A&Q about 350Z
Q:
Thanks to everyone for helping me out. I've also done some more research into this topic, and I've learned a lot. I would like to recap everything with you guys though. Make sure i got everything right .
Here's what I've got, to increase RPM's of an engine one would generally have to decrease the inertia forced on the parts (i.e. weight reduction, shorter stroke) and increase the flow. Am I on the right track?
If so I have some more ?'s for you guys
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1) What are ways that I can increase flow? A better exhaust would help out I know, but what about the valvetrain? What are the technical benefits of a SOHC (i.e. I4's) vs DOHC (i.e. V6's) vs the electronic Hydraulic systems of F1 cars vs any experimental things like the Coates CRVS (I know they have problems with sealing, but I'm talking about the theory behind it)? I know that the F1 systems are the best right now, but why (in technical terms)? And also why aren't there any companies making aftermarket headers using that technology?
2) I understand how Bore/stroke come into play, but are there any formulas for the trade off between them? Ex. if I wanted to keep the same displacement(power) but shorten the stroke, how would I figure out how much to increase the bore? Would it be a 1-to-1 ratio or something like that? Also I know the more you bore out an engine the weaker the walls of the chamber are, but are there any restrictions on shortening the stroke? And is there anyway to reinforce the combustion chambers walls to handle a higher bore?
3) I don't understand why I wouldn't be able to keep the low end power when I increase the RPM's of a low end engine like in my viper ex. It might be because I don't quite understand how an engines powerband is constructed. If someone could explain this in detail that would be AWESOME!!! Even just pointing me to a site with good charts would help me out a lot.
ok, I think that's enough questions to keep you guys busy for a week at least
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Again, thanks to everyone that posted!!! You guys rock.
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while we are on the subject heres a link i think you guys will like. it runs at 102 RPM
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.....
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......108,920 hp at 102 rpm !!!!
now if we could just get the RPMs up to around 2000
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i like the fact it takes 3 guys to install a bearing
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You increase flow by changing intake, head, and valvetrain parameters. Exhaust is also important, but wait to design the exhaust until after the intake side is determined. Technical benefits of OHC designs include simplicity, less weight of moving parts, and longevity (in most cases). The SOHC is fine and allows for the simplicity part. The DOHC adds a little bit of design flexibility as far as where the valves go in the chamber.
You can just do the math. The engine's displacement is the area of one cylinder times the stroke times number of cylinders. The area of one cylinder is pi times radius squared, then multiply by the stroke. That gives you displaced volume of one cylinder. Then multiply it by number of cylinders. You will be able to just use the math to keep the same displacement.
Technically you can make the stroke as short as you want, but you reach a point of diminishing return. I'll talk more about this later, but if you shorten the stoke by half, you have cut your torque in half. You are using the same piston bore to exert the same force on a lever that is half as long. Cutting your torque in half cuts your HP in half. Now you have to spin the engine much faster to regain the HP you lost. In order to do that you have to massively increase breathing capability. I'll put this all together later...
Here's why. I'm sure you've seen a dyno graph. The torque is a curve that usually goes up to its peak (around 4000 rpms in your example) then drops off again. If you just spun the engine faster, things like inertia, friction, and valvetrain events would start being a huge drag on it. The main reason the torque curve falls at higher RPMs is because it begins to require more torque to operate the engine than the engine can make. If you spun it faster, you would lose power. The way to change that is to do things like reduce weight of internal parts, and change how the engine breathes. Just spinning it faster is like asking you to run a marathon while breathing through a soda straw. You would be requiring more air than you could suck through the straw.
I'll give you the long answer now.
