LivingCanvas

iTrader: (4)

Ive heard arguements both ways, and I want scientific proof one way or the other.

Which makes more power?
Which makes more avg. power across the band?
Which one will make the car faster?

Im thinking also that there might be a different answer for what the cars intended use is, so...

Street?
Strip?
All arounder?

I would like some actual data here, not just what you heard under the shade tree

jimmyblue

iTrader: (11)

If you "convert" all your gas pressure to torque before
the exhaust valve opens and open is at / after BDC,
then backpressure opposes the crank and is a loss.
If you have backpressure and an open exhaust valve
before BDC then backpressure aids until the BDC point
but then still opposes. The cam decides the weighting.

Scavenging makes a more potent charge. This wants
its own spark adjustment as it affects burn time.
Over-scavenging wastes fuel, the right amount will
maximize charge drawn in and its quality. Because
the burn rate changes, the same timing that put you
on maximum leverage, maximum torque contribution,
may now be too early and you draw off more of the
pulse energy as heat, less as crank power, possible
net loss despite a better charge to start. This is a
tuning problem, not a mechanical choice kind of one.

In general you do not want pumping losses, and the
exhaust backpressure is.

Your question as posed, "either or", seems off the
mark to me. There's quantities and combinations.
But it's an easy experiment to do at home; take a car
with cam, headers and a good exhaust, run it, jam
a pair of bananas up the tailpipes and repeat.

LivingCanvas

iTrader: (4)

Cool. Very nice reply.

I didnt really mean to pose this as a 'either or' question. What brought it up was, recently, some one told me my car would run stronger if a put a catted Y on in place of my ORY. He said because engines need a certain amount of backpressure. But I thought 'how can a restriction in the exhaust make it run better?'

HOHN

From a certain perspective, you will always have backpressure. The only Zero-Backpressure system out there is in outer space. As long as you are running an engine in atmospheric air, there will be "backpressure" from the static pressure at the end of the tailpipe.

I've always found it odd that people don't give more thought to where their pipes terminate, as some placements are better than others. If you dump the pipe into an area of smooth, hi-speed flow, then Bournoulli tells us that you have lower pressure there, and the exhaust flow will be enhanced because it is essentially being "sucked" out of the pipe.

This is why going to big on the pipe diameter can really kill power, because the exhaust flow has more atmosphere to fight against. Pipe diameter is always a compromise-- the ideal only exists for ONE combination of throttle position, timing, load, IAT, etc etc-- everything else is compromised to some degree.

A good rule to remember is that too small kills power less than too big-- whether it's port size, header diameter, or exhaust pipe size.

JMO

edcmat-l1

iTrader: (10)

Scavanging, no doubt, over back pressure. But as stated earlier by JB, you can over scavange, which will end up reducing power.
Backpressure, in anything other than a turbo app., should be held to a minimum. Excessive back pressure will kill power BAD.
A rule of thumb is less than 1.5 PSI of back pressure under WOT.
We had a Camaro in a few weeks back. Head and cam car. Made crappy power. Put a back pressure gauge in the 02 bung, ran it on the dyno, soon as I hit the throttle, it went to damn near 15 psi!!
Dropped the exhaust and picked up 30 hp at the wheels.

edcmat-l1

iTrader: (10)

I strongly disagree with this statement. I have seen, time and time again, too small of an exhaust, or carburetor, kill a motor.
Now, we're not talking about putting a 4 inch exhaust on a 4 banger, but something like a vacuum secondary 600 on a 383. Swap it out for a 750 and pick up MAD power.
Same with exhaust (see post above)
When it comes to airflow, MOST of the time its better to err on the side of bigger.
This doesnt mean putting 360 CC heads on a 400 inch motor. That unreasonable. And I know others will chime in with the whole velocity issue.
But, even those supporters are talking about using heads with ports and valves SLIGHTLY smaller. Not building 400 inch motors and putting stock heads back on with small valve and ports.

HOHN

I can see your point. I guess it becomes a matter of degrees. Let's frame the discusssion this way: it is better to have a port 25ccs too big or 25cc too small? Or is it more appropriate to think in terms of percentage?

I'd expect the "too big" port to pick up some high end power, but the smaller port will likely have a more area under the curve.

For exhaust, I suspect pipe size mismatch will show up in the power curve, with oversized pipe losing at low RPM and undersized pipe suffering up top.


As for carburetor size, I think a lot of people run a carb that's too small, especially for a dual plane intake. Heck, a 383 with a Performer RPM can run a 1150 CFM carb effectively and make the best power. Swap in a single plane intake and ideal carb size drops to 750 or so.


Personally, I'm a fan of running large vaccum secondary carbs, as I believe they offer the best of both worlds.

That new King Demon RS with the vacuum secondaries is to me the ultimate street carb.

JMO

Alvin

iTrader: (2)

One big thing no-one noted is how back pressure negatively affects volumetric effiecency (not talking about pumping losses, but reversion and residual gas) especially on engines with low compression ratios.

.

edcmat-l1

iTrader: (10)

Whether or not I stated it as such, it drastically affects the VE, or pumping efficiency, or however you want to state it. Point is, you get much over 1 PSI of back pressure and you start to decrease overall power output.