Moved to "Hudson" Category.

Michael, do you have an exhaust leak at the heat riser- yes or no ? If the heat riser is still there, it is probably worn out. Even though the throttle plates are closed, fuel does still flow thru the carb., causing a rich exhaust. If there is a leak at the heat riser, it allows fresh, unused, oxygen rich air into the exhaust, causes it to fire again. This is the same as having an air pump like cars had thru the 70's. What is your MPG now?. remember, the Laughlin Economy run is coming up , and if your in that region, there is a lot to learn from all those guys. I went on that trip with Mike Cherry and Matt Royer a few years ago. Not only was it a great time, BUT was also very interesting to see some of the stuff the participants did. Warren Bactel, among many, had a vacuum gauge taped to the windshield, so he could watch that throttle plate action!! Over inflated tires,Ign..timing and a few tricks on refueling a Stepdown Hudson gas tank, ........ ask PRESS KALE . It was a very fun trip!

Hi Doug - thanks for the reply. The last two tanks of gas (87 octane unleaded regular) gave 13.8 and 14.3 MPG, which i think is quite good, considering that I was getting 10 - 11 some time ago before i started tinkering with the timing (base set 4 deg BTDC) and vacuum advance. I still have my heat riser and occasionally spray in some AC Delco Heat Riser Lubricant and Super-Penetrant to keep the valve moving freely. When the engine is running I can feel a slight exhaust leak coming past the shaft, so it's possible that air could get in there if the pressure in the exhaust system is lower than atmospheric (is that ever possible?). I guess that means that the shaft and/or heat riser body may be worn; I'm not sure which one wears first. Nevertheless, if the rumbling/popping sound is coming from secondary combustion downstream from the exhaust valve then, while mildly annoying, it should have no impact on fuel economy, so that's good. I was just wondering if it's a sign of an overly rich fuel mixture under that specific driving condition. Sounds like not, based on your post.

As for the Laughlin Economy Run, I don't know anything about it. I'll have to read up on that via Google. As for a vacuum gauge, I have a spare that I could hook in without too much trouble, but suffice to say that I drive as if I have an egg between my foot and the accelerator, so I should be getting good fuel economy. Some people claim 17-18, but I have to admit, I'd be happy w/ 15.

Thanks again.

'51 was a transitional year, so there are still a few questions. Is your Hrdtp. an auto or stick?. If it is an auto trans., the '51's had a 3:58 rear axle ratio. This made them a dragging machine, but probably didn't help fuel mileage much! If it is stick, does it have factory overdrive?. factory overdrive had a 4 5/9th's axle ratio, with optional 4 1/9th.. If it is a straight 3 spd. ,no O/D, it has a 4 1/9th. The auto trans needs to have the 3:07 Dana ratio, which was stock with a dual range. The dual range obviously had another gear, but final output ratio was the same. We have been doing this upgrade conversion for some time now, and have always liked the results. Most of us aren't drag racing the cars, so the 3:07 axle is a great fix, and increases fuel economy. I also use premium fuel in most of the cars. I can tell the total difference in the way they run and perform!.

Michael-

Do you have Twin-H on your car? If so, best you could possibly hope for is 15-16 on the hwy with a 308 and Twin-H. And that's all things being perfect. Anyone that tells you they get more than that is either lying, selling something, or skewing their results. To get any more than that, you have to do what Doug was mentioning, blowing up the tires, driving 40mph, doing some timing tricks and coasting downhills. 262 and a single carb, 17-18 is more typical. Most guys I have come across get 11-12 in their Hornets and are generally disappointed. I've known people that got out of the club because of it.

These beasts were always thirsty. One of the "cons" that's mentioned in any review I ever saw was that the cars were lushes (drank too much). Just the nature of the beast.

Hi Doug and Russ - thanks for the additional information. So, the configuration I have is a 308 six with single Carter WGD 2-bbl carburetor and Hydramatic transmission. I usually keep the tires at 30 ish PSI.

I notice, according to this link http://www.automobile-catalog.com/auta_perf1.php that fuel economy isn't predicted to be great. These guys estimate 11.6, so my 14.3 is sounding better and better.

On the other hand, in the Dec 1951 issue of Popular Mechanics, author Floyd Clymer reported getting 16.1, 17.2, 16,7 and 19.8 MPG traveling from Detroit to Chicago, Chicago to Denver, Denver to Salt Lake City and Salt Lake City to Los Angeles (mostly downhill, right?), respectively. That's more like what i was trying to achieve.

I'm going to keep plugging away at this.

Here's the URL to the PM article: [ https://books.google.com/books?id=F9kDAAAAMBAJ&pg=PA114&lpg=PA114&dq=fuel+economy+hudson+hornet+hydramatic&source=bl&ots=ZVEKLKeeLP&sig=q5iCgc3dHU4OzeVo_YsP4b7TIfA&hl=en&sa=X&ved=0ahUKEwiR37iCkY3OAhXG4yYKHa81CucQ6AEIQDAF#v=onepage&q=fuel%20economy%20hudson%20hornet%20hydramatic&f=false ]

They didn't have ethanol in the gas then. Ethanol makes fuel mileage worse.

