I know the answer will rely on much more than just compression, but ballpark it here. How high of compression can I use with super unleaded pump gas? Or, when I rebuild my engine, and I want the highest compression that is "feasible", and still fill up at Wal-Mart, what compression should I shoot for for a mostly streetable and sometimes strip-able motor.
compression i run a 351 punched to 357 w/ edelbrock performer rpm's that are milled. psrt and polished intake to match heads, with about .570 lift cam. its pretty nasty , and i run 93 octane ant sunoco while cruising, and at the track i run 100 octane. havnt had any problems. just drove a total of 300 miles to and from a show including cruising the streets in ocean city, md. the car performed incredibly. by the way i pushing 11.7 to 1 compression. its possible and reliable!
My old Mustang 302 was a good example. It used SRP flat top forged pistons that were pretty light...around 450g if memory serves me right (they WERE in a 393 that I grenaded, bought 2 more pistons to replace the 2 I broke) and made about 10:1 compression with 58cc heads, which were '75 351w heads with 1.94/1.50 valves. It was a cheap build....for the most part, but the motor never spark pinged much if at all on cheapo 87 gas. It was also zero decked....which makes a difference, and the camshaft was a Erson 228 218 (yes...shorter exhaust duration) .514 lift hyd. roller on a 115 LSA. Yep...an old turbo 302 grind I had left over from the turbo maverick. Worked great! Good power from idle to about 6300 where the hyd lifters signed off. You can go as high as 10.5 if you zero deck the block, use GOOD head gaskets, make sure the chambers are deburred and remove ALL sharp edges in the chambers and piston tops. It don't take much to cause detonation when you're on the ragged edge! Of course you already know that the cam timing will have more to do with it than any other variables. You "could" use 12:1 compression and a monster cam and it wouldn't detonate....but it wouldn't make much power down low either
Great info, guys. So it looks like around 11 to 11.5 on the max end? To be safe. Steel vs Alum heads...why the difference? Do aluminums stay cooler or something, keeping detonation down?
aluminum: one reason you can run higher compression is due to aluminum's greater heat dissipation rate. The other reason is because most if not all aluminum heads have MUCH better chamber shape, quench, and improved spark plug location....combine the 3 and you see what I mean.
As touched on already, there are lots of variables: Aluminum vs iron. Aluminum can handle 1 point more because it doesn't hold as much heat. Polished chambers vs stock chambers. Stock chambers have bumps and edges that focus heat that detonates fuel. Polished pistons vs non-polished. Same here... smooth surfaces and rounded edges don't cause hot spots. Chamber shape. Open chambers don't manage the flame front well, swirl chambers and quench chambers work differently, but both tend to eliminate focused heat. Cam overlap. There are cams that are designed to build cylinder pressure, then there are cams that bleed off pressure at low rpms. The LCA of the cam plays a large role here too. Low LCA will bleed off cylinder pressure allowing a higher compression ratio than a high LCA. Detonation is most accute when an engine makes high power at lower rpms. If you bleed off some pressure at low rpms, then let it build as the rpm rises, you can use more compression on the same fuel. If detonation is a problem at high rpms, it is much easier to tune away by carb jetting and such. There are other factors too, but these are the biggies. To just ask how much compression on 93 octane is like asking 'how far away is that' without giving your point A and point B. Oh, ignition timing plays a big role too, as does spark plug heat range. You can detonate like crazy with 8.5:1 compression with too hot a plug, just like you can use a colder plug to stop detonation when you have a touch too much compression. Now that reminds me of the cooling system too... mediocre cooling system, low tolerance for compression, top of the line cooling allows more. Theoretically you could run 13:1 compression on the street with 93 octane if you build your engine for it. Dave
cool. just the kind of information i need. Why is it I cannot get my 74 to ping? I can advance the timing until it hits the radiator hose, regular unleaded, hot plugs, etc. Does living at very close to sea level affect it somehow? I am running hot and lean, but still no detonation. Is the compression so low that I am not able to get the pressures up high enough even with all the other variables present?
The guy that did all of my machine work built a motor for his son to go in a '90 Ford Ranger. 302 block, late 70's 302 heads with lots of port work, stock valves, stock valvetrain, (originally the heads were built to go on a stock circle track car), milled to yield 11.5:1 CR with flat top pistons. It had a solid lifter cam with lift in the high .500's. He ran a Mallory Unilite mechanical advance with a hot coil and Digital multi-spark box. It ran on 93 octane and ran like a top. Talk about a burnout machine. Imagine that light in the butt truck with that motor and a TL 4 spd. So, how high can the compression be, maybe it depends. Seth
i run 9.5:1 flat top pistons (hypers) and i run 91 octane around town and 111 (purple gas) at the track....it runs awesome on the 111 octane.... but budget wise...it is only used at the track...
Mine is 74 with stock bottom end, and I think they dropped it down to 8.0:1 that year. I have heard as low as 7.5:1, but I think the 8 is correct. I figured that is why I can't ping.
Mine is 11 1/2:1 compression and it still doesnt like 92 octane gas....sure does like the race gas???
