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Very much a debate for me of which way to go. I want thick cylinder walls, not barely. also if both are made efficient does anyone know how much gas mileage is effected, I am leaning towards a 460 with AFR heads, with the cam I got planned it would have lots of power and smooth idle. Does anyone have any info how much cubic inch effects gas usage. I say this because the production LT1 motor gets as good or close as the coyote motor yet cubic inch is different. 307 vs. 376. Then we have other issues like bore and stroke, piston speed and how this all affects efficiency. I also believe that a good quench although may not effect WOT power as much as some have found , still has a lot to do with efficiency of a motor especially at part throttle as well as a big help towards pre ignition resistance. There are many things to consider besides peak power at WOT. Some here have experience, would love to hear from.
 

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Very much a debate for me of which way to go. I want thick cylinder walls, not barely. also if both are made efficient does anyone know how much gas mileage is effected, I am leaning towards a 460 with AFR heads, with the cam I got planned it would have lots of power and smooth idle. Does anyone have any info how much cubic inch effects gas usage. I say this because the production LT1 motor gets as good or close as the coyote motor yet cubic inch is different. 307 vs. 376. Then we have other issues like bore and stroke, piston speed and how this all affects efficiency. I also believe that a good quench although may not effect WOT power as much as some have found , still has a lot to do with efficiency of a motor especially at part throttle as well as a big help towards pre ignition resistance. There are many things to consider besides peak power at WOT. Some here have experience, would love to hear from.
1. I don't see how thick or thin cylinder walls are going to affect fuel mileage and I have no idea how one or the other could be made "efficient". You will want enough cylinder wall thickness to quell the vibrations generated by the piston rings skidding up and down the walls to prevent generating air bubbles in the cooling water on the other side of the wall which would then cling to the cooling water side of the cylinder and would prevent cooling water from contacting the walls of the cylinder to carry away the heat of combustion.

2. I have always liked Airflow Research heads, and particularly the smaller intake runners on a stock-type displacement motor where RPM's will be limited and most of the driving will be on public roads and highways. The 280 cc intake runners of the AFR Bullitt heads should make an absolute sweetheart of a street motor when bolted to a nearly stock displacement 460.

3. You'll need to know the exact static compression ratio in the motor before you go looking for a cam. A higher static compression ratio used with a short duration cam can be a detonation-prone beast. On the other hand, a lower static compression ratio motor teamed with a long duration cam may not make enough power to pull the hat off your head. The balancing act is at the closing of the intake valve. You see, the air/fuel slug coming down the intake port and headed for the combustion chamber has both weight and speed. The trick is to allow the intake valve to stay open long enough to get all the slug into the chamber before the piston reverses course and begins to push some of the slug back out of the chamber and up the intake runner toward the mixer (carb or throttle body). Pushing mixture out of the mixer (reversion) can be seen at night with the air cleaner off and a bright light shone across the mixer inlet. It will appear as a fog hanging above the mixer and is the result of the cam holding the intake valve open too long for the rpm's the motor is running at. As rpm's increase, the motor will come "up on the cam" and this phenomenon will disappear as the motor begins to run more crisply. You see, the air slug will have picked up fuel at the venturi as it entered the motor, then when it is pushed out of the motor by the ascending piston, it picks up another load of fuel at the venturi because the venturi does not care which way the slug is going, it just sees a low-pressure area and adds fuel. Now it is fuel-rich and hanging around outside the mixer throat. As the crank spins and the motor creates a negative pressure in the cylinder, the air/fuel slug again goes past the venturi, picking up even more fuel. This is why you will see such a motor having to be blipped on the throttle every few seconds to prevent loading up and dying. The air/fuel slug becomes so rich that the motor cannot burn it.

4. Generally speaking, the larger the cubic inches of a motor, the more fuel it will burn over the same distance.

5. I'm glad you mentioned quench. I prefer the term squish, but just as long as all of us know what we're talking about, it'll be OK. To me, quench is the transfer of heat from one place to another, like a blacksmith quenching a horseshoe in a bucket of water after heating it and banging on it with his hammer. Squish, on the other hand, reminds me of stepping on something soft like a warm pat of butter and squishing it out from under your shoe. It is true that the closer the piston crown gets to the underside of the combustion chamber surfaces, the more heat will be transferred from the piston to the head to be carried away in the cooling system. On the other hand, squish describes the squeezing of the air/fuel mixture between the piston crown and the underside of the combustion chamber to "jet" it across the chamber where it breaks up large clots of raw fuel and creates a hurricane-like environment that homogenizes the air/fuel mixture, making it easier to burn and make power rather than being blown out the exhaust valve as unburned fuel.

