

My last in-town mpg was a measly 17, but that was almost all city stop and go, wait at red light, pull in and out of parking lots driving, plus a spirited run up and down the Cherohala Skyway (not very fuel efficient).

Okay, I only got 9 mpg when we spent the day here:

[www.midamericamotorplex.com]

If I can stay out of the boost, I have seen as high as just over 30 mpg. This was under ideal conditions though such filling the tank in the morning with cold fuel and driving all day in fairly flat terrain.
Normally the highway mileage is 25-27 mpg and 21-22 mpg in town. I'm pretty happy with that, because like most guys I didn't build it for maximum fuel economy!:-) :-)

Bill

I tend not to be able to operate well.
Within the high mile per gallon envelope.
Case in point is the Lovely Lynne's Range Rover.
She can run it day in day out all year long at a 25.5 mpg average. (according to the trip computer)
I get in the thing and do the same trip and I can't crack 17!
To my mind "average economy" is the best measurement.
Even if the "economy" part is misleading.
I don't think that the one time, one way, down hill, with the wind at your back, drafting a semi, miracle mileage is a fair representation.
So I'm still gonna stick to my 20 mpg US average estimate. (24 mpg imperial)
Remember that the SD1 was a pretty aerodynamic vehicle for its time at 0.39.
The B with it's flat front and open cockpit is not as good at drag coefficients.
Probably closer to 0.46 with the top down and a roll bar in place.
Although it is a bit lighter so you gain some around town.
So I might be wrong, but I'm a "show me" guy so you would have to prove it.
And then there's the problem of getting rid of the body.....if you do.
Cheers
Fred



Edited 1 time(s). Last edit at 10/26/2013 01:03AM by DiDueColpi.

Well Fred, all you really need is a deserted lonely road across a very deep ravine where nobody ever goes and a deep dark night... oh wait, you were talking about the car body? Sorry, my mistake, I got off on the wrong track.

I've *never* gotten more than 20 mpg with an MGB. Maybe it's just me. But there was one memorable trip across the heartland back in the 70's with my favorite first car, a 350 Olds Cutlass Supreme where for two tankfuls I got an astonishing 25 mpg, God's honest truth. Never duplicated and never explained. One of those Twilight Zone kind of things. But I have to believe that if a School bus or a big class A motorhome can get 8 mpg while the driver literally stands on the throttle most of the time, that 20+ should not be out of reach for an MGB. Heck, most of the cars of that era were capable of between 18 and 20. So is my 5000 lb crew cab full sized pick up truck. All you have to do is breathe on the gas pedal for a V8 MGB to hold 70 mph and there is no way I'm going to believe that takes as much power as the truck does to hold the same speed. Something about this just doesn't line up right.

Jim

With the stock engine, I got 25-26 mpg, the same as I get with the V8 - hey, what's not to love? 300hp vs 95, and the same fuel efficency.

If I'm dreaming, don't pinch me, let me sleep.

My 4 cyl. never got the fuel mileage the 5.0L Ford does. The 3.55 R/E and .63 OD helps. Similar to Dan's post, Hard not to love it!

Hello my 347 gets about 3 miles per gallon in the city is soon as you get into the main jets car carburetor it eats fuel

out on the highway my T5 trans has a taller fifth gear than stock with my 408 rear end I am at 2900 RPMs at 90 mph as long as I stay between 80 and 90 the car runs completely on the Idol circuit front and rear and it gets about 25 to 27 miles per gallon on the highway

for some reason at the dragstrip with the nitrous on it takes a little over a half a gallon gas to go a eighth mile and that's expensive gas 11 dollars a gallon worth every penny

My car is an Mgb gt and for the life of me I can't figure it out, I'm getting 15-16 mpg combined and I can't see anything readily wrong with my injection. I have messed with timing and everything else I can think of. Contemplating a 3.08 ratio. The tires are 195/50/15. I have put a cleaned set of injectors in, ford 19# injectors but this is a proven swap and can't imagine that adversely affected mileage....

Best Texas drawl..."You felllers can brag, all you want, butt I wounce new a Spaniard got 3,000 miiles to a Galleon". Add a lil' tolulene and neve look back. 2 cents, roverman.

I drive a Mini, a 1960 Austin 7 Mini. When I am ask, what kinda milage do you get with that little thing? I almost always lie and tell them 65 or 70 MPG. The real mpg is not easy to figure since the front wheels always rotate a lot more than the rear ones. The odo is driven from the transmission, the rear wheels just follow the front. I guess I could add a odo to the rear wheel or wheels with a comparitor and then do all the math and calculations to be more accurate, then maybe not. I just keep them spinning and that keeps me grinning. That way those behind me are always wondering whats in that little car? All I have done is made a slight adjustment to the engine and went from 35 HP to 140HP, honest.

Charles, you're a da#%*& liar!!
;-)

Jim

Jim, thanks for the complement. Coming from a person such as youself I consider it a plus to my status rating. I am working on over estimating the facts.

Looks like you underestimated you slight engine adjustment facts. ;)

Well, Carl it is still crossways the car and still a 4 banger. It does have more valves, however all the valves don't work all the time. The only time I can get all my valves working is to stay above 4800 rpm, which is not hard to do, since the gas pedal has a very short throw.
Can't help but wonder what Jim was referring to, maybe the MPG. Well I did exaggerate a little about that, I think it more like 120 MPT. That is miles per tank.

Smiles, did you say?

Jim