Aussie Charger
Well-Known Member
So how does a rocket produce thrust?
Is More Flow Better, Is The Smallest Intake Port That Flows The Most The Best
- Thread starter 273
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Really?
A rocket and a piston engine apply force differently a rocket is a continuous combustion and engine is a bunch of short ones applied to a rotating assembly.
Aussie Charger
Well-Known Member
What moves a rocket?
Aussie Charger
Well-Known Member
Answer: combustion (a force)
What moves a car?
Answer: combustion (a force)
What causes acceleration?
Answer: a force
Is horsepower a force?
Answer: No. It is a measurement of how often a force is applied
So horsepower doesn't move the car?
Answer: No. The force (torque) moves the car. The force is continuously applied to increase acceleration of the mass (car), but it is the force (combustion) that produces the acceleration.
What moves a car?
Answer: combustion (a force)
What causes acceleration?
Answer: a force
Is horsepower a force?
Answer: No. It is a measurement of how often a force is applied
So horsepower doesn't move the car?
Answer: No. The force (torque) moves the car. The force is continuously applied to increase acceleration of the mass (car), but it is the force (combustion) that produces the acceleration.
Combusting Jedi
Aussie Charger
Well-Known Member
Now you're on to something.
So we agree that combustion produces a force which produces acceleration?
So we agree that combustion produces a force which produces acceleration?
no
And it's how often that force is applied is what that matter to us, what powers us down the road and we call it hp. We can calculate how much hp needed for quarter mile , top speed, hwy speed, towing etc...
Done done done
Aussie Charger
Well-Known Member
Well . . . there are other ways of multiplying the force. You could have a lot of torque at a low rpm and still produce a lot of horsepower. V8 vs 4-banger for example. There's also a reason why torque and weight are credited with breaking parts and not horsepower . . . because torque (force) acts agains weight (mass) to produce acceleration.
Anyway, good night. At least we can have a healthy debate without calling each other a "wanker"
dirty white boy
50 yr old Juvenal delinquent
rocket engines produce thrust by the expulsion of an exhaust fluid that has been accelerated to high speed through a propelling nozzle.... ant got nothing to do with a 4 stroke internal combustion engine....DWB!
Phreakish
Well-Known Member
P=fv
V is analogous to rpm.
More revs will always produce more power, all things equal.
Combustion engines produce a range of torque over the rpm range.
A "torque" engine tends to produce more down low and less up high.
A high revving engine tends to produce less torque down low, but makes the same or more torque at higher revs.
Power=torque*revs. Half the revs takes twice the torque to come out equal. Show me a "torque build" that ever doubles a race combo. Yet race combos often run twice the rpm of so called "torque" builds.
All the equations always support the higher revving system making more power, accomplishing more work, or reaching a faster speed within a set time or distance.
The high revver wins the drag race every time because of this.
This t'aint hard. No need for rockets or any other tortured analogies.
Torque is great, but without rpm you have a dumptruck. Sure, it'll get to 25mph in a hurry, or from 25 to 50, but then it's out of steam (power).
V is analogous to rpm.
More revs will always produce more power, all things equal.
Combustion engines produce a range of torque over the rpm range.
A "torque" engine tends to produce more down low and less up high.
A high revving engine tends to produce less torque down low, but makes the same or more torque at higher revs.
Power=torque*revs. Half the revs takes twice the torque to come out equal. Show me a "torque build" that ever doubles a race combo. Yet race combos often run twice the rpm of so called "torque" builds.
All the equations always support the higher revving system making more power, accomplishing more work, or reaching a faster speed within a set time or distance.
The high revver wins the drag race every time because of this.
This t'aint hard. No need for rockets or any other tortured analogies.
Torque is great, but without rpm you have a dumptruck. Sure, it'll get to 25mph in a hurry, or from 25 to 50, but then it's out of steam (power).
Phreakish
Well-Known Member
Yup. Momentum. High heat equals expansion, which is used to propel the mass of the spent fuel at a high speed.
M1v1=M2v2
Use a converging/diverging nozzle to reach supersonic speeds and the expansion results in even greater gains in exhaust velocity.
Since the available momentum is tied to the mass of the propellant, we get what's called "the tyrrany of the rocket equation" (the more mass you launch, the more fuel mass you need, which increases the mass and requires more fuel which..).
@273 the more efficient(better designed) a port is, the higher a velocity it can operate at. High velocity is "free" supercharging, augmented by complimentery runners. They actually work better with smaller valves because more curtain area is used, which allows for less shrouding as well. Poorer ports tend to work better with larger valves deshrouded because they need as much low-mid lift flow as possible to crutch poorer high velocity flows, most OE ports are poor to decent. Very few are great, but some can be modded to be very good/great, so that's where it stands.
Phreakish
Well-Known Member
That's a good generalization.
There's also some practical considerations.
Higher velocity means higher momentum and smaller turning angles. A smooth port that's as close to straight as possible can function well at higher velocities than a port with sharp changes in direction.
