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What octane for 10.8:1 compression?

Performance mods and Advanced Stroker discussion.
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BcurpKodiak
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What octane for 10.8:1 compression?

Post by BcurpKodiak » January 5th, 2018, 6:36 pm

In the middle of getting everything figured out for my build, most of the bottom end is in the block and I've got some work done to the head.

Cam: comp 68-235-4 (technically 68-236-4) with 1.7 RR's intake and 1.65 RR'sexhaust

I have the edelbrock aluminum head ported by Russ with big valves as well. With a .043" head gasket I would have 10.8:1 CR and .033" quench. What octane gas would this most likely require?? Best guess? We only have 90 here and I will probably have to bump it up. Called the local airport to make sure I can get some 100LL avgas.

So my thoughts: Aluminum head will help, much better shaped combustion chamber compared to OEM, forged pistons, tight quench. I'm also running 5.13's and 33" tires with the AW4 auto

Depending on how difficult it will be, I actually want to run 11.2:1

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Re: What octane for 10.8:1 compression?

Post by Alexia » January 6th, 2018, 12:32 am

I have a 10:1 static compression ratio in my stroker and I can run AKI 91 octane. Typically I am using 93 octane since that is what is around here. I have actually ran it on 87 octane and it was fine, but it was only to move it around while the truck was being worked on.

Granted there are more variables to what will work, but it sounds like you will be fine with normal pump gas.

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Re: What octane for 10.8:1 compression?

Post by Russ Pottenger » January 6th, 2018, 7:13 pm

Alexia wrote:I have a 10:1 static compression ratio in my stroker and I can run AKI 91 octane. Typically I am using 93 octane since that is what is around here. I have actually ran it on 87 octane and it was fine, but it was only to move it around while the truck was being worked on.

Granted there are more variables to what will work, but it sounds like you will be fine with normal pump gas.
Just curious.
You're able to run 91 octane with a static of 10.1:1 I'm assuming with an iron head. What are you running for a camshaft and how tight is your quench?

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Re: What octane for 10.8:1 compression?

Post by Alexia » January 6th, 2018, 10:30 pm

Russ Pottenger wrote:
Alexia wrote:I have a 10:1 static compression ratio in my stroker and I can run AKI 91 octane. Typically I am using 93 octane since that is what is around here. I have actually ran it on 87 octane and it was fine, but it was only to move it around while the truck was being worked on.

Granted there are more variables to what will work, but it sounds like you will be fine with normal pump gas.
Just curious.
You're able to run 91 octane with a static of 10.1:1 I'm assuming with an iron head. What are you running for a camshaft and how tight is your quench?
I had to look up my list and get the full numbers, but here it is.

0.060" bored over
55cc combustion chamber
Head shaved 0.020"
Deck Clearance 0.0015", shaved 0.020"
KB944 Pistons - 21cc Dish
9.94 Static Compression Ratio - Sea Level
8.87 Dynamic Compression Ratio - Sea Level
4.2L Crankshaft
4.0L Connecting Rods, Modified
MOPAR P4529229AD Camshaft - 0.440"/0.440" Lift, 248 Intake/Exhaust Duration, 108 Cam Lobe Separation

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Re: What octane for 10.8:1 compression?

Post by marcusofmagna » June 9th, 2018, 12:46 pm

The big question now is, "Why does this matter?" The bottom line is that the DCR is a major factor in whether you can run on regular, midgrade, premium, or race fuel, and not ping. Also, if the DCR is too low, you will lose bottom-end torque. The generally acknowledged safe "sweet spot" is an 8.0:1 DCR for an EFI engine, and it offers good low end and a decent high end, usually on 87-octane regular. Going below 7:1 usually costs too much low-end power. With fuel injection and a good quench dimension (read on) you can sometimes reap the benefits of going higher, perhaps up to an 8.7:1 DCR on regular or midgrade. Dynamic CR above 8.7:1 almost always requires 90- or 92-octane premium. Above 9.0:1 DCR, it's almost always ping city for pump fuel. For reference, the stock 4.0L has an 8.7:1 static ratio and a calculated 8.45:1 dynamic ratio. You can download a DCR calculator from the Internet, but the Keith Black Silvolite Piston site has a good one to play with (see sources). This is taken from an article in 4 wheeler magazine, as you can see it is the dynamic compression ratio that truly determines the octane required to run properly.

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Re: What octane for 10.8:1 compression?

Post by Cheromaniac » June 10th, 2018, 3:33 am

An aluminum head allows you to run up to a full point more compression on the same grade of gas, all else being equal.
With the combination of Eddy head, 10.8:1 CR, 68-235'4 cam, and tight quench, 90 or 91 octane gas shouldn't be a problem.
It's just a shame that CA insists on supplying its consumers catpiss for fuel (as if that's going to solve the smog problem). ;)
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Re: What octane for 10.8:1 compression?

