Exhaust port flow vs intake port flow?

[Xnke]

It seems, from the searches I've done, that most L heads can be lumped into two categories. Those that flow 180cfm++ on the intake, and those that don't.

 

But I'd think the exhaust is just as important, and seems to get forgotten, or maybe it just flows less even best case. 150CFM seems to be considered excellent, whereas the intakes are flowing as much as 2000-210cfm. Doesn't that ultimately mean that the intake can only flow 150cfm, since it's limited by the lesser exhaust flow? What goes in, has to come out, plus some from the combustion process.

 

I guess what I am asking, is how much would it take to get a head that flows as much on the exhaust side as it does on the intake, and why does it not seem to cause a big problem if the flows are that unequal? Would it make a difference?

[m4xwellmurd3r]

not really. that may be so if the air going from the intake, was piped directly into the exhaust, but that's not how an engine works.

on the intake, the piston PULLS the air through the intake ports. the higher the flow, the better suction it can get on the intake to get more air. then it compresses, combusts, then PUSHES the air out of the exhaust ports. the more flow the exhaust ports have, the faster, and more easlily the piston is able to push the air out of the engine.

so intake flow is not really restricted by exhaust flow in the way you're thinking it does.

[Xnke]

Still, you should see some tremendous gain if you could get the exhaust flow up to match the intake. What you can suck in, you still have to get pushed back out, and it takes just as much horsepower to pull something in as it does to push it. And you'll be pushing out even more than you pulled in due to combustion products. Maybe I'm wrong, but

I'm still interested in upping the exhaust port flow in my E88 head. Anyone got some pointers that aren't explicitly outlined in our other head threads?

[ozconnection]

If my high school physics serves me correctly, a descending piston in an engine doesn't suck in air, it is the difference in atmospheric pressure caused by the descending piston that allows the air to be PUSHED in. The intake system therefore has a maximum potential flow into the cylinders of 14.7 psi (1 atmosphere at sea level) at WOT. At part throttle, the volumetric efficiency is less, so less air /fuel enters the engine. Therefore, at part throttle, you're not using the full airflow potential of the intake port/manifold etc. Less air flows into and out of the engine as a result.

 

The exhaust side is different. Camshaft timing of the exhaust valve influences cylinder blow down, so almost all the exhaust gas is blasted out of the cylinder the moment the exhaust valve opens after the power stroke. Like the inlet side of things, it is the differences in air pressure, heaps this time especially as the exhaust valve opens (much more than 14.7psi!!!) and the gasses are very hot and 'excited' that allows the exhaust to work as it should with these 'apparent' shortcomings. Then, as the piston rises on the exhaust stroke pumping losses are reduced. It has also been found through years of experimentation that the exhaust valve needs only be 75% or so of the intake valve size because of the above mentioned. Larger exhaust valves than this didn't gain any more horsepower.

 

I think your confusing the idea that after combustion takes place that there's more 'substance' to get out of the cylinders. No, this is wrong. After the air and fuel is ignited in the cylinder, energy is released, the temperature goes up dramatically and the gasses expand enormously, and it's the expanding gasses that is the element of power production (and also the major force behind the expulsion of exhaust gasses from the cylinder after combustion like I've already said).

 

There is a lot more to this discussion and with my limited high school physics knowledge (It was a long time ago!) I might be cutting some corners.

 

Finally, I do have to say that there may be some use in improving the exhaust port flow but I don't thing you need to lose too much sleep over it, it does a pretty good job of it all by itself. Having said that, there's always room for a well designed exhaust system.

[Calgary280ZT]

I'm very interested in this thread. In a couple of weeks I'll be installing a REbello head that now flows 209 cfm at 25psi. And just received my Lonewolf Perf. ported and polished intake mani that James figures will flow 270 cfm per runner. Got the 60mm TB too. So I'm good on the intake side.

 

But what about exhaust? At a minimum I'll match the manifold to the gasket, open things up a bit. With a 3" exhaust is the exhaust manifold going to be the bottleneck in this system? Should I port the mani more?

[zgeezer]

If my high school physics serves me correctly, a descending piston in an engine doesn't suck in air, it is the difference in atmospheric pressure caused by the descending piston that allows the air to be PUSHED in. The intake system therefore has a maximum potential flow into the cylinders of 14.7 psi (1 atmosphere at sea level) at WOT. At part throttle, the volumetric efficiency is less, so less air /fuel enters the engine. Therefore, at part throttle, you're not using the full airflow potential of the intake port/manifold etc. Less air flows into and out of the engine as a result.

 

The exhaust side is different. Camshaft timing of the exhaust valve influences cylinder blow down, so almost all the exhaust gas is blasted out of the cylinder the moment the exhaust valve opens after the power stroke. Like the inlet side of things, it is the differences in air pressure, heaps this time especially as the exhaust valve opens (much more than 14.7psi!!!) and the gasses are very hot and 'excited' that allows the exhaust to work as it should with these 'apparent' shortcomings. Then, as the piston rises on the exhaust stroke pumping losses are reduced. It has also been found through years of experimentation that the exhaust valve needs only be 75% or so of the intake valve size because of the above mentioned. Larger exhaust valves than this didn't gain any more horsepower.

 

I think your confusing the idea that after combustion takes place that there's more 'substance' to get out of the cylinders. No, this is wrong. After the air and fuel is ignited in the cylinder, energy is released, the temperature goes up dramatically and the gasses expand enormously, and it's the expanding gasses that is the element of power production (and also the major force behind the expulsion of exhaust gasses from the cylinder after combustion like I've already said).