Yeah, yeah, I know it's hard to get me stopped once I get going, but I'm 80 years old and have experienced a lot of stuff in my lifetime.....and I love to teach.
 

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Compression ration should be determined by the cam goy, no the other way around.

The primary determiner of fuel lineage is the weight of one's foot.
 

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Discussion Starter · #5 ·
1. I don't see how thick or thin cylinder walls are going to affect fuel mileage and I have no idea how one or the other could be made "efficient". You will want enough cylinder wall thickness to quell the vibrations generated by the piston rings skidding up and down the walls to prevent generating air bubbles in the cooling water on the other side of the wall which would then cling to the cooling water side of the cylinder and would prevent cooling water from contacting the walls of the cylinder to carry away the heat of combustion.

2. I have always liked Airflow Research heads, and particularly the smaller intake runners on a stock-type displacement motor where RPM's will be limited and most of the driving will be on public roads and highways. The 280 cc intake runners of the AFR Bullitt heads should make an absolute sweetheart of a street motor when bolted to a nearly stock displacement 460.

3. You'll need to know the exact static compression ratio in the motor before you go looking for a cam. A higher static compression ratio used with a short duration cam can be a detonation-prone beast. On the other hand, a lower static compression ratio motor teamed with a long duration cam may not make enough power to pull the hat off your head. The balancing act is at the closing of the intake valve. You see, the air/fuel slug coming down the intake port and headed for the combustion chamber has both weight and speed. The trick is to allow the intake valve to stay open long enough to get all the slug into the chamber before the piston reverses course and begins to push some of the slug back out of the chamber and up the intake runner toward the mixer (carb or throttle body). Pushing mixture out of the mixer (reversion) can be seen at night with the air cleaner off and a bright light shone across the mixer inlet. It will appear as a fog hanging above the mixer and is the result of the cam holding the intake valve open too long for the rpm's the motor is running at. As rpm's increase, the motor will come "up on the cam" and this phenomenon will disappear as the motor begins to run more crisply. You see, the air slug will have picked up fuel at the venturi as it entered the motor, then when it is pushed out of the motor by the ascending piston, it picks up another load of fuel at the venturi because the venturi does not care which way the slug is going, it just sees a low-pressure area and adds fuel. Now it is fuel-rich and hanging around outside the mixer throat. As the crank spins and the motor creates a negative pressure in the cylinder, the air/fuel slug again goes past the venturi, picking up even more fuel. This is why you will see such a motor having to be blipped on the throttle every few seconds to prevent loading up and dying. The air/fuel slug becomes so rich that the motor cannot burn it.

4. Generally speaking, the larger the cubic inches of a motor, the more fuel it will burn over the same distance.

5. I'm glad you mentioned quench. I prefer the term squish, but just as long as all of us know what we're talking about, it'll be OK. To me, quench is the transfer of heat from one place to another, like a blacksmith quenching a horseshoe in a bucket of water after heating it and banging on it with his hammer. Squish, on the other hand, reminds me of stepping on something soft like a warm pat of butter and squishing it out from under your shoe. It is true that the closer the piston crown gets to the underside of the combustion chamber surfaces, the more heat will be transferred from the piston to the head to be carried away in the cooling system. On the other hand, squish describes the squeezing of the air/fuel mixture between the piston crown and the underside of the combustion chamber to "jet" it across the chamber where it breaks up large clots of raw fuel and creates a hurricane-like environment that homogenizes the air/fuel mixture, making it easier to burn and make power rather than being blown out the exhaust valve as unburned fuel.

Yeah, yeah, I know it's hard to get me stopped once I get going, but I'm 80 years old and have experienced a lot of stuff in my lifetime.....and I love to teach.
Thank you for your reply, That was two different topics and I should have separated them better, cylinder wall vs. MPG... with so many people boring their engines out to gain cubic inches I got to the point that it is not worth it if cylinder wall thickness is made too thin, so If I don't have to bore it ,, it will stay stock. I am considering stroking it to 4.14, there are a few pluses I get beside cubic inch. 1. the journals go from 2.5 to 2.2 and I gain a forged crank...2. the piston weight might come down a bit while still have enough piston using 6.8 rods the CH would be 1.438 that is less than the 1.770 the 460 generally uses...Also rod angularity goes down a touch but still real good at 1.6425...........and cubic inch would be 494.49 , I am beginning to like the idea.
 