Post by jsawduste » June 10th, 2018, 6:17 am

marcusofmagna wrote:
June 9th, 2018, 12:46 pm
The big question now is, "Why does this matter?" The bottom line is that the DCR is a major factor in whether you can run on regular, midgrade, premium, or race fuel, and not ping. Also, if the DCR is too low, you will lose bottom-end torque. The generally acknowledged safe "sweet spot" is an 8.0:1 DCR for an EFI engine, and it offers good low end and a decent high end, usually on 87-octane regular. Going below 7:1 usually costs too much low-end power. With fuel injection and a good quench dimension (read on) you can sometimes reap the benefits of going higher, perhaps up to an 8.7:1 DCR on regular or midgrade. Dynamic CR above 8.7:1 almost always requires 90- or 92-octane premium. Above 9.0:1 DCR, it's almost always ping city for pump fuel. For reference, the stock 4.0L has an 8.7:1 static ratio and a calculated 8.45:1 dynamic ratio. You can download a DCR calculator from the Internet, but the Keith Black Silvolite Piston site has a good one to play with (see sources). This is taken from an article in 4 wheeler magazine, as you can see it is the dynamic compression ratio that truly determines the octane required to run properly.
While the concept you write about is on the right track there are many other factors that influence an engines care and feeding. Taking this info verbatim may not give you the results your looking for. Piston crown and chamber design are but two examples your example doesn't touch on.

To add even more to the spectrum. The cold weather areas have different fuel blends for winter and summer. The octane rating may stay the same but not the burn.

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Re: What octane for 10.8:1 compression?

Post by BcurpKodiak » June 11th, 2018, 10:53 am

My setup I originally asked about actually changed a bit. It's a custom Lunati solid cam now and 11.3:1 static.

-Pistons are .010 above deck
-55cc chamber
-17cc dish in the piston
-.027" MLS head gasket/3.950 gasket bore
-3.895 stroke and 3.935 bore

.017" quench! :P

Started by mixing my own gas using toluene and some royal purple octane booster available locally with MMT in it. I'm making about 95 octane. I put it in the jeep before the engine change so it would be in all the lines when I went to fire up the engine for the first time. I am now able to get Torco accelerator and the 100LL avgas if I need to.

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Re: What octane for 10.8:1 compression?

Post by jsawduste » June 11th, 2018, 1:17 pm

at .017 quench I`d be more worried about knocking pistons against the head then octane.

The vapor pressure of avgas is so much different than pump fuel I`d doubt your going to get the results your looking for.

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Re: What octane for 10.8:1 compression?

Post by BcurpKodiak » June 11th, 2018, 2:25 pm

I'd be mixing avgas not using it exclusively. And it's automotive rating is something like 96, not 100. So I take that into account

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Re: What octane for 10.8:1 compression?

Post by Russ Pottenger » June 11th, 2018, 4:56 pm

Brandon,

Are you sure you have a .027 gasket and not a .043?
.010 out with a .043 gasket would be .033 piston to head and that would be perfect.

A lot of factors to consider but .026 to .028 piston to head is the absolute minimum I'd recommend. At .026 you'll start seeing the cylinder head surface marks transferring to the piston. Thats about as brave as I get.

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Re: What octane for 10.8:1 compression?

Post by marcusofmagna » June 17th, 2018, 11:17 am

Jsawduste I would agree there are going to be differences but I don't think basing fuel octane on static compression values is giving the correct information. Th DCR is what will determine the required octane level, which is affected as you say by more than just the number, but it is the correct starting point.

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Re: What octane for 10.8:1 compression?

Post by BcurpKodiak » June 17th, 2018, 6:14 pm

So far I'm running approximately 95 octane. I borescoped a couple cylinders and so far no evidence of the pistons contacting the head. I'm hoping it's a combo of the piston not being quite .010 above deck and a couple thou being added by spraying copper gasket spray on both sides of the gasket. No one usually talks about that, but I have a feeling I added .001 to .002 by spraying that stuff on it. I didn't spray between layers only on top and bottom.

I measured piston depth by running a straight edge across the piston and using feeler gauges to measure between the block deck and straight edge. Not overly scientific.

.017" quench while intentional was a mistake caused by inexperience. I had read multiple places "the tighter the quench the better" not realizing there's an obvious limit. Or maybe not realizing that limit might be as much as .026" (which after research seems common on most engines). So far it's running great and is scary in first gear. Love my Winters shifter with RAD designs shift kit in it. It's badass

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Re: What octane for 10.8:1 compression?

Post by LRSimons » June 19th, 2018, 2:47 pm

I feel bad for you guys without access to E85! Race fuel for 2 bucks a gallon.


Good luck.

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Re: What octane for 10.8:1 compression?

Post by BcurpKodiak » June 28th, 2018, 4:48 pm

So I have gotten to drive my jeep now for a bit and wheeled it. On my 5th tune revision from Ryan at FRP. I have spun it over 6K RPM multiple times.

I have borescoped my cylinders a second time with a much nicer one at work. There is no evidence that I can see where the piston is touching the head. I am not so arrogant that I think I'm the greatest engine builder or luckiest. I suspect a little thickness was added to the gasket with the copper spray, and the piston wasn't quite .010" out of the hole (perhaps my measurements were not accurate).

I'm on 94-95 octane currently and no sign of detonation. It's running very very strong and is quite intoxicating in the best way. Poor jeep now lives a life of throttle blade full open or closed and those are the only options haha

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