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Oct 15, 2012, 09:03 PM
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Yellow Baron's Avatar

Compression question.

I asked this question on the BMW forum site I joined about a year ago but haven't really gotten a response, so I'm asking here.

What is the highest compression I can get on regular 87 AKI gasoline without knock sensors or any fancy equipment?

I'm talking about a single overhead cam, 12 valve, straight 6 on carburettors. My engine right now is 8:1 and I'm thinking of doing an engine build (hopefully starting over winter) to get more out of another engine of the same type, but slightly more displacement and 9:1 compression.
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Jan 15, 2013, 12:45 AM
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nemoskull's Avatar
i assume this is american 87 octane?

i really dont know, but alot of cars run 9:1 stock on 87, but there is more to it than that.
the design if the heads will affect the octane of the fuel, as will timing.

there was a turbo car that ran 91, but could run 87 and the computer would retard the engine.

really you would need to know more about he car and see what others are running.

9:1 dont seem really high. my stock 78 nissan truck ran 8.5:1 stock.
are you shaving the head, or replacing pistions? i always worry when its the head. timing is off, as are head bolts, things that may or may not matter. but you permenantly changing the motor.

fast, reliable, cheap.
pick two.

automotive words to live by.
Jan 15, 2013, 12:47 AM
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nemoskull's Avatar
dont forget most older cars had better heads.
my 86 suzuki has true hemi head. bad for emissions, good for performance. the later models do not.
Jan 15, 2013, 06:28 PM
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Yellow Baron's Avatar
Yeah, US 87octane. I want to swap my '86 BMW 635csi's M30b34 (8:1 comp.) engine for a M30b35 (9:1comp) engine (0.1L more, and another 30hp).

I'm going to leave it fuel injected and run a supercharger; but I was also going to bore the engine out some and was wondering if more compression was possible with out anti-knock sensors and all the modern gadgetry.
Jan 15, 2013, 06:44 PM
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nemoskull's Avatar
its possible. but the EFI system may need to be tweaked if you adding 30 hp.
wait, are you swaping engines, or modding the engine?

if your swaping, check the OEM recomendations.
if your modding the engine, then you pretty much on your own. anti knock sensors just listen and retartd timing. there is always mean of cooling the intake charge, that helps. intercoolers and water or methanol injection.
some run water injection to run advanced timing and avoid knock.

but ive really never mess with compression much.
Jan 22, 2013, 05:27 AM
Registered User
You can run alot more compression than what is ideal for most peoples usage. You have to look at it as a whole. Really depends on the rest of the engine, and what you want out of it. If you push the compression towards the high end, everything else must work perfectly or the engine is history. Also tends to cause lots of undesirable results for a street car. Like burning lots of fuel, breaking drivetrain parts, and quickly balding tires.
I never would want to be the one experimenting, unless I had lots of money. Best to get advice from someone that has built lots of those engines. And, be sure to let them know exactly what you want out of it.
Jan 23, 2013, 12:53 AM
The wind caused that!!
hoghead5150's Avatar
Originally Posted by Yellow Baron
Yeah, US 87octane. I want to swap my '86 BMW 635csi's M30b34 (8:1 comp.) engine for a M30b35 (9:1comp) engine (0.1L more, and another 30hp).

I'm going to leave it fuel injected and run a supercharger; but I was also going to bore the engine out some and was wondering if more compression was possible with out anti-knock sensors and all the modern gadgetry.

you need to search around a little about compression ratios. from what you say here, your are going to supercharge the motor. well doing that you really gotta watch your static compression ratio. for reference, there is static compression ratio, and there is dynamic compression ratio. these are alot different.

for a natural aspriated engine, you will need to know your deck height, the CC of your head, your cam specs including lift, duration, and overlap. with this you could figure out your compression ratios both static and dynamic.

once you dive into forced induction tho, you are really going to have to watch it. i would think 9:1 is going to be pushing the limits with the supercharger unless your going to run some very mild boost.
May 11, 2013, 10:43 AM
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builderdude's Avatar
Hotrodding is an old hobby of mine that I used to do back when things weren't quite so expensive as everything has gotten now. There are a lot of factors that go into octane tolerance, and some things you can do about it.

For one thing, what's your altitude? When I was a mile high in Denver's altitude, I remember I could run my engines at 10:1 compression with iron heads, as long as I used premium gas. Now that I'm at 4000 ft, during the warmer days I have just a little bit of pinging going on with the 10:1 engine I still have (a 283 in a 1952 GMC pickup). On warmer days, a few gallons of Av Gas mixed with premium do the trick. In colder weather, it's okay with midgrade. So I can tell that 10:1 is right on the edge of possible.

I would suggest you go 9:1 on iron heads. You can push it to 10:1 if your altitude is high enough, but I don't recommend it. And increasing compression suffers from the law of diminishing returns also. There is an increase in power going to 10:1, but you can get that same power increase using safer methods, such as improved heads, or better machining in the heads, or improved carburetion, and freer flowing exhaust.

One other thing I have experimented is with ceramic coatings. I have a 630 hp 427 race motor in a 1968 Corvette. The combustion chambers, piston crowns, valves, and the exhaust ports are all ceramic coated. I tried putting 91 octane premium in the tank once to test it, and the ceramic didn't prevent it from knocking. I had to put race gas in it to bring the octane back up. I have 12.2:1 compression in that engine. The pistons were supposed to be 435 horse 11.5:1 pistons, and that might have worked, barely, to allow lower octane. However, what I didn't count on was that the ceramic coating has some thickness, and when I cc'd the cylinder heads, I found that it was actually at 12.2 after doing the ceramic coating, defeating the purpose.
So now I just run pure Av-gas in that motor and it does very well on that.
Anyway, supposedly, ceramic coatings are supposed to make an engine more octane tolerant too.
Apr 04, 2016, 09:01 PM
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Mopar 512's Avatar
Could be as high as 9:1 with well designed cylinder head and reverse flow engine cooling. The head would need good squish characteristics.

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