Airflow in an engine is not a "more is better" proposition. There are two main things to look at in airflow qualities; flow, which is the amount of CFMs is possible to move, and velocity, basically the speed of the intake charge. Let's take the example of the soda straw to demonstrate flow. If you were watching movie on your couch, chances are it would be adequate. That is typical of a stock 5000-rpm engine. If you got up and went for a walk, it wouldn't be enough. So why not just always breathe through a 4" pipe, right? Always enough, but you only take what you need? This is where velocity plays a role. Cams have the valves open during intake an exhaust strokes, but they also overlap. Even a stock cam with 190 degrees duration is open 10 more degrees than one stroke which is 180 degrees. The exhast valve opens early to get rid of spent gasses before the piston starts back up. As the piston gets near the top, the intake valve opens early. Why? The outflow of exhaust is going really fast; fast enough to create a suction and start pulling in air from the intake valve. Intake air starts coming in before the piston starts sucking it in. The piston falls sucking in air/fuel. Again it creates a very fast moving column of air. So fast, that the inertia of the intake charge keeps filling the cylinder after the piston has reached the bottom. The intake valve closes after the cylinder has started back up for compression.
That was a little tangent to help you understand this: At very low RPMs you can see that this "ramming" effect would be small. The intake charge is moving slowly and the piston moves slower. At a certain RPM, this effect is maximized; for example, lets say 2500 RPMs in a mild stock engine. At 2500 RPMs, the stars align and the ramming effect of the intake charge is at its peak. Anywhere below 2500 RPMs, the intake charge doesn't have its peak velocity and doesn't fill the cylinders with as much charge. Above 2500 RPMs, the valve closes too quickly and doesn't get as much charge. This 2500 RPM peak value is where (or near) torque will peak. This is why all engine parts have to be matched together. Putting in a huge cam with lots of overlap will suck at low rpms. The ramming effect will be maximized at, say, 5000 rpms. At 5000 RPMs, the intake flow requirements are huge. This velocity also applies to the exhaust side. Above I mentioned that the speeding exhaust gasses create an inflow of intake. If you increase exhaust size too big, you've killed its velocity, and therefore its ability to suck in intake charge (which is called scavenging by the way)
If you offer up massive amounts of intake air potential to this engine with this cam, you will have a mismatch. Although flow is available, the velocity will have dropped. Without that velocity, the ramming effect that used to peak at 2500 will no longer be there and torque will suffer.
So, basically, the answer to your question boils down to the "straw" answer. Airflow characteristics on your viper engine example are fixed. The intake will keep up with flow to the peak of power, but then they can flow no more. Spinning it faster will be like running the marathon breathing through a straw. Conversely, if you add tons more air, you can spin faster and make more power (like running a marathon with a 4" pipe), but you've lost the ability to make lots of power at low RPMs.
I'm going to shut up now... I'm going to make some fake dyno graphs in a program I have to show you what happens to the power curves. I'll use a single engine, but change heads, cam, etc to show you how low end torque is lost with high-hp designs. Look for it later today or tomorrow.
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Could you combat port flow/velocity by increasing the number of cylinders while retaining the same total displacement.
If so this would mean the displacement of a single cylinder would decrease as would the port flow requirement at equal RPMs to a larger displacement cylinder.
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One way that you can compromise an engine to get the max out of low rpms mid rpms and high rpms is to use different sets of cams and timming. I'm suprised nobody has mentioned variable valve timming yet (although I'm sure it would be on the way). Of coarse a Vipers ohv setup isn't practical to setup this way considering many things (one being the added valvetrain mass making it harder to rev vs. a OHC setup). There is a sticky at the top of the engineering page if you would like to learn more about variable valve timming.
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Yes, however two problems exist here. 1) when you divide the displacement over a greater number of cylinders, the surface area to stroke ratio changes, making the sum pressures of all cylinders combined less than what it was. 2) the smaller cylinders crutch airflow since when you drop diameter of the cylinder, the valve sizes have to be smaller. Although a greater percentage of the piston area can be composed of valve face, the effect is more than offset by the proximity of the cylinder walls shrouding the valves. Basically, your theory would be spot on, except for things like valve shrouding, harmonics, and other factors. The other thing is that (although you could engineer some of this out) the rotating mass in a 350 ci V10 would weigh more than a 350 ci V8.
