What is the highest compression ratio you can go and still run pump gas (92-93 octane) without detonation?

 

That will depend on a number of factors, including:
-combustion temps
-quench volume
-chamber surface & shape
-charge swirl
-dynamic CR (ie VE & valve overlap)

 

A lot of factors come into play and Dynamic CR is a huge factor. With a proper set up and a big enough cam you can go prety high. A LT1 can go higher then most because of the reverse cooling. While I would not sugest it, my Tuner is running 13.5:1 on pump gas (93 octane) with his daily driver LT1.

 

i'm at 10.2:1, and easily run 91 octane with almost zero knock counts, per scans done on tts datamaster engine analyzer. i've run 89 octane with no audible pinging, although i haven't yet hooked up the scanner.
i expect very little knock counts with 89. this is an L98 setup, which is coventional cooling.

 

I was considering bumping up my compression. just wanted to get an idea as to how high i could go without having to add any blue with it.
I c that the new vettes are at 10.9. Was wondering if 11.5 would be too much or maybe 12.
13.5 sounds pretty high, wouldn't want to push the envelope too close.

 

I dont think I would push it past 10.5-10.75 on the GEN I SBC.

 

If the entire engine is desinged and built with high cmpression in mind you can get really wild and run some amazingly high compression numbers on 92-96 octane. What I've found to be the biggest key is camshaft grind and ignition system. With having the time to do some extensive dyno testing, what I've found is anything above 11.5-1 is getting into the range of true "streetability". I just finished a 357ci aluminum headed, flat tappet small block with 13.8-1 compression. The camshaft was custom gound by comp cams with consideration taken for possible use with nitrous.
The engine is in a 1970 nova backed by a turbo 400, 4200rpm stall convertor, and runs 4.11 gears with 28.5 tall tires. The guy came to me wanting to run low 10's naturally and be able to add a little spray and tip into the 9's. We used msd crank trigger with a msd 7 series control box. On the dyno we made the most hp/tq at 37* total advance, but we had to dial it back to 35* total to eliminate any detonation we had running 92 octane. One the dyno, we lost 14hp, 5lb/ft tq at the peak when we backed the timing down from 37 to 35. Considering it made 621hp and 557lb/ft tq we didn't feel this was an issue having to give up so little to be able to rid the detination. The car has gone a best of 9.863@ 146.89mph on 92 octane, without spray in full street trim weighing in at 3470 lbs with driver. The cars is not an easy street driver, but with the high stall speed and ignoring the sound and vibrations it makes, is driven every day to work as long as it is not raining. He drove from Holland Mi. to Morris Il., 212 miles one way, and managed 8.4 mpg at 3800 rpm on the highway.
I've built and run engines with as high as 15-1 compression, but have found that unless you can run exotic fuels, there is absolutely not need for that much compression. I recomend that for true streetability one should keep to 11.5-1 as a max. It all depends on what you want it to run like and if you have patience for street driving a race car!!!

 

I am running about 10.7 /1 on 95 octane ( which is the lowest we have here in the UK ) without any knock counts, see sig below for engine speck....






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A few years back Hot Rod magazine had an article about a 350 running over 11:1 compression. This was not the average 350. It was made from a 400 block bored .030, the crank that was installed was a short 307 crank if I remember right it has a 3.25" stroke. The rods that were used in the engine was from a Ford 300 straight six. These rods were 6.25" or more. Making for an excellent rod to stroke ratio. With such a long rod, the piston stays at TDC for a considerably longer time period than that of a stock 350.

This engine also used a set of 195 CNC ported AFR heads with 58cc chambers. Coupled with a flat top piston and the deck at zero in relation to the piston top, this combo yielded 11:1+ compression. Yet with this combination this engine made over 425hp and 400+ ft/lbs of torque, at well below 5500 RPM. Not to mention that Hot Rod abused the engine by burning pump regular in the engine just trying to make it detonate, all of which was unsuccessful because even at such a high compresison the engine failed to detonate. The cam that was used in this engine was fairly mild. I can't remember the specs on this engine but someone here may have remembered the article. It appeared about 5 or 6 years ago. I have a copy of it somewhere in my archives. Some shop in So. Cal was actually building these engines and selling them. Cost was around $6500 or there abouts. This engine would make a great engine in a C4 and it passed Cal smog sniff tests easily, even with the cam that was in it.