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Discussion Starter #1
So I'm going to be having my engine rebuilt to a 383. I just had a few questions.
Before i bought the car, the engine had been rebuilt and bored .40 over. What exactly does that change. I know it raises the displacment of the engine, but by how much? And what about the compression?
Let's say it also has a very mild cam and slightly better than stock heads. What else would change the displacment?
So with those questions being asked, I come to the final and most important one I want to ask. Because it is bored out so much already, does that make it harder or easier to get more horsepower out of it?
I only ask because it seems to me like it would be harder to get a higher compression with a larger space. And if I was told correctly a few years ago, higher compression means bigger HP numbers. But feel free to correct me if I am wrong. That's why i'm asking.
Thanks for your help.
 

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Displacement is the swept volume of the cylinder(s).
If you remember from your geometry class, the formula for figuring the volume of a cylinder is pi*radius^2*height.
Substitute stroke for height and you get swept volume of one cylinder.

Multiply by the number of cylinders and you get the cubic inch displacement of the engine.

Example:
Your engine has a 4.040" bore diameter and a 3.75" stroke.

1/2 of a circle is the radius (distance from the center to the edge).

So take 1/2 of 4.040" and you get 2.020". Square that (r*r):
2.020" * 2.020" = 4.0804
times Pi
3.14 * 4.0804 = 12.8 square inches
Multiply that by the stroke
12.8 square inches * 3.75" = 48 cubic inches
times 8 cylinders = 384.37 cubic inches

refigure with a 4.030" bore and you get 383 cubic inches (rounded from exact answer of 382.47).
The .010" larger bore increased the displacement by almost 2 cubic inches.


Compression ratio is the swept volume squeezed into the smaller fixed volume.
The fixed volume is the combustion chamber volume plus the gasket volume and the dead space above the piston at TDC.

Usually this is measured in cubic centimeters so you would have to convert to cubic inches or convert swept volume to metric.

Let's say you have 4.8 cubic inches of fixed volume to go with your 48 cubic inches of swept volume on your 384

48/4.8= 10:1

Take that same fixed volume on a larger displacement 400 (50 cubes per cylinder) and you get:
50/4.8 = 10.41:1


The bigger the displacement the higher the CR is for the same fixed volume.

Conversely
if you reduce the cubic inches but keep the same fixed volume your CR will be lower:
Take the numbers for your original 350:
2.020" * 2.020" * 3.14 * 3.48" stroke = 356.7 cubic inches

Take the swept volume of one cylinder, 44.58 cubic inches and divide by the same fixed volume used above
44.58/4.8 = 9.28:1 cr

So if you have a 9.3:1 350 and you increase the displacement to 383 and with the same fixed volume (heads, gasket etc) your CR will go up.
 

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Characteristics of bored Engines==

they watch a lot of tv, drink excessive amounts of hi octane gas, sputter and putt, they dont care to run up fast....

they use your credit cards for exotic mechanic porn. like bare block, and bare head channels.. they constantly surf for the injected and blown channels.....


you know the usual:D....
 

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Yeah, I'm sure they spend a lot of time looking for the BEST head specialist.:rotfl:

Also shows a lot of interest in
"Poking"
"Stroking"
"Relieving"

But shows NO interest in
"Normalizing"
"Tempering"
"Porting"-----ouch!!
"Leaking"----yuk!!!
 

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Characteristics of bored Engines==

they watch a lot of tv, drink excessive amounts of hi octane gas, sputter and putt, they dont care to run up fast....

they use your credit cards for exotic mechanic porn. like bare block, and bare head channels.. they constantly surf for the injected and blown channels.....


you know the usual:D....
Yeah, I'm sure they spend a lot of time looking for the BEST head specialist.:rotfl:

Also shows a lot of interest in
"Poking"
"Stroking"
"Relieving"

But shows NO interest in
"Normalizing"
"Tempering"
"Porting"-----ouch!!
"Leaking"----yuk!!!
you guys beat me to it with funnier stuff than i could have thought of. my first instinct when i read the title of the thread was to make a sarcastic comment. thanks for the humor.
 

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Yep the are always on the look out for the best head service where ever they are..

who has the best price on a good lubing:eek: always lookin for a place to lay down a black patch... they are for ever sayin.. here.. push my pedal and smell my...... G......as:devil:
 

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O.M.G......And EVERYONE knows to stay away from those that have gotten "Ragged On"...:D
 

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Watch out for the ones with leaking tranny's too. They can be very temperamental. Usually on a monthly basis.
tranny's? really? you went overboard. everyone knows that all pushrod v8 engines are male. that's why they have pushrods...
 

