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By Tim Quinn - 5 Years Ago
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Right now I have a Summit 600 cfm on a 312 that is bored 0.90 w/stock cam, aluminum pistons three speed with Overdrive 3:89. Cruise at 70 mph at 2,000 rpm.
Gas mileage is maybe 18 mpg. The primary jets are .67. Would it help to put smaller jets in the primary and secondary for better gas mileage. Planning on heading to Detroit's Dream Cruise.
Is the 600 cfm to big for the engine.
What do ya think, any advice ?
Thanks in advance,
Tim
Hot 'n Humid
Florida 33706
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By charliemccraney - 5 Years Ago
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Smaller jets will lean it out, which may provide better economy as long as it is not too lean but also possibly at the expense of power.
600cfm is probably more than the engine needs but it is vacuum secondary so it should be fine. I don't think it is too big.
It's best to do this type of stuff with the help of an O2 sensor so you can see what is going on in real time. Not something that is good to guess about during a 1000+ mile trip.
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By Tim Quinn - 5 Years Ago
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Right now the primary jets are .67, I'm thinking of replacing them with .64 for the trip to Detroit and put the .67 jets in the secondary position.
Would those jets be too lean ?
Whatda ya think ?
Tim
90 degree Florida
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By charliemccraney - 5 Years Ago
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Impossible to answer on a forum. Bare minimum, look at the plugs and make a judgment call based on how it currently looks. I'm not good at reading plugs so I use an O2 sensor. No uncertainty with that and you can gather more info in a shorter time, speeding up tuning and also no handling hot plugs on a hot engine on a hot day at the side of the highway for an accurate read..
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By Tim Quinn - 5 Years Ago
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Thanks for your time and input.
Tim
Florida
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By Florida_Phil - 5 Years Ago
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My 292 is running a Holley 1848 465 on an iron 1957 intake manifold. I have a 4.11 gear and a three speed with O/D. I like the throttle response of the 465. I've thought about upgrading to a 600. From what I have read, I wouldn't be gaining that much. Like others have said, a vacuum secondary carb is only going to use what it can. I wouldn't change carbs myself.
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By Tedster - 5 Years Ago
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Wideband O2 sensors don't cost that much, the fuel savings alone on a Florida to Michigan trip and back would probably pay for itself, assuming there's anything left on the potential fuel economy table.
You can jet down a fair bit to improve cruise fuel economy, but keep in mind jet size also changes the air fuel ratio when under load, under power acceleration. Getting both of these right is critical if you're going to dig in to carb tuning. Typically involves enlarging power circuit fixed orifices with a pin vise. If too lean under acceleration it will probably burn a valve. As Charlie said a wideband O2 sensor is the best way for most people to tune a carburetor. Make sure the distributor ignition curve is optomized to include vacuum advance before getting too deep into the carb adjustments. If the ignition system is weak, leaner fuel mixtures will expose it. A cooler heat range plug may help too, especially on a long highway run.
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By FORD DEARBORN - 5 Years Ago
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18 mpg is going to be tough to beat, IMO. At least that's the most I can get after careful tuning and as mentioned above, too lean and your going to burn parts. If I can remember right, 16 to18 mpg were figures offered on this forum and those numbers are pretty much what a modern full size vehicle with a V8 will achieve at best also. Unless there are performance issues, your probably in good tune as is. Hope this helps, JEFF..............
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By Tedster - 5 Years Ago
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With the right gearing, a lighter car can do better. Maybe 20 to 24 mpg highway. Though not with corn gas.
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By 62bigwindow - 5 Years Ago
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Put in the jets you want to run then drive it. If you get a surge while at cruising speed then the jets are too small. A wideband is a nice diagnostic tool but not necessary. There is alot of info online on carb tuning. A vacuum gauge is a must in my opinion.
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By Tim Quinn - 5 Years Ago
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Thanks for your info
Tim
Saturday AM
Florida
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By Tim Quinn - 5 Years Ago
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Like your plan
Thanks,
Florida's Tim
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By Tim Quinn - 5 Years Ago
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With the higher cost of corn free gas, would it pay
to burn it for better mileage ?
Let "us" know,
Tim
8:05 AM Saturday
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By Tim Quinn - 5 Years Ago
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Thanks for your input.
Trying to find out if it pays to burn 'corn free' fuel.
Tim---- Saturday AM
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By Tim Quinn - 5 Years Ago
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Have electronic ignition, not Pertronic but full distributor that is electronic.
Almost ready to grin and bear it, but might see if ethnology free fuel is worthwhile .
What da ya think ?
Thnaks,
Tim
Saturday 11May19
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By Ted - 5 Years Ago
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As mentioned, ethanol in the gasoline will cause the fuel mileage to drop. This has to do with the BTU’s per pound for ethanol being less than that of gasoline. Said another way, it takes a higher volume of ethanol to equal the same energy output of gasoline. As the ethanol percentage goes up, the fuel mileage goes down. In my part of the country, the 91-93 octane gasoline has 3-5% ethanol versus the 10% in the 87-89 octane grades. For that reason, I use the premium fuel in all my carbureted vehicles.
The Summit carburetors are jetted reasonably close as delivered. Most of them need no jet changes and only a very small number of them get a two number change. Camming and head work usually dictates if the fuel mixture change is richer or leaner from what’s in the carb as delivered. There are cases where only the secondary jets are changed and the primary jets remain as delivered. Keep in mind that leaning the primary mixture can actually have the reverse effect on fuel mileage in that the engine may be requiring more fuel and as a result, you are having to drive deeper into the throttle to maintain the same speed. Without a wide band sensor, spark plug readings will let you know if you’re grossly rich or lean but may not be the best option for fine tuning.
