Compression vs Boost FYI

[ProbeGT90R]

you just want someone to back you up so you feel like you're right because you're unsure of yourself.

exaxtly. first time doing it and i just want to make sure i do it right. and yeah i'm having a shop do most everything, maybe assembly by me, but them do the work ofcourse. somewhere around 9.2-9.3 sounds good to me, should be enough to get going and enough to add a lot of boost. i'm just not sure what each .1 more would add. i guess it would have to be trial and error, unless it's been done and tested you really wouldn't be able to tell cause each engine and setup would be different. it would have to be tested on the same engine each time. there's a lot of good knowledge being thrown around in this post though, nice to see for once. i think each has their own bebefits. just depends what you want.

[GTRacer93]

Well again, I will say this, if you're not wanting to take a 6 month course of the physics and everything behind the engine internals, then just leave it at the basics. I learned not to ask those questions after my mechanic started asking me like 20 questions about the piston sizes, rod lengths, cylinder sizes, ring used, valve clearance, so much stinking crap that I just said, "What do I need so I can make more horsepower, beable to boost the 12psi that I want, and run a nice dual stage nitrous setup with a 35 shot at the start progressing into a second 50shot totaling 85hp?" His reply. "That's the much better way to go isn't it. Spend the time understanding tuning rather then all the numbers and crunching that goes on to make the parts. It's not worth your time. Understand a basic why the engine is better and leave it at that."

Anyways, good luck to you and don't worry. the .1 won't make that much difference in NA mode but when boosting it starts to add up. Just remember that the whole compression ratio gets severely complicated so I try to explain it to everyone on a basic note. For every 1 dollar you spend, the ratio 9.3:1 makes you spend 9.3 dolalrs. Only in actuality the compression is based on units that all the different ingredients of the formula add in. So when you add compression it adds more ingredients that require more ingredients. The whole cooking example I gave earlier. You add more compression, its going to need more fuel, air, etc. I can't wait to rebuild my motor. I'll probably keep the 10:1 compression as I like the power of this ZE and I'm still going to boost it 10psi. After it melts or blows up, I'll rebuild it with better pistons and rods and probably a higher displacement like 2.6 or 2.7L. Well, enough on this topic. I think its covered enough for everyone.

Enjoy the new engine when its done. You boosting with a supercharger or turbo? I think that depends a little on what you decide on the motor as well as the supercharger will be noticeable off the line so it will make up for the doggy side of a low compression motor or aggressive high rpm cams that lag down low. Thats why I went with a nitrous/supercharger setup.. I want more HP up top but not all the time! And I want more HP down low all the time for getting on the high way and stuff. So boost a little, high compression ZE makes up for the upper end additional HP and then Nitrous for on the track if I want that extra 35 to 75 hp. Again, this is my preference.

[xr8tt]

compression lowering ??

O/k having read ALL the above.How can compression be lowered on KLZE engine ? I only realy want to take heads off as this engine is "very" low mlies. Can some milling be done to chambers to lower c/r ??
Another method is fitting thicker head gasgets but is there any made to do this ?? After market thicker copper head gasgets I guess ??? Thanks in advance....

[ProbeGT90R]

you can get a custom thicker copper head gasket made, that would be the easiest way to lower a few points. otherwise you will need new pistons.

[xr8tt]

What about milling some chamber away on the side where its straight.[Not the area between valves] Cutting it out to gasget as rest of head, making chamber completly round ??? Should get atleast a few cc ????

[veltpak6]

very interesting discussion

has anyone taken into account the rod/stroke ratio of the engine?

for example:
A FSD has a (seemingly terrible) 1.5:1 r/s ratio if my memory serves correctly. What this means is the piston, upon reaching TDC, will accelerate away at a higher rate and that will reduce the chance of detonation and the damage caused by it (less dwell at TDC). KL? well it has a better r/s ratio, more dwell at TDC, slower acceleration from TDC, so the periods of high cylinder pressure is lengthened compared to the FS, which is a formal invite for Mr. Explodey Pants.

My appologies if someone has already stated this... just food for thought.

