Compression vs. Boost (this should be a FAQ)

plev72

Registered User
Hey all, can someone give either a link or an explanation up the advantages and disadvantages required the boost vs compression situation.

I understand that the higher the compression, the less boost you can have.

What are the advantages to going either way? The things I assume intuitively are the following (but could be wrong):

Lower compression (assumably) would require boost more often and to a larger degree because the engine has less HP independant of the supercharger to accelerate or to maintain a given speed. For the speeds that can be supported by the engine with lower compression, when no boost is being used, I would assume the engine would get better gas mileage. Also, assumably it should be possible to advance the timing because there is less likelihood of detonation due to the reduced compression.

Higher compression and lower boost would assumably put less wear on the supercharger and make the engine work more. Assumably this would slightly reduce gas mileage but increase supercharger life. Also, timing would have to be retarded.

Anyway, guess my questions are these...

1) what did I get wrong
2) what did I miss about either the lower compression or the higher compression
3) Since I'm doing a rebuild, which way is a better way to go? I'm assuming that if a go with the thicker headgasker or the bowled pistons that I won't have any problem running the 10% pulley from now on... but is that the best way to go?

Thanks,
Paul
 
OK SCHOOL IS IN!!!!!!

Let's start by expaining the difference between both:


1) Compression is the ratio between the cylinder volume versus compressed cylinder volume.

Look at it this way.......

When the piston is at bottom dead center, the cylinder the piston is riding in is completely empty. This volume for arguement sake is 10 cubic inches. Now I rotate the crankshaft to bring the piston to top dead center and now the air and fuel in the cylinder is a volume of 1 cubic inch.

This would be 10:1 compression or compressing 10 Ci of air into 1 Ci space. This is how an internal combustion engine works.

You compress the air/fuel to an explossive point and ignite a spark causing a controlled burn and explosion of the mixure.

The more COMPRESSION you have, the more torque and horsepower you CAN get.

If you increase the effective compression ratio, (by domed pistons, milled heads or block, both), you increase compression. NASCAR vehicles are a perfect example of this. They have some running 16:1 compression and getting ridiculous horsepower.

Downfall is this: Extreme stress of that much torque on the internals, needing of higher octane fuel since compression causes heat and this will detonate most lower octane fuels upon compression, less timing needed to get proper engine timing, and increased cost of running IE Mileage per gallon.


NOW...........


On to BOOST:


Boost is amount of air measured in PSI (pressure per square inch) that is being forced into the cylinders over the normal atmospheric pressure, (being 14.6 pounds per square inch)



Let me explain (it's been awhile since college physics!!!)

The air around us pushes on us at 14.6 lb/in2. This means that when we breathe in, we don't actually BREATHE air in, we allow the pressurized air around us to enter our lungs!!! (KINDA NEAT HUH!!!!).

Now, in a naturally aspirated motor, the engine runs on the same principal as our lungs. The throttle plate opens allowing the air outside to enter the intake manifold and the cylinders in relation to camshaft lift and duration. Camshaft is like your diaphram, allowing only as much air in as you allow it to. Deep breath and a lot of air, shallow breath and little air.

Now, some cars with turbos have a BAR meter or BARAMETRIC PRESSURE GAUGE. This meaures the atmospheric pressure in BARs. We measure it in PSI and that is the boost we get.


The supercharger and turbo both have the same principal operation, just different power sources.....

TURBO.....EXHAUST DRIVEN

SUPERCHARGER.....MOTOR DRIVEN..


We'll get into the benifits of this later....


The boost we get is the result of the supercharger spinning and forcing air into the cylinders CONSTANTLY!!!!!!!!!!


Now realize this, the blower is always spinning. Always creating boost, JUST NOT ENOUGH TO OVERCOME THE ATMOSPHERIC PRESSURE OF 14.6 LBS!!!!!!!!!

So in laymans terms, boost is not created until the supercharger produces enough air pressure to exceed the normal air pressure.

