Well, after collecting parts and doing all the compression ratio calculations, My old engine was at 7.8:1 with flycut heads and bus deep dish pistons on a 74mm crank. I'm ready to get some more umph out of this engine, and figured while it was apart, my as well get some more compression out of this motor.

This begs the question... what is the maximum compression ratio you can run on 87 octane (cheap gas) with carbs? I'm using this as my daily driver and cannot afford to be putting premium or even midgrade in my car when it's constantly being driven around on a poor college kid's budget.

Thanks, Ethan

i would think 8.5 is safe on a stock cam.

The cam is not stock. Its an almost identical grind to the webcam 86 from Schneider cams here in san diego

10.5:1 can be run on a watercooled with aluminum heads. A lot depends on timing too. 87 octane has more combustion energy than 91octane. 91 octane helps to reduce preignition. if you install a modern EFI system, the computer does all the timing for you and reduces pinging. Most modern cars that say "must use premium gas" is all BS devised by the gas companies and vehicle makers. I will run any car on 87 and not have any problems. The exception is a high comp race engine.

for 87 octane in a 914 I would not go higher then 8.
By 9 you need premium or you will ping - which is bad bad bad.
Even at 8 you MAY need mid grade.

Zach

10.5 on modern engines is due to advances in combustion chamber design and computer controlled fuel and ignition management. there is no way a 1970 engine will with stand that kind of compression.

and yes the modern high compression, high octane engines will run on 87 but will do so at a reduced power and efficiency. running lower octane fuel will cause the ecm to dial back timing and richen the fuel mix trying to keep preigition from happening.

running high octane will not improve the profomance a engine designed to run on 87 fuel, it will likely hurt proformance.

I'm no engine expert
but I've had to use race fuel over 9.0

so your probably pretty much there for low grade pump gas with your 7.8 you already have.

at higher altitude you might go to 8.0 maybe...
but with the quality of gas that is out there these days you may want to leave it alone.

brant

I used 93 pump for 9.7:1, 110 for 10.5.....apparently I'm not as smart as Mike.

A lot would depend on that cam you have (timint events), and how it affects your dynamic compression ratio. Not an authority on type 4, so I'll defer a recommendation to the experts.

Andys

L-O-O-ng thread on this subjuct on the Pelican 911 forum

general consensus in the racing industry is that 8.5 is max for our aircooled engines, and 9.0 for iron motors on 91 octane

yup - some folks run higher - and some folks hole pistons

cam overlap & duration make a big difference - re: prior comment about dynamic CR - but even if ya have a wild cam NEVER "lug" the motor or you will sure kill a piston - ya maybe will hear it ping or detonate just b4 that, but maybe no -

eroded edges of the piston top are the first consquence - and you will usually not know it is going on until enuf of that aluminum decides to melt down onto the sidewall and then the engine will just seize - or break big-time if ya are going real fast

was that on your 6 ?
different combustion chamber shape?

Yes, the 6. No, standard shape, single plug. I did carefully set the timing to 35 total. After 7 years those 9.7 pistons looked nearly brand new. You seem a bit conserative, but there is nothing wrong with that.
I never had a problem with 87 on the stock 2.0L 4.

Yes, the 6. No, standard shape, single plug. I did carefully set the timing to 35 total. After 7 years those 9.7 pistons looked nearly brand new. You seem a bit conserative, but there is nothing wrong with that.
I never had a problem with 87 on the stock 2.0L 4.

There are (experimental) engines out there running 14:1 on 87 octane fuel. They are obviously not 914 engines.

The cam grind, combustion chamber shape, bore size, location and number of spark plugs, altitude, boost level (if any), ignition timing, mixture control, and many more things determine if an engine pings on a given fuel. Not just compression ratio.

Two data points for you: The stock 2.0 engines were spec'ed for regular-grade fuel. (91 RON, 87 AKI which is what the US pumps are labeled in.) The European-spec 2.0 914 engines were spec'ed for mid-grade fuel (I think it was 94 RON, 89-91 AKI). The only real difference was that the US-spec engine was 7.6:1 and the European-spec engine ran 8.0:1 compression. Obviously the cam, chamber shape, bore, and so on were all stock.

I know of people who have run the European pistons and used regular unleaded without apparent problems in the short term. It is possible that the factory left generous safety margins on their fuel recommendations, or it is possible that they got lucky.

--DD

there are gas direct injected [fuel injected directly into the combustion chamber] that are doing this. very high tech tuning here. power and economy/ emissions are outstanding and unprecedented.

Dave hit many points that must be considered. comp alone is not enough to make such statements. there are details that can be done to enable higher comp with 87 fuel. this level of hand work detail is not financially viable in a production engine. cam profile, port velocity, chamber shape , piston design/shape, ring gap, chamber size, valve size, shape, deck height, quinch...
guys that know dont give this stuff out on teh internet for free but their engines can do the talkin.