curtis73 wrote: The same thing I wrote
You got there first! Nice work. Now you can tell angry why the smaller motor takes in less air whilst I go play elsewhere.
curtis73 wrote: The same thing I wrote
You got there first! Nice work. Now you can tell angry why the smaller motor takes in less air whilst I go play elsewhere.
what is this "stall" thing you speak of? people that build small inch, high revving engines don't know or care about that word..
They should... its a critical part of tuning a drivetrain if its an auto tranny.
Airflow requirement is mostly determined by HP.
Let's say you have a 500 hp 383. Now (hypothetically) swap out only the crank so you have a 302. Leave everything else the same. They will both make around 500 hp, but the 383 will do so at 6200 rpm - the 302 will do it at 7700 rpm, and the 383 will make heaps more torque and at a lower RPM.
The airflow demand of both engines will be the same - a larger engine spinning slower vs. a smaller engine spinning faster.
Smaller displacements don't make less power, they make less torque. The parts you have bolted on and in it are what determine HP.
If you have a desktop dyno simulation program, try it. Build yourself a 500 hp 383, then change the engine simulation to a 2.2L 4-cyl. Same HP, but much less torque, and much more revs to get it.
curtis73 wrote:Gimp wrote: I'm looking at building a 500+hp 302 for my C-Prepared car. The smaller motor allows the car to be lighter, which is a huge advantage in autocross.A 302 weighs almost exactly the same as a 383... or a 400... or a 434 SBC for that matter. Same block, the only weight savings is the crank, but when you're talking about the difference between 565 and 575 lbs it won't really help. ... or are you saying that class rules allow for a lighter weight car?
Yes, the entire car can be lighter if I stay under 310ci.
lower cubic inch = lower weight car . . .
Some of these responses should be in Toyman's thread and responses in his thread should be here 
Gimp wrote:curtis73 wrote:Yes, the entire car can be lighter if I stay under 310ci.Gimp wrote: I'm looking at building a 500+hp 302 for my C-Prepared car. The smaller motor allows the car to be lighter, which is a huge advantage in autocross.A 302 weighs almost exactly the same as a 383... or a 400... or a 434 SBC for that matter. Same block, the only weight savings is the crank, but when you're talking about the difference between 565 and 575 lbs it won't really help. ... or are you saying that class rules allow for a lighter weight car?
Ahh, well that does change things. My last post was incorrect then. If you're competing against people with bigger engines, you need to make more RPM to make the same power. Concentrate on valvetrain. Spend a ton of money on springs/pushrods/retainers etc.
curtis73 wrote: Airflow requirement is mostly determined by HP. (snip) The airflow demand of both engines will be the same - a larger engine spinning slower vs. a smaller engine spinning faster.
or in other words, for a given RPM, a smaller engine takes in less air? fascinating. i wish i had thought of that. 
see, there was never a discussion of the amount of power being made at a particular RPM, only the amount of air being consumed by an engine at a particular RPM. it's just a volume relationship. a smaller cylinder holds less air than a larger cylinder because it is smaller. what if i have two engines of identical displacement, but one has 12:1 compression and the other has 6:1. which one uses more air at a given RPM? neither! because their displacements are the same. which one makes more power? the one with the higher compression ratio.
what if i have two engines of identical displacement, but one has 12:1 compression and the other has 6:1. which one uses more air at a given RPM? neither! because their displacements are the same. which one makes more power? the one with the higher compression ratio.
Disagree... The higher compression ratio engine will have a much higher VE, meaning that for each intake stroke it will ingest more air despite having the same displacement.
The 6:1 engine will have a peak VE of about 65% - meaning that for each intake stroke it will only ingest about 65% of the potential mass of air suggested by its volume. The 12:1 engine will have closer to 85-90% VE with dramatically higher airflow requirements.
