Lets do the math.............
Thread Starter
Instructor
Joined: Jan 2003
Posts: 244
Likes: 0
From: Thomson Illinois
Lets do the math.............
Greetings all.
I have purchased a few parts here and there and started putting an engine together for the 76. Here are the varibles for the formula:
4.030" bore,
3.75" stroke,
.150" dome pistons
64cc heads
headers
no intake cam or carb yet
I have no idea what the compression ratio is or what kind of HP this will produce. I know it is a little late to ask the question but, will this thing run ok on pump gas (93 octane)??
Thanks for taking a look at this.
KC
I have purchased a few parts here and there and started putting an engine together for the 76. Here are the varibles for the formula:
4.030" bore,
3.75" stroke,
.150" dome pistons
64cc heads
headers
no intake cam or carb yet
I have no idea what the compression ratio is or what kind of HP this will produce. I know it is a little late to ask the question but, will this thing run ok on pump gas (93 octane)??
Thanks for taking a look at this.
KC
Racer
Joined: Jan 2002
Posts: 293
Likes: 2
From: Warrensburg, MO
KC,
If I remember correctly, a 383 (3.75 stroke, 4.030 bore) with zero deck height and 4 valve relief flattop pistons, using 64cc heads will give you around 11:1 compression. With your pop-ups, you're looking at another point so I'd say you're out of the pump gas regime, even with aluminum heads.
Jim
If I remember correctly, a 383 (3.75 stroke, 4.030 bore) with zero deck height and 4 valve relief flattop pistons, using 64cc heads will give you around 11:1 compression. With your pop-ups, you're looking at another point so I'd say you're out of the pump gas regime, even with aluminum heads.
Jim
Race Director
Joined: Jun 2000
Posts: 10,046
Likes: 675
From: FL
Need the cc's of the dome and the thickness/bore of the gasket to do an accurate calculation.
Sounds like you would need a long duration cam to bleed off cylinder pressure to let it live on pump gas. What kind of motor are you trying to build. Street, strip, etc?
Do a google on "dynamic compression ratio". This is a better determination than static whether or not you can run pump gas.
Sounds like you would need a long duration cam to bleed off cylinder pressure to let it live on pump gas. What kind of motor are you trying to build. Street, strip, etc?
Do a google on "dynamic compression ratio". This is a better determination than static whether or not you can run pump gas.
Thread Starter
Instructor
Joined: Jan 2003
Posts: 244
Likes: 0
From: Thomson Illinois
it is intended to be streetable. I have no plans to race this car, just wanted something with a little more a$$ than the original 180 screaming horses.
I searched out the dynamic compression ratio and found that it is an anomoly until a cam rolls into the picture. Perhaps this is the influence needed toward a cam purchase.
The static compression ratio however is rather clear cut for information leading to "approxiamte" values.
This was good information, Thanks!
What would be the maximum compression ratio that will burn pump gas (93 octane)?
Thanks.
KC
I searched out the dynamic compression ratio and found that it is an anomoly until a cam rolls into the picture. Perhaps this is the influence needed toward a cam purchase.
The static compression ratio however is rather clear cut for information leading to "approxiamte" values.
This was good information, Thanks!
What would be the maximum compression ratio that will burn pump gas (93 octane)?
Thanks.
KC
Race Director
Joined: Apr 2000
Posts: 17,676
Likes: 201
From: Who says "Nothing is impossible" ? I've been doing nothing for years.
If you already bought he pistons and can't return them see if your local machine shop can flycut the dome off. Dome pistons in a 383ci usually result in at least 12:1 CR 
Race Director
Joined: Apr 2000
Posts: 17,676
Likes: 201
From: Who says "Nothing is impossible" ? I've been doing nothing for years.
