what are your specs? do you have pics?

 

Hey Dave I did see your car at Nisei and it is a pretty sick as ride. Love your BBK setup and your cf grill insert. Overall I like it, it just different people have just different taste. Your rims are very aggressive and clean but I'm sure everyone thinks the same with theres. But props to you for having a NICE A@# RIDE...

 

None to my knowledge.

I don't know them off hand, but percent increases in power, vary by each car- for example you'll hear 20%-30% increase with xx S/C (for a specific car at a given CR)! (Then you read the fine print for the boost level, which is NOT universal).

In real world, boost levels that low don't really offer much, so hypothetically speaking, lets say you went with 10psi on two engines, each with different compression ratios, 8.5 and 11.0 respectively. I can't stress it enough, but the example only works with all things being equal; the amount of power you made would still be directly related to how you control heat and burn fuel (thermal efficiency), because higher compression means higher temps, which equal less power after a given point*...so no, the specific percent increase per PSI would not be the same between the two engines, especially when you begin to take into account the other variables. If you had to specificy a percentage increase, this would then mean that your engine running at an 11.0 CR would potentially fall to the bottom of the "percentage increase bracket", while the 8.5 CR engine would place higher, and thus make more power...and continue to do so, within limits, as you raise boost.

*Though since the IS is an N/A car, its actually running more efficiently at this CR, and is able to generate 306hp, while also making decent fuel mileage...at the expense of premium fuel to compensate.

Why? Less heat to cause detonation.

Lower the compression ratio, and you can raise boost, which equals more power..."safely" however, deals with a few other things. You can also use other methods, such as water/alcohol injection, retard ingition timing, higher octane fuel, etc. but what you're really trying to do is cool the air within the cylinders, so that it does not pre-ignite the air/fuel mixture, and cause that kaboom we all love to hate. Everything from your combustion chambers design, to your ignition timing, ECU, even boiling point within your coolant and especially tuning is going to dictate how safe things are.

There is a set amount of displacement, as provided by the bore and stroke, and volume for the cylinders in our V6 engines (and a given level of volumetric efficiency), so when you decrease the compression ratio, you're 'increasing volume', while effectively decreasing pressure (and the work necessary to compress), which decreases heat. Just as if you bore out a motor, FI to a greater extent, allows you to increase the amount of air in that same volume, over what an N/A piston is able to do, which is pull a finite amount of air in, depending on intake system design and efficieny, during its intake stroke. When using FI, its also why in many cases, you invest in an intercooler for example, so that you may further cool the incoming air charge (increase its density), in order to fit more air into that fixed volume, which thus enables you to burn more air/fuel for power.

 

Shinobi-
I'm sorry; I still don't get it. I know that with unlimited money, (No such thing.) the big boys lower CR before turbo/supercharging. Our current cars take in 3.5 liters of air, compress it 11+ times and add fuel. That compression creates a temperature of X degrees, and is considered safe by Toyota. OK, now we add FI, and enough extra air to bring the final pressure from ambient temp. and press. to 13 times press. with a raised temp of Y degrees. This is considered safe by the FI manufacturer. OK, now drop the CR to 6, and add more boost to creat a final CR of 13. Won't this still give you a final temp of Y? The only difference I can see is that the intercooler can act on a greater volume/percentage of the air entering the combustion chamber. Is that the answer? Thanx.

 

I have a spreadsheet developed by a guy that helps alot with this discussion. It calculates the temperature of the compressed mixture in the cylinder. You set the variable for autoignition temperature of the fuel you intend to use, tell it about your intercooler and turbo (it actually has tables for compressor maps), and it will tell you what your maximum boost is.

There are a lot of temperature changes going on with the a/f mixture during the induction process, but there are only two things that are really important. First, does the compressed a/f mixture have enough heat to autoignite? If it does, you need less boost. No ifs, ands, or buts. You can do lots of things to help reduce the temperature - water and methanol injection, better intercooling, cooling system changes, but in the end, it's all about whether that a/f mixture develops enough heat under compression to autoignite.

Second, despite a ton of crap I've seen about ignition timing, there is only one thing to tell you if it is right or wrong. If the peak pressure occurs in the correct crank angle window, your timing is correct. More advance or more retard will not make things better. I've heard lots of guys say they want to run better fuel so they can run more advance. Not true. You run better fuel to fix the first problem. More advance will not necessarily make more power, and if it moves the peak pressure out of the optimum window, it will make a lot less power. If you are retarding ignition timing to suit your fuel, you are probably making a mistake, and you need to revisit combustion chamber shape, cam timing, or any of the host of intake cooling methodologies.