This makes the supercharger so benifical to us!!!! It allows us to produce this boost at low RPM and give us the KICK in the *** we love.

The cost of supercharging is this: Lower compression to start on a normal production car due to unavailable high octane fuels, higher normal fuel economy, engine loss of power due to having to drive the supercharger.

When choosing to build an engine, one must look at many different things......

Higher compression will lead to lower boost pressures and less kick in the *** (this is BAD for us due to heavy vehicle weight), requiring higher octane gas, bigger camshaft to take advantage of higher compression, different spark tables and spark timing for engine throughout the RPM range due to stock tables will make the engine go B O O M !!!!!!!!



Now for gears.....

Gears just increase the multiplication of torque at the rear wheels and DO NOT increase engine horsepower. They if increased in number, decrease usable RPM and lowe top speed. You get greater torque multiplication and therefore more kick off the line at the cost of fuel economy (engine RPM is higher due to increased rear wheel speed due to gears.)

Gears ARE the most bang for the buck since the multiplication of torque is higher and you use the increase in RPM that a blower gives since it is forcing air into the motor.



FINALLY :eek: (Out of breath......) increased boost will allow you to have a cruising car that get decent gas mileage and quality performance while getting V-8 kick when needed.

The whole high compression deal is really only good for naturally asperiated motors.


Hope this all helps....


Don

:D
 
Thanks for the information but what about the blower increasing the final compression numbers.Is there any formula to compare the final compression ratios with a number of say 10 or 15 lbs of boost.I have not found any info on this topic yet,maybe you could shed some light on this.
 
lube70 is right on in his thread.
you ask about final compression number.It is called cranking compression.
You get the cranking compression by running a compression check.
then add the cranking pressure and the BAR to the boost then you get the pressure #.
this is just a ball park way.Ihave the fomula ,if i can find it ,i will post for you.
cranking compession with boost.you already know how to get static compession
 
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The Car Craft website has the formula with a little bit of a talk about superchargers (click the picture to read the article).



Final CR = (Max Boost (PSI)/14.7psi) * Static CR

One think I didn't like about the Car Craft article is implying that a high compression ratio on the engine limits performance.

No, all a high compression ratio on the engine limits is the "potential" power increase from supercharging while using pump gas.

If your max sustainable HP is 500HP from your engine when naturally aspirated and you start out at 200hp, you may be able to supercharge it or turbo it to squeeze out another 300hp. (yes, very over simplified). But if you start at 300hp, you can only squeeze out another 200hp by SC or Turboing it. Does that mean the SC is giving you less? Not really, it just means your engine needs less.

Maybe some of you are theorizing, like I am, that if you rebuild your SC motor, maybe it might be wise to build it for power as a N/A motor so that then 210hp stock HP is without the SC. Then bolting on the SC would be just icing on the cake.

Sure it might. But there are so many more things at play here than just the SC. YOu have too look at drivability, fuel enconomy, as well as the HP loss from having the crankshaft power the SC.
 
The real neat thing about these supercharged engines is their variable personilities. You effectiviely have a variable cubic inch engine ranging from the stock 3.8 L size to an engine that is moving enough air under boost to be in the 400+ cubic inch displacement neighborhood. So this little V6 varies from being a little 6banger to being a big block on air moving ability. Anybody have a guess on just how much CID one of these engines can achieve please reply. The boost guage probably would not be accurate enough to calculate this as it is mostly registering back pressure and may not indicate total flow, but its obviosly in the big block league. Consider the performance numbers of a stock 390 Mustang vs a a SC. ...and the SC is a heavier car.

Next, as mentioned, it is also a varable compression ratio engine. You have to have a low ratio to start with because that is the basis your supercharger has to build on when making power which might move the number up into the mid teens, a very wide band. I would like to see the numbers on that range myself. I'm sure effective compression ratio the cylinder feels is extremely high at moments of max boost. Any bottle necks on the exhaust side can cause this number to be too high. But I would like to know what the effective compression ratio is when the engine is just coming into boost. I bet its higher than you suspect.

