Hi all! I've been out of the A Body Buick game for a long time (currently have a 87 GN and 59 LeSabre), but just got a 71 Skylark for a father-son project. The car is fairly solid for a long time DD. It has 180k miles on it and supposedly had a valve job at 110k. It has 2.73 gears. The block is supposed to be an original low compression 2bbl motor that they put a 4bbl on at some point. The thing is that this motor runs really strong in my opinion given all these factors. I've had a 70 and a 68 that had 4bbl 350's for a short time before I swapped in my BBB, and I don't recall them being anything that impressed me and one of them seemed to run good. This car will light up the tire instantly and do it all the way through 2nd. This throttle response surprises me. So, I cleaned up and looked on the exposed deck of the motor between 1 and 3, 5 and 7, and nothing. If this is the original motor, maybe it was rebuilt and decked. Any other place I can look? Going to do a compression check this weekend. Is there any way by looking by the numbers to see if there is something indicative of it having higher compression than stock? Thanks!
Compression test and a harbor freight borescope will tell you a lot. My 8:1 derpy '76 350 was nasty for what it was. Burnouts at will and sounded like it had double the hp. Bone stock, .030 with a comp 268 and a reworked 800cfm qjet.
Compression tests will not tell much other than if all the cylinders are able to achieve similar maximum pressure. When the engine is running, the piston has only one chance to make pressure, when you do a compression test, you let the piston pump up to 5 times. My 350 is 11.1 to 1 ratio, my compression pressure is 185 lbs. with the roller cam that's in it. If I changed to a smaller cam Im sure Id be over 200 lbs. compression pressure, BUT my compression RATIO did not change.
Yeah I realized that there are too many variables on the compression test to determine the ratio. If compression is consistently above 160 across the board, it will raise an eyebrow. I'll have to get a scope and look at a piston. I need to pull the valve covers and ID the heads too. Can anyone tell me if I'm correct about production code placement? If it is supposed to be on the deck surface where I'm looking, then the block is indeed decked.
How about using the partial VIN to establish the model year car in which the engine originated? Check the front of the engine just below where the cylinder head mates.
I dug a bit more into the deck surface and finally got something to come through, but not much. There is a 282 stamped there. I'll have to remove the power steering pump to get to the serial number on the front of the block. Headcasting number are 1237650. Looks like typical 70-72 heads. I guess all 350 heads are 58 cc combustion chambers?
55-58, but that's mainly production variance. Slightly better flowing than mid 70s heads if I remember right.
Is it just the pistons that makes the motor low or higher compression? any casting numbers that are better than others? Guess it doesn't make a huge difference now that TA has aluminum heads out. I guess if I really wanted performance, I'd head over there and score a set of those giving the budget of course haha.
Pistons change compression ratio. For the most part 58 cc is chamber size. Maybe they did some work on it. But a good tune on an 8.5 makes some run really good. Had 3 77 regals. On had a 400 trans, it would smoke the tires . The other 2 drove well but never had the same power.
All Buick 350 pistons were dished. 68-70 used shallow dish (maybe the 10.25:1 used even shallower dish, the dish in mine were pretty shallow) and the mid 70s to 80 used massive nearly 1/3" deep dish. And the decks were apparently taller. Or shorter pistons.
Found a picture of the stock pistons. Now just need to get a scope and look in the bore to see what mine look like.
I got a buddy who's going to let me borrow a scope tomorrow. Checked compression dry. All cylinders are between 145-150 PSI. Most cylinders hit over 120 on the first stroke. A couple we're not too far behind on that. It seems pretty solid.
That pic is from a 71 motor that I found as a reference. I'll get a chance to check out my bores tomorrow. Not that I know what the low comp pistons are supposed to look like, I'll be able to identify them.
Thanks for that! I figured the SP Pistons would look just like The shallow dish in a Buick big block. Looks like that is true. I'll report back tonight once I get a scope in the cylinders. At this point I think my compression test numbers probably show low compression pistons with lots of carbon on them haha.
Not necessarily, depends on the amount of overlap the cam has, the more overlap the less your cranking PSI will be.
Not sure what to think. It almost looks like they are SP pistons. I thought the dish would be obvious if they were low compression. Thoughts? Sorry the pictures are not very good. The scope I borrowed has no SD card and I don't have a spare.
