Can someone explain why stage II heads are not streetable?
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jasjamz
THS Racer
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I was talking to a local TB guy about this earlier in the week. It basically boiled down to the movement of the powerband being out of the norm on your typical build using Stage II heads.
It has to do with obtaining the optimum port flow velocity to get the best cylinder fill (VE) within a particular rpm range. The port size and volume of production and Stage I style heads are better matched to give the best target port flow velocity for a rpm powerband typically picked for street use. Stage II port sizes and volumes are better matched to give the best target port flow velocity for a rpm powerband that is not typically used for street use. 9,000+ rpm redline.
The proper port flow velocity is at a point where the flow doesn't offer too much restriction to flow, and the velocity helps with fuel/air mixing or fuel atomization. The better the atomization, the more efficient the combustion process.
The proper port flow velocity also gives the column of intake charge in the intake port a momentum that helps with cylinder filling. The momentum of the intake charge in the intake port helps to pack more air and fuel molecules into the cylinder before the intake valve closes.
The reason why smaller heads will not rpm like larger valve and ported heads is because the intake flow velocity becomes too great, moving beyond the optimum port flow velocity, and the speed of the intake charge begins to offer resistance to flow. The higher you try to push the intake port flow velocity past the optimum range, the more resistance to flow that builds up.
Theoretically, the speed of sound is the limit of flow velocity through an intake port. In actual practice, port flow velocity will begin to offer resistance to flow at under half that speed through the intake port.
The proper port flow velocity is at a point where the flow doesn't offer too much restriction to flow, and the velocity helps with fuel/air mixing or fuel atomization. The better the atomization, the more efficient the combustion process.
The proper port flow velocity also gives the column of intake charge in the intake port a momentum that helps with cylinder filling. The momentum of the intake charge in the intake port helps to pack more air and fuel molecules into the cylinder before the intake valve closes.
The reason why smaller heads will not rpm like larger valve and ported heads is because the intake flow velocity becomes too great, moving beyond the optimum port flow velocity, and the speed of the intake charge begins to offer resistance to flow. The higher you try to push the intake port flow velocity past the optimum range, the more resistance to flow that builds up.
Theoretically, the speed of sound is the limit of flow velocity through an intake port. In actual practice, port flow velocity will begin to offer resistance to flow at under half that speed through the intake port.
I see no reason why they wouldn't be streetable, but that probably depends on your definition of streetable. Flow numbers are a small part of the equation.
Here is my opinion on stage 2 heads vs. production: Lets compare two motors everything is identical except for the heads and intake. Both with the same bore size, piston speed (same rpm) and boost. Those three factors define how much force is available to get that charge moving from the runner to the combustion chamber.
The runners on the stage 2 heads are massive. Even the unported versions have at least 50% more volume than a ported Champion or TA head. That volume of air and fuel has mass. When the intake valve is closed, that mass of air and fuel isn't moving. Each time the intake valve opens you have to accelerate that mass to get it into the chamber. The quicker you accelerate it, the more you get into the cylinder.
force= mass * acceleration.
The force for the two motors is the same (same piston speed, bore and boost). So, if the masses of the two are different, the acceleration must also be different. In the stage head, the mass is higher, so the acceleration is lower. In the production head, the mass is lower, thus acceleration is higher. So the production head can get that charge accelerated and up to speed quicker than the stage head. In the limited amount of time the valve is open, that makes the difference.
At some point, no matter how much force (rpm, boost) you put to the heads, the production head is going to run out of steam and stall out. The stage head has the potential to go much further before stalling out.
A street car operates in the low rpm, low boost range most of the time. So there is little available force to move that charge in the runner. The production head is better in this situation. The real difference between the two is debatable. That difference may be acceptable depending on your definition of streetable.
Here is my opinion on stage 2 heads vs. production: Lets compare two motors everything is identical except for the heads and intake. Both with the same bore size, piston speed (same rpm) and boost. Those three factors define how much force is available to get that charge moving from the runner to the combustion chamber.
