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Old 10-14-2005, 03:12 PM   #1
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what's an ideal compression ratio

my question is what's the best ratio for a street performance mustang?
what would be the best/highest ratio to have without needing a higher octane fuel?
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Old 10-14-2005, 03:47 PM   #2
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Re: what's an ideal compression ratio

it depends on what you are doing, power adder? N/A etc etc (what material are your heads made of for instance is another set of questions to ask)
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Old 10-14-2005, 04:01 PM   #3
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Re: what's an ideal compression ratio

hmmm ok. because at redline performance mustangs when you're buying p&p heads and everything, theres a section to fill out of what compression ratio you want.
if i get lucky and can afford a power adder i'll be getting ss valves,strong springs and everything else to go with it to hold up under that extra pressure. i didn't know if 10:1 was too high or what.
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Old 10-14-2005, 06:15 PM   #4
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Re: what's an ideal compression ratio

with supercharger/turbocharger lower compression may be alot better for you. Let some of the power adder people chime in.
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Old 10-14-2005, 09:28 PM   #5
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Re: what's an ideal compression ratio

if you want an all out n/a car you want high compressoin and if your want a power adder of any kind...supercharger/turbo/nitrous... you want to have a lower compresson. you can run higher compression on a car with a power adder but you would have to run race gas or have a serious timing ****** to get rid of the knocking. usually anything below 8.5:1 is good of a power adder car. mustangs come stock with 9.1:1 which is pretty boost freindly. i believe corvettes are 10.1:1, which is ok for boost but not a lot without some serious adjustments.
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Old 10-14-2005, 10:16 PM   #6
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Re: what's an ideal compression ratio

The LS1 is a 10.5:1 motor.
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Old 10-14-2005, 10:28 PM   #7
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Re: what's an ideal compression ratio

the thing with power adders and high compression is that you can run much boost without detonation issues

a power adder is basically changing the compression ratio, there are huge formulas out there that calculate the effective compression ratio as boost is added, if youre really interested in that, i can try to dig it up

a lot of guys with turbo coupes are actually wanting to bump the compression ratio from stock 8.0:1 to 9.5:1, because it gives better off boost torque, and uses less boost to make the power

10.0:1 would be a good ratio if youre not out to get a power adder, but one day getting one, because you will still have power, but then you can add say 8psi, and get a considerable gain, if you were building an engine with boost in mind, go no higher than 9.5:1
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Old 10-15-2005, 12:35 AM   #8
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Re: what's an ideal compression ratio

hey thanks this is really helpful. i'm just trying to get the most volumetric efficiency out of my 6 cylinder. so if i went with a lower compression and didn't run high boost at the time would my performance suck? I want to be in controll and set limits on the boost but don't want to have weak compression to begin with when it's turned down.
I know once i get everything tuned properly i'm gonna want to go all out, but until my car is set up for the higher boost i'm gonna keep it low.

none of this is going to happen soon but i'm planning ahead for once so i have something to work towards. so Tbird if you do find the formulas that would be great, but if it's a hassle then that's no problem.
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Old 10-15-2005, 01:22 AM   #9
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Re: what's an ideal compression ratio

9-9.5 would be good for you I bet...
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Old 10-15-2005, 09:01 AM   #10
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Re: what's an ideal compression ratio

aluminum heads are also different than iron heads so be careful who tells you what (we here know you have aluminum though) the stock 9.3:1 will probably do fine, if you are going blown you could drop it a tad to say 9 even to help a bit with detonation.
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Old 10-15-2005, 10:26 AM   #11
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Re: what's an ideal compression ratio

Taken from www.InductionMotorsports.com:

"What's better, low compression and more boost or high compression and less boost?"

There are certainly reasons to try to raise compression ratio, namely when off-boost performance matters, like on a street car, or when using a very small displacement motor. but when talking purely about on-boost power potential, compression just doesn't make any sense.
People have tested the power effects of raising compression for decades, and the most optimistic results are about 3% more power with an additional point of compression (going from 9:1 to 10:1, for example). All combinations will be limited by detonation at some boost and timing threshold, regardless of the fuel used. The decrease in compression allows you to run more boost, which introduces more oxygen into the cylinder. Raising the boost from 14psi to 15psi (just a 1psi increase) adds an additional 3.4% of oxygen. So right there, you are already past the break even mark of losing a point of compression. And obviously, lowering the compression a full point allows you to run much more than 1 additional psi of boost. In other words, you always pick up more power by adding boost and lowering compression, because power potential is based primarily on your ability to burn fuel, and that is directly proportional to the amount of oxygen that you have in the cylinder. Raising compression doesn't change the amount of oxygen/fuel in the cylinder, it just squeezes it a bit more.