[ProbeGT90R]

Re: very interesting discussion

Originally posted by veltpak6
has anyone taken into account the rod/stroke ratio of the engine?

for example:
A FSD has a (seemingly terrible) 1.5:1 r/s ratio if my memory serves correctly. What this means is the piston, upon reaching TDC, will accelerate away at a higher rate and that will reduce the chance of detonation and the damage caused by it (less dwell at TDC). KL? well it has a better r/s ratio, more dwell at TDC, slower acceleration from TDC, so the periods of high cylinder pressure is lengthened compared to the FS, which is a formal invite for Mr. Explodey Pants.

My appologies if someone has already stated this... just food for thought.

so what are you sying then, that that ratio is bad, or actually helps then against detonation? does that cost any hp to have a 1.5:1 ratio for instance? i'm still figuring out the whole ratio part, like what's good and bad. if it would help against detonation maybe in a turbocharged car it would be better to have the lower ratio if it doesn't mean a hp loss. if it could be made up with the CR or stroke perhaps.

[veltpak6]

Re: Re: very interesting discussion

Originally posted by ProbeGT90R
so what are you sying then, that that ratio is bad, or actually helps then against detonation? does that cost any hp to have a 1.5:1 ratio for instance? i'm still figuring out the whole ratio part, like what's good and bad. if it would help against detonation maybe in a turbocharged car it would be better to have the lower ratio if it doesn't mean a hp loss. if it could be made up with the CR or stroke perhaps.

thats it, you'de have a bit more of a safety window for the turbo application, but but at the cost of high rpm efficiency. maybe not enough to hoot about but u know

[Balor]

Ok I just got finished reading this and man oh man my head hurts...
Now everybody has really good opinions about this subject. But from reading everything I see the same thing over and over from everybody who posts good posts... no matter what side of the fence you’re on.
Here is the jest of what I’m reading
1. a High CR engine with a mid to low boost seems like its going to give you the best bang for you buck (due to the fact that you have a built up motor and just doing nothing more than adding more power on!)However as aid you get more bang for your buck, that’s just it. You have a really good chance of getting more BANG than you asked for. With out the proper tuning you run a really good chance of detonation. Yeah you can add an intercooler, ice cooling. Hell get your self some liquid nitrogen and call it a day... its all about the $$$$$... so this is a use at your own pocket book's exspense cuz if your running this, lets say mid CR (because 14 and up I’d say is high) and running boost and in proper tuning, your asking for trouble..
also the cool thing about turbos (instead of S.C.) you can just simply start running little to no boost at all and have little problem running normal gas (were talking like 10:1 CR here nothing insane) with out the boost and still have the power from the boltons to drive like a bat out of hell.
So let’s summarize the 1st one... High CR + mid-low boost + not the patience or means for tuning = a big fat whampen BOOM!!!
Now High CR + mid-low boost + proper tuning and $$$ = Nice low times at the track
Ok now
2. You can go with a low CR engine and do nothing but cram more air down the throat of the engine running higher boost... all fine and dandy... great your getting the same power that the higher CR engine is doing.. But in turn your doing nothing more than wearing down your Compressor faster... face it ur worken that compressor harder than the other engine... called cause and effect it works on everything.. Yes even engines. Ok now the dealio with this is that your going to be running PEE water in this engine and lower compression... now the lower compresssion is substantialy harder get it to to start detonating so your going to have less tuning problems.. So essentially if you’re looking for a direct bolt on going this way IMO is the best way to go. Because your not going to be looking at a lot of time on the dyno to get it all right. Ok now simply the thing with this here is that daily driving becomes harder with out the compressor because you’re going to have piss poor drivability with such low compression because the engine is relying totally on the compressor...
So here is the summery of the Low CR with high boost.
Your worken your compressor harder, if your not going to run boost your going to have weak power from the engine alone, however you have great play with tweeking and little to no fear of blowing the motor.
So
Low Cr + high boost = more room for playing with tuning with out fear of blowing the motor.

Ok now number THREE...

