N/A Horsepower
03-07-2012, 12:58 AM
#16
Nordschleife Master
03-07-2012, 02:43 AM
#19
RL Community Team
Rennlist Member
Rennlist Member
I thought this thread would be about NA horsepower but it's just about how to increase pollution and give asthma to babies. 
Seriously though, if you want cheap easy horsepower, you can get a brand new high-flow cat for like $50 online. It will be the best of both worlds: more flow and therefore more peak hp, and more backpressure than a test pipe so you might have more low end torque AND it won't make those popping sounds that most catless cars seem to make. The added bonus is it will keep the air cleaner too.
Seriously though, if you want cheap easy horsepower, you can get a brand new high-flow cat for like $50 online. It will be the best of both worlds: more flow and therefore more peak hp, and more backpressure than a test pipe so you might have more low end torque AND it won't make those popping sounds that most catless cars seem to make. The added bonus is it will keep the air cleaner too.
I don't think I have seen you post in 5 years or so.. Welcome..
03-07-2012, 01:44 PM
#23
Rennlist Member
regarding "back pressure" and "exhaust gas velocity", I'll add to the internet melee:
My understanding is that it's really collector placement - where the individual exhaust manifold pipes merge that makes the real difference. Due to the Bernoulli principle, as fluid (air in our case) travels through a pipe with an opening, there will be a vacuum at that opening. The exhaust pulses traveling through the header, when it gets to the collector, creates a vacuum on the other pipes. If the length of the headers are right, and the collector placement is right, and the intake/exhaust valve overlap is right, this vacuum through the exhaust manifold will help suck air thought the open intake valve and into the combustion chamber.
This is called the exhaust scavenge effect - and, in fact, this plays more of a role sucking air into the engine than just the displacement of the piston moving down on the "intake stroke".
Now, because exhaust velocities increase as RPM goes up (because the pulses get shorter in duration), you can tune for the max scavenging effect at different parts of the power band through header design.
My understanding is that it's really collector placement - where the individual exhaust manifold pipes merge that makes the real difference. Due to the Bernoulli principle, as fluid (air in our case) travels through a pipe with an opening, there will be a vacuum at that opening. The exhaust pulses traveling through the header, when it gets to the collector, creates a vacuum on the other pipes. If the length of the headers are right, and the collector placement is right, and the intake/exhaust valve overlap is right, this vacuum through the exhaust manifold will help suck air thought the open intake valve and into the combustion chamber.
This is called the exhaust scavenge effect - and, in fact, this plays more of a role sucking air into the engine than just the displacement of the piston moving down on the "intake stroke".
Now, because exhaust velocities increase as RPM goes up (because the pulses get shorter in duration), you can tune for the max scavenging effect at different parts of the power band through header design.
10-16-2013, 06:15 AM
#24
Well, i know this thread is a bit old, but I think an exhaust scavenge effect with a stock 944 camshaft does not work properly.
Here are the timings of the early vs. late camshaft
EARLY
05 Cam (-01/49 - 43/-03)
Overlap: -4.00 degrees (no overlap)
Intake Duration: 228.00 degrees
Exhaust Duration: 220.00 degrees
Intake Installed Centerline of 115.00 degrees ATDC.
Exhaust Installed Centerline of 113.00 degrees BTDC.
So the early camshaft closes the exhaust 3 degrees before TDC and the intake opens 1 degree after TDC. For the time of 4 degrees in the Exhaust/InTake Cycle both valves are closed. This is rather a sort of second Compression Cylce than a Scavenge Cycle.
LATE
09 Cam (-01/49 - 47/01)
Overlap: 0 degrees (no overlap)
Intake Duration: 228.00 degrees
Exhaust Duration: 228.00 degrees
Intake Installed Centerline of 115.00 degrees ATDC.
Exhaust Installed Centerline of 113.00 degrees BTDC.
The late Camshaft with its longer Exhaust Duration is a bit improved, but still no overlap.
Dimitri Elgin discribes the Scavenge Cycle in his Campaper
:
http://www.elgincams.com/campaper.html
From my opinion regarding the camshaft, there should be some room for improvements. Having said that, provided the prevailing opinion here that the 944 stock camshaft is good, could someone explain to me, why the 944 N/A apparently doesn't need to have intake/exhaust valve overlap?
regarding "back pressure" and "exhaust gas velocity", I'll add to the internet melee:
... If the length of the headers are right, and the collector placement is right, and the intake/exhaust valve overlap is right, this vacuum through the exhaust manifold will help suck air thought the open intake valve and into the combustion chamber.
This is called the exhaust scavenge effect - and, in fact, this plays more of a role sucking air into the engine than just the displacement of the piston moving down on the "intake stroke".
....
.
... If the length of the headers are right, and the collector placement is right, and the intake/exhaust valve overlap is right, this vacuum through the exhaust manifold will help suck air thought the open intake valve and into the combustion chamber.
