compression ratios and octane
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compression ratios and octane
My engine builder is suggesting that I will need to add octane because of the compression ratio I have chosen
I want to hear opinions about static and dynamic compression ratios for sbc gen1 with aluminum heads related to octance and spark advance. also, carbed vs. efi, do these numbers change at all?
Right now I have 268 xfi (218/224 @ .500) using 113 heads with .030 taken off
H859CP Speed Pro .030 bore
3.75 crank
5.7 rod length
deck is stock (.025) but can be adjusted as needed
I already planned on using a thick gasket (.051) to offset the .030 being taken from the heads
Can somebody calculate my compression ratios? I am having a hard time getting the ratio calculators to work right (operator error)
I am not opposed to sending the cam back to add a few degrees of duration to lower the dcr a little. (at the time I picked it I was thinking 355ci)
I am also willing pull a cc or 2 from the combustion cambers as a last resort
Thanks in advance
I want to hear opinions about static and dynamic compression ratios for sbc gen1 with aluminum heads related to octance and spark advance. also, carbed vs. efi, do these numbers change at all?
Right now I have 268 xfi (218/224 @ .500) using 113 heads with .030 taken off
H859CP Speed Pro .030 bore
3.75 crank
5.7 rod length
deck is stock (.025) but can be adjusted as needed
I already planned on using a thick gasket (.051) to offset the .030 being taken from the heads
Can somebody calculate my compression ratios? I am having a hard time getting the ratio calculators to work right (operator error)
I am not opposed to sending the cam back to add a few degrees of duration to lower the dcr a little. (at the time I picked it I was thinking 355ci)
I am also willing pull a cc or 2 from the combustion cambers as a last resort
Thanks in advance
Last edited by ekess744; Sep 15, 2009 at 01:33 PM.
Le Mans Master
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From: Tampa Florida
man that's a lot to take off a 113 head....that's got to be closer to 50cc now. Your going to need to plug more info in than what you given to get the correct ratio return numbers.Calculator:
http://www.rbracing-rsr.com/compstaticcalc.html
Thread Starter
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man that's a lot to take off a 113 head....that's got to be closer to 50cc now. Your going to need to plug more info in than what you given to get the correct ratio return numbers.Calculator:
http://www.rbracing-rsr.com/compstaticcalc.html
Le Mans Master
Joined: Aug 2000
Posts: 8,838
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From: Australia
Quench is important as far as preventing detonation goes.
" The flame front starts at the plug and proceed outward, an expanding ball of fire. That expanding ball of fire "supercompresses" the remaining unburned mixture. If the remaining unburned mixture reaches it's flashpoint and ignites before the flame front reaches it, you have ping (detonation).
In a tight quench area, the mixture trapped there is not burnable, it won't ignite, simply not enough space.
"turbulence" is also important. As the piston reaches TDC and closes down the quench area, the mixture at the edges of the cylinder is squirted back towards the center, kinda like stepping on a toothpaste tube. This helps mix the mixture in the combustion chamber and make it more consistent for a smoother burn."
http://www.chevytalk.org/threads/sho...page/0#1137028
A lower static comp engine with big quench will detonate before a Hi comp engine with low (.040) quench ;all things being equal.
If you have piston stock .025 in the hole , a .015 gasket would give a desirible .040 quench.
" The flame front starts at the plug and proceed outward, an expanding ball of fire. That expanding ball of fire "supercompresses" the remaining unburned mixture. If the remaining unburned mixture reaches it's flashpoint and ignites before the flame front reaches it, you have ping (detonation).
In a tight quench area, the mixture trapped there is not burnable, it won't ignite, simply not enough space.
"turbulence" is also important. As the piston reaches TDC and closes down the quench area, the mixture at the edges of the cylinder is squirted back towards the center, kinda like stepping on a toothpaste tube. This helps mix the mixture in the combustion chamber and make it more consistent for a smoother burn."
http://www.chevytalk.org/threads/sho...page/0#1137028
A lower static comp engine with big quench will detonate before a Hi comp engine with low (.040) quench ;all things being equal.
If you have piston stock .025 in the hole , a .015 gasket would give a desirible .040 quench.
Last edited by rodj; Sep 14, 2009 at 07:37 PM.
Le Mans Master
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Posts: 8,838
Likes: 31
From: Australia
Why did you cut the heads instead of the block?
Using the figures you give
383 ,
12cc dish piston ,
54cc head,
stock .025 deck
.030 gasket I get
SCR 11:1
DCR 9.0 with # -466 cam but with a big 0.055 quench
If you went to the #-467 cam (230/236 .570 ) DCR would come down to 8.6
The 467 cam is popular with 383 owners
Using the figures you give
383 ,
12cc dish piston ,
54cc head,
stock .025 deck
.030 gasket I get
SCR 11:1
DCR 9.0 with # -466 cam but with a big 0.055 quench
If you went to the #-467 cam (230/236 .570 ) DCR would come down to 8.6
The 467 cam is popular with 383 owners
Thread Starter
Pro
Joined: Mar 2008
Posts: 702
Likes: 0
Quench is important as far as preventing detonation goes.