You could keep the same bore and reduce stroke when you add cylinders, but there again, you are decreasing torque on the crankshaft. All in all its usually a wash.
Still working on dyno graphs.... More later.
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comparing dohc to sohc i believe the sohc generaly have more torque but their top end power falls off cause they cant flow as much as a dohc. they get more torque cause they have less stuff to move, half as many cams and half as many valves.
the dohc motors have more stuff to move so get a little less torque but with twice as many valves it can flow more air and exhaust at higher rpms allowing for a more powerful top end.
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Dyno charts. Two for now, more later if I get the time.
The first one pictured here is based on a typical street performance Buick 455; bore 4.313, stroke 3.900.
This next one is based on an engine exactly half its size. I just took the dyno software and changed the 8 cylinders to 4 cylinders. It ends up being a Buick 228 4-cylinder.
Take a look at both. They both make about 300 peak HP, but in order for the 4-cylinder to make that power, you have to more than double the airflow to each cylinder. Take a look at what it did to the torque, too. In order to get 300 hp from the 4-cylinder, I had to simulate 4-valve race heads, a single plane intake, and high flow headers. The important part I want you to get from the first graph is this. Take a look at power output at 8000 rpms. If you could get your engine to run that fast, it would be making zero output. It would be running, but at that point, the engine is making exactly the same power that it requires to overcome its own inertia and friction. Net power to the crankshaft would be zero. You had asked why you couldn't just run a viper V10 faster and have it make more power. There's your answer. After it passes the peak, it keeps making less power the faster it spins. In the graph above, the exponential increase in friction and air restriction meets up with the engine's abilities at 8000 RPMs.
There are obvioulsy pros and cons to each one. They both should produce similar acceleration, but the 4-cylinder needs a loose converter, short gearing, and in the end would most likely return less service life, poorer fuel mileage and less "streetability" It would idle very roughly, you couldn't operate any of your emissions equipment or your power brakes since it wouldn't create enough vacuum.
In addition, you would really notice the lack of torque on the street. With the 8-cylinder graph, you'd be able to drive it like a street car; without much thought. The 4-cylinder would require you to really rev it to get to the power you want.
Tomorrow's lesson --
Just kidding.-- Tomorrow I'll make up a couple more charts that show your V10 in street trim and race trim.
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More dyno graphs
The first is a vague approximation of the Viper V10 in stock form:
This one is a hopped up version:
Its important to note that I exxagerated the parts combo in the second graph to show a particularly poor torque curve. There are ways of adding power without killing your torque curve this severly, but for the sake of clear demonstration I really smacked the air flow to it without properly updating a few key parameters. In the first graph (the stock viper) notice that you get good torque through the entire band. Notice in the second graph that you have more power and a higher powerband, but torque at just-off-idle is below 200 ft-lbs compared to the nearly 500 you had before. In this case you gave up 300 ft-lbs of torque to get 80 hp. In reality you can choose parts that would keep almost the same torque at low rpms while adding power up high... In theory, adding flow without sacrificing velocity. That's the basic idea behind pocket porting heads. Alter the shape of the port to flow more without really adding much volume to slow it down. Fully porting heads is often a means to gain bulk flow with less regard to velocity.
That was an incredibly long answer to your question, nubtuner. You could spin a stock V10 faster in stock form, but as you can see from the first graph in this post, after 6000 RPMs it loses power fast. You'd do best in this car to shift at 5800 or 6000. Going beyond that, your acceleration would be hindered by the fact that you're using the engine in a band where it doesn't make much power. Contrast that with the theoretical second engine in this post, you would do best to shift at 7500. You would also need to reduce your rear gearing to help you get the RPMs up to where the engine makes power. You'll also have to go pick up the pieces of the shattered $9000 engine that just sent 10 pistons careening through the side of the block at 7200 rpms
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Wouldn't compression play a role in the high revs? I heard that F1 has super high compression. Most cars can't rev very high (+10,000) because the fuel can't burn fast enough right? At leased it's that way with deasels so shouldn't it apply to gasoline?
