effective compression.

[RBs93]

-engines are compression. fuel/air + how much is squeezed +spark = power
more:
-displacement
-cylinder pressure
-rpm
= more power

one thing i dont see alot of people talk about when boosting a motor is "effective compression"
my motor has a N/A compression stock of 9.2:1,im running 15psi so that makes my effective compression of 18.5:1

my question is with all things relative 2 motors of the same compression will make about the same peak power.
-if i drop my motors n/a comp to 8.5:1 and raise the boost to 17 i will have the same "effective compression".

I'm still new to all this engine math so just wondering if I'm going to be making about the same power since i will have the same compression (effective compression of 18.5:1) or am i not looking at this the correct way??

with the boost turned to 20psi with 8.5 pistons i would be pushing an e.c. of 20:1 which with a basically stock block is scary.

if im dropping the compression should i bore it out as well?

my mind is a bit jello-ish right now so outside input would be great!

[LU_NA20v]

-engines are compression. fuel/air + how much is squeezed +spark = power
more:
-displacement
-cylinder pressure
-rpm
= more power

one thing i dont see alot of people talk about when boosting a motor is "effective compression"
my motor has a N/A compression stock of 9.2:1,im running 15psi so that makes my effective compression of 18.5:1

my question is with all things relative 2 motors of the same compression will make about the same peak power.
-if i drop my motors n/a comp to 8.5:1 and raise the boost to 17 i will have the same "effective compression".

I'm still new to all this engine math so just wondering if I'm going to be making about the same power since i will have the same compression (effective compression of 18.5:1) or am i not looking at this the correct way??

with the boost turned to 20psi with 8.5 pistons i would be pushing an e.c. of 20:1 which with a basically stock block is scary.

if im dropping the compression should i bore it out as well?

my mind is a bit jello-ish right now so outside input would be great!

compression ratio is compression ratio, what you're looking for is cylinder pressure. The pressure created in the form of PSI when correlating boost/compression ratio.

what I highlighted is a bad idea, boring it out would decrease wall thickness in your motor which would = KABOOM.

the only reason you would bore out your block is to have it sleeved.

[luvmyhondas]

-engines are compression. fuel/air + how much is squeezed +spark = power
more:
-displacement
-cylinder pressure
-rpm
= more power


one thing i dont see alot of people talk about when boosting a motor is "effective compression"
my motor has a N/A compression stock of 9.2:1,im running 15psi so that makes my effective compression of 18.5:1

my question is with all things relative 2 motors of the same compression will make about the same peak power.
-if i drop my motors n/a comp to 8.5:1 and raise the boost to 17 i will have the same "effective compression".

I'm still new to all this engine math so just wondering if I'm going to be making about the same power since i will have the same compression (effective compression of 18.5:1) or am i not looking at this the correct way??

with the boost turned to 20psi with 8.5 pistons i would be pushing an e.c. of 20:1 which with a basically stock block is scary.

if im dropping the compression should i bore it out as well?

my mind is a bit jello-ish right now so outside input would be great!

yes, more displacement will only give you the potential for more power if you keep the compression ratio the same as before the displacement change. technically you could gain displacement by changing the size of the combustion chambers on the head, but it would lower your compression, and most likely would lower your output. More RPM will only make more power if the rest of the motor can handle it. Now a small increase in RPM will sometimes show a gain from stock, but in the performance realm higher RPM will necessitate some other changes if a drastically higher RPM limit is desired. Now, more cylinder pressure from either mechanical means (hi comp pistons, turbo, supercharger) or chemical means (nitrous) will most certainly make more power.

just adding your boost level to the compression ratio does not equal your 'effective compression ratio'. plus, if you really want your brain to hurt, the static compression of an engine can be drastically different from the 'running compression' when you factor in cam profile overlaps...

if you are lowering your compression it won't be necessary to bore it, but it will depend on the condition of your cylinders. if they are not round, or properly finished you may have problems keeping your fresh engine together for very long. I don't know what engine you have, but unless you plan on going to the extreme ends of HP a small over bore (.020", .5mm) should be fine on most Honda stuff assuming it cleans up any ridges/out of round.

here's my advice (assuming Honda B or D series). Build your engine @9.0:1 with solid, high quality components. set a HP target/goal and calculate how much injector you'll need. then let your tuner work the fuel/ignition tables to the desired power level. This does assume that your turbo is properly sized and that the rest of the drivetrain can handle the power increase. don't get hung up on "im running 20 lbs or 15lbs." If you build your engine and can make 300HP on 15psi, and another guy has to build 20psi to get the same power then I'd say the other guy is behind the curve.

