I was wondering if porting a 5300 vortec cylinder heads would actually improve mpg? If there's more air coming in with less throttle therefore less gas used or is my logic off?

 

Grossly oversimplified... more air requires more fuel and lowers economy

 

This problably doesn't belong here but I'm intrested to know too. A more effiecent design will produce better mpg. Less air and fuel to make the most power equals to lless throttle. If runner size is increased by porting it will slow down air port velocity at lower lift and lower rpm which will also mess up the atomization of fuel being sprayed into the cylinder. From what I understand the ls6 heads 243 799 compression chambers burn fuel very effienciently making them better for mpg I believe. Also I've heard higher compression makes for better effiecency. Do a search for better mpg to learn more. There's a lean cruise map u could program into the cpu and there something about natrual egr which supposdly helps with mpg.

 

The maf reads the amount of air coming into the intake and adjusts the amount of fuel from there regardless of throttle position. Horsepower = less gas mileage. if anything a more constrictive head will give you better mileage lol.

 

Ok ported heads = less mpg.

 

I would think due to the increased volumetric efficiency of the ported heads the mileage would increase. As for the fuel not atomizing, even though the velocity would decrease at cruse the amount of fuel being sprayed by the injector is a very small amount that should atomize even at much lower velocity. And compression does increase the overall efficiency of an engine, but there are many variables that could affect the mileage. Just like most would think if you put a turbo on a engine you would loose mpg, but actually if you make the engine more efficient mpg will increase as long as you don't also use the newly added power. But how many of us on here are to pick up a mpg or two, most of us modify our engines to make more power, then use the added power to have some fun!

 

Yes and No,
Way too many factors that come into play.

 

Assuming you were going to gain any volumetric efficiency from it, I would agree. If a head isn't ported properly VE will drop. Even if it's ported properly that doesn't automatically give you an increase in volumetric efficiency. Higher compression does result in a higher burn efficiency, not a higher volumetric efficiency.

If there were ANY increase in MPG, it most likely wouldn't be noticeable. What it comes down to is your AFR - how much fuel goes with a given amount of air that gets pumped into the engine. If the computer is set up to give 1 part fuel to 12 parts air, it will do what it has to to maintain that ratio. If more air goes in, then so does more fuel. The best way to increase mileage would be to have the car tuned. A lot of people are tuning for AFR's of 18:1 or leaner at low RPM.

 

So far as I know the two primary factors of head efficiency are:

1. Volume and cross-sectional size of intake and exhaust runners
2. Degree of angles and turbulence within the intake and exhaust pathways.

--- Which means, ---

1. Porting to larger runners automatically means that air intake speed will be lower. To put it simply, the engine likes to have pulses of quick air forcing its way into the valves when they open, rather than slow air that lazily flows in. This is similar to the effect a large camshaft makes toward a poor idle. By increasing runner size, you decrease low RPM power/torque and increase high RPM power/torque.

2. A high quality head, whether it has pushrods or not, will have optimized (as small as practical) angles. The harshness of the angles that the air must take to make it from the intake manifold, through the head, into the engine, and out the exhaust, are decreased. This means increased volumetric efficiency, and less work that the engine has to waste on sucking in air. Optimized valve angle and turbulence are a huge part of why LS powered sports cars have always had better gas mileage and more power than their European and Japanese sports car counterparts, despite their smaller engines and lower weight.

So, if you are able to get a head setup where flow angle and turbulence are better optimized, then you will see an increase in fuel economy and power at ALL engine speeds. But, if you simply port a head by increasing the runner sizes, you will compromise low RPM torque and power for high RPM flow.

 

this is true,
therefore I would think volumetric efficiency has nothing to do and is completely independent of fuel economy.

if the point is better mpg, then it's simply a matter of using less fuel for a given power output value of the engine.
If you use less fuel, generally you have less power output (because the power comes from the fuel).
If you want more power output from less fuel mass with the fuel being gasonline the only way I'm aware of achieving that is buy increasing compression to have higher cylinder pressures.

Diesel on the other hand, because it is compression ignition opposed to spark ignition, I think is different. The term that comes to mind is torque rise. I don't read much about diesel power anymore but I do know that it is very important to size a diesel motor correctly for a given application and power output required to gain the most fuel economy/efficiency. I am not sure whether the torque rise principle related to diesel engines applies at all to gasoline (spark-ignition) engines.

 

Volumetric efficiency means lower RPM to make the same power.

Cam profile. Ignition timing. Air to fuel ratio. Intake. Exhaust. Head flow characteristics. Bore. Stroke. Rod to stroke ratio. Type of oil. Type of fuel. Gearing. Stiffness of valvetrain.

 

Another myth is the better gas mileage based on heads... Ok we know that there are too many factors that play into gas mileage and your camshaft,tune,gearing and stall have more to do with gas mileage....

Prime example: Just regular driving(like we all drive regular here..ha ha)
holding everthing else constant but auto tranny :
1.LS2 with a TH350 and 3.73 gears 3000 stall 27.5 tire
2.LS2 with a TH350 and 3.23 gears 3000 stall 27.5 tire
3.LS2 with a 4L65E and 3.73 gears 3000 stall 27.5 tire
4.LS2 with a 4L65E and 3.23 gears 3000 stall 27.5 tire

Now at 3000rpms:
combo 1. 66MPH
combo 2. 76MPH
combo 3. 94MPH
combo 4. 108MPH

Gear ratio in final gear....
1.3.73 in 3rd
2.3.23 in 3rd
3.2.61 in 4th
4.2.26 in 4th

Now -/+ for some margin of error, and depending on the quality of the converter but for this we will say this converter is extremely efficient and tight w/less than 5% slippage if any...... The point here is to show how different gearing and and tranny types affect gas mileage, regardless of the head..... You can figure it out from the data given here which combo would probbly have better gas mileage and it would lean towards the combos utilizing less rpm with correlation to MPH..... So if they market to you that brand X head will get better gas mileage above others, if so it would be marginal and so marginal that they cannot do a test or be worth the money spent to prove it.... And as we old racers know, the rest of the combination has more to do with gas mileage, then the head alone.....

