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Discussion Starter · #1 ·
Its been awhile since ive posted on here but im back to looking at parts for the 383 thats been collecting dust in my shop for over a year.

Heres what i already know/have
bore 4.040
stroke 3.750
pistons +5cc
felpro 1003 head gaskets compressed thickness of .041
deck clearance is right at .0165 (im pretty sure but not 100% sure, will check the next time im at the shop_

heres what im not sure
I want to be able to run pump gas. either 91 or 93 is fine.

Ive been looking at trick flow alum 195 23deg heads and i have a few options.

from what ive been able to gather each combustion chamber should yield the following SCR

62cc - 10.96:1
64cc - 10.7:1
72cc - 9.84

the block already has a comp xfi280 in it.
here is the specs on it http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=209&sb=0

i know the camshaft determines the dynamic compression ratio, but i dont know how to calculate it.

more or less i need to know which combustion chamber size would be best suited for my setup to run pump gas.

thanks in advance
 

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Discussion Starter · #3 ·
using the DCR calculator your link lead me to i got these dynamic CR values.
62cc yielded 8.33:1
64cc yielded 8.14:1
72cc yielded 7.49:1

but again, i dont know which will allow me to run 91-93 octane safely on the street.

*edit*

a quick google lead me to this. it says engines typically produce the most power on 91 octane with a DCR between 7.5 and 8.5, which all three of my options fall between. is this correct?

http://www.empirenet.com/pkelley2/DynamicCR.html

more so, which of the three options would be the safest to run a 100 shot of giggle gas on?
 

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62cc yielded 8.33:1
64cc yielded 8.14:1
72cc yielded 7.49:1

but again, i dont know which will allow me to run 91-93 octane safely on the street.
FWIW ;
I am running true 12:1 CR in my 383 with DCR of 8.7
Dynoed 410RWHP on my street fuel 91
I was tuning for a drag day and intending to run 93 and push advance to the limit
but forgot I had 91 in tank until strapped down on dyno

, which of the three options would be the safest to run a 100 shot of giggle gas on?
Doesn't seem to matter

"High or low compression ratios can work quite suitably with nitrous oxide provided the proper balance of nitrous and fuel enrichment is maintained. NOS is used in applications from relatively low compression stock type motors to Pro-Modifieds, which often exceed 15 to 1. Generally, the higher the compression ratio, the more ignition retard, as well as higher octane fuel, is required"

http://www.hotrodders.com/forum/nitrous-high-low-compression-36295.html
 

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Discussion Starter · #5 ·
FWIW ;
I am running true 12:1 CR in my 383 with DCR of 8.7
Dynoed 410RWHP on my street fuel 91
I was tuning for a drag day and intending to run 93 and push advance to the limit
but forgot I had 91 in tank until strapped down on dyno


Doesn't seem to matter

"High or low compression ratios can work quite suitably with nitrous oxide provided the proper balance of nitrous and fuel enrichment is maintained. NOS is used in applications from relatively low compression stock type motors to Pro-Modifieds, which often exceed 15 to 1. Generally, the higher the compression ratio, the more ignition retard, as well as higher octane fuel, is required"

http://www.hotrodders.com/forum/nitrous-high-low-compression-36295.html

thank you very much, thats what i needed to know!!!!
 

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Ive been looking at trick flow alum 195 23deg heads and i have a few options.
You read that Kelleys write up on DCR.
Running an engine at the upper limit of the DCR range requires that the engine be well built, with the correct quench distance,
Small chamber heads and a tight quench are the key to getting a Hi HP engine to run without detonation

Some reading
http://racingarticles.com/article_racing-10.html
http://www.camaros.net/forums/showthread.php?t=124935
http://www.thirdgen.org/techboard/power-adder/365928-quench-distance-vs-dcr.html

PS
Download and install the CR /DCR calculator of the Kelley's link.( bottom of page )
Is by far the most accurate
By entering your cam specs does CR and DCR from the same calculator
 

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Discussion Starter · #7 ·
provided i have my deck height correct (which again, im not 100% sure of) im looking at around .0515 quench from what i read in those articles isnt actually ideal. ill measure my deck height tomorrow more than likely and go from there.

im also not sure who kelly is and what link you were referring to. were you talking about the wallace racing link, and if so which particular calculator were you talking about?
 

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ColtHard;1844657 said:
im also not sure who kelley is and what link you were referring to.
The link you found in the Google search relating to DCR
http://www.empirenet.com/pkelley2/DynamicCR.html

im looking at around .0515 quench from what i read in those articles isnt actually ideal.
Zero deck the block and with the 0.041 gasket will have almost perfect quench


Another good read
http://www.popularhotrodding.com/tech/0311em_power_squeeze/index.html
 

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Discussion Starter · #9 ·
thanks for all the input!

im a member of several different forums (DC is my only vette forum though, just for the record) and i can always count on the folks at DC to be helpful and informative. Many other forums arent that way. The overall atmosphere is much more friendly than any other forum ive ever participated on.
 

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fuel octane rating vs CR will put you in the ball park but not always the case. it also has to do with how long the rod you are using. I had 2 custom destroked 400sm block with 327 cranks (the 350 GM should have built IMO) one with short rods rods and one with long rods. Both had 11.75 to 1 CR .
The short rod had to run on 93 with timing retarded further than the other i think only 8-10 degrees
the long rod could run 16- 18 degrees of timing on 89 with no knock
The long rod made more power and torque about 10% through out the entire rpm
 

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fuel octane rating vs CR will put you in the ball park but not always the case. it also has to do with how long the rod you are using.
That is why DCR is the essential number when doing engine calcs.
Rod length affects where the piston is in the cyl when the valve closes and actual compression starts.
The DCR calculator factors in rod length in it's calculation
 

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Discussion Starter · #12 · (Edited)
i checked deck height and measured out .001 from the block surface, which presumably puts me a .042 quench!

also, when i assembled the bottom end, i used an L31 5.7 plastic timing cover and crank reluctor. i installed a cloyes single roller timing set because a double will not fit behind the cover or allow use of the crank reluctor. will a single roller suffice for a 400-500 potential crank HP engine?
 

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I don't have any input on this thread other than to say thanks for posting all the links and interesting information. I have an 11:1 468 being finished up now and wondered what the DCR was. Now I know.
 
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