I have a 350 Chevy, .040 over with the typical dished, 4 valve relief GM pistons, classic 1970's smogger slugs. Can anyone tell me what my CR. might be with 76cc heads using a shim gasket that adds 3.24cc and a 64 cc head with a gasket that adds 9.1 cc to the chamber. I've done a net search along with on here to no avail. Never was good at math anyway. Thanks!
8.5 open chamber, or 9 to one closed chamber. Roughly. Not enough info given, so we have to guess on stuff....
There's a lot of stuff I'm not sure of. If I had cylinder volume I could calculate as the stroke is stock. I loaned my buret to somebody and can't remember who. I'm just looking for a just about which Squirrel has answered. Thanks!
glad to help. I was guessing 15cc dish volume, .020 deck height, .015 shim gasket, .040 thick gasket. ymmv
Static compression when you just look at bore, piston and chamber, dynamic compression comes into play with camshaft design, you will have to add that into the equation, if heads are not installed, measure how far piston top is down from top of block deck, .010, .025"? You need that dimension, with 76cc heads and mild cam, I prefer flat top Pistons, good luck.
One other thing with the whole package and today's gas on the old SBC like yours is quench volume, too much and you can have detonation problem's , vortech heads maybe a better deal if short block is already done with the lo - comp piston.
The shortblock I have is a stock, .040 over, Pioneer rebuilt. All I did was put the tin on it. It is super tight and I'm just going to run it as is for now. The goal is to get it on the road ASAP and I can build another later and swap it out. For what I want this car for, some seat time this summer, it is actually ideal. The heads I have are a fresh set of 400 heads so they are definitely smoggers. If I get 200 hp out of it with the 650 Holley and the Weiand dual plane I'm just fine. I might run a steel shim gasket, probably not the best ideal for heads that have most likely been cooked at least once, but it would raise the cr. a bit. They are 1.94 heads so the torque should be respectable. Also, this is in front of a three speed and in a pretty light car.
Just start with premium fuel for startup and breakin. Later, introduce regular 87 octane, listen for ping/detonation under load. Enjoy your ride!
JagMech got it exactly right. Don't guess, check it before you select a gasket and put the heads in place. Its not just about compression ratio, you have to be sure you have a proper quench/squish area between part of the piston top and the head or you will have detonation problems. Not talking about just too small, but too large is also a problem. Talked to a guy who didn't set his quench right, and every time he filled his tank he had to add an octane enhancer. It doesn't take much effort to bring the piston to TDC and mic the distance to the piston top edge. Then add head gasket thickness (compressed) and you know what quench you have. https://www.motortrend.com/how-to/ideal-quench-height/
No more than .040", .035" better, for dimension illustrated above. Piston,( flat area at perimeter) compressed gasket, to head surface. Pioneer builds forklift motors, maybe remans at some point, who knows what cam, perfect time to convert to roller, from the picture, deck is not machined , from keyway position it's close to #1, tdc. but I hope that piston can come up a touch more. Good Luck.
KB piston site has a great calculator for compression ratio if you know the piston cc’s, thickness/diameter of gaskets, piston to deck, bore , stroke, etc. As a guess …. 76 cc chamber, a flat top piston, .015 in the hole, a 042 gasket, and no eyebrows or dish is pretty close to 9.5 to 1. Add in eyebrows it right at 9-1.. dish and I’m guessing just over 8-1. This is static. Dynamic is way less if you know the cam info.
is that with small or large chamber heads? And since he has the dish smog pistons, how does that affect things?
Good discussion. Especially the picture showing what quench dimension is. It seems I've heard most rebuilder cast pistons are at least .020 inch down below deck surface. Maybe more even? Bottom line is even with a thinner head gasket it will be tough to get small enough quench.
Do not be in a hurry to slam this gem together with what you have at hand. Do a little more research on the correct combination so that you don't have any regrets, do you know what cam specs are? Goggle how camshaft affects dynamic compression, it would take no time to pull cover and remove cam to i.d. it. With all the problems with cam/ lifter quality today think about it. If you want to go as is, spend the money for the proper heads, again, newer swirl chamber, vortech, something that help minimize detonation, lf you want the 76cc heads, run flat top pistons with 2 valve reliefs, ask a local engine builder with about 50 years of experience on your set up before you go further.
Chevy built millions of these engines with the big chamber heads and shim gaskets, put them in trucks, and they worked great...well, they worked. But yeah, you can always improve things if you want to spend some money. Sounds like you're not real interested in spending any more than you have to, though.
I assumed the 76cc he described in the 1st post. Corrected my values he stated. Since I’ve never cc’d a dished piston but his looked 1/8” I guessed an additional 1 point in compression as I said in my last sentence.
