347
12-11-2010, 05:00 AM
#11
6th Gear Member
Join Date: Apr 2007
Location: California
Posts: 10,468
You can get a 12:1 engine to run without detonation, and have a 9:1 engine ping. It's all about cam timing and how much the cylinder fills. The more efficiently you fill a cylinder, the more likely it will ping. A high compression ratio with a cam that bleeds cylinder pressure off can be safer than low compression with a cam that packs the snot out of the cylinder. Heads, intake, exhaust, they all factor in too. As does load, lighter vehicles with more gear are less prone to detonation. However, a setup that bleeds cylinder pressure off is giving up power that it could otherwise make.
That's why it's hard to say whether any given compression ratio will be safe or not. At least with a custom grind the cam is setup for the particular combination, whether it's high or low compression.
Compression ratio isn't nearly as important as many people think. Keep in mind that before the rules changes, NASCAR Busch engines were 348cid n/a engines limited to 9.5:1 compression and premium pump fuel...and were producing over 500hp with a 6,500rpm limit.
You're better off running the minimal compression you need and packing the cylinders as much as possible by optimizing everything else, it'll make more power that way. Barring that, you need to factor everything in and build a setup that will run on the fuel you need it to. Will 10.5:1 work on pump gas? On some setups, yes, and on others, no.
Here's a more comprehensive, but not completely inclusive list of things that will effect whether an engine detonates or not. Compression, cam, heads, port size/shape, intake, exhaust, piston type, rod ratio, chamber design, valve angles, valve placement, fuel mixture quality, transmission gearing, differential gearing, tire size, climate, vehicle weight, rpm range, transmission type(manual or auto), fuel, driving style......
If you're not sure then go with a known combination. 347 with the Comp XE282 on 10.5:1 with rpm intake, plus all the other crap is one reasonably known setup. Still, the best bet is to match all the parts for what you need, get a good flat top or dish piston to put the compression where you need it for the rpm range, and get a custom cam and not have to worry.
That's why it's hard to say whether any given compression ratio will be safe or not. At least with a custom grind the cam is setup for the particular combination, whether it's high or low compression.
Compression ratio isn't nearly as important as many people think. Keep in mind that before the rules changes, NASCAR Busch engines were 348cid n/a engines limited to 9.5:1 compression and premium pump fuel...and were producing over 500hp with a 6,500rpm limit.
You're better off running the minimal compression you need and packing the cylinders as much as possible by optimizing everything else, it'll make more power that way. Barring that, you need to factor everything in and build a setup that will run on the fuel you need it to. Will 10.5:1 work on pump gas? On some setups, yes, and on others, no.
Here's a more comprehensive, but not completely inclusive list of things that will effect whether an engine detonates or not. Compression, cam, heads, port size/shape, intake, exhaust, piston type, rod ratio, chamber design, valve angles, valve placement, fuel mixture quality, transmission gearing, differential gearing, tire size, climate, vehicle weight, rpm range, transmission type(manual or auto), fuel, driving style......
If you're not sure then go with a known combination. 347 with the Comp XE282 on 10.5:1 with rpm intake, plus all the other crap is one reasonably known setup. Still, the best bet is to match all the parts for what you need, get a good flat top or dish piston to put the compression where you need it for the rpm range, and get a custom cam and not have to worry.
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