DONT DRIVE A PROBE... i mean me i love the probe, love it to death, don’t want to drive anything else... but I’m not the kind of guy who does nothing but want to make this lil 2.5L engine push 500+ whp or something insane like that. U want that... your going to get more fun and enjoyment out of a supra or something like that. Just face it. You can’t get our cars to do that unless you know or own your own engineering company that is willing to dish out says a million dollars or so to redesign our engines to perform like that. (Oh and we aren’t even talking about the $$$ for other components to support that much WHP... such as I know the cases of the trany crack at about 300whp or so... but that’s a different thread)
Just keep it in perspective to the car that you purchased...
Now all the theories I got are from this thread these are all of what I’ve seen and there is no MAGIC number for CR to Boost you should be running. Your going to have to shell out the $$$ to figure that out on your own... just like I said keep it in perspective for the kind of cars we own. There is a limit.

[Necromancer]

Re: compression vs boost

Okay this is just my two cents. I could be completely wrong.

It seems that with more boost and less compression you get one added bonus. Without getting into numbers and everything think of this. With a higher boost motor, while the intake valves are open it seems the air would rush into the cylinder faster as well as help push in the piston. If you're running low boost but high cylinder compression then you wouldn't as much of an advantage while the intake valves are open.

[Richard]

Re: compression vs boost

The engine with high CR is at it's limit and will break if you push it any more but a low compression engine can take a-lot more boost there for getting a higher total hp..If ya want your car to last and have a turbo go with low cr unless your only running low boost like 9psi or less. I ya want to hit 15-30 psi it's not going to happen with anything over 8.5:1 and water/alcohol injection + a really big intercooler, really good valves, timing retard, racing gas, and one h*** of an end gap on your rings.

[cadvis3d]

so what have we learnt from this children?.....

is high or low compression best.......yes or no answers please....

[Richard]

The only way to make more power is to increase cylinder pressure and burn more fuel.

You need more fuel and air (volume not presure) more air and fuel on a low cr engine will give you more power without the risk of detnation. The air and fuel the bigger the bang. The reason you can get more air/fuel volume in a low cr engine is because it's not going to have detnation problems. If you want to run 30 or 40 psi you can not do it with 10:1 or 9:1.....to get to 40 psi you would have to have like 7.5:1....It all comes down to the fact you need volume not presure. That's one reason running a t25 at 15 psi is not as good as a t3/t4 at 15 psi. If both turbos are running 15 psi the t3/t4 will have more valume and a lower temp then the t25

[Nelson93]

How can I get my turbo to push valume???

[Richard]

How can I get my turbo to push valume???

get a bigger one.... A larger wheel(t3/t4e)+ a bigger A/R will give you more volume at a cooler temp which is the whole point of turboing a car. A turbo charged car that has the most volume at the coldest temp produces the most Hp. This is true even if you have high or low cr. Haveing a low cr makes it safer and last longer, and also gives you more overall potential. Air charge temp is the most important part....Colder air gives you more volume. This is the concept behind Nitrous, and water injection. This is why useing n2o and a turbo together is a really good combo. That is if your engine is "built" to handle lots of hp..cause that is what your going to get when you put FI and n2o which injects at 260 some degrees below 0. If you have a really big turbo and you spray it will drop your air charge temp really low(colder = more volume), give you a really dence air charge without any presure at all. This gives you much more exhaust leaveing the moter even though your turbo hasn't spooled yet. Becuase there is not much presure in the intake becuase your turbo has not yet spooled the extra exhaust spools your turbo twice as fast. So you could have a turbo that is really big and still get it to spool like a medium sized turbo.

Most of these ideas work best for drag raceing not street racing. Street racing you speed up and slow down. Drag raceing you get the high rpm's and keep it there till the end. All the facts are the same with street racing just get a smaller turbo, or if possible one smaller turbo and one bigger turbo "twins". This will give fast spool up and the top end you want for drag racing all in one set up.

The bottom line is it's all about your air charge temp. and who much volume you can get into the moter before detination. High cr just brings you closer to denation. Doesn't matter what formulas you come up with to make your self sound smart. All the answers are pretty basic.