 

There is a lot more to this discussion and with my limited high school physics knowledge (It was a long time ago!) I might be cutting some corners.

 

Finally, I do have to say that there may be some use in improving the exhaust port flow but I don't thing you need to lose too much sleep over it, it does a pretty good job of it all by itself. Having said that, there's always room for a well designed exhaust system.

 

I've heard that 75-80% rule before: the reason (I thought "excuse") that my brother spend so much time and money on slicing up the exhaust side of '70 Ford Cleveland 4 barrel heads and then bolting an aluminum plate on to raise the exhaust port roof and straighten the flow of the exhaust ports. Only clean up work on the intake ports. It worked, but those heads are infamous for huge direct short intake ports and a sharp turn down in the exhaust ports. For a while it seemed that every third Cleveland had an exhaust plate to balance the flow between intake/exhaust. The percentage to shoot for seemed to be an exhaust that flowed 75% of the intake port.

 

g

 

g

 

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[zgeezer]

If my high school physics serves me correctly, a descending piston in an engine doesn't suck in air, it is the difference in atmospheric pressure caused by the descending piston that allows the air to be PUSHED in. The intake system therefore has a maximum potential flow into the cylinders of 14.7 psi (1 atmosphere at sea level) at WOT. At part throttle, the volumetric efficiency is less, so less air /fuel enters the engine. Therefore, at part throttle, you're not using the full airflow potential of the intake port/manifold etc. Less air flows into and out of the engine as a result.

 

The exhaust side is different. Camshaft timing of the exhaust valve influences cylinder blow down, so almost all the exhaust gas is blasted out of the cylinder the moment the exhaust valve opens after the power stroke. Like the inlet side of things, it is the differences in air pressure, heaps this time especially as the exhaust valve opens (much more than 14.7psi!!!) and the gasses are very hot and 'excited' that allows the exhaust to work as it should with these 'apparent' shortcomings. Then, as the piston rises on the exhaust stroke pumping losses are reduced. It has also been found through years of experimentation that the exhaust valve needs only be 75% or so of the intake valve size because of the above mentioned. Larger exhaust valves than this didn't gain any more horsepower.

 

I think your confusing the idea that after combustion takes place that there's more 'substance' to get out of the cylinders. No, this is wrong. After the air and fuel is ignited in the cylinder, energy is released, the temperature goes up dramatically and the gasses expand enormously, and it's the expanding gasses that is the element of power production (and also the major force behind the expulsion of exhaust gasses from the cylinder after combustion like I've already said).

 

There is a lot more to this discussion and with my limited high school physics knowledge (It was a long time ago!) I might be cutting some corners.

 

Finally, I do have to say that there may be some use in improving the exhaust port flow but I don't thing you need to lose too much sleep over it, it does a pretty good job of it all by itself. Having said that, there's always room for a well designed exhaust system.

 

I've heard that 75-80% rule before: the reason (I thought "excuse") that my brother spend so much time and money on slicing up the exhaust side of '70 Ford Cleveland 4 barrel heads and then bolting an aluminum plate on the raise the roof and straighten the flow of the exhaust ports. Only clean up work on the intake ports. It worked, but those heads are infamous for huge intakes and a sharp turn down in the exhaust ports.

 

g

 

g

[ozconnection]

I've heard that 75-80% rule before: the reason (I thought "excuse") that my brother spend so much time and money on slicing up the exhaust side of '70 Ford Cleveland 4 barrel heads and then bolting an aluminum plate on to raise the exhaust port roof and straighten the flow of the exhaust ports. Only clean up work on the intake ports. It worked, but those heads are infamous for huge direct short intake ports and a sharp turn down in the exhaust ports. For a while it seemed that every third Cleveland had an exhaust plate to balance the flow between intake/exhaust. The percentage to shoot for seemed to be an exhaust that flowed 75% of the intake port.

 

Cleveland 4V heads were overkill for a street driven car. They were designed for the racing cars of the '70's. It's not so much the problem with the exhaust being a poor flower, it was more of a case of the intake valves and ports being too big, poor low speed flow was the result. Guys tried to correct this problem by using aluminium tongues in their intake ports to make them substancially smaller and flow better. Eventually the 75% thingy returned and the engine woke up at lower speeds. Power at the top end wasn't adversely effected by the tongues either. And the story continues but lets not bore the Datsun guys with Ford stuff. Lets move on from this shall we.....?

[woldson]

If my high school physics serves me correctly, a descending piston in an engine doesn't suck in air, it is the difference in atmospheric pressure caused by the descending piston that allows the air to be PUSHED in.

 

Yes the out side air pressure pushes the air into the vacuum.

Its kinda like there is no such thing as darkness and not such thing as cold. Except absolute zero, if you want to consider that cold.

 

The ignited gas air mixture is of a greater volume potential, (not mass) so more volume needs to be evacuated from the cylinder.

[Nigel]

Yes the out side air pressure pushes the air into the vacuum.

Its kinda like there is no such thing as darkness and not such thing as cold. Except absolute zero, if you want to consider that cold.

 

That reminds me of my Science of Flight course in University, where, when explaining lift, the wing was always shown as stationary, with the AIR being in motion...

 

Nigel

'73 240ZT