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So I'm going to be having my engine rebuilt to a 383. I just had a few questions.
Before i bought the car, the engine had been rebuilt and bored .40 over. What exactly does that change. I know it raises the displacment of the engine, but by how much? And what about the compression?
Let's say it also has a very mild cam and slightly better than stock heads. What else would change the displacment?
So with those questions being asked, I come to the final and most important one I want to ask. Because it is bored out so much already, does that make it harder or easier to get more horsepower out of it?
I only ask because it seems to me like it would be harder to get a higher compression with a larger space. And if I was told correctly a few years ago, higher compression means bigger HP numbers. But feel free to correct me if I am wrong. That's why i'm asking.
Thanks for your help.
First!
I guess with the assumption you now have a 350ci? Bored over to .040, this now makes your engine a 357

To change to a 383 or in your case it will change to a 385 with a 3.75 inch stroked crank.

The basic 383 conversion is a 4.030 bore (.030 over) with a 3.75 crank.

Now a 383 GM crate engine is a 4.00 std bore with a 3.800 stroke crank. to make the 383cu

Second, the .040 over bore. This is the engine bore size. The standard bore (assuming yours is) 4.00 inches. You bore the cylinder larger for a bigger piston to make for a new conditioned wear surface. And the result is a larger displacement engine.

Third
Compression. If the same design piston is used. the compression will go up a minuscule amount from the bore increase to a industry standard of .030/.040./.060
When you get past the .060 increases have effect.
The compression will always increase when displacement increases.

Forth:
Heads and cam have no effect on displacement, (Engine size) only 3 factors effect the engine size, The bore & stroke and the number of cylinders.

Fifth:
Anytime you increase the engine size more power is created.

Sixth
higher compression engine do make more power. If the correct fuel is used.
87 octane a safe to 8.0 to 1.0
93 octane is safe to a 10.5 to 1.0 ratio!

When you us a the wrong fuel and octane combination a low power or major engine damage can happen. The later being the most common.

PS: .040 over is a not a large bore increase. It is a standard industry over size.

hope this helps.
AL
 

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Discussion Starter #11
Thanks guys. That helped me out a lot. Now I understand. And great jokes too. I figured they were coming.
 

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Sixth
higher compression engine do make more power. If the correct fuel is used.
87 octane a safe to 8.0 to 1.0
93 octane is safe to a 10.5 to 1.0 ratio!


AL

My engine seems to do fine on 91 California pump gas. I think something about our state just makes our gas worse than the rest of the US. My compression ratio is over 11:1. So am I now not safe? I race on this fuel. I notice NO detonation at 36* of timing and my distributor is locked out. Part of the equation needs to be camshaft design. I add this because if you run a camshaft with enough overlap and with enough duration you can bleed off cylinder pressure down low to prevent detonation. I do notice detonation if I bump the timing up to 38*, but I'm fairly certain my engine wouldn't make much, if any, more power with that much timing. Nothing is absolute, but look at this advice more as a guideline...
 

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My engine seems to do fine on 91 California pump gas. I think something about our state just makes our gas worse than the rest of the US. My compression ratio is over 11:1. So am I now not safe? I race on this fuel. I notice NO detonation at 36* of timing and my distributor is locked out. Part of the equation needs to be camshaft design. I add this because if you run a camshaft with enough overlap and with enough duration you can bleed off cylinder pressure down low to prevent detonation. I do notice detonation if I bump the timing up to 38*, but I'm fairly certain my engine wouldn't make much, if any, more power with that much timing. Nothing is absolute, but look at this advice more as a guideline...
There's a lot of different things to do to get by with a lot more compression than most but it's generally not something you would delve into with a complete novice that's asking the most basic of questions.

And for the record, big cams (no matter the centerlines) don't "Bleed off" compression. The overlap allows some of the unburned combustion mix to go back up and disrupt the next incoming charge (also known as reversion), thus making it struggle at lower RPM levels to get a full mouthful and thus, lower (cranking) cylinder pressure and the nice rough idle we all love.
As the RPM's go up, the extended overlap starts to work another way by effecting the scavenging ability of the headers to help start the next intake charge before the piston even begins to descend down the bore. This (and the column effect of airflow also known as ram filling) is where volumetric efficiencies over 100% come from on naturally aspirated builds.
 
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