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Old 09-01-2016, 11:51 AM   #1
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Does boxer exhaust valve rythm reduce exhaust scavenging effect?

I've been thinking about all the posts about the lack of gains from exhaust and intake upgrades. Frankly I find it hard to believe that it should be so difficult to get more than 204hp from this engine. After all Renault reliably get 197hp from the 4 cylinder normally aspirated Renault Sport engine in my Clio. The boxer is abour 20% larger. The Renault is only 2.0 liters with a similar basic on-off variable cam system.

I noticed when timing the cams on the boxer engine that all four cams are "parked" at the same time, meaning all the valves are closed for one "beat" of the cycle. Whereas on my 944 one cylinder will always have valves open when in TDC, keeping the "beat" constant

Could it be that the 1,2,3 and, 1,2,3 and rythm is decreasing the effectiveness of the scavenging putting more emphasis on other design factors of the exhaust such as back-pressure?

If this is a cause for the lack of response to exhaust upgrades could some kind of X-pipe perhaps improve the scavenging?
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Old 09-01-2016, 12:16 PM   #2
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Old 09-01-2016, 12:44 PM   #3
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Camshafts are far from my cup of tea, but looking at the numbers, you can see that the Clio's engine has less peak torque, and at a higher rpm (215Nm @ 5500rpm for the clio and 245Nm @ 4500rpm for the boxster).

To achieve the same horsepower from a smaller engine, the clio has to have more aggressive camshafts profile that relies more on the scavenging effect to make power.
While the boxster relies on its displacement and not so much on the scavenging effect to achieve the same hp numbers, but with more torque and a wider powerband.
The clio RS also has a very good header from factory too compared to the boxster 2.5!

Also, I think that most headers available for the boxster are too big in diameter, that's why they don't show much gain. The size of the primaries on mine are as big as what I usually see on big V8s.

If I remember, on the 101 projects book, there is a 2.5 with camshafts that makes around 240hp? I have to check!

Feel free to correct me if I said anything wrong
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Old 09-01-2016, 01:09 PM   #4
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Some important thoughts here. Personally I want to maximize on-track performance. I don't mind losing a bit of torque as long as there is an overall gain. I can see why most people would be dissapointed by a loss of torque.

I agree that the boxser engine has more torque but the usable area under the curve does not feel that much larger since the Clio revs higher. This is on track. In traffic the Renault requires more work than the boxer engine to access its potential, so you are definitely right there.

FWIW the "kick" when the cam timing changes feels about the same on both engines. Also, the Clio can be easily and very affordably remapped to remove the environmental flat spot the factory put in.The Clio revs to 7500 and feels very quick. Of course it has a close-ratio 6 speed box which makes it easy to stay in the power band.

You are right about the 4-2-1 headers. They are a work of art. Renault also has a racing header for this engine. In race trim it delivers around 250hp. This brings me back to the original thought that the collectors on a 4 cylinder are more effective at scavenging than on a 6 cylinder boxer. The pulses will arive closer together in time and ther will be no pause per cylce which, as far as I can tell, there is on the 6 cyl. boxer.
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Last edited by Bebbetufs; 09-01-2016 at 01:15 PM.
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Old 09-01-2016, 01:26 PM   #5
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Nevermind what I was saying about the 2.5 boxster with cams...
Porsche Boxster Dyno Testing - 986 / 987 (1997-08) - Pelican Parts Technical Article

my knowledge stops there...
Maybe a cross pipe would help, but where to put it exactly to be effective? Plus there's no room.

Anyways, our engines will never be horsepower beasts :/
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Old 09-01-2016, 01:39 PM   #6
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Thank you for the information. My knowledge on this is virtually non-existant. At the moment I'm simply thinking out loud, hoping someone more knowledgable will pick up on it and move it along further. Perhaps we should ask this over in a 911 forum. There seems to be many engine "gurus" in them.

Were to put it can probably be calculated, but the lack of room may make it impractical. I did see a test on x-pipes vs. h-pipes. The difference wasn't great, but the effect was similar. I belive this was on a BMW straight-six. Perhaps an h-pipe could be crammed in at the right spot.
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