If you want to get technical, the reason is also partly due to BSFC. The 6:1 engine will be far less thermally efficient; meaning that it won't effectively extract as many BTUs from the fuel to convert to heat/power. So, the airflow requirements per HP will be a bit higher than the 12:1 engine... but not enough to require the same mass flow. That is to say, if the high compression engine makes 70% more hp, it won't require a full 70% more mass flow to make that additional HP since its doing more with what you give it.
the point remains that HP goes proportionally with airflow. It makes sense since fuel carries a certain amount of stored BTUs per unit. More power will require more air/fuel all things being equal.
curtis73 wrote:what is this "stall" thing you speak of? people that build small inch, high revving engines don't know or care about that word..They should... its a critical part of tuning a drivetrain if its an auto tranny.
and why would someone put an auto tranny behind something like a Chevy 302? Chevy only put 4 speeds and 3.73-4.56 gears behind them in the 67-69 Camaros and silly stuff like air conditioning wasn't even on the option sheet with that engine..
and why would someone put an auto tranny behind something like a Chevy 302? Chevy only put 4 speeds and 3.73-4.56 gears behind them in the 67-69 Camaros and silly stuff like air conditioning wasn't even on the option sheet with that engine..
Autos are for wussies
You're right about auto trannys not being an option on the COPO 302s, but you could get an auto behind a 302 in a Z28. It came with a 3100 stall converter too.
curtis73 wrote:what if i have two engines of identical displacement, but one has 12:1 compression and the other has 6:1. which one uses more air at a given RPM? neither! because their displacements are the same. which one makes more power? the one with the higher compression ratio.Disagree... The higher compression ratio engine will have a much higher VE, meaning that for each intake stroke it will ingest more air despite having the same displacement. The 6:1 engine will have a peak VE of about 65% - meaning that for each intake stroke it will only ingest about 65% of the potential mass of air suggested by its volume. The 12:1 engine will have closer to 85-90% VE with dramatically higher airflow requirements. If you want to get technical, the reason is also partly due to BSFC. The 6:1 engine will be far less thermally efficient; meaning that it won't effectively extract as many BTUs from the fuel to convert to heat/power. So, the airflow requirements per HP will be a bit higher than the 12:1 engine... but not enough to require the same mass flow. That is to say, if the high compression engine makes 70% more hp, it won't require a full 70% more mass flow to make that additional HP since its doing more with what you give it. the point remains that HP goes proportionally with airflow. It makes sense since fuel carries a certain amount of stored BTUs per unit. More power will require more air/fuel all things being equal.
Curtis- you are mixing your efficiencies up.
Upping the CR doesn't change your volumetric efficency, it changes your combustion efficiency. From 6:1 to 12:1- it changes a LOT.
Yes, you do get a lot more heat-energy conversion at 12:1, but that's not voleff.
Oh, and technically, you have it backwards- gernerally, airflow defines power- but that's being nit-picky.
Eric
Curtis- you are mixing your efficiencies up. Upping the CR doesn't change your volumetric efficency, it changes your combustion efficiency. From 6:1 to 12:1- it changes a LOT.
Sorry, but 6 years of engineering edumacation and 15 years of engine design and building tell me otherwise.
It dramatically changes VE. Try it on some dyno sim software.
In reply to curtis73:
my "fascinating" comment was a poke at tuna55, not you. see, tuna disagreed with my statement that a larger cylinder is larger than a smaller cylinder, and asked you to explain it to me since i was too stupid to understand his explanation. then you said exactly what i said, and tuna agreed with you. that gives me LOLs, so i shared.
because i don't have empirical data, i will agree in theory that higher CR engine may have slightly higher VE due to better scavenging / cylinder filling in overlap as the exhaust leaves the cylinder. but i think you might be mixing VE with BMEP when you say the high-cr engine will have a "much higher" VE.
anyway, i appreciate the discussion.
Since BMEP is theoretical, its tough to have quantitative numbers that we can actually use to prove anything, but if we had those two engines you discuss, and they had the same VE and used the same airflow, they would make the same torque (and therefore same power). We know they don't, therefore the VE logically has to be different.
I wish I had my DynoSim on this computer. I used to run a website that was Q&A for this exact purpose; engine VE theory and design. I had hundreds of screenshots of dyno sims that demonstrate all of this, even a few videos that showed the exponential rate at which all of these curves relate.
