I have Chevy question for the guys who have played with some different head/piston combos. I have a .040 over cheapie rebuilder 350, 2 bolt, nothing special. Cam is a stock rebuilder piece. I've put a Melling high volume pump and a decent baffled pan on it. I also have a good ignition and a 650 Holley on an old Weind dual plane. All decent street stuff. My question is, what heads should I run to get about a 9-1 compression ratio? I have .040 over dished pistons and they are about .050 down from the deck at TDC which is actually pretty low in the bore. I'm thinking some 492 or 291 heads, (64cc), should be pretty close. Due to the mild nature of the motor 1.94 valves should be OK. This is not meant to be a max horsepower piece, just a good peppy, reliable street motor. All suggestions appreciated greatly. There is no shortage of just about any heads around here I can probably get whatever I need.
I haven't played with SBCs so this is just general knowledge. Pistons .050 down in the bore plus a dish is not great for quench. I'd go with the smallest combustion chamber you can find. There are a ton of resources online for determining combos like this, do some research.
Are you sure they are 0.050" down in the hole? I think that would be hard to achieve 9:1 if that's the case. Are you measuring to the edge of the piston or to the dished area of the piston to get that value? Is the block standard deck height?
This is a typical AutoZone, Advance, etc. type rebuild. Probably all EngineTech or equally high quality parts. Pistons are definitely .050 down in the bore. Measured from the outside flat of the piston. I'd like to see them up a bit but I can accept some compromise. I've calculated this out and I should have about 8.7 to 1 at .040 over. The motor had some 400 76cc heads some really thick gaskets when I got it so I'm assuming it was a real pig at best. The motor has almost no miles on it so would like to salvage as much as possible.
You could pull the rotating assembly out and zero deck it.... that would get you back to a 0.025" range or so. Then you could do a 0.015" shim gasket and some 64cc heads and be doing ok.... You wanna try to be in like 0.035"-0.040" quench area to get a good burn. That's my thinking anyway. I have some similar type stuff going on. Late model Vortec blocks that are in great shape - trying to avoid a full rebuild. My thinking is to zero-deck the block (which for me brings the pistons up to ~0.000"), then getting some heads shaved down to bump the compression a little more.
one of the places you should be looking at https://butlerperformance.com/n-12872-compression-calculator.html Just remember, shaving block and heads means the intake should be checked, too. https://www.hotrodders.com/threads/...restore bolt hole alignment and port mismatch.
Not sure you will get to 9:1 with those pistons, but a 64 cc combustion chamber is about the smallest common one to try to find to get as much compression as possible. There are some slightly smaller ones but not as common. I like running any of the old 461, 462, 186, 291, etc. double hump heads on small blocks.
To remain thrifty, I'd look for the compression you want in the heads over shaving the block and heads. Smaller chambers and a shim head gasket. Just my 2 cents. (My two øre - in Norway )
It's going to be difficult to get to where you want to be without some machine work or replacing some parts. You could deck the block 0.025" and get some Vortec heads and take some off them. Might be cheaper to just replace the pistons. With a stock capacity oil pan, a standard volume / standard pressure oil pump would be a better choice. And depending how much zip you expect, that rebuilder cam may not get it for you.
I'm now thinking the pistons may need to go. Keith Black makes some decent pistons for this combo. If I'm tearing it down it needs to get a decent cam as well. I just don't want to have 5G in a motor for a whole car that is worth 5G. Done that way too many times already. I should be able to be in around $2,500.00 on doing what I need to do and still have an OK motor. The block is a two bolt 014, which is the same as the desirable 010 block so it is worth doing a little bit to if it runs pretty decent. Still not bad for a fresh motor. Time to kill the Visa again.
Get some 58cc or even 49cc from EQ…. I’d dump the High Volume oil pump in a heartbeat unless you have had the crank ground with extra clearance..
One option (won’t help quench) is to look for the 416 305 heads. They are 58cc if memory serves, have okay flow. Seems like “rebuilder” pistons were used with a standard deck. The only proper way to achieve CR you want with 64cc heads, and a somewhat proper quench with a dished piston, would be a standard pin height piston. with the pistons .050 in the hole, and a decent head gasket, your quench area (if we can call it that) with those pistons will be around .075 or so. All my babble aside, it’ll run fine, won’t be “peppy” but probably more like the early smog era stock engines in the ‘70’s.
As you are aware and have many previous replies, the problem is pistons too far down. The 305 heads with 58 cc chambers are probably not a bad idea. But given your larger bore, you could go with bigger 1.94 valves to improve breathing a little. Even 64 cc heads will work, just a bit lower static compression ratio. I agree that it will run fine with either type heads, but will not be optimized since the quench space is too large.
My second option would be to throw some 305 heads on and run it next summer, shredding it over the winter for an upgrade. This would give me some time to collect parts. Budget 36 has a good perspective on this. If it got me down the road I'm OK with it for one summer. I really don't want to acquire debt to build a motor right now and I really don't want this project to be off the road another year. Thanks for the help guys!
