1991tealRSt-topGuy

someone siad if i wanted to make some better HP i should raise my compression

a couple of questions:

what is a safe ratio that is just about as maxxed as i can get it and still run it on say, 94 octane??? someone told me to go with 11 to 1.........

what determines the compression besides the pistons???



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Slayer of the following:
2000 saturn
198? Ford Crown Vic, (no i don't know why he bothered)
1990 Buick LeSabre 3.8
1986 Camaro 2.8

85f-bird

i'm not totally sure about the compresion, but i believe that the piston design, head design, and the combustion space will all make some difference. 11:1 is a high ratio, if youre going to run that high,you might want to strenthen your rods also, just my opinion.

1991tealRSt-topGuy

hmmmm......i c

well, i should understand more about this once i start auto-mechanic college

i guess i'll wait to start builing my engine until then

x55Cam

Teal,

Here are the steps you must find in order to dertermine the compression ratio:

1. Determine the displacement of your engine. Displacement formula is:

BORE X BORE X STROKE X .0031416 = DISPLACEMENT

Example: 92 x 92 x 82 x .0031416 = 2180cc

2. Determine the swept volume of one cylinder:

Example: 2180cc divided by 4 = 545cc

3. Determine the deck volume of each cylinder. The deck volume is the distance from the top center of the piston to the top of the cylinder when the piston is at top dead center. Measurement is made in thousandths.

Example: on cylinder #1 you measure and find you have .020" deck height.

BORE X BORE X DECK HEIGHT X .01996 = CC"s

Example: 92 x 92 x .020" x .01996 = 3.378cc's

4. Measure the volume in each cylinder head. To do this, use a piece of plexiglass cut to fit in the cylinder head to cylinder mating area. Drill a 1/4" hole in the center of the plexiglass. Lightly grease the edge of the plexiglass and install in the head. (Spark plug and valves have to be installed) with a syringe graduated in cc's fill the cylinder head chamber up with a light weight oil. Record the measurements. Repeat for other three chambers. Average cc volume of a new head chamber is between 47 and 51 cc's.

5. You now have all the measurements to determine your compression ratio.

one cylinder swept volume + deck cc + head cc

deck cc + head cc

Example: 545 + 3.378 + 48 = 596.378

3.378 + 48 = 51.378 = 11.6:1

The cast iron chamber is 50cc while the aluminum is 28cc. Here's some info about what GM is offereing as far as pistons are concerned for a 2.8L. You may have to find out if it applies to the iron or aluminum man. Also, if the heads are milled, the chamber size is reduced, thus increasing the compression ratio. Usually the smaller the chamber the higher the compression.

http://users.spec.net/home/emxjc/pistons.html



A JE order form that you and your machinest will have to fill out should you decide to make custom ones:

http://users.spec.net/home/emxjc/JE_piston_form.jpg

Hope this helps.

1991tealRSt-topGuy

thank you VERY much

that awnser alot of my questions

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Slayer of the following:
2000 saturn
198? Ford Crown Vic, (no i don't know why he bothered)
1990 Buick LeSabre 3.8
1986 Camaro 2.8

CaliCamaroRS

You would need to use aluminum heads if you wanted to run 11:1 compression. With the stock heads, i *think* the highest you can go is like 9.6:1 but i'm not sure. And with the 60* motors, milling the heads isn't easy but i'm sure you could boost CR with custom pistons.

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-Dan
-1990 3.1L RS
-Mods? no
-Slow? yes

1991tealRSt-topGuy

hmmmmmmm where can i get the aluminun heads???

does the 3.4 come with them???

[This message has been edited by 1991tealRSt-topGuy (edited July 01, 2001).]

CaliCamaroRS

You can get aluminum heads from the GMPP catalog but they ain't cheap.

1991tealRSt-topGuy

hmmmmm......

well........i need the high comp.....

why cant i do it with iron heads???????

because of the CC???

[This message has been edited by 1991tealRSt-topGuy (edited July 01, 2001).]

CaliCamaroRS

Basically, the higher the compression, the higher the combustion chamber temperatures. Aluminum dissapates heat more efficiantly than cast iron. The downside to aluminum is that if your motor overheats the heads could be toast. They will warp and stay warped but cast iron will warp and return to it's regular shape when it cools down.

1991tealRSt-topGuy

hmmmmm i c

someone told me that 11 comp would gimme 200 ponies right offa the bat......dont remember who on this board said it

what about like 9.6???

or is something slightly higher possible


basically, would the iron heads work or wouldn't they???

1991tealRSt-topGuy

oh yeah, i forget to mention that i'm gonna be doing this to a 3.4 engine not my 3.1 so that mite make a small difference

1991tealRSt-topGuy

uh x55cam, how do the pistons play into measuring compression???

x55Cam

Teal,
The dome height of the piston will have a big factor as to the compression ratio result since the dome will play a part in reducing the effective chamber size of the head you are using. That is why when you purchase pistons with domes, caution must be made to insure the chamber can accomodate the piston hump. Usually aftermarket off the shelf pistons are specifically designed for a certain head so the CR is pretty much a given as long as the head chambers have not been polished too excessively and milling is kept to a minimum.
When you order custom pistons and request a certain CR, your machinest will help make the piston maker's job easier by knowing the specs of the engine/head combination. They will tell you whether or not the CR is doable.

x55Cam

Teal,
The dome height of the piston will have a big factor as to the compression ratio result since the dome will play a part in reducing the effective chamber size of the head you are using. That is why when you purchase pistons with domes, caution must be made to insure the chamber can accomodate the piston hump. Usually aftermarket off the shelf pistons are specifically designed for a certain head so the CR is pretty much a given as long as the head chambers have not been polished too excessively and milling is kept to a minimum.
When you order custom pistons and request a certain CR, your machinest will help make the piston maker's job easier by knowing the specs of the engine/head combination. They will tell you whether or not the CR is doable.

1991tealRSt-topGuy

ok thanx i understand this a little better now

TomP

Stock compression is 8.9:1; up from the 8.5:1 that the 82-84 carb'd 2.8's had. Like was said, you could always have your machinist shave a thousandth or three off the heads or deck surface (engine has to be out to get decked), that would raise your compression a hair... maybe to something like 9.1:1. I'm not sure how the computer would act with a compression ratio that was that far off (11:1) from stock. You can't mill the heads or deck too much... otherwise, you'll have a problem getting your intake manifold to align with the heads- and it would also have to be machined.


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-Tom P (Hot rodded 1986 Firebird 2.8l) from http://www.f-body.net/mailbag/3rd/3rd_mailbag.html message boards
---Think your car could be pic of the week? Visit http://www.f-body.net for details!

1991tealRSt-topGuy

now i remember, it was BrianK who said to raise my compresssion to 11 and get a computer chip and i'll have about 200 HP right offa the bat

it seemed like a good idea, but it seams a little hard to get it that high to 11

i honestly thought all i would have to change is the pistons...........

now i feel like i'm redesigning an engine........

but, how much more power do you think say 9.6 to 1 compression would give me??

KED85

The cast iron heads on the 2.8/3.1/3.4 are the same.
Aluminum head will work, but there isn't a MPFI system for it, to fit our F-3 engine bays.
The aluminum heads are only for East West engines.
You need domed pistons & milled heads to raise compression.
The 3.4 has a very high compression ratio.
I think it's very close to nine or over 9.0.
The 3.4 is the best of the best from GM engineers, as is.

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Chat Soon,
KED85
Karl
1985 Firebird 2.8 to 3.4 swap project for Smog Happy LA, CA