You already have the 600 cfm model which is adequate for most warmed over Y’s. I lean heavily towards the 750 cfm versions as the vacuum secondary function simply insures that the engine only receives only the amount it requires. With the 750 version, most Y’s will never be under-carburetted. I have the 500 cfm Summit carb on my own 272 and it is on the small side but it is a good driver. There’s very few times that engine exceeds 4500 rpms so it’s not on the top of my list to get a larger carb for that particular engine.
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By Tim Quinn - 5 Years Ago
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Ted, thanks.
I'm going to leave the jets alone and try "corn" free gas and see if the mileage improves.
Again, thanks
Tim
Hot 'n Humid, FL
33606
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By Tim Quinn - 5 Years Ago
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Hey Tted,
I found this report about ETHANOL from THE MINNESOTA BIO-FUELS ASS.
So are our old Y-blocks going to be more efficient (better mileage) with 93 octane or will they just run better ?
Tim
Florida
'56 Crown
This is where ethanol comes in.In terms of its octane rating, ethanol has a rating of 113. As mentioned above, fuels with a higher octane rating reduce engine knocking and perform better. Also, almost all gasoline in the US contains 10 percent ethanol. When you mix 10 percent 113 octane ethanol with 85 octane gasoline it increases the octane two points to the normal 87 octane most consumers use. So the higher the ethanol content, the higher the octane. The octane rating for E15 (15% ethanol) is 88 octane and E85 (85% ethanol) is 108 octane.In addition, as Argonne National Laboratory states, ethanol reduces greenhouse gas emissions between 34 to 44 percent compared to gasoline.Moreover, since ethanol is cheaper than those synthetic aromatics, gasoline blended with ethanol reduces the price at the pump. As detailed in a study released earlier this year by the University of Illinois, ethanol is 35 cents to $1 cheaper than benzene, toluene and xylene.In other words, consumers not only get to save at the pump, they get to ensure their vehicles run smoothly while reducing harmful greenhouse gas emissions. Talk about a win-win-win solution. Moving forward, as car manufacturers increase the number of models equipped with high compression engines to maximize performance and efficiency, higher octane fuels will be required and ethanol is set to play a crucial rol
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By charliemccraney - 5 Years Ago
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Everything equal and where a typical pump gas octane works just fine, ethanol blended fuel is less efficient and produces less power. Where ethanol (E85) can be a benefit is in custom built performance vehicles that require higher octane than is typical from pump gas and the engine has been built to take advantage of it.
A problem with ethanol free is, every time I've seen it, it's more expensive so you might get better mpg but at a higher cost per g. You also have to plan the entire trip around the availability of ethanol free gas which can mean driving farther out of the way for gas and using more expensive gas to do so. So, is it actually more cost efficient?
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By Ted - 5 Years Ago
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Tim Quinn (5/12/2019)
Hey Ted, I found this report about ETHANOL from THE MINNESOTA BIO-FUELS ASS. So are our old Y-blocks going to be more efficient (better mileage) with 93 octane or will they just run better ? Tim Florida '56 Crown This is where ethanol comes in.In terms of its octane rating, ethanol has a rating of 113. As mentioned above, fuels with a higher octane rating reduce engine knocking and perform better. Also, almost all gasoline in the US contains 10 percent ethanol. When you mix 10 percent 113 octane ethanol with 85 octane gasoline it increases the octane two points to the normal 87 octane most consumers use. So the higher the ethanol content, the higher the octane. The octane rating for E15 (15% ethanol) is 88 octane and E85 (85% ethanol) is 108 octane.In addition, as Argonne National Laboratory states, ethanol reduces greenhouse gas emissions between 34 to 44 percent compared to gasoline.Moreover, since ethanol is cheaper than those synthetic aromatics, gasoline blended with ethanol reduces the price at the pump. As detailed in a study released earlier this year by the University of Illinois, ethanol is 35 cents to $1 cheaper than benzene, toluene and xylene.In other words, consumers not only get to save at the pump, they get to ensure their vehicles run smoothly while reducing harmful greenhouse gas emissions. Talk about a win-win-win solution. Moving forward, as car manufacturers increase the number of models equipped with high compression engines to maximize performance and efficiency, higher octane fuels will be required and ethanol is set to play a crucial rol
The Pro-Ethanol groups will tend to put their own spin on their marketing in promote ethanol use. What’s not mentioned very often when discussing ethanol laden fuels is the increase in engine wear that goes along with that increase in ethanol.
If an engine does not require a higher octane fuel to prevent detonation, then using an increased octane fuel will have no benefit in increasing the fuel mileage. If you have an engine that runs fine on 87 octane gasoline, then going with a higher octane fuel will have a zero effect on the fuel mileage assuming the fuels in question do have the same density. What does have a direct increase on the fuel mileage is the density of the fuel. The higher the density, then the smaller the jets in the carburetor must be in which to properly meter the appropriate amount of fuel to the engine. This goes back to the energy content of the fuel per pound or BTU’s. While I don’t check pump gasoline for density very often, I do check the race fuel density and when changing from one fuel to another that has a density change, the jet size can be accurately predicted based on that density change.
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By Tedster - 5 Years Ago
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That's interesting, I recall that the official BTU content of gasoline has been defined down, without respect to additives like MTBE or Ethanol. If you read older primary source documents from say, the 1950s or 1960s it seems to me it was listed as ~ 120,000 BTU per US gallon. Today, it's something like 111,000 BTU. What happened?
I thought higher octane fuels would return a bit more mileage if the ignition timing was advanced to take advantage of it. One thing I've never quite been able to understand, if the air fuel mixture is leaned out too far, efficiency or fuel mileage actually goes down.
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