Hi Ray - really good point; from what I've been reading, the reduction in MPG could be as high as 6-7%, so, leaving out the outlier trip of SLC to LA and taking the average of the other three trips Detroit-Chicago-Denver-Salt lake City I get 16.7 MPG. Reducing that by 7% leaves 15.5. Seems like I'm getting closer. B)

You can go on the internet and they have listings of ethanol free gas stations for each state.

REALLY good point Ray made.

[quote="Michael Cohen" post=18021]Hi Ray - really good point; from what I've been reading, the reduction in MPG could be as high as 6-7%, so, leaving out the outlier trip of SLC to LA and taking the average of the other three trips Detroit-Chicago-Denver-Salt lake City I get 16.7 MPG. Reducing that by 7% leaves 15.5. Seems like I'm getting closer. B)

I think you're thinking is good on this.

Well, Russ, I'll have to pipe in and disagree about potential mpg. On my way to/from Chattanooga this year, over 1300 miles, mixed flat and mountain driving, 65-73 mph, A/C running whole time, I got a very consistent 17 mpg highway as determined by filling tank and calculating miles between gas stations. I'm not selling anything and my log doesn't lie. I did not calculate city mpg while in Chattanooga. Measured every tankful during the long legs so I could ensure no variability in determining when tank is full. Odometer compared well to GPS. Speedo actually runs a tad slow so error, if any, would be in direction of under-estimating mpg.

53 HH sedan, fresh 308 bored to 318, stock iron head, TwinH carbs prepped by Walt Mordenti (he set jets per my set-up), Pertronix 12v ignition, time 4 deg advanced, Clifford headers with dual exhaust, DR Hydro. I credit the carbs and ignition for the result, which I assumed, up until now, was typical of Hudsons.

Michael,

I think the sensation you get when slowing down is normal. At idling,there is a rather strong vacuum in the intake manifold, and the right amount of gasoline is being sucked in for the car to run at idle speed. When slowing down from high speed, the engine is turning much faster than idling speed, but the amount of gas being sucked in is only slightly more than it is at idling. Therefore the mixture is too lean, and tends to misfire. the products of the misfiring (unburned gasoline) can collect in the manifold and occasionally burn, as Doug said.

It is possible to get extreme results under carefully controlled conditions. My first car was a 1951 Hornet with hydramatic. I was lucky to get 11 miles per gallon in local driving in downtown New York City. One time I made a test when I drove from NY City to Philadelphia on the completely level New Jersey Turnpike. I filled up at a gas station near an entrance to the turnpike. I filled up again at a gas station near where I exited the turnpike. I did this both directions. In one direction, I drove between 40 and 45 mph. In the other direction I drove between 60 and 65 mph. I did not stop, and I kept the speed as constant as possible. The car was already warmed up each direction. The turnpike is about at sea level. At the slower speed, I got 20 mpg. At the faster speed, I got 14 mpg. Note that both of these were more than I usually got in daily driving. I felt at the time that the motor was turning unnecessarily fast when going 60 to 65. Doug's point that the next year Hudson slowed the engine a lot is valid. I later had a 1952 with hydramatic. I felt that it was perfectly geared.

The good gearing at highway speed can be achieved just by changing the rear end from 3.56 to 3.07. The additional benefit of the "dual range" hydramatic introduced in 1952, is that you can keep the car from shifting all the way to 4th gear. This is very convenient if you do a lot of slow local driving. With the 1951 transmission, the car shifts into 4th at about 25 mph, and the moment you apply a little gas, it shifts back to 3rd. I found this very annoying driving in the city. With the 1952, I was able to shift it to the local drive position, so it would shift normally from 1st through 3rd, and stay in 3rd. It would only go to 4th if I put it in the highway drive position.

Per

Hi Dave and Per - thanks for the great information. First, one of the common themes for the high fuel economy from both of your posts is the rear end ratio. Mine is a stock '51 Hydramatic (not dual range) so I assume it has the 3.56 ratio and yes it seems as if the engine is turning a bit fast at highway speeds. Since i'm not really into jackrabbit starts, I might swap my rear end out for a 3.07. I assume that a different gear for the speedo is also available?

Going off of Dave's comment about the Petronix ignition, I'm also curious to see how a rebuilt distributor might impact things; the one in the car is OK, with the base timing set at 4 BTDC and the dwell right at 38, but I have no way to verify the accuracy of either the mechanical or vacuum advance curves myself. I just received back a spare distributor that I sent to a gentleman in Phoenix who has one of those Sun testers. On the distributor that he just sent back to me he said the centrifugal advance was working fine, but the vacuum advance was way off, and now set to factory specs by swapping out the spring and spacer(s). I'm going to put that one in the car and send him the one that's in the car now to have it checked out/adjusted. Since I just filled up, I'll drive on that disttributor for this tank of gas.

A for the explanation for the popping on a steep downward grade, that sounds right to me. My only question is now whether or not others out there also observe this. So far I've heard some very good theoretical explanations, but no one saying "Oh yeah, my car does that all the time". Just sayin'

Thanks to all for such information-rich posts.