Exhaust Duration, Boost and Elevation

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ptuomov
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Re: Exhaust Duration, Boost and Elevation

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CamKing wrote: Sun Sep 12, 2021 10:35 am
BLSTIC wrote: Sat Sep 11, 2021 7:18 pm
CamKing wrote: Sat Sep 11, 2021 3:39 pm With a Turbo, or a blower, the engine doesn't know it's at high altitude. It only knows the "Altitude" in the plenum.
You would cam them the same way you would at sea level.
If the pressure in the plenum is 46", that's what the engine sees.
Would the lower pressure seen in the exhaust system at altitude affect much for the blower engine?
If you overdrive the blower to get the same pressure in the intake manifold, the pressure in the exhaust would be the same.
The lower tail pipe exit atmospheric pressure has to impact the exhaust port pressure at least to some extent, but the question is whether that’s practically relevant.
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Re: Exhaust Duration, Boost and Elevation

Post by F-BIRD'88 »

CamKing wrote: Sun Sep 12, 2021 10:33 am
F-BIRD'88 wrote: Sun Sep 12, 2021 7:02 am Here's your cam.. low cr 383 cid 6000 rpm. supercharged street power where it matters on a street car.
Comp XE284H-10 12-250-3 Can use 1.6/1.5 rockers.
Install on 104/116 centers (110LSA)
That is a ridiculously horrible recommendation.
These cams work very well in street 383's.
The blower just essentually amplifies whats there.
The slight added advance from cam card enhances the blowers performance overall on a low cr 383 that does not want to go much beyond 6000 rpm.
The valve events (on 104/116 phasing) are street blower friendly on a simple 383. It will GLH.
Last edited by F-BIRD'88 on Sun Sep 12, 2021 10:44 am, edited 1 time in total.
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Re: Exhaust Duration, Boost and Elevation

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ptuomov wrote: Sun Sep 12, 2021 10:37 am
CamKing wrote: Sun Sep 12, 2021 10:35 am
BLSTIC wrote: Sat Sep 11, 2021 7:18 pm

Would the lower pressure seen in the exhaust system at altitude affect much for the blower engine?
If you overdrive the blower to get the same pressure in the intake manifold, the pressure in the exhaust would be the same.
The lower tail pipe exit atmospheric pressure has to impact the exhaust port pressure at least to some extent, but the question is whether that’s practically relevant.
You're talking about the atmosphere, the tail pile dumps the exhaust into, after the collector, and after the mufflers.
The engine will never see any effects from that.
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Re: Exhaust Duration, Boost and Elevation

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CamKing wrote: Sun Sep 12, 2021 10:43 am
ptuomov wrote: Sun Sep 12, 2021 10:37 am
CamKing wrote: Sun Sep 12, 2021 10:35 am
If you overdrive the blower to get the same pressure in the intake manifold, the pressure in the exhaust would be the same.
The lower tail pipe exit atmospheric pressure has to impact the exhaust port pressure at least to some extent, but the question is whether that’s practically relevant.
You're talking about the atmosphere, the tail pile dumps the exhaust into, after the collector, and after the mufflers.
The engine will never see any effects from that.
The turbo situation is interesting where my first instinct is that it could be very close to the same pressures in the intake and exhaust ports for a turbo compensated engine at different altitudes. For a belt driven supercharger, I’d guess that tail pipe pressure does alter the exhaust port pressure trace, especially for low pressure periods. The question is whether that’s a practically relevant change in terms of magnitudes.
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Re: Exhaust Duration, Boost and Elevation

Post by BLSTIC »

A vaguely related question here:

Say I had a turbo blow-through carb engine with a turbo that could come on boost before the stall converter, so it's effectively always on boost at full load...

If I set it up so that it ran 5psi at sea level (for 20psi-ish absolute) and at 6000ft I run 8psi (ambient pressure at 6000ft is 3psi lower, so it would be close enough to the same absolute pressure), would the carb calibration be the same? And if I had a MAF sensor somewhere in the piping, could I just raise boost with altitude until the MAF read the same?
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