DOHC Swap
01-05-2011, 12:31 AM
#1
Burning Brakes
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DOHC Swap
In 951's they came SOHC with 8 valves, I know this is great for making a good mid range engine, has anyone swapped a DOHC head on a 951? I know it would create more power in the upper RPM's.
01-05-2011, 12:54 AM
#3
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I'm not sure it would create more power in the upper rpms. Fuel/air/Spark that is what makes power. So you add as much fuel as possible, more air, and ignition. By adding another camshaft now you have twice the spark and more precise control over the ignition which improves efficiency. But it's not that simple, there are many changes needed to be made to the motor besides a new head. It's easier to take an S2 or 968 motor and turbo charge it .
01-05-2011, 01:15 AM
#4
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But it's not that simple, there are many changes needed to be made to the motor besides a new head.
01-05-2011, 01:26 AM
#5
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I'm not sure it would create more power in the upper rpms. Fuel/air/Spark that is what makes power. So you add as much fuel as possible, more air, and ignition. By adding another camshaft now you have twice the spark and more precise control over the ignition which improves efficiency. But it's not that simple, there are many changes needed to be made to the motor besides a new head. It's easier to take an S2 or 968 motor and turbo charge it .
From there, you deal with thermal efficiency, which is how efficiently the engine turns that air and fuel into power (the term is "indicated power," which does not account for friction). And that is compression ratio and chamber shape. That stuff is defined by bore size (unchanged) and spark plug location. The ideal place is in the dead smack middle of the chamber. Which is where it is on the 16V, but not on the 8V.
Finally, there is mechanical efficiency, which is how much power it takes to move things around. Dominated by ring swept area. The other cam may take a little bit of power to turn, but it iis pretty miniscule.
So yeah, I'm POSITIVE it will make more power in the upper RPM band. Unless you design it to make more torque in the low and midrange. But because of the thermal efficiency, you can do that, too. Of course, seeing the 8V intake adapted to a 16V head certainly isn't ideal.
Last edited by 67King; 01-05-2011 at 01:31 AM. Reason: 968 info.
01-05-2011, 01:58 AM
#6
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I'm not sure it would create more power in the upper rpms. Fuel/air/Spark that is what makes power. So you add as much fuel as possible, more air, and ignition. By adding another camshaft now you have twice the spark and more precise control over the ignition which improves efficiency. But it's not that simple, there are many changes needed to be made to the motor besides a new head. It's easier to take an S2 or 968 motor and turbo charge it .
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01-05-2011, 02:44 AM
#8
Nordschleife Master
It all starts with volumetric efficiency. You can shove a whole lot more air through a pair of 37mm valves (or 39 with a 968 head) than a single 45mm valve. A whole lot more air. Fuel is secondary, but you just add it to match the air you breathe.
From there, you deal with thermal efficiency, which is how efficiently the engine turns that air and fuel into power (the term is "indicated power," which does not account for friction). And that is compression ratio and chamber shape. That stuff is defined by bore size (unchanged) and spark plug location. The ideal place is in the dead smack middle of the chamber. Which is where it is on the 16V, but not on the 8V.
Finally, there is mechanical efficiency, which is how much power it takes to move things around. Dominated by ring swept area. The other cam may take a little bit of power to turn, but it iis pretty miniscule.
So yeah, I'm POSITIVE it will make more power in the upper RPM band. Unless you design it to make more torque in the low and midrange. But because of the thermal efficiency, you can do that, too. Of course, seeing the 8V intake adapted to a 16V head certainly isn't ideal.
From there, you deal with thermal efficiency, which is how efficiently the engine turns that air and fuel into power (the term is "indicated power," which does not account for friction). And that is compression ratio and chamber shape. That stuff is defined by bore size (unchanged) and spark plug location. The ideal place is in the dead smack middle of the chamber. Which is where it is on the 16V, but not on the 8V.
Finally, there is mechanical efficiency, which is how much power it takes to move things around. Dominated by ring swept area. The other cam may take a little bit of power to turn, but it iis pretty miniscule.
So yeah, I'm POSITIVE it will make more power in the upper RPM band. Unless you design it to make more torque in the low and midrange. But because of the thermal efficiency, you can do that, too. Of course, seeing the 8V intake adapted to a 16V head certainly isn't ideal.
Meaning, you would give the engine an 80mm intake valve (just for arguing) that would flow the same as 2 34mm valves, but the air would be moving so slow that you would need incredible engine speed to be able to fill the cylinders effectively.
For one, the 2.5L 16v heads are rare as straight men at a Cher concert.
