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Compression Ratios / Short Block
I know this was covered in an older thread, but neither the questions or answers gave me the info I was looking for, so here goes.
I'm currently running a Ptuning Stage 1 Turbo with water meth. At peak 15psi Boost I get 365hp to the wheel and 250 ftlbs torque.. That said, I'm not going there very often. Nonetheless, I'm fully aware the FA20 engine has high compression and the engine was designed for NA. I'm also hearing enough stories about bent or broken rods that it may be cheaper to be proactive... So, in the interest of having a really solid setup I've started looking at short blocks. Some shops that make their short blocks build them with 12:1 CR because they say it can make more horsepower, others say 10:1 gives you the ability for more boost, and more torque and more hp. In a perfect world, if this engine had 400hp with 400 ftlbs torque and was robust and reliable that would be pretty awesome. Hell, I'd be happy with 350/350. So- does anyone have sage advice about the pros/cons of 12:1 and 10:1? |
The higher your CR the higher octane fuel you will require to run. If you are boosting 12:1 expect to run E85 99% of the time unless you have low boost. You can get more pump gas with a lower compression and not be stranded if no E85 pumps are near by.
The main issue with our engines and rods is not the power its the tuning. People have run 500 HP on a bone stock engine for over a year with the right tune, but if your timing is too advanced to make power and your octane is too low you will see knock and detonation. This bends and snaps rods. Safety factor is lower compression and pump gas. E85 is not reliable from a pump unless you have a flex fuel setup to automatically adjust timing based on ethanol content sampling. At the end of the day. Flex Fuel setup and ECUTek with a proper tune and you are all set. Mix and match E85 and any pump gas with peace of mind. |
JE Pistons sell 13.5:1 CR pistons ;D
If/when I build my engine, I'll be trying to stick as close to stock CR as possible because a lot of the tuners these days know what they are doing with this engine now to not blow it up as easy. There is a tuner here in Aus that regularly does at least 300kw at the wheels (on his dyno that would be about 470hp on a dynojet) on stock internals. Unfortunately he doesn't (refuses) to use EcuTek. IMHO going 10:1 would be a step backwards as this engine's fuel management strategy allows us to run quite high compression from stock etc. Going 10:1 would be like putting an EJ engine in. Good, but old tech ;D In conclusion, higher CR is good if you are able to run E85 mostly, and/or have a very competent tuner. Otherwise 10:1 isn't terrible, but it's a backward step. |
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So, you make high peak HP, and much lower peak torque, and you don't get to high RPM's very often. You want HP / TQ to be roughly square, and you want a lot more torque. Horsepower is just a math equation. It comes from torque. Specifically (Torque x RPM) / 5252 = HP What that means for you, is you have to hit peak torque below 5252 RPM to have nice square TQ / HP numbers. Your gears reverse the equation and turn horsepower back into torque at the wheels. (if the output is a constant, and you lower speed, you raise torque.) Torque at the wheels is what accelerates the car. There are many ways to achieve this, and it depends on many variables. I'll be honest, The GTX 3076 in that Ptuning kit is a little big for that goal. You're going to want to retain as much compression as you can, because if you're looking for a "torquey" motor, I guarantee you're also looking for quick transient throttle response. Lower compression slows transient throttle response, both in and out of boost. People say high compression spools a turbo faster, but that's an erroneous conclusion from a chicken-and-egg scenario. Whatever. To the driver, that statement is true, that's all you need to worry about. I can continue, but in order to sort out the inputs, like compression, you need to refine your output goals. |
:popcorn:
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You made 710 WHP "Around" 6600 RPM. For easy math's sake, let's say it's 6658. That allows us to have a 1.0 transmission, 4.10 gear, and 24" tire. Stock. That means you're making 560 Ft. Lbs @ 6658 RPM. That's nowhere close to your peak torque though. By the time your power peaks, your torque is a fair way along on the downhill slide. How am I doing so far? Your torque peaks at.... Hrm... 5340 or so. Right at about 628 Ft. Lbs. I could draw the curve if you like. :cheers: /drunkmath |
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;) humfrz |
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nah, who am I kidding, Im gunna investigate what this might take... edit1: If I had the equation for the piston (my plane) , I'd be able to give a cross product of the torque, assuming all the combustion equations are equal for all 4 pistons. Then dip into some airflow equations because I would love to see what kind of airflow the stock units get (insert someone's reference here). My first assumption is that since its a turbo, the best way for it to make more power would be to make it breathe better. Exhaust would be fun to think about too. ...didnt think about it too long. saturday night saved :thumbsup: |
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and you are correct my torque did fall off before then...... UNTIL we adjusted the cam timing a little then strange things happened......... |
The problem with these cars is going to trying to maintain adequate oil pressure to even support 300+Hp with these FA20's, from what i've seen and read if you aren't in the 60+ PSI range your rod bearings will be the first thing to go out.
