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Compression Bump

3K views 35 replies 11 participants last post by  Coach62 
#1 ·
I think I already know the answer, but on the off chance there's another way, here goes...
Are there any other tricks to bump up compression on a BBC? I have the heads off right now. I know about these, but are there any others that I'm not thinking of? I have a calculated ratio of 8.33:1 and I'd like to increase it at least a point.

1. Thinner head gaskets (already done. .020 metal)
2. Mill heads (tempted. .006 per cc?) Problem is the heads aren't worth anything and I don't want to put any more money in them.
3. Deck block. (Don't want to disassemble short block)
4. Higher compression pistons (Same as 3)

My current specs:
Bore & Stroke 4.155 x 3.76 (.030 over 402)
Heads 112cc 148 truck heads (could be 122cc... I need to cc them tonight)
Pistons - Silvolite 1445 +30 (-8)
Head gaskets - .020 x 4.37 metal
 
#2 ·
Longer stroke also builds more compression than a shorter stroke. In other words you will have a high compression with a 4.5 inch stroke and flat top pistons at zero deck than you will with a 3.76 inch stroke and flat top pistons at zero deck. A BBC needs to have a dome to fill the huge combustion chambers. The problem is the bigger the dome the more it interferes with flame propagation.

One solution to this problem is a second spark plug on the lee side of the dome. Shame that none of the head manufactures have though of this as Chrysler has done this with the Hemi's for years. Of course if the manufactures read the tech boards they wouldn't need to invest as much in R&D with fewer failed projects.

You can build nearly twice the power of a 396 (402) with a SBC 400 for about half the cost of a BBC, if built with modern heads and a modern roller cam. The BBC requires a large displacement in excess of 540 cubes to build comparable power if peak horsepower is your goal in life. I wouldn't put anything but rings and bearings into your rebuild and look for a 4.500" to 4.600" bore block to build instead if you want those big valve covers.

Big Dave
 
#3 ·
Thanks Dave. I'm just trying to work with what I have for right now. Eventually I'll get some bigger cubes to play with. I'm not looking to build a race motor with this, but I'd like it to build some respectable power without putting much more money into it if possible. If it isn't, I'll live. Just trying to exhaust possibilities before I put it back together this weekend.
I'm tempted to mill the heads, but I'd need to shave off .0057" to get up to 9.5:1. I don't know if that's doable or cost effective with these heads.
 
#11 ·
Thanks a lot guys. There's definitely a lot of options if I want to spend more money, but I don't right now. I like John's idea if I were worried about taking someone in a race or something right now, but I'm just looking for day to day power.
I talked to Holbrook Racing last night. Good guy to talk to. I didn't realize that I'd have to mill the intake also if I milled the heads that much. Makes sense. I don't really care about these heads so I don't want to ruin my intake by matching it to junk heads. It would have run ~$400 too. $250 for the heads and another $150 for the intake. Not worth it.
I do have a set of virgin 290 (101cc 2.06/1.72) heads from my born with motor, but I decided I didn't want to mess with them. That's the purpose of me running this motor. I don't want to hurt my born with engine. I could easily throw a cam in that one and add some 2.25/1.88 valves and it would be plenty of power for what I'm looking for. It's already 10.25:1.
Eventually I'll just find something with bigger cubes and build it with the right pistons and heads.


You're right. I added an extra zero...

Find the smallest closed chamber oval port heads you can and install the 2.19/1.88 valves, have them pocket ported while putting the valves in, or find a set already done, nobody wants the iron oval ports, should be cheaper
I'd do that, but I haven't seen any decent heads for reasonable money in a while. If you know of any give me a holler. I won't be putting them on until this weekend. I might shoot up to see you on Sat if you'll be around.
 
#5 ·
Milling the heads was my first thought, the other was changing pistons and there is the thought of a small street blower. But all these thoughts get more expensive as the ideas came to me. When I thought of pistons, I recall guys using the Speed O Motive pistons. I recall using them in race cars, I know they were very cheap in material as one guy said. But then you are basically doing expensive stuff again and you said you just wanted to get by for now.

All you want at the moment is a seat of pants feel of power. Can you find a used 454 somewhere? Decking block as Dave mentioned could work but you need to tear down entirely. Did you say once you had access to smaller chamber heads? #290 head with a 105cc chamber?

Used 502?
 
#6 ·
Back in the 70-80's we used to angle mill the heads on big blocks for higher compression, and to help the entry angle for the intake ports. More was milled off the exhaust side. You had to mill the intake flange to compensate for the intake manifold. Exhaust flange angle needed to be watched/altered too.
It was a good strategy-check with your machinist.
 