Basically what I'm trying to do is get something together that will run through the summer. I don't mind tearing it down over next winter to upgrade. I pretty much work 7 days a week and really have no time to build the kind of engine I'd like to. Also I'm getting a little edgy over how much I want to put into this car. If you pop over to my Vicky build thread you'll see this is actually the third engine I've built for this car. The others were pretty well done and ended up being so expensive I just passed them on at way less than I had in them just to cover what I still owed on parts. This car is no beauty queen. Most cars like this have 307's sourced from a Craigs list ad for $75.00. (I'm not that cynical yet!). This motor is not my idea of perfect but is a decent start. All of the comments above are excellent info but the determining factor is the budget. It doesn't make a lot of sense to put 10K into an engine in a car that would flip for maybe 5K if I was lucky. This is no deuce. I've built at least a dozen cars over the last 12 years or so and ended up flipping every one just to recupe hard costs by the time they were finished. I don't want to be in so far I need to do that again. Not bitching, just wanted to clarify where I'm coming from. Oddballs don't have a lot of value around here and to be honest, I'm kinda tired of building and not driving. If I was going to do it up I probably wouldn't go with a .040 block because of overheating issues. A good set of flattops with good heads and a decent roller cam would do the trick. With machine work a quick 5 grand. Maybe next year. If we still can drive these in a year or two I can easily replace it. https://www.jalopyjournal.com/forum...-vicky-body-a-build-thread-with-pics.1197541/ My Vicky build thread for reference.
Throw some 305 HO heads on it with 58 or 53cc chambers. Pick up about a point of compression with no other changes. It won't pull past 5k, but you'll gain a ton of torque with the compression. Devin
The later centerbolt ones have a lump in the intake ports. A cheap HF die grinder, a carbide burr, a sharpie, and a gasket to trace will see those gone.
Understand that you just want to get it running as inexpensively as possible. I'm not suggesting that you spend any more money at this point. I'm just saying that you need to measure the distance from the engine deck to the piston top and see if there is a head gasket that gives you a proper quench area, No money involved there, but you do need a depth mic or a dial indicator to check it. The parts you have may work just fine, but you have to check to have even a rudimentary idea if it will work properly. No one can tell you what your compression ratio will be without knowing how far down (if any) the piston is below the deck.
Great replies here guys! I think you've answered my questions. Even if I end up with around 8.0-1 it will still be serviceable for the summer. the cost of a set of heads will pretty much pay for what I need for the rest of the car. Better to drive it with 200 horses than look at it rust in the driveway. Actually I think with the Holley or the Sam-O-Ram it will put out in the 250 range with enough torque to make it fun with the three speed and the well done 9" in there. Rear is 3.55 9" with Detroit Locker and Moser axles. One concession to my vanity. Car weight should end up around 2,200lbs. at most including the overweight driver.
I just checked how far the piston is down in the bore using a stop bolt, Starrett machinist square and Starrett feelers gauge, (remember, I'm a fabricator not an engine builder). Using this rather crude method the piston is down in the bore .051. Seems like a lot but it is damn close I'm sure.
If thats right, and you add the thickness of a compressed head gasket, thats too much. That sounds like a lot more than I would expect to find. Don't want to beat a dead horse here, but I hope it works for you.
Seems rebuilder pistons were used, came into the same thing on a friends 327. The pistons have a higher pin height to account for decking the block. Nothing you can do short of taking it apart and changing pistons or decking the block. I’ll have to check with my friend and see if he ever got the engine in and running.
You can just put it together and run it, too. Won't be the most powerful old truck engine around, but it'll get you down the road until you win the lottery.
I'm having great reservation on this motor. The fellow I got it from got it from a friend who had it under the bench for ages and he acquired it from a friend who knows nothing about it. It is a relatively fresh short block, and is actually tight as all hell. No idea why somebody would pull a fresh motor and strip it to a short block. It was definitely running at some point. Even if the car was wrecked why would you pull a fresh motor apart? I could see if they snagged an intake or even the heads but the pans and tin, oil pump etc. was all gone. Except for garage dirt it was as if it just came off the pallet. It still bears the Pioneer rebuilders tag. Pulled a couple rods and mains and it looks new so they didn't run it "dry" or anything like that. I'm going to put it together and fire it on the floor. We'll know soon. Unfortunately, I've sunk a bunch into it already. Perhaps they had issues with it and they were to return it and it never got back?
Done a ton of research on all the things we talked about in this thread and came to the following conclusion, (right or not). In stock engines the quench is much more than on performance engines. The .050 or so the piston is down in the hole is not unusual for a smogger as they were chasing maximum flame travel at that time. With an .024 head gasket, which are supposed to compress to about .015, I get a quench of .066-.075. Not performance territory but also, run regular all day long with no detonation. According to numbers I've pulled on line for the pistons, gaskets, and head volume I am right on for an 8.40 static compression ratio. Probably not that bad for a car being built as a highway cruiser and trips to the breakfast spot. It looks like the rebuilder knew exactly what they were doing as this motor was intended for a grocery getter or a pickup or the like. I have seen some decent motors come out of production line rebuilders and I don't think this is one of them but it will serve my purposes. It does have a set of "pink" rods which is really unusual unless they just had them and used them out of convenience. Comparing what I know now to similar factory engines, I should pull about 260-275 hp out of this as it is. Plenty for what I'm doing. I say this cautiously as I'm not sure of the cam as it is unmarked but I had it in a Blue Racer box also unmarked, almost no run time on it as the black nitrating is still on so it was never even broken in, maybe never run. I assume it is an RV grind as I used to run a load of them in mild motors. They do idle nice and pull like a sombitch out of the hole, (but not for long!). I'm pretty sure I pulled it for a stouter version for somebody. The dual plane and the Holley along with the overbore will help as the 250 horse of similar spec was a 2V motor. I can easily do a tad of port matching and general cleanup on the heads to stay closer to the higher end of the scale. As these are 1.94 heads that will help as well, (still shitty heads!). I really appreciate everybody who jumped in on this thread as all of you pointed me in the right direction and what I needed to look for. I love bouncing things off the HAMB crew. Back to work with me!
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