Max pump gas compression
Thread Starter
Racer
Joined: Jan 2017
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From: San Francisco Bay Area
Max pump gas compression
I know we have some savvy engine builders on the forum and I am requesting your opinion on max static compression for a 91 octane, (California),pump gas, aluminum head, carbureted, small block Chevy. Cam will be very mild with 218/220 duration, 495/512 lift and 112 lobe separation. Don’t want to get too greedy and have to use octane boosters or mix in race gas, or retard timing to reduce/eliminate pinging, but at the same time don’t want to leave some usable compression on the table
Opinions please. Thanks much
Opinions please. Thanks much
Had a 1976 L-82, 4-sp
Joined: Mar 2011
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From: Some days your the dog and some days your the hydrant.
Royal Canadian Navy
Plug your engine numbers into a DCR calculator. If its below 8,5, you MAY be ok. My DCR is about 8.3 with a 10.7 cr and it runs on 91 octane up here in Canada
Last edited by resdoggie; Nov 20, 2020 at 04:39 PM.
Advanced
Joined: May 2019
Posts: 66
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From: Kalmar, Sweden
I know we have some savvy engine builders on the forum and I am requesting your opinion on max static compression for a 91 octane, (California),pump gas, aluminum head, carbureted, small block Chevy. Cam will be very mild with 218/220 duration, 495/512 lift and 112 lobe separation. Don’t want to get too greedy and have to use octane boosters or mix in race gas, or retard timing to reduce/eliminate pinging, but at the same time don’t want to leave some usable compression on the table
Opinions please. Thanks much
Opinions please. Thanks much
Team Owner
Joined: Oct 2004
Posts: 81,241
Likes: 3,063
From: altered state
St. Jude Donor '05
w/joystick...Get an AFR 180 head or 190 vortec....those combustion chambers are awesome for detonation resistance. You can run a lil more compression and timing for sure. It does help...imo more than the "only adds xx %
OP if you like I can pm you the nuber of a member who is an afr dealer, get it cheaper than Summit
AFR is having a price increase Jan 1 think its 50+ perset.
OP if you like I can pm you the nuber of a member who is an afr dealer, get it cheaper than Summit
AFR is having a price increase Jan 1 think its 50+ perset.
Last edited by cv67; Nov 21, 2020 at 04:08 PM.
Le Mans Master
Joined: Apr 2011
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From: Hermosa
https://www.jeepstrokers.com/forum/calculator/
use this calculator to computer you DCR with that cam.
keep it under 9.5 : 1 for aluminum
under 9.0:1 for cast iron.
I currently run up to 9.2:1 DCR with 91 and no issues.
Edit: Sorry fat fingered those. 8.5 aluminum and 8.0 cast iron and I run up to 8.2.
I am running aluminum heads. AFR's
use this calculator to computer you DCR with that cam.
keep it under 9.5 : 1 for aluminum
under 9.0:1 for cast iron.
I currently run up to 9.2:1 DCR with 91 and no issues.
Edit: Sorry fat fingered those. 8.5 aluminum and 8.0 cast iron and I run up to 8.2.
I am running aluminum heads. AFR's
Last edited by REELAV8R; Nov 21, 2020 at 03:51 PM.
Le Mans Master
Joined: Aug 2017
Posts: 8,336
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From: Cool Northern Michigan
Everything I read on Dynamic Compression Ratio states that ideal DCR is between 8.1 - 8.5.
Lower than that is loss of power. Higher than that is loss of everything. (grenade)
Lower than that is loss of power. Higher than that is loss of everything. (grenade)
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Team Owner
Joined: Jan 2006
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From: Crossville TN
9.5 to 10.0 should be achievable with aluminum heads.
I have 10.5:1 Pontiac engine with iron heads and it runs fine of 93 Octane. It likely would ping with 91 octane fuel.
Some gyppo fuel stop sold me Premium fuel for my Pontiac...ONCE...and it ran like total crap. They were putting 'regular' in every tank and just charging folks the premium price, figuring that the modern electronic controls would still make the engine run OK. Didn't work on my Pontiac.
