can you run 11:1 compression with 93 octane??? - 460 Ford Forum
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post #1 of 137 (permalink) Old 03-15-2011, 09:58 AM Thread Starter
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can you run 11:1 compression with 93 octane???

I am curious if you can run an 11:1 engine with cast iron heads on it on the street with 93 octane gas. If a cam grinder custom grinds a cam will it be able to or is it just to much compression?

engine is a 552 ci, fully ported D3 heads, headers, 1150 dommy, new TFS intake, zero decked, flattop pistons, fox mustang car, 4.11 gears, 3,000 stall, don't have cam yet.
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post #2 of 137 (permalink) Old 03-15-2011, 10:27 AM
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It's technically possible with just the right parts combination, including addtional details such as polished chambers, fire slots, among other things.

It's not something I normally recommend nor would I normally build such an engine for a customer that must run pump gas...but it's technically do-able with iron heads.

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post #3 of 137 (permalink) Old 03-15-2011, 10:31 AM
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Quote:
Originally Posted by tiger View Post
I am curious if you can run an 11:1 engine with cast iron heads on it on the street with 93 octane gas. If a cam grinder custom grinds a cam will it be able to or is it just to much compression?

engine is a 552 ci, fully ported D3 heads, headers, 1150 dommy, new TFS intake, zero decked, flattop pistons, fox mustang car, 4.11 gears, 3,000 stall, don't have cam yet.
Static compression means absolutly nothing bc the engine will never see that, static compression will increase/decrease as dynamic compression changes (if you were to change it). What you want to figure out is dynamic compression. There are many tools on the net that can figure this out for you. However you will need the cam specs and all your motor specs to complete the formula. static compression is the full compression at TDC if the intake valve is closed from the bottom of the stroke. As we all know cam duration will affect the closing time of the valves so essentially it will bleed off compression, depending on the cam compression could just start when the piston has already moved 1/4way up the bore! As the intake valve is still closing the compression will bleed off so your static compression isnt accurate at all! Dynamic compression will NEVER change throughout the engines life unless you change things like cams, stroke, rod size, head gaskets, head chamber size.

My 521 should have 10:817:1 compression STATIC but my dynamic compression is only 8.872:1. This is the number you want to figure out.

heres what i used
http://kb-silvolite.com/calc.php?action=comp

My question is that I CAN NOT find any solid information on what is the highest dynamic compression that can be run with 93 octane. I know that alot of things affect this, head material is one, also quench, engine temp, timing etc. Since mine is aluminum heads i should ward off detonation more than you, and i will keep my engine temps down as much as possible.

So the real question is... can ANYONE tell us how much dynamic compression can be run with 93 octane?!? i would like to know if ill blow my motor up on startup

maybe randy could jump in or carl?

Last edited by customblackbird; 03-15-2011 at 10:34 AM.
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post #4 of 137 (permalink) Old 03-15-2011, 10:38 AM
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D3's, 11:1 and pump gas will only work together with a BIG cam.

If only somebody made a low cost aluminum head that could be used in place of the D3.....

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post #5 of 137 (permalink) Old 03-15-2011, 10:42 AM
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Quote:
Originally Posted by CarsByCarl View Post
If only somebody made a low cost aluminum head that could be used in place of the D3.....
You had to go there, didn'tcha?

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Last edited by Paul Kane; 03-16-2011 at 09:59 AM.
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post #6 of 137 (permalink) Old 03-15-2011, 12:30 PM
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Quote:
Originally Posted by customblackbird View Post
Static compression means absolutly nothing bc the engine will never see that, static compression will increase/decrease as dynamic compression changes (if you were to change it). What you want to figure out is dynamic compression. There are many tools on the net that can figure this out for you. However you will need the cam specs and all your motor specs to complete the formula. static compression is the full compression at TDC if the intake valve is closed from the bottom of the stroke. As we all know cam duration will affect the closing time of the valves so essentially it will bleed off compression, depending on the cam compression could just start when the piston has already moved 1/4way up the bore! As the intake valve is still closing the compression will bleed off so your static compression isnt accurate at all! Dynamic compression will NEVER change throughout the engines life unless you change things like cams, stroke, rod size, head gaskets, head chamber size.

My 521 should have 10:817:1 compression STATIC but my dynamic compression is only 8.872:1. This is the number you want to figure out.

heres what i used
http://kb-silvolite.com/calc.php?action=comp

My question is that I CAN NOT find any solid information on what is the highest dynamic compression that can be run with 93 octane. I know that alot of things affect this, head material is one, also quench, engine temp, timing etc. Since mine is aluminum heads i should ward off detonation more than you, and i will keep my engine temps down as much as possible.