2. There is quite a bit of modification needed to make it work. New intake, New exhaust, then you have to tune the whole setup, and have a way to tune it....
01-05-2011, 02:55 AM
#9
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keep things simple; instead of taking an undersized displacement engine and adding a few cubic inches via improved airflow why not just add about 180 cubic inches ? take a fugly, 1975 Chevy Vega w/ ****box 4 cylinder motor.... add sbc. result: world's fastest 0-100-0 car mod for the 1970s....
the Monza, with chasis based on the Vega, but about 150 pounds heavier, to support the hachback and larger displacment engines, came with 5-speed manual, and 4.5, 5.0 and 5.7 litre V8 options... amazing the similarities between the Chevy Monza and Porsche 928, or 944 V8.
can you imagine a Vega (like the car in the photo) with a properly built, 377 c.i. sbc, lowered suspension, modern wheels and wide, Y-rated tires, and swapped in manual gearbox and 2.29:1 rear diff ? you'd be talking about a 170 mile per hour car for short money.
not bad for the 1970s and just bolt-in parts. 1st photo is a Vega. 2nd photo is the Monza.
.
the Monza, with chasis based on the Vega, but about 150 pounds heavier, to support the hachback and larger displacment engines, came with 5-speed manual, and 4.5, 5.0 and 5.7 litre V8 options... amazing the similarities between the Chevy Monza and Porsche 928, or 944 V8.
can you imagine a Vega (like the car in the photo) with a properly built, 377 c.i. sbc, lowered suspension, modern wheels and wide, Y-rated tires, and swapped in manual gearbox and 2.29:1 rear diff ? you'd be talking about a 170 mile per hour car for short money.
not bad for the 1970s and just bolt-in parts. 1st photo is a Vega. 2nd photo is the Monza.
.
Last edited by odurandina; 01-05-2011 at 03:55 AM.
01-05-2011, 03:01 AM
#10
Burning Brakes
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There is a lot more too it than just "opting" for the DOHC setup.
For one, the 2.5L 16v heads are rare as straight men at a Cher concert.
2. There is quite a bit of modification needed to make it work. New intake, New exhaust, then you have to tune the whole setup, and have a way to tune it....[/QUOTE]
So essentially a new stand alone EMS would be a must?
For one, the 2.5L 16v heads are rare as straight men at a Cher concert.
2. There is quite a bit of modification needed to make it work. New intake, New exhaust, then you have to tune the whole setup, and have a way to tune it....[/QUOTE]
So essentially a new stand alone EMS would be a must?
01-05-2011, 03:17 AM
#13
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The key phrase was "have a way to tune it". And I'll add, or know someone who can tune it.
The DME is quite capable - it is simply a matter of updating the software.
The DME is quite capable - it is simply a matter of updating the software.
01-05-2011, 09:44 AM
#14
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Very true. Still effective too. Aftermarket stuff however is increasingly more common and simple to install and use. I think primarily of Megasquirt, but there are many others as well. If you were going to go through the hassle of swapping in a 16v head, you have enough already invested that you may as well have total control over your fuel and ignition systems. I really liked how I could tune the car on the dyno, then I could tweek as much as I wanted to when ever I changed something else on the car. Even got rid of some of the "old tech" sensors. No more Bosch Flapper style air flow meter! All replaced with modern off the shelf sensors. No this was not on my Porsche, but on my 280Z with OEM Bosch L Jetronic fuel injection. Same idea though.
To the OP. I believe that swapping a 16v head to be more involved than most are prepared, and if you find a head, it is likely already on an engine, and you may as well swap the whole motor. It would be a great project IF you had the parts already available to you, and you had some spare time.
01-05-2011, 10:44 AM
#15
With the 16v motor I don't think you could use the stock turbo setup off a 944 turbo. As far as I know the S motor block has different water jackets than the 8v block. Correct me if I am wrong. That is why people go with the larger 2.7 or 3.0 for a 16v motor.
If you could make the stock turbo setup with manifolds fit onto an S motor with lower compression then you just need to do it. This is a very general idea, but it is what is needed to be done. It would be a fun project. $$$$$$ would be involved thought. 16v heads are costly, and then there is the whole idea where you can't just stop at a 2.5 motor you will want something bigger and so on and so forth.
If you could make the stock turbo setup with manifolds fit onto an S motor with lower compression then you just need to do it. This is a very general idea, but it is what is needed to be done. It would be a fun project. $$$$$$ would be involved thought. 16v heads are costly, and then there is the whole idea where you can't just stop at a 2.5 motor you will want something bigger and so on and so forth.