In fact, the VE and BMEP curves exponential relationship to SCR are exactly why there are larger changes in power when going from (for instance) 9:1 to 10:1 than there are when you go from 12:1 to 13:1.
curtis73 wrote: You're right about auto trannys not being an option on the COPO 302s, but you could get an auto behind a 302 in a Z28. It came with a 3100 stall converter too.
No.... no, I don't think you are right there. In fact, I'm pretty sure you are wrong. Not 100%, but pretty sure. Time to go digging through my Z/28 notebook with 1000+ pages of printed documents in it, but I'm pretty sure the 4 speed was a REQUIREMENT to order a Z/28. And I have no idea what a COPO 302 is (I understand what it COULD BE, but I also do not recall anything of the sort ever existing). If anything, the auto would be in a COPO considering the ordering process. The COPO cars were ordered to get around the 400ci ban in midsize or smaller cars at the time (chevelle, le mans, camaro, nova). I am not saying that there isn't such a thing as a COPO 302, but it wouldn't make much sense at all.
And like was said, whoever puts an auto behind a high-winding small block has some serious issues. That kind of defeats the purpose in my mind...
AngryCorvair wrote: In reply to curtis73: my "fascinating" comment was a poke at tuna55, not you. see, tuna disagreed with my statement that a larger cylinder is larger than a smaller cylinder,
If that's what you think I said than even THIS post is useless.
I didn't mean to start a riot 
I am having trouble remembering the last time I saw a desktop dyno roaring down the track. Everyone becomes an engine authority with software. Where's the perf?....hey Eric, what did the Alfa go last time out?.....233 and a new 231 two way record or something insignificant like that if I remember correctly.
BTW, a 'crank swap/cube change' is not equal if you don't 'fix' the intake tract, the one will not work with the different engine size. Runner length and plenum volumes don't match....not that it could be important. Also, the higher rpm engine needs a change in CR and different cam timing....again, not that that could be important.
curtis73 wrote:and why would someone put an auto tranny behind something like a Chevy 302? Chevy only put 4 speeds and 3.73-4.56 gears behind them in the 67-69 Camaros and silly stuff like air conditioning wasn't even on the option sheet with that engine..Autos are for wussies
the only engine available in the Z/28 from 67-69 was the 302, and the only transmission was a Muncie 4 speed. i can't remember if they got the M21 or M22 Muncie- i'm thinking M22 just because that was the beefiest one- but an auto wasn't an option with that engine.
the only reason they built the Z/28 was for homologation into the brand new SCCA Trans Am sedan class, and the reason they chose 302ci was because it came in under the 5.0 liter engine limit using radily available parts out of the GM parts catalog. literally the only special part they had to design for the engine was the pistons- everything else was the best stuff they put in the fastest production small blocks of the day.
the 4 speed manual trans was a part of the RPO Z/28 package- why would a race car have an auto trans??- along with a 12 bolt rear end with either 3.73, 4.10, or 4.56 gears, and front disc brakes in either manual or power.. Chevy had a rule about not putting AC on cars with solid lifter engines, but power steering was an option.
the SCCA dropped the engine limitation on production cars after the 69 season, so the 302 was dropped and replaced with the LT1 350 in the 70 (or 70 1/2 if you want to be that guy..) Camaros. you could get an auto trans in the Z/28 starting that year, but AC wasn't an option with that engine until the next year when they went to a smaller cam with hydraulic lifters and less static compression..
curtis73 wrote:Curtis- you are mixing your efficiencies up. Upping the CR doesn't change your volumetric efficency, it changes your combustion efficiency. From 6:1 to 12:1- it changes a LOT.Sorry, but 6 years of engineering edumacation and 15 years of engine design and building tell me otherwise. It dramatically changes VE. Try it on some dyno sim software,
I would, but we use real dynos here.
That, and we sell cars. Not dynos, and not simulation software.
Think about it- what about compression ratio changes the breathing? Your ports don't change, cams nor valve movements don't change, and the volume doesn't change. So how, physically, does compression ratio change volumetric efficiency. The part that a lot of people miss is that too high of compression will kill combustion efficiency in the real world due to the availability of real gas....