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You don`t build big blocks for fuel economy. G.M. crap engines cut cylinders at cruise, thats why their m.p.g. is high. Comparing carbed motor with modern e.f.i. factory engineering is like a sword versus a m-16.
 

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Discussion Starter · #8 ·
You don`t build big blocks for fuel economy. G.M. crap engines cut cylinders at cruise, thats why their m.p.g. is high. Comparing carbed motor with modern e.f.i. factory engineering is like a sword versus a m-16.
You don't mind if I try to make as efficient as I can vs. not, I am not expecting 20 or 30 MPG but I would like to do the best I can, is it OK if I try and do things that would make it better? and if You say yes , I say thank you.
 

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Very much a debate for me of which way to go. I want thick cylinder walls, not barely. also if both are made efficient does anyone know how much gas mileage is effected, I am leaning towards a 460 with AFR heads, with the cam I got planned it would have lots of power and smooth idle. Does anyone have any info how much cubic inch effects gas usage. I say this because the production LT1 motor gets as good or close as the coyote motor yet cubic inch is different. 307 vs. 376. Then we have other issues like bore and stroke, piston speed and how this all affects efficiency. I also believe that a good quench although may not effect WOT power as much as some have found , still has a lot to do with efficiency of a motor especially at part throttle as well as a big help towards pre ignition resistance. There are many things to consider besides peak power at WOT. Some here have experience, would love to hear from.
Great post. I have thought about this a lot myself, and is a bit relevant to me as I will be building a 521 inch engine in the not-too-distant future.
The comments you get when asking questions about improving fuel economy in big block applications are sometimes..........interesting. Nothing wrong with building a powerful engine while at the same time making it as efficient as possible.
Hopefully you'll get more input relevant to your concerns.
 

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On the other hand, a lower static compression ratio motor teamed with a long duration cam may not make enough power to pull the hat off your head.
Impressive.... Could you please elaborate a little more on this quote?
Not new to 385's, but new to stroked 385's. I'm also building a 521 but am shooting for a 9'ish range compression ratio. What in your opinion would be a long duration cam on a 521 with a lower compression ratio?
 

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The most critical event on a camshaft lobe is the intake closing point at some number of degrees After Bottom Dead Center. The intake valve has begun to open at some point Before Top Dead Center on the exhaust stroke of the piston and will remain open as the piston reverses at Top Dead Center and begins a trip down the bore to create a negative pressure in the cylinder, allowing atmospheric pressure.to facilitate filling the cylinder with a combustible mixture of air and fuel. As the piston approaches Bottom Dead Center, the intake valve has begun to close. Since this combustible mixture has inertia, it will continue to move past the intake valve and into the cylinder as long as there is a negative pressure in the cylinder. At Bottom Dead Center, the piston reverses itself and begins a trip back up the cylinder to compress the combustible mixture which was just pushed into the cylinder by atmospheric pressure. This is where we reach a juggling act in the whole mess, by choosing an intake valve closing point that will allow all the combustible mixture to be pushed into the cylinder that physics will allow before we close the intake valve and begin pressurization of the air/fuel mixture with the piston headed up the bore on its compression stroke. If we close the intake valve too soon, we may not fill the cylinder completely and power may be down compared to what it could have been if a full charge were allowed to pass by the intake valve. On the other hand, if we delay the close of the intake valve too long, the piston will have reversed direction and can begin pushing some of the combustible mixture back past the intake valve, up the intake port and out past the mixer to atmosphere, again reducing power. In this scenario, if using a carburetor, as the slug of mixture goes past the venturi in a reverse direction, the venturi will see a negative pressure condition and add more fuel to the mixture. Then, on its trip back into the motor on the next piston stroke, the venturi again adds fuel to the mixture and the motor is in a tailspin of rich mixture. I know you have witnessed motors idling that will die if the driver does not blip the throttle every little bit to clean out the rich mixture. Now, we have only discussed what happens at idle or low rpm's so far. Too much cam at idle does not necessarily mean too much cam when the tach reads several thousand rpm's or more. The more r's, the cleaner the motor will get. Of course, the displacement of the motor plays a role in this scenario also. A cam which might be horrible in a 300 inch motor and might require a very loose converter and short gears in the diff might be just right in a 500 inch motor. Again, it's all a balancing act where the builder will get better and better at it the more motors he or she builds.
 

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I just realized that I repeated what I posted earlier and didn't really answer your question. Sorry, but it appears that I can't make heads or tails out of anyone's catalog anymore.
 
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