Also, I noticed the other day that Summit now sells a 'medium' compression cast piston. I haven't calculated anything other than dish size, which is 12.77cc. Should come to about 9:1 on an unmachined block and heads. EDIT: just calculated it. With a .030 overbore, .070 below deck, and 58cc heads with .040 head gasket, it comes to 8.94:1. It's a 'v6' piston, but comes in a set of 6 and can be ordered individually. Comes in overbores up to .060, and has a 1.805 CH, which means it'll sit .070 below deck with a 10.187 DH. Shaving .010 off the heads and .010 off the deck will net you around 9.44:1 with a .060 overbore, just right for a 58* IVC camshaft like the Crower level 2, while permitting the use of the stock pushrods and no milling of the intake. Now we have another choice for an off the shelf direct fit piston that won't require tons of machining to get 9.5:1 compression. Gary
Thanks Gary. I didn't order yet just in case someone posted on this thread. I've decided to leave my bores stock so the CR will be a bit lower than 9.44. These medium compression pistons may be just right for a mid grade gas. I had talked myself into the 10.25 pistons. Now I'll be thinking about this all weekend. Ponch, As for the head porting, is there any data that would prove your theory that ported heads in an otherwise stock engine will net better MPG? I'm thinking the stock ports are large enough and flow enough for low rpm mileage type service and I won't see a gain from porting and maybe see a loss. I can see where getting rid of major casting flaws could be a benefit.
I am seeing a few blocks with pistons down hole .090 and felpro head gaskets are .043 crushed. .note for compression calculations
Yeah I just used the default 10.187 deck height. I've heard it's typically taller than this though. Also, the dish in the piston is spherical, so the downward slope into the dish is nice and curved...which is good, but it puts the calculations off when measured strictly from diameter to depth, like I did. It came to 12.77cc with that, but in reality is probably closer to 10cc. It has a 1.805 compression height though, which will put it into the hole a ways (.070 according to the deck height previously mentioned). You'll have to measure everything to get a more accurate reading for compression, but with those 'medium' pistons, you have some head surface and deck height to play with, and won't have to shave off a huge chunk to zero the deck like you would with the low comp deep dish pistons--or end up with comp too high with the deck and heads resurfaced with the 'high' comp pistons. Gary ---------- Post added at 05:23 PM ---------- Previous post was at 05:20 PM ---------- I would just clean up the heads and not worry about taking out too much material. That should net you the best mileage you can get with max velocity. Gary ---------- Post added at 05:29 PM ---------- Previous post was at 05:23 PM ---------- I've read elsewhere that it was .043 also. Good to know. As long as your dynamic compression is LESS than 8:1, you should be fine with premium gas, or around 6.5:1 for regular gas. You can be off half a point below those numbers and still be good to go power wise with the aforementioned compressions and corresponding grades of fuel. Gary ---------- Post added at 05:41 PM ---------- Previous post was at 05:29 PM ---------- A static compression of 8.92:1 gives 7.5:1 dynamic compression, which is decent for a 58* IVC cam like the Crower level 2. I wouldn't go any lower than that though for use with premium or you're just unnecessarily losing power. So with a 58* IVC cam, 9-9.25:1 actual compression is what you wanna shoot for (with premium gas), which is minimal material taken off the heads and block, even if the deck height is taller. Otherwise, about 7.5:1 static (for about 6.3-6.4:1 dynamic) for regular gas, which would require the low comp pistons. (Bear in mind that higher compression engines are more efficient, and I hear 2 MPG more is not uncommon for premium vs regular engines. This means if the car gets 20 MPG with regular, it'll get 22 with premium, which is a 10% increase. The cost of fuel for premium over regular is about that, so you're not spending any more money on gas.) Gary
What is the part number on these medium compression pistons? I hope they are available in 3.800 bore size. One other potential problem is the piston country of origin. I refuse to install Chinese pistons in my Buick.