Naturally, I went back to have a "talk" with the owner.....
I have 10.5:1 Pontiac engine with iron heads and it runs fine of 93 Octane. It likely would ping with 91 octane fuel.
Some gyppo fuel stop sold me Premium fuel for my Pontiac...ONCE...and it ran like total crap. They were putting 'regular' in every tank and just charging folks the premium price, figuring that the modern electronic controls would still make the engine run OK. Didn't work on my Pontiac.
Naturally, I went back to have a "talk" with the owner.....
Last edited by 7T1vette; Nov 21, 2020 at 05:46 PM.
Advanced
Joined: May 2019
Posts: 66
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From: Kalmar, Sweden
There is a little bit of apples and oranges in this thread now.
I would say, do NOT go above 8.5:1 DCR, whatever that will be SCR all depends on the cams IVC.
As mentioned above, run a DCR calculator.
I would say, do NOT go above 8.5:1 DCR, whatever that will be SCR all depends on the cams IVC.
As mentioned above, run a DCR calculator.
Le Mans Master
Joined: Oct 2007
Posts: 7,455
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From: RI, Now Franklin/Nashville TN
I have AFR 180's with Howards Roller Cam (Duration 219/225, Lift .525/.525, duration 110) with 1 7/8 inch headers with straight through C4 Borlas Type S muffles with 34 total timing, Aluminum L-82 intake and holley 4175 Vaccum secondary 650 CFM Qjet spreadbore type carb (lots of details here). Total static compression is 10.2:1. Engine runs perfect with Zero detonation with 89 octane......
Had a 1976 L-82, 4-sp
Joined: Mar 2011
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From: Some days your the dog and some days your the hydrant.
Royal Canadian Navy
The SCR is independent of the ivc point but its the DCR that is calculated using the ivc point. For higher scr's, longer duration cams are needed to bleed off cylinder pressure and reducing the risk of detonation. As mentioned, a DCR for aluminium heads on pump gas is 8,5 max and 8.0 for iron heads.
Le Mans Master
Joined: Apr 2011
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From: Hermosa
Another thing that is just as important is the quench distance. Keep that between .035" to .040" and you'll reduce the possibility of detonation a lot.
So if the block is still stock height then the pistons will reside .025" in the hole with stock compression height pistons. Use the felpro .015" shim head gasket and you'll get .040" of quench. Too big of a quench and you'll not get the advantage of a good tight quench increasing the likelyhood of detonation.
I run .038" of quench 10.6 CR with, like I said, a max of 8.2 DCR and I can still run 87 octane with no detonation. That being said the cooling system needs to be well sorted as well, and the timing and the jetting and I have a true cold air intake to keep the fresh intake charge temps down. So it all matters in the end, but having the CR, DCR and the quench right is the place to start.
So if the block is still stock height then the pistons will reside .025" in the hole with stock compression height pistons. Use the felpro .015" shim head gasket and you'll get .040" of quench. Too big of a quench and you'll not get the advantage of a good tight quench increasing the likelyhood of detonation.
I run .038" of quench 10.6 CR with, like I said, a max of 8.2 DCR and I can still run 87 octane with no detonation. That being said the cooling system needs to be well sorted as well, and the timing and the jetting and I have a true cold air intake to keep the fresh intake charge temps down. So it all matters in the end, but having the CR, DCR and the quench right is the place to start.
Last edited by REELAV8R; Nov 22, 2020 at 10:22 AM.
Le Mans Master
Joined: Apr 2011
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From: Hermosa
If I assume that the advertised duration of your cam is 272* intake and a 4.0" bore with a .040" quench (.025 in the hole and .015 gasket) and a 7 cc flat top piston, 64cc heads, 4* advance on the cam (standard) then I get 8.06 DCR and 10.03 CR for your cam at sea level.
Bringing the bore up to 4.030" gives 8.16 DCR and 10.15 CR
So 10.0:1 CR will put you in the safe zone with good power.