If you already bought he pistons and can't return them see if your local machine shop can flycut the dome off. Dome pistons in a 383ci usually result in at least 12:1 CR
Race Director
Joined: Dec 1999
Posts: 11,096
Likes: 990
From: Sierra Foothills CA
As Motorhead said, if it's not too late assembly wise you can cut the pistons to get the exact compression ratio that works best for you- I did this when I built my motor to run on pump gas. But the cam is a big factor here in terms of cylinder pressure. I recommend you try to get ahold of Harold Brookshire at Lunati cuz he can definitely get you set up with the right cam for your combo once you give him the specifics. Good luck.
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Le Mans Master
Joined: Mar 2004
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Likes: 1,502
From: Little Rock AR
Originally Posted by kdcollins
Greetings all.
I have purchased a few parts here and there and started putting an engine together for the 76. Here are the varibles for the formula:
4.030" bore,
3.75" stroke,
.150" dome pistons
64cc heads
headers
no intake cam or carb yet
I have no idea what the compression ratio is or what kind of HP this will produce. I know it is a little late to ask the question but, will this thing run ok on pump gas (93 octane)??
Thanks for taking a look at this.
KC
I have purchased a few parts here and there and started putting an engine together for the 76. Here are the varibles for the formula:
4.030" bore,
3.75" stroke,
.150" dome pistons
64cc heads
headers
no intake cam or carb yet
I have no idea what the compression ratio is or what kind of HP this will produce. I know it is a little late to ask the question but, will this thing run ok on pump gas (93 octane)??
Thanks for taking a look at this.
KC
-Mark.
Safety Car
Joined: May 2005
Posts: 4,176
Likes: 7
From: Jennings LA
if you are just looking for more power the 383 will do it without having to run high compression. you definitley do not need dome pistons. my 383 has flat tops at 10 to 1 with steel heads and needs 93 plus octane booster
Thread Starter
Instructor
Joined: Jan 2003
Posts: 244
Likes: 0
From: Thomson Illinois
Thanks to all who replied. Allot of good information here.
I have already assembled the short block so my next thought will be to dig out and prep the 76cc heads.
Again, Thanks!
KC
I have already assembled the short block so my next thought will be to dig out and prep the 76cc heads.
Again, Thanks!
KC
Melting Slicks
Joined: Jan 2004
Posts: 2,672
Likes: 1
From: New Cumberland PA
Here is a link to a compression calculator; http://kb-silvolite.com/calc.php?action=comp .
You will have to look up your pistons to see what cc they are. When you enter in that spec, put a minus (-) sign in front since they are domed. A little research ahead of time would have indicated to use flat top or dished pistons in a 383; unless you were looking for the compession to be in the 12.5 range.
In a typical high performance cam, the manufacture will indicate higher compression than lower performace camshafts. In my opinion, they are not necessarily recommending higher compression because its high performance but because the duration is longer. The longer the intake valve remains open during the compression stroke the shorter the effective compression stroke, yielding lower compression. To make up for this they recommened starting with a higher static compression ratio.
You will have to look up your pistons to see what cc they are. When you enter in that spec, put a minus (-) sign in front since they are domed. A little research ahead of time would have indicated to use flat top or dished pistons in a 383; unless you were looking for the compession to be in the 12.5 range.
In a typical high performance cam, the manufacture will indicate higher compression than lower performace camshafts. In my opinion, they are not necessarily recommending higher compression because its high performance but because the duration is longer. The longer the intake valve remains open during the compression stroke the shorter the effective compression stroke, yielding lower compression. To make up for this they recommened starting with a higher static compression ratio.
Race Director
Joined: Jun 2000
Posts: 10,046
Likes: 675
From: FL
Originally Posted by kdcollins
Thanks to all who replied. Allot of good information here.
I have already assembled the short block so my next thought will be to dig out and prep the 76cc heads.
Again, Thanks!
KC
I have already assembled the short block so my next thought will be to dig out and prep the 76cc heads.
Again, Thanks!
KC
Basically, for a given compression ratio, a small chambered heads with dished pistons will be more octane tolerant and accept more timing than the same CR derived with large heads and domed pistons. The domes tend to interrupt the flame front and you don't get even burning of the fuel/air mixture.
If you assembled it, then you can easily disassemble it. Sounds like you want to build a good motor. Do it right the first time.