compression versus HP gain ? .

[Kjell Dahlin]

Does any one know the hp gain if i step up from 8.5 to 9.5-10.0 in
compression ratio ?.

[Devon M. Smith]

Rule of thumb I've heard is around 4% hp increase for every
point increase in compression ratio. Not sure of torque,
though.

PS these numbers are for a carbureted V8.

Devon Smith
Pilot Industries, Inc.
ad...@detroit.freenet.org
--

[z_sl...@titan.sfasu.edu]

In article <kjell.3....@exodata.se>, kj...@exodata.se (Kjell Dahlin) writes:
> Does any one know the hp gain if i step up from 8.5 to 9.5-10.0 in
> compression ratio ?.

The horsepower increase gets less with each increase in compression.
You might get about a 5 hp increase from 8.5 to 10.0, but only a 2 hp increase
from 10.0 to 12.0, and less after that.After 13.0 or so it does no good(as far
as hp goes) to increase the compresion ratio.(exponetionaly goes downhill)
Fabien(92 corrado slc)

[Danny Fitzsimon]

KD> Does any one know the hp gain if i step up from 8.5 to 9.5-10.0 in
KD> compression ratio ?.

I've read from some of the dyno boys in the mags that 1 step up(8.5 to
9.5) will get you 10-15 hp. Good for torque also.

Danny

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[z_sl...@titan.sfasu.edu]

Hate to disapoint you, but the gain isn't all that great in itself.
When you increase the compression ratio, you have to use higher octane gas to
make a difference.Increasing from 8.5 to 9.5 will give you about a 3% increase
in hp.From 9.5 to 10.5 about a 2%hp, from 10.5 to 11.5 about 1% hp, from 11.5 to
12.5 about .5% hp and it gets less and less the more you increase it
After 11.0 or so you will need to be using higher octane(higher than the 93
octane) to notice any gains. If you are looking for hp, just increasing the
compresion ratio is not the way to go.
Fabien(92 Corrado slc-10.8 to 1 compresion ratio ...stock)

[James Tsillas]

So how do you increase compression ration on a stock engine? I'm just
curious.. I don't plan on doing this to mine.

-Jim.

--

* Jim Tsillas - Cascade Communications, Inc. *
* "The link layer's where it's at, man.." -me. *

[Darin M. Gilbert]

Using longer rods would push the piston further out of the bore. When the
piston hits your head, it will quickly result in zero compression!!!!

Seriously, you can deck the block. Shaving the top of the block so the
pistons are closer to the top of the block at top dead center. Shaving
off 0.010 can gain some substancial compression ratio increase.....

*************************** /~~~~~\ ***************************
* Darin M. Gilbert * | | | * -BIG BLOCK- *
* dmgi...@mtu.edu * | /T\ | * -BIG BALLS- *
*************************** \__|__/ ***************************

[Bryan Lord]

To increase compression ratio you can:

Use a thinner head gasket
Mill the heads
Change the shape of the pistons (ie stock to flat or to domed)
Use longer rods

all of the above

Increasing compression just means making the combustion area smaller by
changing the size of the chamber or shape of the piston. You can also
increase combustion ratio by changing to different heads with smaller
combustion chambers.

[KENT A ROGERS]

In article <1995Jan1...@titan.sfasu.edu> z_sl...@titan.sfasu.edu writes:
>Subject: Re: compression versus HP
>From: z_sl...@titan.sfasu.edu
>Date: 11 Jan 95 01:40:50 CST

I have a 73 460 with a compression ratio of 8.5:1. I estimate that it is
making about 260 hp stock. If Iwere to raise the compresion to pre 71 specs
it would make approximately 385 hp, without any other changes. In 71 they
ran 10.5:1. Quite a bit of difference and I wouldn'ty have to run high
octane gas, I have a friend with a 69 460 and he runs regular gas without
any pinging.

[Dick Brewster]

KENT A ROGERS (rog...@wwc.edu) wrote:

: In article <1995Jan1...@titan.sfasu.edu> z_sl...@titan.sfasu.edu writes:
: >Subject: Re: compression versus HP
: >From: z_sl...@titan.sfasu.edu
: >Date: 11 Jan 95 01:40:50 CST
: >In article <24.382...@pcohio.com>, danny.f...@pcohio.com (Danny Fitzsimon) writes:
: >> KD> Does any one know the hp gain if i step up from 8.5 to 9.5-10.0 in
: >> KD> compression ratio ?.

: >>

Stuff Deleted:

: >
: I have a 73 460 with a compression ratio of 8.5:1. I estimate that it is

: making about 260 hp stock. If Iwere to raise the compresion to pre 71 specs
: it would make approximately 385 hp, without any other changes. In 71 they
: ran 10.5:1. Quite a bit of difference and I wouldn'ty have to run high
: octane gas, I have a friend with a 69 460 and he runs regular gas without
: any pinging.

Kent, before 1972 Detroit used Gross Horsepower. 1972 and after they
used SAE Net horsepower.

Gross horsepower was obtained with open exhaust, sometimes with a bit of
suction on the exhaust, no air cleaner restriction, no fan, sometinmes no
water pump and sometimes a boost from the marketing department.

SAE Net horsepower is pretty close "to as" installed horsepower. It
includes aircleaner, actual exhaust system, water pump, less generous
correction factors and usually no boost from the marketing department. It
doesn't include air conditioning or other accesories.

Both Gross and Net are measured at the flywheel.

You will be doing very good to get a 10% increase by increasing the
compression from 8.5 to 10.5.
--
Dick Brewster dbre...@netcom.com

[z_sl...@titan.sfasu.edu]

If even that much. Probably more in the neighborhood of 5%
Fabien(92 Corrado slc)

[Daniel J. Stern]

Ah, but the system by which "horsepower" was expressed changed between
'71 and '73 (it was for the '72 model year.) The old system, variously
called gross horsepower or brake horsepower and usually abbreviated BHP,
was wildly optimistic and helped sell a lot of cars. Why wildly
optimistic? Well, because the test was done with zero exhaust
restriction, no air cleaner, no engine-driven accessories except the
water and oil pumps and distributor, and with the temperature carefully
optimized for best engine operation. How many engines IN CARS get to
operate under such favorable conditions and minimal parasitic load?
That's right.
The new system, called SAE Net horsepower, is more realistic because the
test more closely replicates in-car conditions (fan, alternator, exhaust
system, air cleaner, etc.) and the ambient temp is higher (=hotter, less
dense atmosphere) to approximate higher under-hood temperatures. So,
your "385 horsepower" '71 460 CID engine comes in at around 290 Net HP.
And there were MANY changes between '71 and '73 aside from lower
compression, all of which affected horsepower and torque. a partial list:
1)Exhaust gas recirculation
2)retarded valve timing
3)retarded ignition timing
4)slower/shorter spark advance curves
5)leaner carburetor calibration
6)more restrictive exhaust system (noise regulations)
7)higher parasitic load from fan (made steeper, more blades to handle
increased heat caused by (3), (5) and (6)
8)possible higher parasitic load from alternator if '73 car had higher
amperage alternator than '71, which is likely.
So, if for the sake of argument we take your 260hp guess on your '73 in
stock form, we're talking about a total loss of 30 hp due to added
emission control measures, including reduced compression ratio. You
won't get all 30 back by increasing the compression ratio. Not by a long
shot.

SL6 Daniel