So the real question is... can ANYONE tell us how much dynamic compression can be run with 93 octane?!? i would like to know if ill blow my motor up on startup

maybe randy could jump in or carl?
according to this site a Dynamic Compression Ratio (DCR) of somewhere between 7.5-8.5 is best for 91octane so it seems your 8.87 is on the higher end of their scale and im sure the aluminum heads and couple extra points in octane help a bit.
http://www.empirenet.com/pkelley2/DynamicCR.html

other things ive read say that old school all iron engines would be best to stay closer to 8.0 DCR. im far from being a pro though but thats just info ive come across and just parrotting.
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post #7 of 137 (permalink) Old 03-15-2011, 12:54 PM
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Static compression is a calculation of volumes, and has nothing to do with valve events. Dynamic compression changes throughout the entire rpm range of the engine, it changes with temps, it changes with lash, it changes with air density...it changes all the time, and that's why they call it "dynamic".
There are 12.5:1 engines that run 93 and are fine, and there are 10:1 engines that can't run 93. Forget about dynamic compression and build the engine with the static in mind. If you're going to run 11:1 on 93, you need to minimize any chance of detonation. Polish chambers, blend piston domes, keep the quench as tight as possible, good ring seal (any oil in the chamber is death when on the edge of detonation) and most of all, good timing and mixture. You should look into a cam grind that will work best with your combination, but preventing detonation when on the edge of your octane rating is going to be more in the prep. and maintenance of the combustion process than anything. You could also drop your compression half a point and not give up anything.
FWIW, I just finished a pump gas 540 (top end) that made over 900 HP on 10.7:1 compression @ 6200rpm. I'm not saying you can do that with iron heads, just illustrating the potential for pump gas if everything is right.
As usual, this is just my opinion. YMMV

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post #8 of 137 (permalink) Old 03-15-2011, 01:07 PM
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Quote:
Originally Posted by customblackbird View Post
Static compression means absolutly nothing bc the engine will never see that, static compression will increase/decrease as dynamic compression changes (if you were to change it). What you want to figure out is dynamic compression. There are many tools on the net that can figure this out for you. However you will need the cam specs and all your motor specs to complete the formula. static compression is the full compression at TDC if the intake valve is closed from the bottom of the stroke. As we all know cam duration will affect the closing time of the valves so essentially it will bleed off compression, depending on the cam compression could just start when the piston has already moved 1/4way up the bore! As the intake valve is still closing the compression will bleed off so your static compression isnt accurate at all! Dynamic compression will NEVER change throughout the engines life unless you change things like cams, stroke, rod size, head gaskets, head chamber size.

My 521 should have 10:817:1 compression STATIC but my dynamic compression is only 8.872:1. This is the number you want to figure out.

heres what i used
http://kb-silvolite.com/calc.php?action=comp

My question is that I CAN NOT find any solid information on what is the highest dynamic compression that can be run with 93 octane. I know that alot of things affect this, head material is one, also quench, engine temp, timing etc. Since mine is aluminum heads i should ward off detonation more than you, and i will keep my engine temps down as much as possible.

So the real question is... can ANYONE tell us how much dynamic compression can be run with 93 octane?!? i would like to know if ill blow my motor up on startup

maybe randy could jump in or carl?
How does the dynamic compression change ? plus how does dymamic change static compression ?

Last edited by 2slow; 03-15-2011 at 01:12 PM.
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post #9 of 137 (permalink) Old 03-15-2011, 01:25 PM
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How does the dynamic compression change ? plus how does dymamic change static compression ?
cam duration/timing.
dynamic compression doesnt change static compression but calculating it gives you a better picture of your engines characteristics.
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post #10 of 137 (permalink) Old 03-15-2011, 01:27 PM
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post #11 of 137 (permalink) Old 03-15-2011, 01:33 PM
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Well, I don't know about any one else but that explains it pretty clearly for me....
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post #12 of 137 (permalink) Old 03-15-2011, 01:39 PM
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A local character runs a 460 Bronco on the strip. I'm not sure what his compression ratio is but he said he couldn't run pump gas or he got into serious preignition problems. He then put on an MSD ignition box and that cured it with no other changes. I've never tried it but has anybody else found this?
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post #13 of 137 (permalink) Old 03-15-2011, 02:31 PM
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Originally Posted by MauriSSSio View Post
cam duration/timing.
dynamic compression doesnt change static compression but calculating it gives you a better picture of your engines characteristics.
Thanks , but I know that , static and dynamic doesnt change , they are fix to a given build .. only thing that can change is . ......leave that up to the bird for the answer

Last edited by 2slow; 03-15-2011 at 02:36 PM.
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post #14 of 137 (permalink) Old 03-15-2011, 04:33 PM
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Originally Posted by CarsByCarl View Post
If only somebody made a low cost aluminum head that could be used in place of the D3.....
I don't believe changing to an aluminum head really changes the octane requirement. I believe that is an old wive's tale that's been told for decades. I'll listen to an opposing view though. I've been wrong before.

paulie

Last edited by Paul Lovett; 03-15-2011 at 04:58 PM. Reason: messed up quote
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post #15 of 137 (permalink) Old 03-15-2011, 04:56 PM
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Originally Posted by CarsByCarl View Post
If only somebody made a low cost aluminum head that could be used in place of the D3.....
Or are you talking about the chamber design of the low cost aluminum head, rather than the fact that it is made of aluminum?

paulie
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