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Home > Performance Mods > Engine Mods > Compression vs. Boost....what to know.

Compression vs. Boost....what to know.

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Web


Reg'd: Apr 17, 2006

THE SHADOW

Quote:
Almost as fast as a supercharger (or turbo) can be bolted on, the question of how much boost can be run is sure to come up. When building up a motor to be supercharged, you've got the issue of just how much compression to run. Both of these questions relate to essentially the same set of equations. Assuming that all of the other requirements of the motor are satisfied, the compression -vs- boost aspect is not all that difficult.

All things being equal, there are only three ways to make more power - increase displacement, increase cylinder pressure, or increase rpm. It is the primary responsibility of a supercharger to increase cylinder pressure, while also, to a degree, increasing the effective displacement of the engine. As the supercharger supplies the motor with a more dense air charge, it allows for the ability to burn additional fuel. By adding a supercharger, additional air should no longer be a problem, which will also help to increase rpm. Ensuring that there will be enough additional fuel to maintain the proper air to fuel ratio, as well as controlling spark timing, will be the key to getting the most from a supercharged combination.

All motors have a static compression ratio based on the components (heads, pistons, etc...) its made of. This is the amount that the air inside the cylinder is compressed. It is a ratio of the cylinder volume at BDC to the volume at TDC. When a supercharger is added, additional air is forced into the cylinder effectively raising the compression ratio. The result of this is called effective compression. The formula for finding the effective compression is very easy:

((boost psi / 14.7) + 1) x motor compression = effective compression.

The effective compression formula allows a supercharged motor to be compared to a normally aspirated motor. For the most part, a supercharged motor with the same effective compression as a (similar) normally aspirated motor with the same static compression should have about the same overall power.

This may bring up the question that if the overall power should be about the same, why go with a supercharger? The main advantage of the supercharger is that it allows for a moderate compression level during normal driving (off boost) while allowing for very high compression levels when needed. Obviously a high compression motor of about 14:1 makes a lot of power, but it would never survive daily driving. A lower compression motor is great for daily driving, but greatly reduces the potential for power. The supercharger allows for higher compression levels than could be used without a supercharger, while still offering the benefits of a standard compression motor. Many street supercharged systems will go beyond 18:1 effective compression under peak boost. Under race conditions, many supercharged race motors will go well beyond 25:1 effective compression. Both of these levels are far beyond what could be done reliably or cost effectively without a supercharger.

This brings us back to the question of just how much boost or compression can, or should be run. Obviously there can't be a simple number that could be used for every application. This is why it's so critical to chose the proper components. It's not necessary to build a low compression motor to use a supercharger, but the correct parts are still necessary. The biggest factors will be in things like head bolts (or preferably studs), gaskets, and the strength of the other engine components. It goes without saying that the incredible power that a supercharger can add, can easily start breaking things. It is very important that as the boost levels rise, the need for a stronger crank, rods, pistons, etc... becomes very critical. Many people forget this as the motor itself is relatively mild, while the supercharger pushes it well beyond the practical limits it was intended for.

Now, back to the compression issue. Anyone who has looked into supercharging has heard that you need a low (static) compression motor. This may have been true once upon a time, when roots type (positive displacement) superchargers and carburetors ruled the land, but it's not so necessary now. The problem with a low compression motor is that it relies heavily on the supercharger for its power. An 8:1 motor is definitely not going to be a power house. Sure, you can throw 18 lbs of boost on it and get some real power, but why? A higher compression motor of 9:1 or even 10:1 will have much more power without the blower. Then, with less boost you could easily have the same overall power - only it would be much more usable. Both of the motors (8:1 with 18 lbs boost and 9.5:1 with 12 lbs boost) will have almost the same effective compression and about the same peak power. The big difference will be where you see the power, and how much of a demand will be placed on the supercharger. Obviously, the 9.5:1 motor is going to have far greater torque and low end power as the boost is only starting to come in. It is also going to be much easier to find a blower to survive at only 12 lbs of boost -vs- one that would have to put out 18 lbs of boost. It is now very easy to see why a higher compression motor with lower boost is becoming so popular.

Please understand that when I say higher compression and lower boost, there are limits to each. Going over about 10:1 will make the amount of boost that is usable drop quickly to the point that the supercharger is somewhat wasted. In my opinion, anything less than 8 lbs of boost is a waste of a supercharger. Going over 10:1 will also make daily driving with pump gas much more difficult. In this same way, compression levels much under 9:1 will require substantial boost levels to make massive power gains. This would require boost levels that are very demanding of a supercharger. This is truly unnecessary. This isn't to say that the lower compression / higher boost set-up doesn't have a slightly higher potential for power, because it does. A lower compression motor has the ability to contain more volume. This can be an advantage, but is such a minor one that it's not necessarily worth the effort - unless it's for an all out race motor. Even then there are limits for the same reasons as the street / strip motor.