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Yes and No. Diesels inject fuel directly into the cylinder and it burns slower, but is also injected through part of the power stroke. Gas gets it all at once, and once its burned its done. That is a BMEP discussion for another day.
Static compression ratio plays a big role as well in the airflow characteristics of the engine, but with the changes we're talking about, the big factor is dynamic compression; the actual compression based on static ratio and cam timing, VE, and other stuff. In the case of the second viper engine that I messed up, changing the static ratio between 8:1 and 11:1 wouldn't change power more than about 10hp since its mismatched.
You are correct, though. Provided all other factors are matched, a higher static compression ratio will provide better power at higher RPMs. This is mostly due to combustion efficiency flattening out the torque curve a touch. I won't get into the fuel burn rate discussion again; I've been flamed out of that too many times
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A DOHC four valve head has less mass to move than a two valve head, and especially when comparing to a push rod engine. Designed for a given power output the DOHC four valve head will also produce a higher torque throughout the whole rpm range while also reducing the stress in the valvetrain offer a better reliability and a lower cost in the long run.
The current F1 engines are designed for a life of 800 km, they are three litre V10 engines, each cylinder having a displacement of 300 cc from a bore of 96 to 99 mm. This means that the stroke is around 40 mm. The conrod has a length of about 2.5 times the stroke.
The engines use four gear driven camshaft, the valves are actuated through DLC coated finger followers. The engines has no conventional valve springs, they use pneumatic springs instead which allows the use of such high engine speeds. Compression ratio is somewhere around 13:1, to make the combustion chamber smaller than that would be difficult.
A F1 engine produces the same torque output like most three litre engines, but the high rpm make it possible to extract such high power levels, around 950 hp for the last of the 2004 engines. Idle is around 4000 rpm, max torque at around 15000 rpm and max power at around 18-19000 rpm (low 17k for the least powerful engines).
The engine manufacturers spend together around one billion dollars a year if I remember correctly, this is almost only for engine developement. The production cost is very low.
An F1 engine is designed for maximum power output, torque (curve) is only for driveability reasons. Engine weight is about 90-100 kg.
The engines use a small displacement because they are designed to be light and small to suit short and tricky tracks. This was decided after WW2 and has stayed the same, the regs just changes from time to time to reduce the power output for safety for example.
To be able to use engine speeds this high much developement work is spent to reduce friction, decrease combustion time, increase volumetric efficiency, optimise burn and so on.
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This is all very correct information except for one fact. The main limiting factor in incresing revs is piston acceleration. This is a factor that is determined by the rod stroke ratio. And is also one of the main factors in low speed port velocity. I don't have my notes on me, but i will try to bring formulas to post.
Pretty much an engine with a large bore stroke ratio (something typical of small block/big block v8's) have a long rod and stroke but a ratio of about 1:1. This produces a piston that accelerate and decelerates faster, which is a factor limiting speed because there is a limit to how fast you can accelerate the piston.
This also has an effect on the velocity the piston accelerates away from top dead center very quickly while something with a smaller ratio (F1 engines) move away from top dead center much slower.
Now don't curse me if i got my large/small ratios mixed up but i'll bring my notes in and pos a lot more good stuff.
And on a side note, i've done a lot of reaserch on fuel and flame speed and come up with a whole lot of nothing. One thing i do know is that F1 uses fuel that is very similar to pump gas (you can even run this stuff in your car). F1 fuel meets all European Union Standards for pump gasoline wich means that it is indeed very similar. At lower combustion temps (still trying to figure out actual temps,) the flame speed has very much to do with the volatility of fuel, but at higher temps it is more dependant upon the chemical nature of the fuel and much of the information on this reaction is still unknown.
hope this helps