So the big question becomes, how much boost do we gain for X amount of compression? The best method we have found is to calculate the effective compression ratio (ECR) with boost. The problem is that most people use an incorrect formula that says that 14.7psi of boost on a 8.5:1 motor is a 17:1 ECR. So how in the world do people get away with this combination on pump gas? You can't even idle down the street on pump gas on a true 17:1 compression motor. Here's the real formula to use:



sqrt((boost+14.7)/14.7) * CR = ECR

sqrt = square root
boost = psi of boost
CR = static compression ratio of the motor
ECR = effective compression ratio


So our above example gives an ECR of 12.0:1. This makes perfect sense, because 12:1 is considered to be the max safe limit with aluminum heads on pump gas, and 15psi is about as much boost as you can safely run before you at least start losing a significant amount of timing to knock. Of course every motor is different, and no formula is going to be perfect for all combinations, but this one is vastly better than the standard formula (which leaves out the square root).

So now we can target a certain ECR, say 12.0:1. We see that at 8.5:1 CR we can run 14.7psi of boost. But at 7.5:1 we can run 23psi of boost (and still maintain the 12.0:1 ECR). We only gave up 1 point of compression (3% max power) and yet we gained 28% more oxygen (28% more power potential). Suddenly it's quite obvious why top fuel is running 5:1 compression, that's where all the power is!!

8.5:1 turns out to be a real good all around number for on and off boost performance. Many "performance" NA motors are only 9.0:1 so we're not far off of that, and yet we're low enough to run 30+ psi without problems (provided that a proper fuel is used).
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Old 10-15-2005, 07:49 PM   #12
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Re: what's an ideal compression ratio

Problem with the above thoery is that low compression sux off boost. Sux is too mild a word though.

The idea compression ratio is limited by your fuel's ability to resist detonation. Run Propane or alcohol and you can push 11:1 no problem. Run water injection and you can boost a high compression motor a lot higher and have more cake to eat.
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Old 10-15-2005, 10:34 PM   #13
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Re: what's an ideal compression ratio

my car is my dd so i'm not going to be running alcohol. maybe 100 oct. but that will be seldom since its about 4.69 a gallon now.
I think i'll just try and keep my compression ratio where it is so i can run low boost and not lose performance. then later on when my car is built up (engine,chasis,brakes,fuel) i can bump up the boost to 10 plus and not have to think twice about it being too much.

from what i've been told i can run 10 psi on my stock motor but i know it won't last long so i'm gonna wait a while and do it smart.
thanks again for all your help
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Old 10-16-2005, 12:41 AM   #14
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Re: what's an ideal compression ratio

with your compression ratio, ive seen 14psi run on a 3.8L, the biggest thing is getting a proper tune

for example, the 5.0 guys swear up and down that the block splits at 500rwhp, currently, there is a guy pushing 680ish at the wheels on a stock block, and he's been driving it almost every day for 2 years, the biggest thing is that he has the car tuned VERY well, it runs efficiently, doesnt generate any more heat than a stock engine, and has a great combination

boost is only a measurment of restriction, boost isnt what breaks you car, its the tune that makes or breaks it
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Old 10-16-2005, 06:47 PM   #15
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Re: what's an ideal compression ratio

whats your definition of a good tune? i plan on going through justin using the xcal and the wideband sensor, and email him the info so he can give me the right tune. what more needs be done with the tuning process besides making sure my car is running properly before adding boost and doing the usual maintinance?
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Old 10-16-2005, 07:19 PM   #16
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Re: what's an ideal compression ratio

while justin can get an xcal pretty decent for an N/A setup, theres a lot more involved, and worse consequences with a forced induction setup

a big thing is to build the car, say, install the turbo kit, talk to people who have done similar setups and find out things like what fuel injectors work well for your goals, what MAF is the best for what youre doing, youll need a good fuel pump back there, adjustable fuel pressure regulator, and a bunch of other things, get them pretty close, and then get it on a dyno, and tune it on the dyno

what they can do, is they can take the RPM's, the HP, torque , and the A/F, overlay the graphs, and figure out possibly at which RPM you could be lean and give some detonation, where you need more fuel, where you might need less fuel, etc

they can dial in your setup to what you want to do basically, especially if they can interface an xcal, or if theyre burning you a chip for your setup specifically
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