OK so I was wondering about the tradeoff between compression ratio and boost in there relation to power. I have been told that there is no trade off and that power is power. So if you have a engine running 12.1 and 5psi or a 8.1 compression engine running 25psi you will get the same power, Theoretically of course. I had my 3.3 liter engine rebuilt for my z. We decided to fill in a recess in the piston and the heads and block were decked. The stock comp. ratio was 9.1. We still have to calculate what the exact comp ratio is but we are thinking that it will be 9.5 to 1 or a little higher. As far as power goes am I better off with the higher ratio and a little less boost or should i have gone lower and ran more boost?
Background. 3.3 pathfinder motor, bored .020 over, weisco pistons, Jim wolf s2 cams and valve springs, haltech engine management.
I guess what I am asking is what scenario is better for making power high comp/low boost or low comp/High boost.
grade of fuel, cylinder head design (pentroof vs a “less efficient for high comp” design, type of management to be used & the tuner’s skill, type of racing/driving to be done, tranny, gear ratios, where & how you want the power, which turbo, manifolds (intake & exhaust) all will factor in / make a difference.
with high compression you will reach a point where detonation is very hard to avoid without race gas, that is the point of lowering compression.
let me use the SHO engine as a example since it is the only engine I know well. Stock compression is 9.8:1 it will safely take 15psi all day. bump it to 17psi and you will be detonating if your intake charge is anything but really cold. 20psi not much you can do on stock pistons as you will blow the ring lands out and have a melty pile of goo in your cyliner.
if you drop the compression to 9:1 you can run 25psi with a good charge cooling setup is detonation is minimal, and you will make more power.
the point of lower compression is to reduce the chance for detonation. thats it. run the highest compression possible without detonation.
the point of lower compression is to reduce the chance for detonation. thats it.
which is m my point.
you’re not considering:
fueling
load
gross weight
comb chamber design
rod ratio
intended use
tuning
climate
gearing
power adder
etc, etc.
But, yea, you’re right, run as much compression as you can until you hear detonation. When you do, pull out the slugs & run less DCR. or, add a thicker headgasket.
all i’m going to add to this is, 15psi is not a universal factor for all setups. 15psi on a 14B does not have the same characteristics as a GT42R @ 15psi…
all i’m going to add to this is, 15psi is not a universal factor for all setups. 15psi on a 14B does not have the same characteristics as a GT42R @ 15psi…
Ok well I think that I understand better. I am planning on running a 93 tune, 100 octane tune, and a c16 tune. I have a haltech standalone engine management system that should be able to handle the tuning with the help of mike at innovative. drivetrain is a spec clutch, tt blueprinted tranny, heavy duty drive shaft, clutch lsd, and I believe it is a 3.7 rear end. It will be for straight line and 1/4 acceleration. I will end up tracking it at the glen but that isn’t till next summer. I was trying to say that using same turbo for each scenario what would be able to make more power. 9.5:1 shouldn’t be that big an issue with all forged internals and weisco pistons, race cams, and valve springs.
motor one has 9.5 compression, motor 2 has 12.0 compression.
both motors are exactly the same except compression ratio (yea ik, but lets just say they are the same… fine. they have the same brand parts, but the low comp pistons and hg in motor one, regardless)
which motor should make more power n/a? obviously #2 (ask anyone who has driven a turbo car with low comp w/ a blown coupler)
now add boost and a good tune to both motors. lets say 12psi each. which will make more power under the same conditions? again, #2
a higher compression ratio in either case allows you to get more air and fuel in the combustion chamber…
low compression is easy to tune, dont bitch out and go low comp. jack that comp up!
OK now you see you’re talking about a 4 stroke motor, known as the Otto cycle. So you’ve got your AF intake, adiabatic compression, heat addition, adiabatic expansion, exhaust, and you repeat. Now from there you go back to the first line of the first post and forget everything that followed, including this post. Well, except that a gallon of boost is a gallon of boost. Not because it’s correct, but because it made me chuckle. :mamoru:
Both from a theoretical analysis and practical experience for pump gas motors low compression/high boost is the way to go. But there are so many variables and real world limitations that the discussion can get unfocused quick. Assuming you are starting with a fresh build of a given engine family, there will still be significant limitations in terms of the available heads, pistons, bore and stroke, gaskets, and so on. Then you need to consider the blower choices, intercoolers and so on. How this all pans out for the typical SBC, SBF, and BBC’s that I am familiar with is that 8-8.5:1 is where you want to be for compression. Using pump fuel, you then size the blower to produce ~15psi and that is going to be pretty close to optimal. With a decent method for lowering the air charge temp you can expect ~1.7hp/ci at the wheels with that kind of setup (with good heads and exhaust, etc.) and it will run on 93 octane. interestingly, this is about the same hp as a really optimized NA setup will produce on the same fuel - the limitation is the fuel. But while it is POSSIBLE to get the same peak hp NA it is going to be MUCH more difficult and expensive to achieve and the torque curve/engine bevaior may not be something you want to live with. Most people don’t enjoy cruising a V-8 at 4,000rpm running 4.10’s or for that matter buying a $5,000 set of cylinder heads and paying for a few days of dyno time while they figure out what is going to work or not. And so on. It’s not that the blower setup is going to be exactly plug and play either, but it is an order of magnitude simpler and will produce a car with much better real world performance.
I have a rudimentary knowledge of why lower CR/more boost is the way to go. If I get time later, I will try to explain it.
Low compression high boost is not a lot of fun for a street car that is mainly driven out of boost. You will be better off for a street car to leave it the way it is and boost it because you will make more N/A power while in lag. What I can say from experience is leave it at the 9.5:1 and boost it you will definitely have a lot of fun.
Low compression high boost is not a lot of fun for a street car that is mainly driven out of boost. You will be better off for a street car to leave it the way it is and boost it because you will make more N/A power while in lag. What I can say from experience is leave it at the 9.5:1 and boost it you will definitely have a lot of fun.
i was at 8.2:1SCR. i am upping it to ~8.6:1, keeping a tight quench, and a little less dish. on pump gas, boost will be down, but the car will be more responsive. if i want to have some fun, a little race gas and some more boots will commence :). timing will be low. boost>timing. i was going to go a little higher with the SCR. With LS1s, alot of guys are starting to bring their SCR back up, and they are having good luck doing so. a lot more power on the table, car is not as sluggish, so on and so forth. only peoblem is, at about 15psi, race gas is needed. but i think that will be a vice that is going to help ME out as it will keep me on less boost when im driving the car to work in the summer and what not. :lol:
there is ALOT more to be said, but its different for every type of motor, so its pointless to talk about it. Cubes, ALTITUDE, head design, quench, gas, timing, VE, CFM, IAT, purpose of the build, etc etc all have something to do with it