Once again, the compression -vs- boost issue. For a car that will see the streets (actually for most applications), the best thing to do is start with a motor compression that is high enough to make the horsepower you want for normal driving. Don't rely on your supercharger to make all your horsepower. With a good motor compression, add as much boost as is safe for your particular application. Decide on a final effective compression, and work your way back through the formula to find your maximum boost level: ((effective compression / motor compression) - 1) x 14.7 = boost. With the proper fuel system and related engine components, an effective compression of 16:1 to 18:1 should be more than workable. For heavily modified cars, effective compressions over 20:1 should be very carefully considered. Remember, even Indy cars only run about 18 Lbs of boost and reasonable static compression levels. Technology has come a long way and modern day supercharging should take full advantage of this.

While these opinions are not exactly the most popular, they are based on facts and real world performance. While there will always be those who continue with tradition and stick with what was done in the past, it is those who reach for something more that are winning races. Often times, some of the best advice can be found from those who have done what you want to do. All too often it is those who know the least that offer the most advice. After having been involved in supercharging for many years, I have heard it all. Most of it was worthless. It was often the least mentioned things and trail and error that have been the most rewarding. Hopefully this information will help to explain some of the most misunderstood aspects of supercharging.


http://www.motorsportsdigest.com/tech/forced2.htm


I know not with what weapons World War III will be fought, but World War IV will be fought with sticks and stones. A.E.
[#] Mar 20, 2007 03:09pm
dewey

Reg'd: Feb 22, 2006

Vehicle Designer

nice explanation should i get nology wires?

[#] Mar 20, 2007 04:55pm
Web


Reg'd: Apr 17, 2006

THE SHADOW

No, you have a DIS (direct ignition system....plug on coil) so nology wires would be pointless. They don't do anything for OUR engine.


I know not with what weapons World War III will be fought, but World War IV will be fought with sticks and stones. A.E.
[#] Mar 20, 2007 05:46pm
Mugetsu


Reg'd: Oct 11, 2006

Scion Guru

well according to this the TRD is a waste of a super charger... lol a better question would be CAN the TRD S/C be ran with an after market EMS such as E-manage or SAFC to acheive higher boost(other than swapping the pulley)safely? or is it limited to 9 mechanically?

[#] Mar 20, 2007 06:43pm
Web


Reg'd: Apr 17, 2006

THE SHADOW

I believe it could BUT, a pulley would increase the boost potential easier.


I know not with what weapons World War III will be fought, but World War IV will be fought with sticks and stones. A.E.
[#] Mar 20, 2007 06:45pm
andino


Reg'd: Mar 05, 2006

Godlike Advisor

good piece of reading...i guess my choice for the S/C is still in the running. S/C + pulley and management system probably make quite a bit of power.





Are you in SoCal? Come to our CStC meets!
Located in Socal? Need something Installed?
Have a question and no one is willing to help? Shoot me a PM or hit me up on AIM.
[#] Mar 20, 2007 07:16pm
Web


Reg'd: Apr 17, 2006

THE SHADOW

by pulley i mean the S/C pulley, not the crank pulley.

Just so no one's confused


I know not with what weapons World War III will be fought, but World War IV will be fought with sticks and stones. A.E.
[#] Mar 21, 2007 07:51am
aDelcoTc


Reg'd: Jan 25, 2007

Scion Guru

Web wrote:
by pulley i mean the S/C pulley, not the crank pulley.

Just so no one's confused


my goal is to have a NA tc that beats S/C tc's.

but i just thought about it, when i do get my hi comp pistons, i should probably have headwork done (pnp, maybe titanium valves and retainers and springs and ARP studs) at the same time... 1) so the new hi comp motor doesn't blow off the head 2) save myself some labor and have it all done at a good shop upfront, to avoid having the pistons done, then lifting the head 4 months later and having to go back.

side note: i really don't know how all these boosted tcs are lasting. my stock basically NA tc motor screams and sounds like its about to blow up when i rev real high in 2nd or 3rd. can't imagine what it would be like under boost.

i wouldn't trust it.

Scion tc: Its essentially a 2 door Camry that spins up the tires quicker.....
[#] Mar 21, 2007 10:05am
Web


Reg'd: Apr 17, 2006

THE SHADOW

Head work is key. Upgrading valves and valve springs along with pistons, rods and the head itself will raise the rpm range and also the power range......it won't peak at 5700 but more around 6300 with Stage I head and 6600 with Stage II. Stage II increases redline to 7200 .