OTOH, knowing that the Otto cycle is very dependant on compression and expansion ratio, you don't even have to know the voleff to know that the CR will change the otto cycle efficiency, aka- combustion efficiency.
And Wheels- it was 233, but the car blew up. Thankfully, it ran a two way average of 230 in a different class before the next attempt. Still working for 240.
Think about it- what about compression ratio changes the breathing? Your ports don't change, cams nor valve movements don't change, and the volume doesn't change. So how, physically, does compression ratio change volumetric efficiency.
You're forgetting volume/mass ratio and adiabatic heating.
Think of it this way. Let's say you have an old school bike pump; one of those talk skinny manual pumps with a 20" stroke. In this example, lets simulate the low compression engine by stroking the plunger at the top 10" of travel. To simulate the high compression engine, stroke it at the bottom 10" of travel. Exact same displacement. Which one pumps more air? If you are moving it slowly and there are no restrictions to flow, niether one does. Now do the same experiment with the pump hooked up to a tire at 3 psi and pump as fast as you can. Ever pump so fast that when you reach the bottom you can hold it and it continues to flow as the pressure equalizes, or you let it go and it kicks back? That is a perfect model for VE and adiabatic effects on SCR. At its most basic level, operating the pump in the top 10" of travel generates very little pressure. Low pressure means less force to generate flow against restriction. Higher compression ratios create much higher pressure differentials. Even if they have the same displacement, the pressure drop during the intake stroke and the pressure spike during the exhaust stroke are much greater in the high compression engine. Greater pressure differential means more available force to overcome inertia and resistance.
Its entirely because of dynamic pressures inside the cylinder. A 12:1 engine actively pressurizes the exiting exhaust much more than the 6:1 engine, and it also positively displaces far more volume/chamber ratio. If you have a 12:1 engine with 50cc combustion volume, and a 6:1 engine with a 100cc volume (just example numbers, not math), the 12:1 engine actively expels 50 more cc using the same displacement.
One more analogy. Let's say you have two theoretical engines, both 100cc displacement. For the sake of argument, we'll operate these engines at slow speeds with no restrictions. One engine is a 2:1 compression. Total combustion space is 100cc. Let's make the other one infinity:1 compression. Total combustion space is zero.
During one cycle of the high compression engine, it draws in 100cc and expels 100cc which is 100% of its total volume. The low compression engine also draws in and expels 100cc, which is only 50% of its total volume. Now start spinning them faster. Since air volume is fluid, the low compression engine relies on a piston dancing around way down in the volume. The actual flow of air will be based on a pressure wave that is travelling at a fixed speed. As you increase RPM, the actual amount of air that is being cycled is dramatically reduced compared to the infinite compression engine; hence reducing VE
The other factor that is often overlooked with the higher compression is the increased inertia created by the exiting exhaust and its effects on scavenging.
Now, combine that with the fact that people commonly mistake the actual definition of VE. VE includes factors like BMEP, STP corrections, and the kitchen sink.
One more analogous example... Take two air tanks - one is 5 gallon at 100 psi and the other is 10 gallon at 50 psi. They have the same mass of air in them; the same number of molecules. Open a 1/4" valve on each one. Which one empties first? The 5 gallon does because it has more pressure to overcome the resistance to flow through the 1/4" valve. As you increase the valve size (flow area) the difference in drain times will be reduced, all the way down to an identical drain time if you just did something like remove the top of the tank
In a static or slow moving engine (for instance, turning over by hand) compression ratio doesn't affect VE much if at all... but it has a HUGE effect on a running, breathing engine given the fluid dynamics of the air its moving. You have to think of airflow in an engine like a rubber band.
I would, but we use real dynos here. That, and we sell cars. Not dynos, and not simulation software.
So far this discussion has been pretty adult and mature, but its getting a bit childish and unobjective. If you are truly attempting to learn, I hope I helped. If you are just saying, "I don't believe you, prove it douchebag," then I hope you have a very painful papercut and get lemon juice in it 
The reason I suggested you use dyno simulation software is because it calculates an estimated VE and you can physically watch all of the factors involved that make the engine perform the way it does. Its true that we don't race dynos, and we don't race peak HP numbers, but they are all tools and part of the formula.
