Ok, I'm getting ready to set the compression ratio by cutting the deck on the 430 . My question is what would the octane requirement for a 9.5:1 motor? I wanna run 89, or 91 octane most of the time. I'm going to be using the TA413 cam. As it stands, my compression is 8.2:1, so I have alot of options. Would the motor run with out a hitch with 10.0:1 on 93? I'd rather run 89 or 91, so help a guy out. Whats the max compression I could run with 91? Thanks
I just did some research on this after i found out what compression ratio i have after my rebuild. I think it's 8 3/4 which im not to happy about because my mechanic said i needed to have this ratio to run premium pump gas. Well it turns out I could have kept 10:1 and run premium just fine so im a little pissed. With the ratio i have now i am running regular without a problem. Ive been told 10:1 is probably the highest youd want to go and still be able to run premium but at 9.5:1 you might be able to get away with plus but im not certain. You could deffinitely get away with premium though. Im pretty sure but im a newb so you might want to get this confirmed.
There are many things that can affect an engines ability to effectively use a particular fuel mixture without detonation. Detonation being too much heat in the cylinder for a given fuel mixture. The most obvious thing is the "static" compression or mechanical compression ratio, which is fixed by the physical characteristics of your particular engine combo, bore, stroke, combustion chamber volume, deck hieght, piston type and head gasket thickness are key. Other things such as the valve timing events of your particular camshaft also play a part in how much cylinder pressure your engine will produce at a given rpm. In simple terms "Larger" cams that tend to move the torque numbers up into the higher rpm ranges tend to bleed off cylinder pressure. This can help to lower octane requirements. On the other hand a high static compression ratio combined with a small "stock type" cam is going to want good fuel. Aluminum Heads will also lower any given engines octane requirements as they dissipate more heat from the combustion chambers than cast iron... and polishing the combustion chambers to remove any potential hot spots can also help to lower the possibility of detonation. And of course there are the initial ignition timing, mechanical or vacuum advance curve speed, and total timing numbers to think about. So you see there can be a lot to consider especially when your trying to get optimum performance on pump gas. There are some pretty sharp Buick engine builders on this board, perhaps they can give you some idea of how your particular combo will react to pump gas. Hope this helps a little.:TU:
just up your octane till it stops knocking! air temp is a big factor also, my 11 to 1 moter will run fine on 91 if it under 70 degrees, and if it is over 90 im usually running 96+.
Mine was built w/ about 9.8 compression and I have no problems running 93 octane. And that is verified by the builder, Jim Weise. Brian
My compression is 10.13 to 1. I have run 89 fuel in mine with no pinging at all. I did totally polish my combustion chambers and the tops of the pistons to eliminate any hot spots. It takes about an hour each chamber, but worth the time and effort.
I run a stock 70 bottom end and use 93 octane without a problem. Compression was advertised at over 10, but the pistons are acutally in the hole from the factory, but still high to todays standards.
What Mr. Big said. To add to that, it's important to have a good cooling system if you're pushing the limit. Although cam timing has a lot to do with it, there is a limit on static compression that you shouldn't pass, no matter how big your cam is. Everyone I talked to about this recently said to stay at or below 10.25:1 in an iron headed motor. I've heard in an aluminum headed motor that you can get away with nearly 11:1.
Timing is the key. I can run a 12-1 motor on pump gas with the timing is retarded. Or rattle the pistons out of an 8.5-1 motor with too much timing. I would recommend lower comp on the street and adjust the timing just to where it pings and back it off a bit. We had a 71 GS conv with 455 running 12.00 with just headers, intake, Thermoquad, 113 cam, converter......with 8.5 comp (heads never off, sm valve).....timing was optimized for race gas but back it off and it was used as a tow vehicle to the GS Nats twice....and it's still alive. (used to belong to Andy Livermore) Bruce
I'm at 3500 feet so I run a little more compression to make up for the fact our air pressure is low.....I have run a static 9.8 to 1 on regular 89 octane with no problems (288-94H cam). later Tim
