Duster
Well-Known Member
What would be the maximum safe compression ratio for a street use big block with aluminum head on 91 octane gas? Thank you.
Robert
Robert
Maximum Compression Ratio
- Thread starter Duster
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omahamoparguy
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10 to 1 or if you got a large cam maybe a bit higher. [you can bleed off compression with a big cam]. I'm being on the safe side here IMHO. Big blocks tend to get touchy in the street with too much compression.
UDUST81
Well-Known Member
id be interested in hearing what would be an acceptable cranking psi/dynamic compression ratio for 91 octane as well seeing 93 is fairly hard to find in my neck of the woods
Jlcaptain24
Well-Known Member
I'm running 9.5-1 with my iron headed 440, 91 octane fuel with no issues. Dynamic compression is around 8-1 if I remember correctly......
Duster
Well-Known Member
So 10:1 static would be a ball park. So what I was wondering is what would be a safe dynamic ratio? There must be a # for any gas engine in general?
Jlcaptain24
Well-Known Member
Can't give a good answer on that one brother...all my research in building my engine pointed to dynamic compression being the most influential factor. 8ish was the general consensus. Can't remember if that's exclusive to iron head only or aluminum also? I'll defer to the more knowledgable builders........
Duster
Well-Known Member
Thank you very much for your reply, I'm that much closer to what I'm looking for, I have a real life ratio.
Dan Stratton
Well-Known Member
It depends also what altitude the car is being ran at
I am at 4500 ft and I have a 440 with 12.5:1 compression and aluminum heads and run pump gas and have no problems
What cam are you using, going to use?
A larger cam bleeds of compression, as stated above. So, a 10-1 on one cam will be fine while the 10-1 on a smaller cam pings like hell. With 91 octane available, 10-1 is a safe range area. Also assuming you have a decent performance cam in there and not a stocker or a very mild upgraded cam.
My 360 small block is 10.82-1 and runs a med. size ([email protected]) solid with no problems. I'm at sea level. Never more than a couple hundered feet above it. The Island isn't that high up.
A larger cam bleeds of compression, as stated above. So, a 10-1 on one cam will be fine while the 10-1 on a smaller cam pings like hell. With 91 octane available, 10-1 is a safe range area. Also assuming you have a decent performance cam in there and not a stocker or a very mild upgraded cam.
My 360 small block is 10.82-1 and runs a med. size ([email protected]) solid with no problems. I'm at sea level. Never more than a couple hundered feet above it. The Island isn't that high up.
TrailBeast
AKA Mopars4us on Youtube
I was going to mention that and glad someone did.
At 5,500 here with 9:1 Iron heads and can run 87 just fine, but sometimes I go to Phoenix which is a few thousand feet lower and I have to pull a couple of degrees or run 91.
Duster
Well-Known Member
Maybe I should be looking at max compression pressure which depend where the intake valve close? I'm curious of the maximum(safe) dynamic compression a gas engine can be build on 91 octane gasoline without having premature detonation. Thank you.
Robert
Robert
Dan Stratton
Well-Known Member
I have used the Mopar 509-292 and Edelbrock 478-308 car runs low 11s in quarter
Dan Stratton
Well-Known Member
also you can retard timing a few degrees
Duster
Well-Known Member
I will choose after I was looking for that number to decide for the rest. If it doesn't make sense let me know I will gladly take advise. Thank you.
Contact AJFourms. he is good with the math thing. Even better if you know the cam and it's specs.
I hate when I have to do that since I'm also taking power out of the engine and a gd amount is allways found in a few degrees. BUT, if it makes it drivable....in a pinch, I'll do it.
Hey Dan, I'm interested in the combo specs by the way.
moper
Well-Known Member
If your builder knows what he's doing with modern parts - ask him. If he doesn't, do go past 9.5:1 static. Pretty much anything above that is subject to the parts chosen, the fuel being run (not all 91 octance is the same) and the tuner's ability. Some can run upwards of 11.8. Some will get ping at 9:7.
The amount of mechanical advance in the distributor can make or break any combo.
I am running 10.81 static in a small block with aluminum heads, I tuned it to run on 93 octane.
Intake closing will make a difference in psi and dynamic compression.
I am running 10.81 static in a small block with aluminum heads, I tuned it to run on 93 octane.
Intake closing will make a difference in psi and dynamic compression.
Exactly!
There is as safe Max but it just changes with every combo, that now, the question is answered with a general statement of..... Ratio. And in and on error to be safe. What can be run and what is run as well as what is available to the fuel tank where the next guy lives is all part of the generic answer that is slightly lower than what could be possible.
Is it possibleto run a 13-1 iron headed engine on the street swill of 93 octane gasoline?
(Young guns and novices, careful, loaded question above!)
But seriously folks, standard pump swill is what we run so......
Many years back, in the MP books, Larry Shepard gave us tips on what to do and how to do it. His ugide lines were oin the safe zone. This way, everyone stays happy. Could you push his published numbers? Sure. Would you be happy. Probably.