Bringing the bore up to 4.030" gives 8.16 DCR and 10.15 CR
So 10.0:1 CR will put you in the safe zone with good power.
Le Mans Master
Joined: May 2007
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From: North tonawanda NY
If I assume that the advertised duration of your cam is 272* intake and a 4.0" bore with a .040" quench (.025 in the hole and .015 gasket) and a 7 cc flat top piston, 64cc heads, 4* advance on the cam (standard) then I get 8.06 DCR and 10.03 CR for your cam at sea level.
Bringing the bore up to 4.030" gives 8.16 DCR and 10.15 CR
So 10.0:1 CR will put you in the safe zone with good power.
Bringing the bore up to 4.030" gives 8.16 DCR and 10.15 CR
So 10.0:1 CR will put you in the safe zone with good power.
Actually I had a related question which this thread has helped answer here since I have been considering swapping out the crank with a scat 383 crank which should raise my static to 11:1 and my dcr to 8.2:1 sounds like I would still be ok even with my iron eagle heads.
Last edited by augiedoggy; Nov 22, 2020 at 10:39 AM.
Le Mans Master
Joined: Apr 2011
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From: Hermosa
I have a similar setup that you actually advised me on building 6 years ago... after digging up the exact measurements, I have a .037 quench 10.3:1 static and 7.6:1 dcr with the voodoo 262 cam im running which bleeds off pressure and allows me to run any pump gas. Although I run 89 or higher to be on the safe side and also because I use the non ethonal gas whenever possible...
Actually I had a related question which this thread has helped answer here since I have been considering swapping out the crank with a scat 383 crank which should raise my static to 11:1 and my dcr to 8.2:1 sounds like I would still be ok even with my iron eagle heads.
Actually I had a related question which this thread has helped answer here since I have been considering swapping out the crank with a scat 383 crank which should raise my static to 11:1 and my dcr to 8.2:1 sounds like I would still be ok even with my iron eagle heads.
With your current set up I ran a few numbers to get a 10.3 CR and I come up with about 8.6:1 DCR with the 262 cam at sea level. Is your DCR drop due to running at about 5000 feet?
Le Mans Master
Joined: May 2007
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From: North tonawanda NY
If I was to go to a 383 with 11:1 CR, I would want a cam with more duration or narrower LSA (both likely) to take advantage or the rev potential. With the 262 cam on a 112 your gonna limit the power due to low RPM potential.
With your current set up I ran a few numbers to get a 10.3 CR and I come up with about 8.6:1 DCR with the 262 cam at sea level. Is your DCR drop due to running at about 5000 feet?
With your current set up I ran a few numbers to get a 10.3 CR and I come up with about 8.6:1 DCR with the 262 cam at sea level. Is your DCR drop due to running at about 5000 feet?
Oh yeah.. 4.030 bore
Last edited by augiedoggy; Nov 22, 2020 at 12:27 PM.
Le Mans Master
Joined: Oct 2007
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From: RI, Now Franklin/Nashville TN
I think I know what you were trying to say about the 383 power potential but a 383 with a longer stroke than a 350/355 makes more power at the same rpms as the smaller displacement 350/355 at that same rpm. The 355 actually will rev more easily than a longer stroke 383 to make similar TQ at a higher rpm. Any shorter stroke engine will rev much easier to high rpm than the longer stroke motor of the same design and cylinders.....
Last edited by jb78L-82; Nov 22, 2020 at 12:13 PM.
Le Mans Master
Joined: Aug 2017
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From: Cool Northern Michigan
I was referring to Dynamic 8.1 - 8.5 With the max being 8.5 for safe operations.
Static, on the average is two points higher.
Thread Starter
Racer
Joined: Jan 2017
Posts: 496
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From: San Francisco Bay Area
Thanks to all for the great feedback. I ran the numbers through a DCR calculator and came up with 8.6. I am running some really nice AFR aluminum heads so I am assuming I should be good. Thanks again