Find out why the pistons are .050" in the hole in the first place. Is it those chepo rebuilder slugs? As long as the machine work done on the crank and rods was kosher another set of standard compression height flat top pistons might be a better starting point, bringing the quench dimension back into the ball park, and giving you more cylinder head options. Understand the problem before deciding on corrective action. or....just slap something on there and drive?
How difficult or expensive is it to get longer rods to make up the difference? I just did this on a VW build, pistons were 70 thou in the hole, dialed in my deck height (40-60 thou)with longer rods and a head gasket. Flat top pistons too. Some cast summit pistons can't be that expensive?
I’m running a dished mid 70s 350. Put 461 heads on it. 194s Small cam. Does good in my truck. Larger valves make sense for higher RPM stuff. Not much improvement for a cruiser. The smaller cc 327/early 350 heads are probably better choice than the 76cc later ones. If ya just want to drive, slap some heads on it. If you’re trying to squeeze as much as you can then a better piston to get the quench correct would be a good choice. I’ve never seen a need for a high volume oil pump on the street.
VW is a bit of a different beast than a SBC. I’m thinking there are no rods that would be 5.725 or so long. Pistons would be the only feasible option, but since it’s not a performance engine (imo) screw it together and enjoy the ride. It won’t “not run”.
My 327 has some smogger 76cc heads on it, and it runs fine and has been running fine like that for 30 years. I'll bet the CR is 8:1 or less. I'll be pulling the heads soon so I'll find out for sure. So yeah, for a street motor, run what ya got.
One last thing to think about (yes I squeak when I crap) is later on when you take it apart. Right now going through a set of smaller CC heads will cost a few bucks. But then you will probably be looking for another set of heads when done putting it back together. If it were me and I had a set of heads that I could bolt on, get on down the road, I’d do that and defer the cost of everything until I take it apart later and make the engine as I wanted.
Checking around for some reasonably priced heads to get it running for now. This is a fresh engine that was pulled for some reason, my assumption is with the 76 cc heads it was a real pig and whoever had it thought it was just a trash motor. The bottom end looked great when I got it along with the bores and everything else. Also the block appears to have been caustic dipped or some other similar method as the water jackets are spotless bare metal. I forgot who the rebuilder was, there's a metal seal epoxied to the block. They appeared to have done a great job, except for the cheap pistons. The crank is .010/.010 with Federal Bearings that spec dead in the middle of tolerance, rods and mains. It still has a nice crosshatch and absolutely no ridge in the bores so it doesn't have a lot of run time. I started this thread because I was hoping somebody had some experience with this sort of dilemma having never had one with the pistons that far down in the bore. I can't believe a reputable builder would have done something like that if it wouldn't run but I've seen worse! Also I might add, I threw the high volume pump in there because I had it on hand along with the correct pickup. I would have just used a stocker otherwise. I wouldn't expect cavitation problems on this motor, it will just dump any excess back in the pan.
The potential issue with a high volume oil pump and a standard capacity oil pan is if the drain back to the pan through the valley gets impeded it's possible to have a lack of oil in the pan while sailing along the interstate. So if you have the engine apart clean up any excess casting flashing in the valley.
Find some 58cc heads, bolt them on and run it. See how you like it and adjust from there. Remember your car is pretty light to begin with.
Excellent point and already done as partly shown in the pic. I didn't see any point in screening the drains in this motor however. If it grenades it's a gonner.
It just occurred to me that maybe this motor ran like all hell and whatever it was in met an untimely demise? Nah, not with my luck!
Is the crank .010 or .020 under? Could be the crank is offset ground by accident I. E. poor machining. That could account for some of the piston short fall. I’ve seen that before.
Run of the mill oversize rebuild pistons were (probably still are) "destroked" the same amount of height as the overbore, i.e. a .030" O/S piston had the compression height lowered .030", and proportionally for other oversizes. This was to keep the compression ratio the same, but it is a terrible idea because increased quench increases tendency to knock. I think Silvolite does, but Keith Black does not "destroke". Anyhow, the catalogs or online listings should tell you the actual compression height. The high volume oil pump will just waste a couple horsepower.
I had a .040" 350 that I built with some cheap $89/set H345ACP hyper rebuilder pistons. With pistons down .055"and a .039" head gasket, "quench" was .094". Solid roller 251/261 114lsa. 57cc Brodix -10sp. Cranked less than 100psi on the starter. It got 20+mpg on the highway, sprayed it to 700hp on 87 octane pump gas. 5.73et in the 1/8mi, 1.308 60' with a manual trans. Liked that low compression 357 so much that I built a basically identical flat top 355 for the car with .084" quench. Forged pistons, aluminum rods, -11RI heads. It gets sprayed much harder, still gets 20mpg on the highway with 3.73 gears and 28" tires, no overdrive. I've got many friends with street/strip cars that stay in the garage because it costs them $100 for a 5 gallon pail of gas. I get a lot more seat time than them, as I can buy gas for mine basically anywhere. Grant
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