[RBs93]

yes, more displacement will only give you the potential for more power if you keep the compression ratio the same as before the displacement change. technically you could gain displacement by changing the size of the combustion chambers on the head, but it would lower your compression, and most likely would lower your output. More RPM will only make more power if the rest of the motor can handle it. Now a small increase in RPM will sometimes show a gain from stock, but in the performance realm higher RPM will necessitate some other changes if a drastically higher RPM limit is desired. Now, more cylinder pressure from either mechanical means (hi comp pistons, turbo, supercharger) or chemical means (nitrous) will most certainly make more power.

just adding your boost level to the compression ratio does not equal your 'effective compression ratio'.

the equation that i was told for effective compression is:
((boost/14.7)+1)xengine compression=
for my car this would be ((15/14.7)+1)x9.2=18.5:1 effective compression

I don't know what engine you have, but unless you plan on going to the extreme ends of HP a small over bore (.020", .5mm) should be fine on most Honda stuff assuming it cleans up any ridges/out of round.

i dont know whats "extreme". shooting for 450-500whp on the new setup.

my advice (assuming Honda B or D series). Build your engine @9.0:1 with solid, high quality components. set a HP target/goal and calculate how much injector you'll need. then let your tuner work the fuel/ignition tables to the desired power level. This does assume that your turbo is properly sized and that the rest of the drivetrain can handle the power increase. don't get hung up on "im running 20 lbs or 15lbs." If you build your engine and can make 300HP on 15psi, and another guy has to build 20psi to get the same power then I'd say the other guy is behind the curve.

im already running 9.2:1 so spending 600bucks on new pistons to drop the compression that little bit isnt cost effective on my side. i could,as you siad mess with the head to drop the compression a tad.

im not worried about how much boost im going to run. all i care about is that if im spending money...its being invested properly and not put me behind the curve as you said.

[RBs93]

i guess it would help if i posted the set-up:
b18b
255 pump
660cc injectors
rods
LS head
stock cams/gears
BC valves springs,retainers
water/meth kit
garrett 35r turbo
cast iron turbo manifold (ram horn when i get it finished)
new 3" charge pipe turbo-intake

[5thgen]

If you really think that equation for eff. compression works.....why don't we see n/a people running 17:1 daily drivers on pump gas?

Just a thought.


And you cannot gain displacement by removing material in the chamber. Displacement is bore x stroke.....always. You have to mess with 81x89 to get more displacement.


And on your ls setup....more rpm won't do anything but let your torque drop further. Ls powerbands are roughly 4700-6500 rpm....unless you get some decent sized cams to keep your torque curve from dropping like a rock.


I also would not want 450-500 on 660cc injectors.....unless you are running a high base pressure.

[RBs93]

effective compression is purely a reference number. it is to show how much added power the sc/turbo is supplying the engine. it is to say an engine on boost would have to run XX:X compression to make the same numbers naturally aspirated.

my original question still remains: if i drop my compression and raise my boost my old setup and new setup will have the same effective compression. all things relative in an engine..your compression is your power...so will i be making the same relative power on a lower comp/higher boost setup just in a different power band?

[rickles8099]

http://www.rbracing-rsr.com/compression.htm

[5thgen]

effective compression is purely a reference number. it is to show how much added power the sc/turbo is supplying the engine. it is to say an engine on boost would have to run XX:X compression to make the same numbers naturally aspirated.

my original question still remains: if i drop my compression and raise my boost my old setup and new setup will have the same effective compression. all things relative in an engine..your compression is your power...so will i be making the same relative power on a lower comp/higher boost setup just in a different power band?

Wouldn't a dyno show how much power you added?.....that seems a lot easier to me. 120whp stock...up to 330whp is a gain of 210whp?.....

And I don't see how your powerband is going to change....you will lose some power without a pressured charge obviously.....but just lowering compression won't significantly change where your power happens.

I think you are thinking way too much into this. Search honda-tech and find comparisons between low comp. and hi comp. setups. Everything on a honda has already been done before....it shouldn't be hard to find what you are looking for.

[Bwill9886]

Rbs, it sounds like you may be thinking in WAY too complicated of terms right now! haha

Let's establish a few things first so that people may be able to give you more help. You have a good start by posting your current setup.

-What are your whp goals? Obviously you wanna go for as much as possible, but realistically, what are you shooting for?

-Budget? You may not know exactly right now, but maybe you could help us by establishing whether your going to really BUILD this engine (as in almost every component or maybe going LS/VTEC), or are you gonna go for some solid upgrades that aren't going to run more than a several hundred to a few grand? Nothing wrong with either, but this can give us a good idea of how you need to build your car to maximize its function.

Quick example: Robert's Civic can be really fast if the boost got turned back up (it already is fast BUT its not taking full advantage of its power potential), but it is in no way the most streetable car in the world. Are you shooting for that, or something that can be driven more easily?

Oh and 5thgen gave good advice on checking out honda-tech. There is a wealth of information in the forced induction section that you should definitely check out anyway.