But don't complain and ***** when you are not.
TrailBeast
AKA Mopars4us on Youtube
Like David Freiburger said on Road Kill.
High compression engines are fun, but it isn't that fun to wonder if your junk is going to blow up all the time.
High compression engines are fun, but it isn't that fun to wonder if your junk is going to blow up all the time.
AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
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Rumble, thx for the nod
Robert
This subject is not well understood by us commoners yet. I am still compiling data from the threads here, and have not yet ,perhaps I never will, got a definitive answer. The best answer might not even be the best answer. Some guys are willing to walk the fine line, and modify things as they go. Some guys just want to slam something together and never think about it again. Some guys are racing, and don't want to tell, or for them it really doesn't matter, cuz they're not limited to pump gas. On the street, the every-day guy just cruises around and wants a nasty sounding engine and doesn't care if it's really nasty,sorta nasty, or incredibly nasty; as long as sounds nasty.
Then there are some guys like me, who are or were very curious, cuz our car's are long-term projects. We find it challenging to explore the limits. Fun to swap stuff. Exciting to be engineering on the new Dcr frontier.
I think you might be one of these guys. So Il tell you what I remember. These are all aluminum heads and all at under 1000ft.One guy here on FABO, I heard, is running a Dcr over 9.3 and 91 gas. Others are running right around 9.0 still on 91. I myself am running 8.6,but I run 87E10 exclusively. So if you look at the numbers, one might think that the spread is from 8.6 to 9.3, and from 87 to 91. A little math then says in the middle should be in the middle, or 8.95 should be 89, and that completes the aluminum range.
I have heard that for iron heads, both compressions need to be cranked back about a half a point.This also lines up with the scuttlebutt around the forum.
One thing that has to be remembered is that some physics never change. The maximum Static compression that can be run still has to be kept in mind. One cannot run the Dcr up to 9.3, with a big cam that requires a 12.0 Scr to get the 9.3 with. No! the static has been well established for iron for years,to be: safe at 10, maybe safe at 10.5 borderline unsafe at 11, probably will shatter stuff at 11.5. So, depending what the application is and what cam size is going in, the Dcr can be moot, cuz maybe 11 is your Scr limit!.
Another thing to remember is that Dcr is a tool,not a target,per se. We use it, to figure out how a particular combo will behave at low rpms to early midrange. Low Dcrs tend to be soggy off-the-line; and need extra work to get moving, like a hi-stall, a very low starter gear,and some creative timing work.Whereas a too-high Dcr leads to detonation and broken parts, same as too much Scr. This is very helpful stuff for a streeter to understand; not so much for a dragracer. Dragracers don't spend any time running around in traffic looking for gas stations. They, by and large, don't need to care about Dcr.
Another thing is that; adjusting the Scr to achieve a certain Dcr, gets expensive. Usually we are targeting Zero decks, tight Q, and small head-chambers. This all costs money, sometimes more than necessary.
And finally, the quest for a high compression ratio is,for most, kindof misunderstood. Rusty has driven this point home more than once. The difference going from 10/1 to 10.5/1 is around 1% in the horsepower department, but represents a TOTAL chamber volume difference of 5.2 cc on a 440. Sometimes these 5.2 cc can get very expensive to get. Especially if you factor that cost as dollars per horsepower. We are talking about 4.4 hp on a 1hp/cid 440, 5.28hp in a 1.2hp/cid 440.
So you can see how complicated the answer to your question can be.
Nobody, me included, want's to be responsible for your engines demise because you built it to a number that FABO said was safe.
All we can tell you is what worked for us.
But we are very eager to hear what worked for you, cuz one day that theoretically absolute number you wanted today, might just have been established; and your results may go down in history, right here on FABO..
So far I have heard of exactly one guy here on FABO,who wrecked his engine by detonation. And he did it by accident, using the wrong fuel, and ignoring the detonation. He just ran the wrong fuel.There may be others, I'm not here 24/7,lol
I think Moper said it best, in post #16. And IQ52 in #19
The builder is the guy who sees the traffic. He is probably the most qualified guy to say what breaks stuff. And when you know that, then you also know what doesn't.
Robert
This subject is not well understood by us commoners yet. I am still compiling data from the threads here, and have not yet ,perhaps I never will, got a definitive answer. The best answer might not even be the best answer. Some guys are willing to walk the fine line, and modify things as they go. Some guys just want to slam something together and never think about it again. Some guys are racing, and don't want to tell, or for them it really doesn't matter, cuz they're not limited to pump gas. On the street, the every-day guy just cruises around and wants a nasty sounding engine and doesn't care if it's really nasty,sorta nasty, or incredibly nasty; as long as sounds nasty.
Then there are some guys like me, who are or were very curious, cuz our car's are long-term projects. We find it challenging to explore the limits. Fun to swap stuff. Exciting to be engineering on the new Dcr frontier.