#8 ·
Find the smallest closed chamber oval port heads you can and install the 2.19/1.88 valves, have them pocket ported while putting the valves in, or find a set already done, nobody wants the iron oval ports, should be cheaper
 
#12 ·
Just so you know it's only worth roughly 4% more horsepower for every full point increase up to 10:1 and around 2% for every full point increase above 10:1. Where you're at now is too low for sure. There's really only one way around it and you know what it is......new heads. Did you check our racingjunk.com as I mentioned before?
 
#14 ·
I'd just like to get the compression up somewhere where the cam needs it to be. Above 9:1. I did check racingjunk and go back once in a while. I'll keep looking.

"I do have a set of virgin 290 (101cc 2.06/1.72) heads from my born with motor, but I decided I didn't want to mess with them. That's the purpose of me running this motor. I don't want to hurt my born with engine."


What I don't understand is that you have these 290 heads from you born with motor and you don't want to run. This motor has the peanut pistons with matching heads.

These heads don't have an expiration date on them. Sounds like you are saving the born with motor also for ??

This is why I suggest finding a 454, 502 or 572 to play with. Maybe you can find a 427 motor complete with square ports and rebuild it as you will have more compression to play with and good flowing heads. Sounds like you are willing to spend extra bucks to get around running this peanut motor.
You are spending dollars to same dimes. Just my 2¢
"Peanut pistons"? I've never heard that term before. Only peanut plugs and that's not what that motor is. Are you referring to something else?
Not too much to understand on the heads and motor. It's irreplaceable if I break it. Sure I could get something date coded correct again some day, but this is the virgin VIN #s matching motor to an L34 X66 Camaro we're talking about. 1 of 2,018 built and a lot fewer than that with 4 speeds like I have. Think about all the guys going crazy and dropping insane amounts of money to track down and get their original motors back. I have mine. I just don't want to screw with it unless I decide to go back to a bone stock trailer/garage queen some day. Until then, I don't want to be afraid to side step the clutch at 6 grand... >:)
I always keep my eyes open for big cube motors and I'd absolutely love to find a 427, but they're big bucks for just the blocks everywhere I see now. I'm in no rush and I keep my eyes open. The wife and our 5 and a 16 year olds have the priority with my money these days.
 
#13 ·
"I do have a set of virgin 290 (101cc 2.06/1.72) heads from my born with motor, but I decided I didn't want to mess with them. That's the purpose of me running this motor. I don't want to hurt my born with engine."


What I don't understand is that you have these 290 heads from you born with motor and you don't want to run. This motor has the peanut pistons with matching heads.

These heads don't have an expiration date on them. Sounds like you are saving the born with motor also for ??

This is why I suggest finding a 454, 502 or 572 to play with. Maybe you can find a 427 motor complete with square ports and rebuild it as you will have more compression to play with and good flowing heads. Sounds like you are willing to spend extra bucks to get around running this peanut motor.
You are spending dollars to same dimes. Just my 2¢
 
#16 ·
The real answer is to return the car to stock with the original motor in it. The block I understand, the heads would be tine to use, as they don't blow up to often, blocks yes,

I will be around this weekend, call me
 
#17 ·
Where's the fun in stock? Aesthetically I want stock/day 2, but I want to be able to play a little...
I found a set of 290 heads locally that look like they're in great condition for $200. I think I'll grab them if I can and give them a shot. Almost the same dates as my originals too so I'll have some spares.
 
#18 ·
I ended up getting the 290 heads for $200 last night. I think I did pretty good all things considered. Now the question is, what to do with them? Most of the seats are perfect, but two have a couple of small pits that I'd like to see gone. Should I replace all the seats with hardened seats and should I go to 2.19 or 2.25 valves? What would I be looking at in machine shop costs if I went that route?
I compared the factory pistons in my 350hp 396 to the replacements that are in the 402 I'm playing with. There should be zero clearance issues. The replacement pistons are supposed to be a stock replacement piston, so I figured that was the case.

My virgin born with 396 piston compared to the replacement Silvolite 1445





New Heads 3964290 (H 11 9 & I 23 9 A month newer than mine)







 
#20 ·
They are. I was replacing the springs with the springs from my cam kit so I wasn't worried about that. I was thinking about re-using my valves, but it's only ~$150 for Summit intake and exhaust valves. The guides look good. Maybe a hone? It's really only the seats I'm worried about. I might hand lap them and see how they turn out. 14 of them I'm sure will be perfect. The other two I'm not sure about. They might be ok. If that's the case I can probably just throw them together with what I have.
 
#22 ·
I'd love to have a machine shop check them out, but I'm having a lot of trouble finding one near me. They all seem to be out towards Detroit.
 