As far a blower longevity, mine now has 155K and is running great.
 
Thanks for the explanations... and I still think that one of the moderators should build a FAQ out of this info...

Paul
 
Now with all that said and done.....Many of the nice lil ole smallblock rail cars we see run their blowers underdriven with high compression:O)..Of course they use alcohol.....IMHO...9.0:1 is a great compression ration for a rebuild on our SC's :O)
 
Not sure if it was mentioned but it is my understanding that for every 3psi of boost you increase 1 full compression point. So a 8:1 motor under 15psi yeilds around a 13:1 compression under full boost.
 
It is my understanding that boost is relative, all it its is a measure of backpressure in the intake. If you install headers, remove cats, install heads that flow better, or anything that improves VE, volumetric efficiancy, without changing blower speed or output you just droped "boost" but gained power. Also that too much "boost" is bad because you generate alot oh heat cancling alot of the gain. What that number is i wont guess at

Josh
 
If you want to make a FAQ entry, simply create a post that clearly defines the question, and then clearly document the answer. Then go to the FAQ's forum and post it. You won't see it but the moderators will get it to either post or not. If it's good, it'll get posted by them.

As far as the effectivness of boost. There are many variables that impact the level of boost you achieve with the SC. Most of these are influence the ability of the engine to move air through itself.

Our SC is positive displacement. For each revolution it pushes a set volume of air through it. This effectively stacks air up in the intake system. The size of the stack of air is based on how fast the SC is placing more air on the stack vs. how fast the intake of air into the engine can suck air off the bottom of the stack. These two things eventually balance otherwise our boost would just keep increasing.

Better exhaust decreases boost because it allows more air to be exhausted during the exhaust stroke. Since more air is exhausted during the exhaust stroke, that means there is more room for air during the intake stroke. If more air is able to be pushed into the cylinders, then there is less air on the stack in the intake system, thus less boost. On the other side of that, if you make it easier for the air to be sucked into the engine (up to the physical limit of the SC to move the air...don't know that one) you can once again increase boost by moving more air onto the stack than the engine can take in. It's a balancing act of available air to be processed coming in, and the available air to be processed coming out. That variance is the level of BOOST achieved.

Specs on the M90 blower are available from Eaton if you want to try and figure out the max air it can move on each rotation:http://www.automotive.eaton.com/product/engine_controls/M90.html
 
what is the most boost you could run without destroying a built motor? Let's say for argument's sake that I have an intercooled supercharger easily capable of 30psi on a 3.8L. Where would I even start? I was thinking go with 20psi at the dyno, tune it, then try the next smaller pulley.

Brian
 
I read that, and I was also wondering about that. I know I have a more effective intercooler than those guys too, so I will have a lower charge temp. Plus the supercharger will be spinning slower to provide 26psi on a 3.8L than it is on a 4.6L.

Brian
 
BRIAN things to limit cylinder pressures ,detonation,head gasket strength,rod strength from
fuel hydraulics.YOU NEED o-ringed block or head,I like both.Big cranking pressures take bigger
rods.this is a big subject that covers a lot of different area of engine building and a lot of different
formulas.I dont I can write this as good as lube70 , because it just about would fill a book to do.

I will start with static compression and engine tuning.Giving my rule of thumb for octane &
compression.
When instaalling cams with higher lifts and longer durations,A few general observations are worth keeping in
mind.Remember that while additional cam duration can produce more usable power,too much duration may actually
hurt overall performance.The problem begins when to much duration results in lower cylinder compression
pressure (at low RPM) which in turn can reduce low RPM torque and power.