There are upper limits to port velocity even when perfectly straight, and it can get complex. Especially as it flows across the valve and into the chamber and expands.
Basically: it depends.
The carb, heads, and intake all need to match the intended flow capability of the shortblock.
Rat Bastid
Dunamis Metron
If you are going to port heads velocity management is imperative.
Just standing at a flow bench and getting more flow and running the velocity up can get you in trouble.
A running engine isn’t static, and adding fuel to air changes everything.
Ive seen way more heads with a bunch of velocity lose power because once you add fuel you change everything.
Yep. I am trying to steer this thread back onto the original track, lol...
Agreed, it's not an automatic, & even wet-flow testing can let you down. I used to high-speed videotape through an acrylic cyl. while dusting white flour over the intake manny, watching where the major flow was taking it. It was half-assed I guess, but at the time, it was still eye-opening some of the things it showed Me.
Phreakish
Well-Known Member
Yup.
The only reason fuel vaporizes without heating it in a carb is the pressure drop which results from the increase in velocity. Local pressure drops under vapor pressure and poof, gas vapors.
Vapors turn corners better, but sharp corners have stagnation points which can push the local pressure over the vapor pressure and then fuel condenses out.
Boundary layers are also slower, and the gradient is related to the Reynolds number, which is a complex subject but suffice to say slow flow will have a slower velocity gradient next to a wall which makes it even more difficult to keep fuel evaporated. A liquid in the flow will also introduce turbulence and can totally bone the flow. Not always, but it makes it more likely.
It's also not just that vaporized fuel is a good thing, it's that a good port will tend to keep fuel in vapor form. The conditions that make it precipitate out also steal energy from the flow and will reduce flow potential as a result. Vaporized fuel is easier to burn, but the pressure loss and recovery across a valve will almost always condense fuel out in certain conditions - typically at higher manifold pressures. The chamber and valve job should have more effect on this than the port though.
Another consideration is that smaller/faster ports can limit the available "supercharging effect" of overlap, since the pressure wave is going to significantly increase velocity. If the port is already choked, the effect will be diminished. This is why it's foolish for people to choose parts based on matching flow numbers (not to mention flow is tested at depressions that don't adequately match an operating engine!).
Part choice and design need to match intended goals.
I agree velocity is important but I think over worrying about leads to strangling your engine, say you have a zero decked 367 with a xe285hl cam, for most their choices for heads is what is available, at the very least stock 1.88 heads and max for most trick flow. It comes down to $$$ and how much power for most they have little control over it.
Velocity like everything else about balance and compromises, too much of anything can be bad.
Rat Bastid
Dunamis Metron
Thats cool. Any testing you can do that helps visualize what a mostly invisible entity is doing is awesome.
Phreakish
Well-Known Member
Doesnt seem to be many SBM heads that are "too slow" to begin with either.
I think people worry themselves with port velocity to rationalize not going to greater lengths with their porting. Over-porting seems mostly to be a worry when people focus on the wrong areas and wind up with wide and narrow spots along the port or just blow out areas that are easy to reach.
Or they just ruin really important spots thinking they're improving things...
Also because people are cheap and think offset rockers are too complicated.
Rat Bastid
Dunamis Metron
AMEN TO THAT!! The SBM is cross section limited.
340 is slightly on the small side and a fairly low power with reasonable powerband, never heard anyone say 340 would be better off with smaller port now most start with a 360 thats 20+ cid over a stock 340, ideally at minimum you think you want to start with a head slightly more port then a 340 even for a mild combo nevermind something with some teeth to it. Even a 318 is only slightly smaller than a 340 especially with an overbore, it has heads originally designed for a 273.
lead69
hopeless car junkie
You better be careful or @Rat Bastid will die of elation that someone else get's it lol. Seriously, it's nice to see some smart, articulate people who understand what the hell is going on. I'm guessing some of these guys are getting bad drinking water or something because the stuff they come up with is mind boggling.
Rat Bastid
Dunamis Metron
Think about this.
The W2 head was designed for 340 inch engines. Look at the cross section of that head. And that was 1975.
That head is too small for 340 inches turning 8500.
Ive seen heads that were said to be Glidden’s heads but year later I was told if they weren’t brazed up they weren’t his heads.
I’ve seen fully brazed, record setting 292 GM castings by Lee Shepard (he had 2 pairs and a spare and the receipt for them so I know they were legit) and they were brazed up about everywhere.
And those heads were for a 287 inch engine turning 9100 in the lights.
lead69
hopeless car junkie
I was just having a conversation with rb last night and was telling him about how I used to make the port just big enough to reach a number but still keep the port small for velocity. I had one set of j heads that I experimented on and got 234 @.300, 242 @.450 and down to 232 @.500, at the time I was really impressed and they worked great for the low lift cam the guy was using. Now I look back and relish the experience but kind of shake my head at how I will never bother with that again, kind of like my full port a 318 head experience, now I just hit the speed master button, mild port work, cam it right and make more power everywhere.
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