#23 ·
Aren't you in Saginaw area? Check out Terry Rosebush. Saginaw is only 90 minutes away from Detroit. I would think a short drive is worth it for quality work. Total Flow Products out of Troy MI is excellent. I know they will do it right.
 
#24 ·
Unfortunately I'm down around Ann Arbor. I'm sure there is something around, but I'll be damned if I can't find it! If worse comes to worse, I can go to Detroit. There's quite a bit out there.
I found some prices from a machine shop in OH on line. Even if I went with everything including replacing valve guides and regrinding valves, it still would only wind up around $600.
 
#25 ·
My shop here in Vegas quoted $600-$700 to replace guides, seats and valve job on Double Hump SBC heads. And they'd still be crappy heads. Sold them for $200 and bought new Alum Heads..
 
#26 ·
As Vegas said, you're throwing good money after bad but if you insist, Booth-Arons is in Berkley, MI (75 minutes from Ann Arbor) and Total Flow is another 15 minutes from them. I have used both and they both do QUALITY work.
 
#27 ·
The thing is, BBC 290 heads aren't crappy heads. That's why I grabbed them. Let's say I end up with $600 into them. That's still 25% of the cost of a good set of aluminum heads. 1/2 the cost of the cheap Pro Comps. Most of the aluminum heads are too much for my motor also. They're mostly 300cc runners and above. Brodix has 270cc runner options, but even those are really too big I think.
Thanks for the leads Chet.
 
#28 ·
Well that just made things a lot cheaper... I was just talking to an engine builder about my heads and he was saying you don't put the hardened seats in these heads unless it's really necessary. They're extra close to the water jackets apparently. Now all I'll really need to do is cut the intakes for 2.19 valves and that will take care of the couple small pits I didn't like. I think these heads will wind up perfect for what I need for less than $500 all in.
 
#29 ·
Before you start read Dave Vizard's book on head porting, and another of his infinite series on engines on how to build a BBC.

https://www.amazon.com/Chevy-Big-Bl...8&qid=1475260926&sr=1-3&keywords=david+vizard

https://www.amazon.com/David-Vizard...8&qid=1475260926&sr=1-4&keywords=david+vizard

I learned after building engine after engine how to do it cheaper by reading a book after I had spent several years and $200,000 in today's money on engine builds that the author John Thawley knew as much as I did. Could have saved thousands reading the book first instead of writing my own.

That insight was one of the motivating factors that got me to finish my engineering degree after I had changed majors from Chemistry to Astronomy and couldn't finish those degrees. (I discovered the hard way that I was highly allergic to certain organic enzymes and the astronomy department had left Tampa to go to north Florida in search of darker skies).

Book learn' is cheap and do you good. Just look what it did for Jethro Bodine of the Beverly Hillbillies.

Big Dave
 
#30 ·
I love it. I should have gotten my engineering degree instead of a business degree. Still could, but c'est la vie... I'll give those books a look. I've been doing a lot of reading on "grumpy's" site just to get a good idea of what I should be looking at.
The more I look at it, the more I think I'll tackle at least polishing the ports over the winter. I can practice on the truck heads I'm taking off. I know all the horror stories...
Thanks for the tips and stay away from those organic enzymes!
 
#31 ·
Maybe I missed it. But what are you going to be using your car for? There is nothing on this planet (no astronomy-pun intended) that isn't more fun with aluminum heads compared to iron. Especially the time saved when NOT having to visit the physical therapist for a ruptured spine. And they knock 75 pounds off of the top of your front tires. If they could talk they's give you a great big kiss. Oh wait, wrong analogy,,,,
 
#33 ·
Maybe I missed it. But what are you going to be using your car for?
Street/strip grins...

Glendale auto on orchard lake just north of grand river has a machine shop as well.

I have a set of comp cams 930 springs you can have along with the retainers that I just took off, they were not right for my cam, but may work for you
Thanks for all the help this weekend! I plotted the curve in a free on-line program on my phone. The graph actually came out halfway decent.

 
#32 ·
Glendale auto on orchard lake just north of grand river has a machine shop as well.

I have a set of comp cams 930 springs you can have along with the retainers that I just took off, they were not right for my cam, but may work for you
 
#35 ·
Glendale auto on orchard lake just north of grand river has a machine shop as well.
What do you think about them compared to Best Racing? I stopped by Best today and got a quote of $525 labor only to cut the 2.19 intake valves, tank them, surface, and do a valve job including setting up the springs etc.
 
#34 ·
I think that will work with your 4 speed, by the time you start to move, you will be around 1800 at a minimum
 
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