This is called the exhaust scavenge effect - and, in fact, this plays more of a role sucking air into the engine than just the displacement of the piston moving down on the "intake stroke".
....
.
Here are the timings of the early vs. late camshaft
EARLY
05 Cam (-01/49 - 43/-03)
Overlap: -4.00 degrees (no overlap)
Intake Duration: 228.00 degrees
Exhaust Duration: 220.00 degrees
Intake Installed Centerline of 115.00 degrees ATDC.
Exhaust Installed Centerline of 113.00 degrees BTDC.
So the early camshaft closes the exhaust 3 degrees before TDC and the intake opens 1 degree after TDC. For the time of 4 degrees in the Exhaust/InTake Cycle both valves are closed. This is rather a sort of second Compression Cylce than a Scavenge Cycle.
LATE
09 Cam (-01/49 - 47/01)
Overlap: 0 degrees (no overlap)
Intake Duration: 228.00 degrees
Exhaust Duration: 228.00 degrees
Intake Installed Centerline of 115.00 degrees ATDC.
Exhaust Installed Centerline of 113.00 degrees BTDC.
The late Camshaft with its longer Exhaust Duration is a bit improved, but still no overlap.
Dimitri Elgin discribes the Scavenge Cycle in his Campaper
:http://www.elgincams.com/campaper.html
...The Scavenge Cycle: Intake Valve Opening to Exhaust Valve Closed
The scavenge cycle occurs during the overlap period, when intake and exhaust valves are both open at the same time. The intake valve is just opening. The exhaust is closing but not yet seated. Overlap is what the cam and valves are doing, dictated by the combination of total cam duration and the locations of lobe centers. Scavenging is what the engine is doing with that.
A good number of engine processes (and a few unsolved mysteries) are going on now simultaneously. The most important are (1) scavenging the last of the exhaust gases as much as possible from the clearance volume, where the piston cannot reach to push them out, and (2) initiating intake flow into the cylinder without wasting very much of it out the open exhaust valve.
Overlap duration increases as total duration increases, and it also increases as the lobe center decreases. Increasing the time for Overlap makes more time for scavenging at high rpms. Residual exhaust gases kill power twice over: they displace their volume in incoming charge, and later during combustion they absorb heat that should have gone into making power. At 5000 rpm an engine with a high-performance cam carrying 55 degrees of overlap must complete the entire scavenge cycle in less than two thousandths of a second.
In standard engines, valves are open together for only 15-30 degrees of overlap. In a race engine operating between 5000 and 7000 rpm, the overlap period is more like 60-100 degrees. The penalty for so much overlap in a street engine is very poor running at lower rpms, when a lot of the intake charge has time to sidetrack directly out the open exhaust valve.
.....
The scavenge cycle occurs during the overlap period, when intake and exhaust valves are both open at the same time. The intake valve is just opening. The exhaust is closing but not yet seated. Overlap is what the cam and valves are doing, dictated by the combination of total cam duration and the locations of lobe centers. Scavenging is what the engine is doing with that.
A good number of engine processes (and a few unsolved mysteries) are going on now simultaneously. The most important are (1) scavenging the last of the exhaust gases as much as possible from the clearance volume, where the piston cannot reach to push them out, and (2) initiating intake flow into the cylinder without wasting very much of it out the open exhaust valve.
Overlap duration increases as total duration increases, and it also increases as the lobe center decreases. Increasing the time for Overlap makes more time for scavenging at high rpms. Residual exhaust gases kill power twice over: they displace their volume in incoming charge, and later during combustion they absorb heat that should have gone into making power. At 5000 rpm an engine with a high-performance cam carrying 55 degrees of overlap must complete the entire scavenge cycle in less than two thousandths of a second.
In standard engines, valves are open together for only 15-30 degrees of overlap. In a race engine operating between 5000 and 7000 rpm, the overlap period is more like 60-100 degrees. The penalty for so much overlap in a street engine is very poor running at lower rpms, when a lot of the intake charge has time to sidetrack directly out the open exhaust valve.
.....
Last edited by H.F.B.; 10-16-2013 at 09:10 AM.
10-16-2013, 07:46 AM
#25
Race Director
H.F.B., you are correct that there is room for improvement in the camshaft. In fact, the lack of overlap that you point out is the key reason the 944 engine with the stock camshaft does not respond well to modifications such as a long tube cold air intake or a header. It's hard to improve the volumetric efficiency with no overlap on the camshaft.
I used to race in a class that did not allow for changing the camshaft and that made producing more horsepower exceedingly difficult.
I used to race in a class that did not allow for changing the camshaft and that made producing more horsepower exceedingly difficult.