... You can sell me a dozen eggs and I'll believe there are 12 of them, but I like to know how to count to twelve and see how you determined that. Same with engine VE theory. Anyone can look at dyno numbers and drool, I want to know the math and the formulas behind it.
wheels777 wrote: I am having trouble remembering the last time I saw a desktop dyno roaring down the track. Everyone becomes an engine authority with software.
I'm not an engine authority because of software, I'm an engine authority because I've built several thousand of them over a 15 year period, and my mathematical background on it must have some merit since I actually consulted on several dyno sim programs, including the one used by Comp Cams on their website.
... but you're right, I probably don't know what I'm talking about.
BTW, a 'crank swap/cube change' is not equal if you don't 'fix' the intake tract, the one will not work with the different engine size. Runner length and plenum volumes don't match....not that it could be important. Also, the higher rpm engine needs a change in CR and different cam timing....again, not that that could be important.
Again, disagree... if the only thing you change is displacement, the airflow demands are the same, its just that the smaller displacement engine will now make the power at a higher RPM. "fixing" the rest of the engine means its no longer an apples/apples comparison.
I agree with that statement if you are decreasing HP, or attempting to keep the same torque peak, but this comparison is between equal HP, different displacements
In reply to curtis73:
Why are you including combustion volume in part of the gas exchange?
If it's 100cc or 50cc, it's still in the chamber- not pumped in or out. The swept volume is the displacement- valve opens, gas gets pumped out, combustion volume stays, intake valve opens, back flow generally happens, pressures are equalized- the whole time, the combustion volume is not being swept. So I'm not sure how that volume increases or decreased the amount of air that you can or can not get in.
And you are still ignoring the thermodynamics of higher compression/expansion ratios- which increases combustion efficiency.
As for the dyno- that's being an ass just as much as your "my model says so" point. I've seen bad information come out of computer models more than once, so forgive me for being quite cautious that your model isn't telling you more than what is really happening. Math and models are good, but reality is what we sell. Drives me nuts that we can out do model predictions, and them modellers question reality.....
Again, I think you are putting the combustion benefits into VE, and it should not be.
But if you are convinced you are right, you should come work here. That's no joke- there are plenty of opportunities.
Have fun.
Eric
curtis73 wrote:wheels777 wrote: BTW, a 'crank swap/cube change' is not equal if you don't 'fix' the intake tract, the one will not work with the different engine size. Runner length and plenum volumes don't match....not that it could be important. Also, the higher rpm engine needs a change in CR and different cam timing....again, not that that could be important.Again, disagree...
Okay. All the plenum sizing effort and intake and header length work everyone does is not right according to you. I can't argue with you. You are way smarter then me.
alfadriver wrote: In reply to curtis73: Why are you including combustion volume in part of the gas exchange?
I'm not, I'm merely using it to demonstrate the fact that unswept volume has a large effect on flow in and out of the cylinder due to the pressure effected behind it. I thought that was crystal clear with the bike pump analogy. If you are sweeping the upper 10" of travel, you're not generating as much pressure and therefore not moving as much mass of air.
If it's 100cc or 50cc, it's still in the chamber- not pumped in or out. The swept volume is the displacement- valve opens, gas gets pumped out, combustion volume stays, intake valve opens, back flow generally happens, pressures are equalized- the whole time, the combustion volume is not being swept.
Combustion volume stays, but at what pressure? There may be more or less mass (volumetric equivalent) than is suggested by the volume. If the cam timing events, RPM, exhaust diameter and flow, intake overlap, exhaust intertia, and several other factors are working together, there could actually be negative pressure in that chamber at TDC exhaust which is what creates scavenging. Rev past that point and you have positive pressure. At idle you have closer to neutral pressure.... hence the entire theory behind having a VE peak at a certain RPM. Its where all the stars align and the most mass gets trapped in the cylinder.