Kistler make some very useful devices for helping tune an engine. The device in the link will allow the tuner to make a pressure/volume map for the engine to determine correct ignition timing without needing a lot of dyno time. You just don't want the engine to detonate with one of these sensors installed. It would be a costly mistake.

 

There are two kinds of compression ratios. Static and dynamic. Static is the one you can easily calculate from physical dimensions. Dynamic takes into account boost and volumetric efficiency (VE). Dynamic compression ratios will change based on rpm because VE is a moving target.

In your example, yes, if you reduce the static compression ratio to 6:1 and raise the dynamic compression ratio back to the original amount, you should see similar power, but it will depend a lot on combustion efficiency. Lower static ratios tend to be a lot less efficient. Brake specific fuel consumption for FI engines is always higher than for NA engines because they are less efficient. You can see this when you go to RC Engineering's site and size fuel injectors.

Another thing - for those who think sticking a thicker headgasket to reduce compression is a good idea. BAD, BAD, BAD. You will lose a LOT of detonation resistance with this approach. I can explain that, but it will take a separate, and pretty long post to do it.

 

Thanx for the comments, and please don't get the wrong idea of what i said; wasn't trying to be cocky (if i did i would've ***** out the pix, but there's a reason why i haven't post anything about it yet on any forums); and when i mean "aggressive" i mean the wheel/tire stagger setup, size & width of the rims, offset to even disc of the rim, size and width of the tires, stretch... even to the factor of how it may affect the coilover setting and the height of the car, and brake clearance V.S. having lips...etc espects; but in this case of this thread probably the first couple espects i mentioned here matter to what that other dood said earlier about the stock vs aftermarket wheel on dyno.

 

lobux is there anywhere i can subscribe to your 'clliff notes' ? lol

 

Three things:

As lobux stated, there are two types of CR's. To get at your example, lowering the static CR would require deep pockets, as that would apply to actual changes to the physical dimensions of the engine...this is the compression ratio you see on paper, and the one which Lexus offers.

Dynamic CR (DCR), is a calculation of the actual volume determined when our intake valves are closed, relative to the pistons position...which can be changed, based on the cam you choose to use for example, or be affected by variable valve timing (or VVT-i as toyota calls it). This however, is where boost comes in to round out your figures (as you suggest to 'add' 7 psi to 'make' 13), as you begin to alter the compression ratio actually taking place within our cars cylinders. If DCR is raised too high (too much boost), you then begin to produce excess amounts of heat, which when uncontrolled, can cause detonation.

You then have chamber pressure (which is read in PSI), which is what constantly changes based on throttle position, intake/exhaust efficiency, rpm etc. Its what you refer to when you get your motors 'compression' tested (not ratio). Its what you affect when you break in the motor (you seal it properly, to make good 'compression'), and its potentially what's affected when your engine begins to lose power. While driving, romping on the gas will cause this to go up or down (as you alter the load and rpm), and affect your gas mileage, as this is the actual area of where the air/fuel burning process is being conducted. This is the actual changing pressure being created in the cylinders.

As for the intercooler, all of the heat created from FI occurs at the power adder, so yes, the intercooler can and is acting directly on the incoming air.

 

This is turning into a brutal hijack. Should we start a new thread on FI considerations for the 2GR?

Some cliff notes by me.

 

Total Hijack. Removed all posts regarding the supercharger.

 

little late...

 

As hijacks go, I don't see how this was that bad. You can't post about a new product without argument about it's worth, and after that, there was just some discussion about FI theory. If anything, the theory discussion just helped clear the air about why the gains weren't what some expected (me included).

As I said in my first post (#44), "I'll wait to post judgement until I see a dyno with the right pulley." If rt350 is offended, I apologize. I would love to see an SC kit for this car that performs well, and I'm glad someone has been making headway on it.

 

I swear you type stuff on this forum just to be typing. It adds nothing to the discussion except for white-noise, until people become so irritated they just leave.

Just out of curiosity, how much HP gains did you get from your Eibach springs and new tires?? What's the cost vs. HP rating on those??

 

Who are you referring to?