Mileage calculations

[edward ericson]

Here, friends, is my tale of woe: I’ve been doing my best to figure Bridget’s fuel efficiency ever since the swap in. I was expecting to see something between 32 and maybe 38 mpg. 

So far? Just under 26.

That is actually a little better than the old engine, but obviously not what it should be. 

And the engine is running very well, so far as I can tell. 

This last test is especially distressing. I mean, yeah, I got the car on the track at Lime Rock. But only two laps! And never above 50 mph! And she never even got out of third gear. In fact I went to second just to hear the dang engine!

The ride up–68.5 miles–was very gentle. And yeah, maybe I played a little bit with a Triumph on the way back home. But not, like, in a crazy way. We never got over 80, I swear.

And then the next day I just went a few miles on I-95 with my mother in the passenger seat. Very, very gentle drive there.

On the VA next month I hereby pledge to drive for mileage, as I imagine David Stroud, the Subified Speedster owner from Ontario, does. But meanwhile–anyone have any thoughts?

[billnparts]

Your mileage of 26 is double what I got from my last tank. And that was an improvement over the tank before that. Yours is hardly a tale of woe.

Bill Ascheman
Fiberfab Ford
Modified 5.0, 5sp., 4:11
Autocross & Hillclimb
"Drive Happy"

[sreynolds]

For what it’s worth, I use to own a Sub Legacy station wagon and never got over 25 mpg at steady highway driving. Granted the MGTDr is lighter, but that was my experience.

Sam

[Paul Mossberg]

Ed,

What was the EPA rating for the car the engine came out of? Temper that number a bit, since real world never matches the EPA numbers.

And then there are the aero vs light weight factors. Yup, Bridget is probably a thousand pounds or more lighter than the donor car. But you also have those big air dams up front (the fenders) and a flat wind screen. I’ve no clue how to account for that. But it’s gotta have some level of effect.

Paul Mossberg
Former Owner of a 1981 Classic Roadsters Ltd. Duchess (VW)
2005 Intermeccanica Roadster

If you own a TDr and are not in the Registry, please go to https://tdreplica.com/forums/topic/mg-td-replica-registry/ and register (you need to copy and paste the link)

[john barry]

I get @ 25 mpg in my MGTDr around town with the 1776cc type 1. This has pretty much been my  mileage since I got my green machine  4 years ago ..I am   happy with that 

 now my ’06 Civic EX gets 41mph hwy, 35mpg in town .

jebarry2015-09-20 14:45:47

[KentT]

Speed costs! How fast do you want to go? 😉

26 mpg with your gearing is not really a surprise, IMO…

Early FF TDr on 69 VW pan
Slowly coming back from the ashes...

[edward ericson]

OK, guys!

I think we can improve though. The high-30s figure comes from David Stroud, a Speedster guy who installed a 2.2 Suby in his car and promptly drove to Florida with his wife in it. He ran stock 3.88 transaxle and a two-barrel progressive Weber carb. He reported high 30s mpg. I am going to ask him to confirm.

I also did some aero calculations on Bridget. It is truly a sad shape for slip-streaming but (and I don’t have the math and formulas in front of me now, but I can dig them up if there is interest) the frontal area of Bridget appears to be smaller-enough from the Legacy Wagon that, by my rough estimates, the wind resistance would not be all that much more, even with the windshield up. 

Remember too: my gear ratio is much better than Stroud’s. My final drive ratio is about 3.20 to 1. It’s not quite the overdrive range that the Suby had but it’s very chill in comparison to normal VW gearing.

Keep those cards and letters coming. I will update this thread as (or if) I get more data.

edsnova2015-09-20 21:20:02

[Bob]

My old Subaru Impreza with 2.5 Auto (NA not turbo) averaged around 26 MPG for the 10 years I had it.

This 1679(?)cc VW engine is getting only 22 to 24 MPG from 10% ethanol 87 octane, but once it calculated at 27 on 89 octane. That was while trying to eliminate pinging I was getting from ‘pedal to the metal’, and it seemed to do the trick.

Reset timing to less advance and never got rid of the pinging with 87 octane, MPG staying fairly consistent at 23.5 plus or minus <1.

Just now trying 89 octane again to confirm increase or not.

I drive mainly highway and freeway getting to places, a fraction of the time on lower speed roads or in-town traffic.

At first I had trouble from one of the old drum brakes dragging but that’s been fixed, was before I began following the MPG. I think my car is not at it’s best yet.

Hopefully you’ll be able to track down all the possible causes if there is a problem. For example, right now I need to recheck compression, had variance of 90 to 110 PSI on initial check, and switch to ordinary spark plugs (VW parts dealer sold me a special type). Seems to run rough, uneven, something not quite right anyhow.

[Royal]

LRH.  If your compressions are 90 to 110#, and assuming your timing is set at 30 degrees BTDC at 3000 rpm, you should not ping with 87 octane unless you have significant carbon buildup in the combustion chamber or perhaps the wrong spark plug.  90 to 110 is about a 20% variance which is pointing towards the need for an engine rebuild so maybe carbon buildup is what is causing the pinging.  It’s true that a “colder” spark plug may reduce pinging. 

Our VW engines should not require more than 87 octane fuel unless someone has raised the compression ratio during a rebuild. 

You may want to fiddle around with your carb adjustment.  Too lean will increase temperatures in the cylinder and therefore increase the likelihood of pre-ignition.   

I burn 89 octane only because that is the only octane of pure (non-ethanol) gas that I can get.  My mileage is pretty much rock solid at 25mpg.  (1600cc, ported polished, 009 dist, 3487 exhaust, stock cam, NGK BP5HS plugs, Holly Bugspray 2bbl carb, 7.7/1 static [mathematical] compression ratio.  All compression pressures within 5# of each other.)

Keep us posted. 

And Ed, I don’t think you have a problem other than insatiable curiosity and a screwdriver in your hand.  Tune for best running and then see what your mileage is, rather than tuning for best mileage and hoping for best running. 

Royal2015-09-21 08:11:44

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