Something that might help you grasp this... Stop saying volume. Say mass. Mass is a better model because it is relative to volume, temperature, and pressure. You're assuming things based on volume (which is fixed, you are correct) but the amount of mass and temperature that fills that volume is what I'm discussing. The amount of pressure behind the flow is what changes VE. Having more pressure in the cylinder can't not affect VE. Its the whole explanation about how cam duration shifts cylinder peak pressures (and therefore torque, ergo HP) up in the RPM band. Increased SCR is specifically used to replace the lost cylinder pressure in the lower RPMs when a larger cam is used. Increased CR provides a linear increase in VE across the band to replace the lost VE lower in the RPMs, and the resulting increased cylinder pressures found in the higher RPMs happen during a time when fixed flame front speeds are overshadowed by increased piston recession speed.
So I'm not sure how that volume increases or decreased the amount of air that you can or can not get in.
Because swept volume at low pressure doesn't move as much mass as swept volume at high pressure. Its not a can or cannot deal. Its how much is actually ingested and expelled due to the dynamics involved. Higher pressure air has more molecules per cubic feet than lower pressure air for a given temperature. If the port/valve/runner/whatever is capable of flowing 200 cfm, then higher pressure air will flow more mass within that flow constraint. Lets not overlook time as well since this is all happening in milliseconds. If its operating at 1 RPM, you'd be spot on, but when you're talking about 6000 RPMs and mere milliseconds when this all happens, the amount of air trapped in the cylinder is very different. In fact, if what you are arguing is true, then VE would remain constant throughout the RPM range of the engine, but it doesn't. As the engine sweeps through its RPM range, cylinder pressure and gas velocity are all changing but cam timing and flow path restrictions are fixed which causes VE to change. This same principle (and very similar mathematical formulas) is demonstrated in the construction of subwoofer boxes. The suspended air inside the box is tuned to the port volume. That is why smaller boxes are tuned to higher frequencies. If this were just an open tube where pressure wasn't a factor, you'd be spot on. Given the fluid dynamics of the actual airflows that are happening inside the cylinder and the pressure differentials on each side of the valve, its a huge factor.
Back to that bike pump analogy...
Let's say you quickly push the plunger down and let go. It pops back up some because despite the fact that you displaced 100cc doesn't mean that a volumetric equivalent of 100cc left the hose on the pump. Asserting that VE in an engine doesn't change because of compression ratio neglects that principle. Assuming that VE doesn't change with compression means that you are assuming that the entire swept volume is expelled or ingested equally regardless of the pressure generated in the cylinder. If that were the case, all engines would be the same VE and we wouldn't be having this discussion.
And you are still ignoring the thermodynamics of higher compression/expansion ratios- which increases combustion efficiency.
I'm not ignoring it at all... its an entirely separate discussion. I would be glad to discuss delta-Ea's and adiabatic heating if you would like, but that's not part of this discussion.
As for the dyno- that's being an ass just as much as your "my model says so" point. I've seen bad information come out of computer models more than once, so forgive me for being quite cautious that your model isn't telling you more than what is really happening. Math and models are good, but reality is what we sell. Drives me nuts that we can out do model predictions, and them modellers question reality.....
I'm not questioning reality at all, merely explaining the formula and theory behind the reality. Assuming that these computer models are faulty is a fine skepticism, however these are formulas and known facts that have been in existence since the first internal combustion engines were built. They are also used all the time by auto manufacturers, race teams, and top engine builders, so they might have a little more actual validity than you lend them. They're all based on Boyle's gas laws which have been part of gas physics for a few hundred years.
Again, I think you are putting the combustion benefits into VE, and it should not be.
Absolutely not in any way, shape, or form. I fully understand both dynamics and I am not combining them in any way.
But if you are convinced you are right, you should come work here. That's no joke- there are plenty of opportunities.
You can't afford me
I've really appreciated the discussion, but I feel like I'm arguing age-old physical truths and you're either disagreeing because you truly believe differently or because I'm not explaining it clearly. Either way, let's keep it constructive and objective. I'd like to hear your proofs to the contrary.
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