I know not with what weapons World War III will be fought, but World War IV will be fought with sticks and stones. A.E.
[#] Mar 21, 2007 10:07am
aDelcoTc


Reg'd: Jan 25, 2007

Scion Guru

i'd probably just have a custom head job done at beezer built engines. but how much is the stage 1 head package? out of curiosity....

Scion tc: Its essentially a 2 door Camry that spins up the tires quicker.....
[#] Mar 21, 2007 11:09am
Web


Reg'd: Apr 17, 2006

THE SHADOW

Pistons:
http://www.zeropointindus...=28_80&products_id=63

Stage I head:
http://www.zeropointindus...=28_80&products_id=29

Head studs:
http://www.zeropointindus...28_80&products_id=161



Stage II head:
http://www.zeropointindus...28_80&products_id=187

Stage II head parts:
http://www.zeropointindus...28_80&products_id=292


Stage I uses stock head parts but Stage II has oversized valves and ports.



I know not with what weapons World War III will be fought, but World War IV will be fought with sticks and stones. A.E.
[#] Mar 21, 2007 11:13am
aDelcoTc


Reg'd: Jan 25, 2007

Scion Guru

i'm not really too sold on ZPI. and thats a lot of hassle for the headswap.

i'd rather just take the car to a machine shop, have them crack open the block, work on the head while they're replacing the pistons, then put it all back together and be done with it.

Scion tc: Its essentially a 2 door Camry that spins up the tires quicker.....
[#] Mar 21, 2007 11:42am
Web


Reg'd: Apr 17, 2006

THE SHADOW

True


I know not with what weapons World War III will be fought, but World War IV will be fought with sticks and stones. A.E.
[#] Mar 21, 2007 12:00pm
southpole12

Reg'd: Apr 13, 2007

Window Washer

s/c tc's are pieces, its turbo or nothing lol... s/c is junk in my eyes... you will got alot more power out of a turbo.

[#] Apr 13, 2007 11:47pm
TongMan


Reg'd: Dec 04, 2006

Master Mechanic

Long, but informative read...

TRD Supercharged Release Series 2.0 tC
[#] Apr 13, 2007 11:57pm
aDelcoTc


Reg'd: Jan 25, 2007

Scion Guru

southpole12 wrote:
s/c tc's are pieces, its turbo or nothing lol... s/c is junk in my eyes... you will got alot more power out of a turbo.


twin charged tc FTW!!! now who's gonna do it???

Scion tc: Its essentially a 2 door Camry that spins up the tires quicker.....
[#] Apr 14, 2007 05:35pm
nickah


Reg'd: Oct 28, 2006

Vehicle Designer

is there a more aggressive cam kit for our tc too?

You can't learn me nothin...
[#] Apr 15, 2007 03:06am
Web


Reg'd: Apr 17, 2006

THE SHADOW

Not right now....Del said someone is working on them but still not in production.


I know not with what weapons World War III will be fought, but World War IV will be fought with sticks and stones. A.E.
[#] Apr 15, 2007 10:37am
aDelcoTc


Reg'd: Jan 25, 2007

Scion Guru

Web wrote:
Not right now....Del said someone is working on them but still not in production.


brian crower is developing them. from my standpoint, the best aspect of aftermarket cams may be for FI. since our vvti on operates on the intake cam, the exhaust valves won't be able to flow enough exhaust gases at high engine speeds under boost. ie. the excess exhaust causes backpressure and robs power. with aftermarket cams, the exhaust system would have much less trouble expelling gases with the longer duration and lift on the exhaust valves.

obviously, also the intake valves would be allowing for a much greater intake flow capacity as well. but i really think they may be more tuned for FI apps. we'll have to see....



Scion tc: Its essentially a 2 door Camry that spins up the tires quicker.....
[#] Apr 15, 2007 03:15pm
trdmaster


Reg'd: May 28, 2008

Detailer

i think ill have a say in this matter

I've had the nology hotwires, they dont do anything, u only think they do, i took them out as soon as i tried driving around then realized i had a missfire

Compression is the total volume inside a cylinder being pressed and squeezed as the piston goes up, boost is raising that number significantly

Stay w/ a turbo, u can get way more boost and more power, supercharger looses its power at high end

-=TRD Master=-
[#] May 28, 2008 05:16pm
Web


Reg'd: Apr 17, 2006

THE SHADOW

No need to bump old threads.....thread is over 1 year old.

Information provided in first post encompassed what you already said.


I know not with what weapons World War III will be fought, but World War IV will be fought with sticks and stones. A.E.
[#] May 28, 2008 06:10pm

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