Trying to get a BBC build to right at about 10:1 static and about 8:1 dynamic compression. Option 1 are 120cc open heads giving me about 9.6:1 and 7.72 compression. Option 2 are 115cc open heads for 10.03 and 8.07 compression. Oh, and option 2 costs a not insignificant $1,500 for what might end up being nothing more than 10 horsepower. Tell me I'm nuts to consider it

 

On my small block, shim type head gaskets were good for a half point in compression.
Perhaps this or another end around is possible on your big block?

 

Many ways to skin this cat. Head milling, gasket thickness, cam specs, cam installed centerline, and probably a few more. Depends on your choices. Picking a different cam would be a good consideration.

 

I am using a set of the BB heads made by GM and they have been great for making compression and power. The heads I am using are the large Oval Port closed chamber heads and they have a 100.9cc chamber cylinder head. They are listed as the GM part: #3931063 and both were cast in 8/67 and the heads were used on 396 and 427 engines back in the day. The closed chamber keeps it from pinging as badly as the early open chambers did. I have pushed the overall timing up to 38* and still no detonation or pinging with my engine. The pistons I am using are L88 pistons and the valve pockets in the pistons match the ones in the head.

Before re-assembling the cylinder heads they were sent out for a general port cleanup and they came back. The heads were stripped them down and then we replaced every valve and spring with new modern components. I also had them install the hardened seats on the heads while the machining work was done. Then we spent many days installing the new springs to get the proper seat and open tension. Due to the compression I went ahead and put play dough on all 8 piston crowns and then bolted the heads on and rotated the engine and then removed the heads. It was close but I have right at 1/8" of clearance when the pistons are at TDC.

Compression makes power and lots of it. I like using the highest C/R that I can safely run on pump gas. I have a water/methanol Injection system that is set up to activate when the engine is hot so that it has extra protection from detonating. I can also retard the timing in small increments if I have to.

Life with a higher compression engine is FUN!

 

I think you have that backwards. The closed chamber heads were the earlier design (my '67's are similar to yours: oval & closed) but my '72's are rectangular & open. The open chambers were, in part, a way to lower compression.

 

Thank you all for the great responses. I didn't even consider head milling or thicker gaskets. Those options seem like low hanging fruit to get me there.

 

All I will say is "buy once, cry once". If the more expensive heads flow better, then you are looking at more than 10 hp. Of course, if the lower cost heads flow more, they could more than make up for the 1/2 point of compression.

 

if you want performance go with the 10:1. I have 11:1 on my 427 with aluminum oval ports and no pinging on 93 pump gas. You are over ****-izing this to death. You obviously arent looking for that next tenth of a second in the quarter mile so go with the best you can afford and move along...the summer is nearly over. never go with less compression unless you are planning on a blower/ turbo

 

If you want the power, go with the best flowing heads out there, like AFRs.
They can be worth 60-80HP over OEM heads, at the same CR.
Then go with whatever piston you can find to get it in the ballpark with those heads, 9.6-10.5
The good flowing heads are worth way more than a little CR.

 

60-80 more than 781s with bowl work and bigger valves?

 

I have OEM L88 Aluminum 074s with very mild port work and that's what my builder and his head porter buddy both said when I had them flowed.
They do so much work with race motors, they said they forgot how bad these old heads really flow.
"They really suck don't they"
Mine dyno'd at 490, with this 60 year old head design, they said it should have been 550-560+, with newer better heads.

 

The cam needs to be matched with the heads. Choosing heads based on a particular cam and funky DCR calculators will cause a much less than optimized combo.

Which heads are you looking at? As mentioned..compression is great....and the right chamber will allow you to safely run more. But flow at all lift points will make the power. Not just peak....but all through the curve. Having a big difference in IN/EX flow isn't a big deal....just pick the cam to work with it.

Heads and cam are THE biggest choices you'll make....you'll want to do it right.

JIM

 

Thanks all. I was making decisions with my wallet. Not necessarily what is optimal. Back to the drawing board!

 

No, but 30+ is reasonable. That’s not factoring in the weight reduction in front end of the car which is significant. A pair of aluminum heads is roughly 70 lbs lighter than iron heads.

 

Bowl work and bigger valves gets you part way there on the old heads.
The chamber shapes and the port shapes in the new designed heads are light years ahead of our 60 yr old originals.
Heads are the biggest single upgrade you can make.

 

Once again, thanks to all for the education. No substitute for the years of experience on this board!

 

Another way to look at this is how much power and in what RPM range are you planning? What's the combo? Use? Trans and gearing?

With some cubes a well ported set of iron oval ports can go 650 HP or more....500-550 doesn't take much of anything.

JIM

 

That's the thing. I'm not looking to break any speed records. I want the engine to have daily driver civility with good vacuum. 4 or 5 speed manual. Just a fun, pump gas friendly car for Sunday drives. The cam I had in mind tops out at about 5,700 rpms. Board member Rescue Rogers was very helpful but I got stuck on compression. My original plan was just a stroked 383 but I don't think 525+hp is reasonable with a mild cam and heads. So then I though a BB might work. A 454 hits the spot when figuring dollars per horsepower. And that's where I thought reworking old 781s would save a few bucks.

For giggles, I spec'd out a 427 to be different. It's $1,000 more than a 454 build while giving up cubes and power. That doesn't make mush sense. Anyway, here are three potential builds. Still open to the 383:

383 - 224/230 hydro roller cam with .502/..510. 1.6:1 rockers, flat-tops with 7cc relief. Topped with AFR 195, 65cc chamber heads and an Edelbrock Performer RPM. Just don't think that will get me to 525+ HP. No idea what it would be.
454 - same cam in BB form. 224/230 roller with .510/.520 lift, 1.7:1 rockers, 27cc domes. head choices are reworked 781s or AFR 265, 112cc heads with Eddy Performer RPM. I have no idea what the HP would be.
427 - same as 454 except for 3.766 stroke rotating assembly. 28.2cc domes and either 781s or the AFR 265s. No idea what the HP would be but it's $1,000 more than the 454 to build.

As above I got stuck trying to maintain a friendly 10:1-10.5:1 CR on the 427 but given the sacrifices, it's not high on the list.

 

I have no idea about your above 383 build but a typical mild 383 will make 430 HP with ease. If you already have the brackets and accessories for the SBC then that is a good option. Cheaper and lighter is good.

JMO - If you require more power than the stroker provides, I would be advising more cubes and more money.