Here they are, and they are available in standard 3.800 bore. http://www.summitracing.com/parts/uem-1736-std/overview/make/buick Part number 1736-(insert size) The singles are the same part number, only with an 's' before the number: http://www.summitracing.com/parts/uem-s1736-std/overview/make/buick They make them all the way up to .060 from what I've seen. Dunno where they're made, other than they're Keith Black silv-O-lite cast pistons. Gary
I dropped off the crank, block, heads and rods at the machine shop today. I decided to bore it as the bores were worn some after 190k miles. I haven't made the final decision on the pistons but I'm leaning towards 10:1 for the extra torque. I kept saying I was going stock stock stock but I also think I might regret not going high comp. After all, it was an option I realize that in general, a higher compression engine is more efficient but I wonder in real life, in the 1970 Buick world, would a 10:1 engine really get significantly better mileage than a 9:1 engine all else being the same. Has anyone tried both engines or compressions in the same car or had identical cars with different compression?
There is some info from KB about the topic of compression vs fuel mileage. One of the last attachments has a few notes about better fuel mileage with higher compression. Personally it all comes down to the camshaft. If you are using a stock cam then stick around 9:1, if using a TA 212 cam go to 9.5:1, and if using a large cam go to 10.5:1.
Most newer cars run higher compression so thats kind of an indicator.with the 3.0 v6 flat tops I think measured out to just short of 10 to 1 with being .090 down hole .043 head gasket and stock 58 cc heads which should be good for you depending on cam. Alittle polishing of chambers could bring it down a bit and can go well with most cams.I did have a77 regal with a stock 350 and when it blew up put a 72 350 in it and did notice a change but I assumed mostly it had more power to get up to speed so you let off accelerator sooner.
What was the purpose of casting these bosses into the combustion chamber? Thanks for that Sean. Their comparison was between 8:1 and 10:1 engines and they said 1-2 MPG better. From that I'd say .5-1 better MPG between 9:1 and 10:1. That's not much but it's something. AFAIK, the GS 350 engine, SP code, 10.25:1 could have been ordered as an option on the Custom and that seems like a good excuse to sneak in the 10:1 pistons. :Brow:
the guy that owns the machine shop is good guy and does good work but he's in no hurry. The block is bored and he ordered the "engine kit" last week so it's getting close. I told myself this was going to be a cheap stock build but I've fallen off the wagon a little. It's turned into more of a balanced and blueprinted GS350 motor. The rotating assembly will be balanced. Which brings up a question. Should he have the harmonic balancer? I'm thinking yes, and that means 2 hours on the road. The mains are getting align honed and the decks squared. The heads are getting whatever they need. All in all 11 C notes in labor. I was shocked and pissed at myself for spending that much just in machine work on a street drone but I tell myself it's going to run so smooth that it'll be worth it.
Just pretend you did not have to pay for the labor and move on LOL... It takes some $ to do it right and it is worth it to have piece of mind. Yes I think the machine shop will need the balancer and flexplate in order to balance the engine. The factory balancer and flexplate both contribute towards the balance of the engine. The only time that the balancer and flexplate are not needed for balancing is if the shop is doing an internal balance job and in that case you would need to use a neutral balance balancer and flexplate but these are high dollar pieces.
Looking at piston spec and found hi comp cast piston is supposedly .030 taller compression height with a 10 cc dish. Nets around 9.3 depending on measurements. I am considering going like Taulbee with 43 cc chambers too. I replaced my timing set with a double roller and advanced cam 4degrees and ported heads with. 025 shaved from head. Thinking I need more than that.so I yanked intake again .will pull a head when I have time. Just swapped in a 10 in convertor and have 3.73 setup ready to go in
I dropped off the balancer and flexplate today and there was my little 350 hanging in front of the boring machine. When I was locating the balancer in the totes I packed seven years ago I found brand new Clevite 77 bearings in the boxes as well as some new gaskets and seals I forgot about. All new stuff was just ordered so I'll have to sell them. I'm sure there are more surprises packed away.
Paul, I measured everything today. The machine shop ordered the engine kit for me. He told me I was getting the Silv-o-lites but I got Sealed Power 340Ps instead. It's funny that the compression came out to 9:1 with 10:1 pistons. Unless I goofed something up? The block and heads were decked for straightness and nothing more and I left the bosses in place in the chambers. The Fel-Pro head gasket measured .047 and I guessed it could compress .002". I wonder what the actual compression ratio was from the factory on the 9:1 engine?
Do you want just 9 to 1. My block measured .028 over spec which is a lot in compression then add that the 9 to 1 engine used steel shim headgasket at .020 and felpros are .043 crushed.