I think you might be one of these guys. So Il tell you what I remember. These are all aluminum heads and all at under 1000ft.One guy here on FABO, I heard, is running a Dcr over 9.3 and 91 gas. Others are running right around 9.0 still on 91. I myself am running 8.6,but I run 87E10 exclusively. So if you look at the numbers, one might think that the spread is from 8.6 to 9.3, and from 87 to 91. A little math then says in the middle should be in the middle, or 8.95 should be 89, and that completes the aluminum range.
I have heard that for iron heads, both compressions need to be cranked back about a half a point.This also lines up with the scuttlebutt around the forum.
One thing that has to be remembered is that some physics never change. The maximum Static compression that can be run still has to be kept in mind. One cannot run the Dcr up to 9.3, with a big cam that requires a 12.0 Scr to get the 9.3 with. No! the static has been well established for iron for years,to be: safe at 10, maybe safe at 10.5 borderline unsafe at 11, probably will shatter stuff at 11.5. So, depending what the application is and what cam size is going in, the Dcr can be moot, cuz maybe 11 is your Scr limit!.
Another thing to remember is that Dcr is a tool,not a target,per se. We use it, to figure out how a particular combo will behave at low rpms to early midrange. Low Dcrs tend to be soggy off-the-line; and need extra work to get moving, like a hi-stall, a very low starter gear,and some creative timing work.Whereas a too-high Dcr leads to detonation and broken parts, same as too much Scr. This is very helpful stuff for a streeter to understand; not so much for a dragracer. Dragracers don't spend any time running around in traffic looking for gas stations. They, by and large, don't need to care about Dcr.
Another thing is that; adjusting the Scr to achieve a certain Dcr, gets expensive. Usually we are targeting Zero decks, tight Q, and small head-chambers. This all costs money, sometimes more than necessary.
And finally, the quest for a high compression ratio is,for most, kindof misunderstood. Rusty has driven this point home more than once. The difference going from 10/1 to 10.5/1 is around 1% in the horsepower department, but represents a TOTAL chamber volume difference of 5.2 cc on a 440. Sometimes these 5.2 cc can get very expensive to get. Especially if you factor that cost as dollars per horsepower. We are talking about 4.4 hp on a 1hp/cid 440, 5.28hp in a 1.2hp/cid 440.
So you can see how complicated the answer to your question can be.
Nobody, me included, want's to be responsible for your engines demise because you built it to a number that FABO said was safe.
All we can tell you is what worked for us.
But we are very eager to hear what worked for you, cuz one day that theoretically absolute number you wanted today, might just have been established; and your results may go down in history, right here on FABO..
So far I have heard of exactly one guy here on FABO,who wrecked his engine by detonation. And he did it by accident, using the wrong fuel, and ignoring the detonation. He just ran the wrong fuel.There may be others, I'm not here 24/7,lol
I think Moper said it best, in post #16. And IQ52 in #19
The builder is the guy who sees the traffic. He is probably the most qualified guy to say what breaks stuff. And when you know that, then you also know what doesn't.
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Your welcome AJ. It know you good with da numbers. And your double right. No one wants to give bad advice.
I know how things work without the numbers due to making the mistakes. Having been there and done that on a few things, having friends and club members on the same traveled road helps speed things up. No internet to post on. Word of mouth from the local good fellas and wisdom from the machinist were the guides.
Then there are the 3 best books I ever picked up.
MP engines
MP chassis
How to rebuild your small block MoPar
The in "General Rule of Thumb" we followed for iron headed mills for compression and performance was 9-1 for easy builds, 9.5-1 for mild builds, 10-1 for snotty builds. 1 pt. more for aluminum. All 93 pump. Any stouter, the local 100 cam II was used and not to bad to add to the snotty mills ether.
I know how things work without the numbers due to making the mistakes. Having been there and done that on a few things, having friends and club members on the same traveled road helps speed things up. No internet to post on. Word of mouth from the local good fellas and wisdom from the machinist were the guides.
Then there are the 3 best books I ever picked up.
MP engines
MP chassis
How to rebuild your small block MoPar
The in "General Rule of Thumb" we followed for iron headed mills for compression and performance was 9-1 for easy builds, 9.5-1 for mild builds, 10-1 for snotty builds. 1 pt. more for aluminum. All 93 pump. Any stouter, the local 100 cam II was used and not to bad to add to the snotty mills ether.
Duster
Well-Known Member
First I would like to thank all of you for your reply I really appreciate it. The reason for this post is exactly to have a basic understanding (or warning) from people with knowledge in order to make an educated decision as to where to go with the buildup, which is what I always do before I start a project. I do not intend to point the finger at nobody for my mistake since I will make the final decision on what will go in. That being said I got answers and food for though to my question, AJ thanks for the long post. Again thanks everybody.
Robert
Robert
moper
Well-Known Member
I broke a 340 when I was younger by running a nice lady's old lawnmower fuel in it because I ran the car out of fuel in the middle of nowhere. Ah to be 17 again... anyway the bits of ring land and piston I keep in a little plastic baggie over my work bench. It's been moved from my parents house to two shops to my current home. Sometimes the best learning comes from making mistakes.
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