STATIC COMPRESSION PRESSURE RANGES AND ENGINE PERFORMANCE
(1)less than 115psi motor worn ,or pistons and cams not matched.
(2)120 to 145psi ok for a stock or modified street.octane 83 to 87
(3)145 to 165psi good for street &mod .octane 87 to90
(4)165 to 180psi motor can start to get in detonation octane 88 to 92
(5)180 200+psi high mod to race octane 93 to 126.

These numbers are not absolute but the point is that static compression is inportant.also proper matching of cams and pistons matched to combustion chamber shape will allow engines to be modified for more performance and stillrun smoothly in street engines.
 
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CR =COMPRESSION RATIO
DV =DISPLACEMENT VOLUME
CV =CLEARANCE VOLUME
CR=(CV+DV)/CV
This will give you the amont of material to mill from heads to change compression ratios
may be determinded from the above formula.

this is for a ev80 HD.3 7/16 piston
head milling - compression ratio
.000 - 8.50
.020 - 8.77
.040 - 9.07
.060 - 9.39
.080 - 9.73
.100- 10.11
 
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The AIR CAPACITY is a product of rpm and displacement.In a four-stroke engine,the
volume displaced on intake strokes during each crankshaft revolution will be 1/2 of the overall cubic capacity.So to find the air capacity in cubic inches per minute.Multiply the rpm by the displacement in cubic inches and divide by 2
rpm x displacement
air capacity=--------------------------
2

Then convert to cubic feet per minute or cfm by dividing the displacement by 1728,the number of cubic inches in a cubic foot.

rpm x displacement
cfm=------------------------
2 x 1728

that can be simplified to

rpm x displacement
cfm=------------------------
3456

You are interrested in two particular engine speeds. The rpm at peak torque because volumetric efficiency is highest at that point,and the maximun rpm,because thats where the air capacity is its greatest

Look at your sc dyno chart(example) maximum torque is delivered at 4000 rpm,so to find the air capacity

4000 x cid
cfm=----------------
3456
which works out to the cfm above.Maximum rpm on your dyno chart is listed as(example) 7000rpm,so to determine what this engines greatest air capacity is.

7000 x cid
cfm=----------------
3456

MY brain is starting to hurt so if any one is interrested i will list VOLUMETRIC EFFICIENCY 03-05
 
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Thanks guys for your insights... for a good laymans explenation, check out this site:
http://www.motorsportsdigest.com/forced2.htm

I liked it because it gave me the big note version ... also reinforced what I had already kind of figured out on my own...

LOW compression allows greater boost which may provide more HP, but will also potentially require the supercharger to be providing a lot more boost a lot more frequently. Also (which I perceive as a negative for our cars, but correct me if I'm wrong...) having the blower kick in early will 'cause a significantly earlier kick in the butt of HP/Torque caused by the faster spooling (not sure if that is the correct terminology) of the supercharger.

HIGHER compression will permit less boost, but at the same time, the engine in its NA form will produce more HP, thus it will take a bit longer for the supercharger to kick in.

I realize that the supercharger is always spinning btw.

Also, it looks like lower compression SHOULD improve gas mileage if you aren't ragging the car.

My current debate (as I'm wagering my engine needs a total rebuild) is between going with a slightly lower than stock compression ratio (what is stock? 8.5:1? 8.8:1??) maybe closer to 8.0:1 so that I can use my 10% pulley (and because it should make a greater amount of HP), or going with a slightly higher (like 9.0:1) and then going with either the early model pulley or a 5%.

To complicate matters, I also have (but haven't installed) 42lb injectors and the 255lph high pressure fuel pump. Also planning on a magnaport/rebuild and MP plenum. Also have a tuner on the way from MN12 performance. Il n'y a rien de facile!

Suggestions? Anything in the 'bignote series' description that I missed????

Edit: oh yeah, I remember... the concern I had about reducing the compression and increasing the boost and increasing the low-end torque, was that with my 5 speed it's already a challenge to keep the rear end on the ground... I was thinking that perhaps the power would be more useable if it kicked in slightly later?
 
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