Also I would just stay stock compression if you have E85 readily available in your area, if not go with which ever compression your tuner recommends. Since my tuner recommended me to stay with the stock compression on my built block I just did, since I only run high boost 16+ PSI on E85 and less then 11 PSI on 91 octane. |
You can make 300 whp on 10 PSI on pump gas and stock compression with great drivability. I dunno what everyone's whining about.
With adequate cooling and the right viscosity, you can support any hp you want. The oil pressure freak out was a bit of a "chicken little" type event. For an actual racecar, time attack, for instance, it's always been an art form to keep a wild power turbo setup running. |
Lower compression lets you run more boost, but it also requires you run more boost to make the same power. More boost = more heat, meaning you might need to run a more efficient IC.
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I'll ask a few annoying, mythbuster-type questions. Why does it require you to run more boost? Does it necessarily require you to run more boost? How much boost offsets a drop in compression? If you had a magical detonation-proof fuel, why would you still need an intercooler? Why would an intercooled turbo still make more power? (Because it would, by the way. That's not a trick question.) Another. You have 2 engines. In the first, you leave everything alone. In the second, you drop compression and raise boost, maintaining an equivalent effective compression ratio. Both engines have the exact same effective compression. Which one makes more power? I'll give you a hint. Which engine has more potential energy in each cylinder? One more. Say you take an engine and raise the compression 1 point. Is there any more air or fuel present to burn? No. So there's no more energy there. In fact, it took even more energy to compress it than before. So, why does it make any more power at all? (Again, not a trick question. It does make more power, just not much.) Oh, and 1 more. The turbocharger doesn't drive anything. Why does it increase the thermal efficiency of an engine? Why can a supercharger, which robs power in a parasitic way, also increase the efficiency of an engine? We haven't even addressed cam timing and Miller cycle yet. |
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I spoke to the Ptuning folks and they said I'd be much happier all around with the OEM block, the 12.5:1 CR, and swap out to the new, smaller GTX28. I'll lose a little at the top end, but at the lower end, boost will start around 3300rpm instead of 5000rpm. They had the same misgivings about the lower compression. |
Nobody answered so I'll give it a go. Pulling this out of my ass. Learn me some.
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Necessary: Maybe not if a bigger CAC is an option. How much: Lots of math, ideal gas law, not truly adiabatic system, blah blah... Enough so that the air mass you're stuffing in there is equal? Quote:
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Supercharger? Hmmm... not sure about that. Does it have anything to do with the CAC? |
Oh now this is fun lol
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You've reduced the thermal efficiency of the system. To get the same amount of output work, you'll need more potential energy (fuel, Q in) and the only way to do that is to increase the amount of air input. Quote:
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As for the supercharger, part of your output work is used to drive the supercharger. Your total efficiency is reduced (work input increases for the same work output). |
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I'm going to start by saying Oh My! An intelligent debate on the internet. Achievement unlocked.
I'm going to follow by saying I saw the thread about the updated Ptuning kit. All I can say is I'm impressed. OK. On to the fact-straightening exercise, slash debate. Quote:
First. Displacement volume. Both of you addressed this, so I'll address that first. Quote:
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NEXT. Intercoolers. Let's have a thought experiment. Quote:
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Next up, thermal efficiency and work! Expansion ratios! This covers the rest of the questions and answers. Aaaaand I'm leaving it until tomorrow. I'm too tired to type the rest of this. It's been barrels of fun. Stay tuned for another exciting episode. I want to talk about PV diagrams, and Q. No, Otto and Carnot's Q, not Ian Fleming's funny Englishman with the cool toys. |
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Yes, you're absolutely correct about volume in the chamber. It would indeed be an increase in volume. Once again though, very small to the point I really wouldn't consider it for anything worth thinking about. The pressure drop you're referring to in the intercooler is the loss due to friction (a rather large increase in surface area), turbulence, etc. which is of course only a part of the total pressure drop from all the curves, bends, transitions, etc. in your charge air system. I'm sure I have the complex fluid dynamics equations somewhere (I actually had a MathCAD file written to calculate this). If this were a closed system, a temperature change would cause a change in pressure but we're talking about a system that is allowed to change. And don't forget, time is a very important factor in the consideration of what is considered isobaric. The heat rejection in the intercooler is a relatively slow process (when compared to a turbo or the internals of an engine) with ample time for the system to equalize any pressure transitions (Vdot does not change, but vdot does). The turbo however is a very rapid addition of kinetic energy with a very rapid deceleration as it approaches stagnation exiting the outlet. |
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