10-16-2013, 07:59 AM
#26
Race Director
regarding "back pressure" and "exhaust gas velocity", I'll add to the internet melee:
My understanding is that it's really collector placement - where the individual exhaust manifold pipes merge that makes the real difference. Due to the Bernoulli principle, as fluid (air in our case) travels through a pipe with an opening, there will be a vacuum at that opening. The exhaust pulses traveling through the header, when it gets to the collector, creates a vacuum on the other pipes. If the length of the headers are right, and the collector placement is right, and the intake/exhaust valve overlap is right, this vacuum through the exhaust manifold will help suck air thought the open intake valve and into the combustion chamber.
This is called the exhaust scavenge effect - and, in fact, this plays more of a role sucking air into the engine than just the displacement of the piston moving down on the "intake stroke".
Now, because exhaust velocities increase as RPM goes up (because the pulses get shorter in duration), you can tune for the max scavenging effect at different parts of the power band through header design.
My understanding is that it's really collector placement - where the individual exhaust manifold pipes merge that makes the real difference. Due to the Bernoulli principle, as fluid (air in our case) travels through a pipe with an opening, there will be a vacuum at that opening. The exhaust pulses traveling through the header, when it gets to the collector, creates a vacuum on the other pipes. If the length of the headers are right, and the collector placement is right, and the intake/exhaust valve overlap is right, this vacuum through the exhaust manifold will help suck air thought the open intake valve and into the combustion chamber.
This is called the exhaust scavenge effect - and, in fact, this plays more of a role sucking air into the engine than just the displacement of the piston moving down on the "intake stroke".
Now, because exhaust velocities increase as RPM goes up (because the pulses get shorter in duration), you can tune for the max scavenging effect at different parts of the power band through header design.
First of all, very few engines have greater than 100% volumetric efficiency (sucking in more volume that the swept volume of the cylinders). I believe this is true with F1 engines and I know it's long been true with GP bikes. That of course is going to be true only at a narrow rpm range in which the engine is designed to operate. Of course, the 944 engine is far from it.
As for header and intake tuning, the pulses reflect back at every opening and collection point. This is why 4-2-1 headers on most cars have a broader power band, but produce less overall hp typically. They have two (but weaker) pulse reflections as opposed to one stronger one in a 4-1 header. Ideally, the header should be designed so the reflected pulse bounces off the exhaust valve just before opening. This creates a lower pressure area just ahead of the exhaust pulse, helping with the scavenging. Placement of the collector(s) affects the timing of the reflected pulses, thus affects the rpm or operating range that the header works most effectively. On a street car engine that is going to tend to be toward the middle to upper end of the rev range. On a race engine that generally is going to be only at the upper limit of the rev range as that is where a race engine lives.
10-16-2013, 08:05 AM
#27
Race Director
I should point out that this same pulse reflection (resonance tuning) is how a long tube cold air intake makes power, NOT the induction of colder air.
I've had friends instrument 3 different cars (2 different platforms), including the 944, with thermocouples under the hood. The reality is, at speed under hood temps are near ambient. The long tube CAI tunes the intake pulse. AEM typically makes the best power producing CAIs. This is because they tune the length on a dyno. They will make their base run on a dyno, then install an extra-long CAI. They will make a pass and cut 1/4" off the length and repeat. Cut another 1/4" off and repeat, and so on. They produce a CAI at the length that produces the most hp.
I've had friends instrument 3 different cars (2 different platforms), including the 944, with thermocouples under the hood. The reality is, at speed under hood temps are near ambient. The long tube CAI tunes the intake pulse. AEM typically makes the best power producing CAIs. This is because they tune the length on a dyno. They will make their base run on a dyno, then install an extra-long CAI. They will make a pass and cut 1/4" off the length and repeat. Cut another 1/4" off and repeat, and so on. They produce a CAI at the length that produces the most hp.
10-16-2013, 08:57 AM
#28
Team Owner
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amen.
ahh, the 4 cylinder engine....
he accomplished increasing flow (exhaust velocity baby) at wot.
and while the pipes never were optimized for the mid-rpm range, but more to the side of wot, the engine was never making much power in the mid-rpm range anyway.... and now with the cat gone, the difference between the mid-range power and the power at wot has increased even more, while mid-range has stayed about the same.... thus making the engine appear even more crappy in the mid-rpm ranges. but in reality, it was crappy then and it's still crappy now.
and all the dyno-queen cars are only a little less pathetic below 4,000 rpm.
10-16-2013, 08:59 AM
#29
... In fact, the lack of overlap that you point out is the key reason the 944 engine with the stock camshaft does not respond well to modifications such as a long tube cold air intake or a header. It's hard to improve the volumetric efficiency with no overlap on the camshaft.
...
...
thank you, that makes sense to me.
10-16-2013, 11:25 AM
#30
Three Wheelin'
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Some VE improvements can be had while performing routine work on the car with some "while I'm at it" enthusiasm/time/money, but most require careful planning, and are best done when a full teardown/rebuild is in the works. Significant improvements can be made.