The more throttle or rpm used will affect gas mileage..... The less throttle that has to be used will have better gas mileage, assuming everything is equal..... But you have been pounded over the head that brand X will produce better gas mileage and have better crisp throttle response...... its not brand X that gives you better throttle response, but if you properly assemble the combination stall,gearing,camshaft,tune, etc... will affect throttle response, not just a head by itself.....

And all of the guys talking about VE, explain how a cam with more overlap, will often have better VE, but sucks on gas mileage.....? I know the answer.....

Bozz

 

Overlap does privide better VE... at a given RPM. Cams that have a radical overlap usually put this point of high VE at the very upper RPM spectrum. At the lower RPM's the duration can actually have a negative effect on VE thus resulting in inefficient filling of the cylinder with air and fuel, thus possibly having crappy fuel economy at lower RPMs (where most people spend most of their driving time). Key is finding the sweet spot for that specific sized engine and and having all the other elements work together to make it work properly.

Heres a cool little formula to play with when it comes to VE. If you know your exact displacement and power at a given RPM you can figure out the VE at that RPM.

VE= (HP*5600)/(RPM * Displacement)

So, if we have a naturally aspirated 346 CI engine making peak power of 445 HP @ 6100 RPMs, you can plug those numbers into the formula and get the VE of that engine at 6100 RPMs (which happens to be max power), which is 118.07%. Same engine made 400 HP@5252 RPMs has a VE of 123.27% @ 5252.... interesting.

 

At low airflows you care more about atomization and
such, for economy. Aggressive porting often sacrifices
swirl features for flow. Opening up runners sacrifices
velocity & low-RPM cylinder filling (torque) for high
RPM cylinder filling.

You can bet that if GM had any idea about economy
improvements on the stock casting, they did it.

 

I bet this is the best answer.

As far as fuel economy is concerned, you are principally concerned with lower flow, relatively low throttle, higher vacuum states. I don't see how you can presume that porting the heads would actually improve the efficiency of filling the cylinder in this state.

If you want to imagine that the improved flow means that you get more VE and more power at low throttle and that somehow translates into efficiency, I do not think in this scenario it is necessarily the case. Say I want to cruise at 60mph. That will take a predetermined amount of power. Perhaps that is at 10% throttle on whatever stock car. If porting the heads increases the flow at 10% throttle (which it very well may not) and whatever vacuum then all that will happen is that you will end up using 9% throttle with perhaps more vacuum to make the same amount of power. The higher vacuum from the closed throttle blade may in turn rob any of the efficiency you have gained. In addition, the point about fuel atomization and swirl is important. What is your ported head going to do to those variables?

I do not think that you will find that the porting really improves cylinder filling in a way that results in greater efficiency. If you look at AFM (formerly Displacement on Demand) the whole way it get's its limited efficiency gains is by cutting half the cylinders so that you can run the same RPM but with a wider open throttle blade and less vacuum, which means more efficient cylinder filling for the remaining cylinders.

 

Certain chamber work can reduce the probability of knock thus helping your mpg because you can run the tune a little more optimal.

 

Explain please?

 

Yes,porting can improve MPG if done correctly with that as an objective but you must complete the equation as the motor has 3 different variables(Intake-Combustion-Exhaust)

If you have excessive back pressure from the exhaust side you will see small gains from the changes on the induction side and vice versa

Flow distribution over the valve and average velocity in the port are the tuning factors and shape of the port will overrun the size of the port for the most part.

Combustion chamber shape is important as is the efficiency of it,the LS series engines are good but can be improved-A 0.040 Head gasket and bumping up the compression will help you greatly

You must look at the induction side as a whole unit from the air filer and its location in the engine compartment to the margin of the valve

 

Quote:
Originally Posted by 180ls1
Certain chamber work can reduce the probability of knock thus helping your mpg because you can run the tune a little more optimal.

Not to put words into 180ls1's mouth but I think I now what he's getting at.

From a tuning standpoint you will typically see more MPG gains from running more timing. Alot of people play around with leaner mixtures and don't see any tangible results.

If you can "soften" the chamber (this also includes sharp edges on pistons around valve reliefs) it will move you away from the knock threshold and allow you to run slightly more ignition advance.

 

I had the same thought in mind when I was researching ported cylinder heads on my LT1 Trans AM. I guess we are suckered into thinking that performance ONLY oriented modifications can help improve our motor's efficiency when really it is only PERFORMANCE that benefits, not the fuel economy. After reading this thread, I realize I should have gone to college to become an automotive technician and/or an engine builder. I came close, but I wanted to go to UTI or Wyotech and back in 2008 I couldn't have afforded the tuition, dorm and board. So one childhood dream died but the child playing with his Trans AM, remained.

I finally understand what a mechanic's shop tried so hard to drive into my naïve head, it's best to keep the motor stock if fuel economy is my concern. Since my factory LT1 is nearing it's end, I am going to have a remanufactured LT1 from Jasper, installed. I will request the shop to install a set of hooker ceramic coated street performance--50 state emissions legal--shorty headers that I bought awhile back in my pursuit for performance and efficiency. From what I have read, the headers will help my motor out with efficiency and POSSIBLY fuel economy, no guarantees naturally.