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Highest compression ratio for 91 octane?

Castiel

Castiel

Golden Member
Friend is considering building a N/A high compression 396 for his Chevelle SS. Whats the safest compression for 91 octane? Would 12:1 be pushing it?

We have 93 octane at a lot of the pumps here but he just wants to make sure in case he gets stuck with 91 octane at some random pump
 
Compression ratio isn't the be all, end all for determining the required octane. Combustion chamber design plays a big role, too.

You would have to ask people that are familiar with the engine in question.
 
Max safe comp ratio with 91 octane would about 10 to 1 but with a BBC it might be a bit lower around 9.5 to 1 as the iron heads on a big block are not all that good in chamber design... With alum heads you could push to 10.5 to 1 and be ok but control of engine temps and timing make a big difference with alum or iron... Detonation will occur and you cannot hear it at times which is not good... BTW 12 to 1 with pump gas is asking for severe damage in a short time...

About 4 years ago I ran an iron headed 427 BBC with 10.5 to 1 on pump gas (93 octane Exxon was the only pump gas that would work)... Had to back the timing off to 8 degees BTC and had a Griffing alum racing radiator and enough electric fan power to basicly pull it down the highway by themselves... Worked quite well but it was a weekend toy and was ran on 100LL AVgas with additives on the track... The truck was a 1969 C10 and ran mid 12`s so I was quite happy with it...
 
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Modern engines are pushing 11:1 with 91. Timing plays a factor to, compression ratio doesn't exist in a vacuum, many concurrent parameters to factor.
 
Cam timing plays into it as well. A cam with monster duration will bleed off some of the compression and allow you to run a higher compression ratio. Bore size also plays a role. The bigger the bore the further the flame front has to travel from the spark plug and the higher the chance for detonation. Quench area is important too.

If your buddy is sticking with iron-heads and a carb with a medium-sized cam (somewhere in the 235-245 degree range @.050), I'd stay in the 9.5:1 range to be safe.
 
exdeath said:
Modern engines are pushing 11:1 with 91. Timing plays a factor to, compression ratio doesn't exist in a vacuum, many concurrent parameters to factor.
Click to expand...

modern 4 cylinders are pushing 11:1 with 87 octane (well, GM ecotec LAF 2.4L is 11.2:1)
 
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As others have mentioned, the engine management system plays a MASSIVE role in the ability to avoid knock. Is he going FI or Carbd?
 
RU482 said:
modern 4 cylinders are pushing 11:1 with 87 octane (well, GM ecotec LAF 2.4L is 11.2:1)
Click to expand...

That's because of direct injection, not because it's 4 cylinders. Direct injection pretty much bypasses the whole detonation problem as there isn't a bomb preloaded into the cylinders during the compression stroke, just air.
 
As many others have posted it depends on alot more than just compression ratio, but if you're running all iron and carburated I would say 10.5-11 is about the limit, I've run my 327's up to ~12.5 but up there you definately need 110 octane, or some form of liquid lead additative. Right now I'm 'suffering' with ~400 horses and 9.5. Not perfect, but it's not overheating anymore.
 
exdeath said:
That's because of direct injection, not because it's 4 cylinders. Direct injection pretty much bypasses the whole detonation problem as there isn't a bomb preloaded into the cylinders during the compression stroke, just air.
Click to expand...
So basically it works like a diesel?

I think I started a thread about that before. I asked why we don't use gasoline like diesel then change gas engines to ridiculously high compression ratios. The answer was basically the same as why we don't all use diesel engines - cost of building such an engine is not worth doing in most cases. An engine that can handle 15x or 20x compression is more expensive than just building a bigger engine.
 
I'm pushing 12.3:1 compression on 87 octane, with port injection Ford Fusion Hybrid.

But correct me if I'm wrong but I believe that is because of my car running the Atkinson cycle.
 
My echo is running 10.5:1, runs on 87.

What matters is the dynamic compression, not so much the static compression ratio.

ShawnD1, basically like a diesel, except it's still the spark plug that ignites the mixture, not the compression.
 
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