The runners on the stage 2 heads are massive. Even the unported versions have at least 50% more volume than a ported Champion or TA head. That volume of air and fuel has mass. When the intake valve is closed, that mass of air and fuel isn't moving. Each time the intake valve opens you have to accelerate that mass to get it into the chamber. The quicker you accelerate it, the more you get into the cylinder.
force= mass * acceleration.
The force for the two motors is the same (same piston speed, bore and boost). So, if the masses of the two are different, the acceleration must also be different. In the stage head, the mass is higher, so the acceleration is lower. In the production head, the mass is lower, thus acceleration is higher. So the production head can get that charge accelerated and up to speed quicker than the stage head. In the limited amount of time the valve is open, that makes the difference.
At some point, no matter how much force (rpm, boost) you put to the heads, the production head is going to run out of steam and stall out. The stage head has the potential to go much further before stalling out.
A street car operates in the low rpm, low boost range most of the time. So there is little available force to move that charge in the runner. The production head is better in this situation. The real difference between the two is debatable. That difference may be acceptable depending on your definition of streetable.
Matching the proper port flow velocity to a particular engine application is why you will see multiple intake port volumes offered with popular aftermarket cylinder heads. It gives the engine builder the chance to match the proper cylinder head to the engine application. Low rpm torque, street use, street/strip, or competition. The difference in all those applications typically being the rpm powerband range that the engines will be operating in.
Sure, a street application engine will run with Stage II heads, but from an efficiency stand point, Stage II heads are a poor match for an engine that will be used primarily for street use. Especially if the redline of that street engine is setup to be only 6000-6500 rpm.
Changing the cam opening and closing events won't directly increase port velocity. But by opening the valve later on the intake stroke when there is more piston speed, there will be more "vacuum" in the cylinder. That will help accelerate the charge quicker.
Primarily, it is the intake duration spec that sets the rpm operating range of the engine. Lowering the intake duration in an effort to increase intake port velocity of a large port head is counter productive. It doesn't increase the volume of flow through the port, and just chokes off top end performance. It may affect the flow velocity through the port at some point during the intake stroke, but I don't see the advantage to it as far as trying to tap into the true potential of what a large port head is supposed to be used for.
I think we should check with radical.
IMO, the only reason to run a Stage II head on a street engine with limited cam timing and rpm operating range is simply for one reason only. The under the hood, looky loo, WOW factor. It will not enhance the performance of an engine meant primarily for street use.
So your implying that since i am converting to S2 heads I will not make any more power over TA/Champion heads at the same, cam specs, and rpm level lets say 6000 rpm max and the same boost level. So its just for appearance? I can;t wait to get on the dyno see where this goes
A good example of a port actually being too large to provide optimum performance is the case of the Ford Boss 302. Back in the day, manufacturers were just increasing port size without understanding the importance of proper port velocity. Tuners quickly learned that to increase performance they had to close down the volume of the port. In the case of the Boss 302, it was the exhaust port, IIRC.
If you're looking at a 6000 rpm redline, then my opinion is YES.
If you're rpm redline were to be 7800 rpm, you would start seeing an advantage with the Stage II heads.
If you were to choose a 9,000 rpm redline. Then Stage II heads would be the ultimate bomb.
Keep in mind that those higher rpm limits would demand MORE intake duration.
We could do a comparison between your engine setup to redline at 6,000 rpm with large valve and port Stage II heads, and my engine setup to redline at 7800 rpm using small port M&A heads with 1.835" intake and 1.5" exhaust valves. I agree. This should be very interesting.So your implying that since i am converting to S2 heads I will not make any more power over TA/Champion heads at the same, cam specs, and rpm level lets say 6000 rpm max and the same boost level. So its just for appearance? I can;t wait to get on the dyno see where this goes
Yes it would be interesting, but we both would have to have the same turbo, too many variables. Mac in SD has or had a S2 car with a small turbo,/cam he says it works well on the street, and is has influenced my decision to go this route.
Same person. I did the trans for him.
Like I stated. The car will be streetable.
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