Need some advice on determining (or at least reasonably estimating) compression ratio in my engine. Here's the facts: It's a 1965 model 289 with factory heads, block has been bored .040 over, and it has aftermarket 10.5:1 comp. pistons. According to the guy who built the engine (2 owners back), he didn't mill the heads any, just "resurfaced" them to, as he said, "clean them up". Cam is a Crane and the builder can't remember exactly the specs...but said he "seems to recall" that it was around .508 lift/292 duration (couldn't find one matching this in the Crane catalog, though!). Anyway...what I'm not sure of is how much the overbore has changed the comp. ratio. Any ideas out there on what the actual CR of this engine might be?

 

The overbore won't change the compression ratio in any significant way. Also, most aftermarket piston manufacturers will actually cast the piston with a somewhat lower deck in order to offset the compression differences from the overbore. Since we don't know what the re-surfacing "cleanup" did to the heads, it is not possible to accurately predict actual compression; however, my calculations would indicate around 10.1 or so.
Best,
Al

 

Thanks, Al. Sounds like a "reasonable" guestimate. One more question...here in Puerto Rico, the best gas we can get is 91 octane. What's likelihood of detonation issues with this combo? Maybe I should always run some octane booster mixed in?

 

IMHO, with iron heads, the optimum ratio for your pump gas should be around 9.2+- to 1.0. Anything over that ratio will most likely cause detonation. Timing can be retarded some to prevent too much detonation, but you will then lose the power you were going for with the high ratio.
Best,
Al

 

Thanks, Al....well, there's not much I can do about it now, and don't have the $$ to switch to aluminum heads or change pistons (at least not for a good long while). So...any thoughts on the utility of running some kind of octane booster to help? And if so...any recommendations on brands,etc. There's a bunch of them out there...but I have no idea as to which ones (if any) actually do what they claim.

 

I don't personally have any experience with octane boosters. In my world, they fall into the category of snake oil. That doesn't mean necessarily that they don't work, but I just don't have any comfort zone with them. Can't hurt to try, seeing as if you are a bit backed into a corner. Sorry
Best,
Al

 

yeah, I tend to be skeptical of such products, as well..hence my question. I have just learned though, that there's a new Sunoco gas station in San Juan (about 20 miles away) that sells racing gas (100 octane) straight from the pumps. Will definitely check it out, although I'll actually have to burn a bunch of it just getting back and forth, with all the heavy traffic and all. But hey...if it works...! Then...there's the price...I'm SURE it ain't cheap (well..No gas is cheap anymore!).

 

Hi,
Since every engine is different, your best avenue is to test different timing settings to determine how much is too much. Also, determine if your dist advance is operating as designed. There are too many variables to hazard a close guess, what will work for you and what ever octane is used.
Also, test different dist vac canister settings (if your is adjustable) this will opimize your mid-range performance. Keep track of your adjustments, especially the base-line.
Good Luck!

 

10:1 is fine, dont get excited. The 289 K code was advertized at 10.5:1.

Also. octane boosters work. But you may not need it, do you hear a lot of spark knock? Sometimes you can just adjust the advance curve or overall total advance to deal with that.

Also you could mix in some E85 if you have the rest of the car set up to handle it. Or you could run straight E85. You can do things like get a stainless tank and gas line, and an electric fuel pump and a carb that is OK for it, you will get much more octane with it than you do with premium gasoline.

I am starting a 289 build soon with a goal of 10:1 at a minimum and I have no concerns.

 

Thanks to both of y'all for the comments and suggestions. I've been fiddling around a little with timing, but since there's no timing pointer, I've used a manifold pressure guage to get some idea of whether it was on the "high"or "low" side of the range. At first, it was pretty low (and overheating real quick) and only pulling about 7" of mercury. I adjusted it up a little until it was reading around 17-18". Haven't run it on the road since that change, due to the ignition switch going out right after that (just now replaced), and the fact that I'm also replacing the water pump/radiator this weekend. The distributor is a stock Duraspark off a '85 Granada. I'd like to order a new custom Duraspark unit from Performance Distributors in Memphis, TN., but since I don't know my cam specs (former owner said it's a Crane but lost the paperwork on it and "thinks" it is .508/292), I can't give them enough info to properly curve it.