" The flame front starts at the plug and proceed outward, an expanding ball of fire. That expanding ball of fire "supercompresses" the remaining unburned mixture. If the remaining unburned mixture reaches it's flashpoint and ignites before the flame front reaches it, you have ping (detonation).
In a tight quench area, the mixture trapped there is not burnable, it won't ignite, simply not enough space.
"turbulence" is also important. As the piston reaches TDC and closes down the quench area, the mixture at the edges of the cylinder is squirted back towards the center, kinda like stepping on a toothpaste tube. This helps mix the mixture in the combustion chamber and make it more consistent for a smoother burn."
http://www.chevytalk.org/threads/sho...page/0#1137028
A lower static comp engine with big quench will detonate before a Hi comp engine with low (.040) quench ;all things being equal.
If you have piston stock .025 in the hole , a .015 gasket would give a desirible .040 quench.
" The flame front starts at the plug and proceed outward, an expanding ball of fire. That expanding ball of fire "supercompresses" the remaining unburned mixture. If the remaining unburned mixture reaches it's flashpoint and ignites before the flame front reaches it, you have ping (detonation).
In a tight quench area, the mixture trapped there is not burnable, it won't ignite, simply not enough space.
"turbulence" is also important. As the piston reaches TDC and closes down the quench area, the mixture at the edges of the cylinder is squirted back towards the center, kinda like stepping on a toothpaste tube. This helps mix the mixture in the combustion chamber and make it more consistent for a smoother burn."
http://www.chevytalk.org/threads/sho...page/0#1137028
A lower static comp engine with big quench will detonate before a Hi comp engine with low (.040) quench ;all things being equal.
If you have piston stock .025 in the hole , a .015 gasket would give a desirible .040 quench.
Thread Starter
Pro
Joined: Mar 2008
Posts: 702
Likes: 0
Why did you cut the heads instead of the block?
Using the figures you give
383 ,
12cc dish piston ,
54cc head,
stock .025 deck
.030 gasket I get
SCR 11:1
DCR 9.0 with # -466 cam but with a big 0.055 quench
If you went to the #-467 cam (230/236 .570 ) DCR would come down to 8.6
The 467 cam is popular with 383 owners
Using the figures you give
383 ,
12cc dish piston ,
54cc head,
stock .025 deck
.030 gasket I get
SCR 11:1
DCR 9.0 with # -466 cam but with a big 0.055 quench
If you went to the #-467 cam (230/236 .570 ) DCR would come down to 8.6
The 467 cam is popular with 383 owners
This was the chart I used to figure I was ok on compression ratio
http://www.northernautoparts.com/Pro...tModelId=15101
Are these numbers off?
I am really considering the exact cam you mentioned
It's kinda looking like I am going to have to pull a cc or 2 from the cambers even with a cam switch
Last edited by ekess744; Sep 14, 2009 at 08:20 PM.
Le Mans Master
Joined: Aug 2000
Posts: 8,838
Likes: 31
From: Australia
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Le Mans Master
Joined: Aug 2000
Posts: 8,838
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From: Australia
Thread Starter
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im getting 10.57 when I calculate with a .051 gasket... But I realize that my qunech is going to be horrible with .025 deck that the piston supposedly comes with. I'll know more once I actually measure it.
Last edited by ekess744; Sep 15, 2009 at 01:35 PM.
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After looking at the hot-rodding article posted by rodj and this article, I would have to conclude that the requirement for 104 octane with 11:1 compression would refer to dynamic compression (vs SCR). For that kind of dynamic compression to occur, static would need to be much higher.
(Edit: Otherwise even our stock engines would have trouble running on pump gas -- by the chart I linked!)
For the OP, I like the chart in the article rodj linked -- because of the addition of water temp!
I've seen several people running pump gas with 11:1 compression but most (if not all) are running flat-tops. If you can find a piston to knock things down 1/2 pt, I gotta think life would be a lot easier for you.
(Edit: Otherwise even our stock engines would have trouble running on pump gas -- by the chart I linked!)
For the OP, I like the chart in the article rodj linked -- because of the addition of water temp!
I've seen several people running pump gas with 11:1 compression but most (if not all) are running flat-tops. If you can find a piston to knock things down 1/2 pt, I gotta think life would be a lot easier for you.
Last edited by GREGGPENN; Sep 16, 2009 at 01:42 AM.
