Compression ratio question

[jmlcolorado]

So I picked up an oldsmobile 350 that a guy had built in 1998 but never finished. He let me take the heads off to inspect, let me take the pan off for the same reason. This thing is clean as a whistle. Brand spankin new!
It's got big block aluminum heads on it milled down to shrink the combustion chambers a bit. All his paperwork shows the combustion chambers were 67cc.
Out of my own curiosity, I wanted to check the compression ratio.
I CC'ed the heads to find that they are 67cc. It has 20cc dished pistons (I CC'ed these as well), and with all the numbers plugged in, I have 8.3:1 compression on this engine.

My question is, I know lower compression ratios are better for boost, but I don't plan on forcing air into this right now (due to financial reasons).....
So will 8.3:1 be good enough for a good performing street engine?
I have the engine torn down to the short block right now, so I could opt for flat top pistons and that would take me to 10.1:1. I would just need to check valve clearance to make sure I have the room for flat tops.

 

[Tripper]

For sure it should run great on the cheap bogus gas they're peddling these daze but if u can jump up close to 10:1... I'd do it but I'm a HP junkie!!! [ddd[ddd[ddd

BoB

 

[jmlcolorado]

For sure it should run great on the cheap bogus gas they're peddling these daze! [ddd[ddd[ddd

BoB

Sweet!
So you think I could leave the CR alone?
I'm thinking if I bumped it up to 10.1, I'd likely have to run better fuel, maybe not premium, but certainly not regular. Good call, I didn't think about that.

 

[Tripper]

Sweet!
So you think I could leave the CR alone?
I'm thinking if I bumped it up to 10.1, I'd likely have to run better fuel, maybe not premium, but certainly not regular. Good call, I didn't think about that.

You should be able to run regular easy with 8:1 but if u keep those slugs I'd at least make a bumpstick change!

BoB

 

[jmlcolorado]

You should be able to run regular easy with 8:1 but if u keep those slugs I'd at least make a bumpstick change!

BoB

Well. The only thing I DON'T know about this engine is what cam is in it.
I have documentation on everything else including the machine work.
The heads do have much larger valves in them.

I did send comp cams my info and they recommended a Dual Energy, 275DEH
High Performance Street cam with duration @.050” 219/233, lift is .476/.508 on a 110 LSA.

I'm getting the impression that I should stick with the current setup on the CR, but change to a cam.

 

[Old Iron]

The specs or part number is stamped on the end of the cams or, you can check the lift with a dial indicator. You can also use a dial caliper.

 

[donsrods]

If you go to 10 to 1 you WILL be running premium, regular is going to ping on you for sure. But that is ok, I have to run premium in my 27 and also my daily driver, it just costs a little more at the pump. They do make pistons with less of a dish than the ones you have that would get the compression up into the 9 to 1 range, and those will be more streetable.

One thing that I would definitely check is how much they cut off of the heads because on an Olds (or a lot of other engines too) it changes the angle of the intake mounting surface. We played h*** getting a good intake seal on the 455 Olds in my Son's rpu and had to have the intake cut to seal properly.

Also, do not use the tin gasket that they used from the factory, use something like Mr Gasket 404 composites with The Right Stuff sealant in place of the rubber end seals. Olds intakes are harder to seal in our experience than lots of other engines because of the way they cast the manifolds. They are a little thin in the areas where the bolts go through into the heads. If you want to keep hot oil off the bottom of the intake, Mondello sells a tin shield that bolts into the lifter valley and will do that.

If you go with a cam change consider the Thumper series. My Son is running a Comp cam 280H and our engine guy said he would have preferred a Thumper in there for the split overlap characteristics and cooling benefits.

Don

 

[jmlcolorado]

Well I pulled the cam out. I googled the legible numbers and found nothing of any help.
I did fond some castings in it and it seems like this cam might a NS cam. NS meaning Nothing Special
So, Im thinking of leaving the pistons as is. And sending the rest of the specs to Comp Cams again (I think I now have mor accurate info) and let them recommend a cam for me.
Here's some picture fun.

 

[bob w]

The general opinion is 4% increase in horsepower for every point in compression. Weigh that against the cost of pistons and installation. Although it appears you already made that decision.

I've had good luck with Comp cams.

 

[jmlcolorado]

The general opinion is 4% increase in horsepower for every point in compression. Weigh that against the cost of pistons and installation. Although it appears you already made that decision.

I've had good luck with Comp cams.

So from 8.3:1 up to 9.1:1 I should expect roughly 32% increase in HP?
That's pretty substantial.
It's funny you mention the ratio vs hp cause this ties into my next post.
I'm up in the air on cams. I already have matching edelbrock performer rpm parts, and their website lets me choose a cam with all the parts working with each other, but they suggest 9.1 compression ratios with the cam I'm looking at to work with the heads and intake I have now.
They are a stimating 396hp and 400ft/lb.
realistically, I wouldn't want to go much above this for this truck. It would be a handful.

 

[jmlcolorado]

So here's a thought. Since I already have Edelbrock performer rpm heads and intake, maybe it's an option to utilize an edelbrock cam as well so everything is matched together.
Cam specs:

Camshaft: Performer RPM
Part #: #7112
Vehicle Type: OLDSMOBILE
Engine Application: 350-403 V8
RPM Range: 1500-6500

DURATION AS ADVERTISED INTAKE: 290° EXHAUST: 300°
DURATION @ .050 INTAKE: 224° EXHAUST: 234°
LIFT @ CAM (Lobe) INTAKE: 0.31" EXHAUST: 0.325"
LIFT @ VALVE INTAKE: 0.496" EXHAUST: 0.52"
LOBE SEPERATION: 112° INTAKE CENTERLINE: 107° IDLE VACUUM @ 1000 RPM: 12"

The information below is for verification of
opening and closing events.
INTAKE TIMING @ .050" LIFT: Opens: 5° BTDC
Closes: 39° ABDC
EXHAUST TIMING @ .050" LIFT: Opens: 54° BBDC
Closes: 0° ATDC
OPTIONAL VALVE SPRINGS: #5712

CAM FEATURES: 397 HP & 400 ft.-lbs. with #7111 intake & stock iron heads.
INSTALLATION NOTES: Use Edelbrock Performer-Plus Timing Chain and Steel Gear Set #7813. Do not use late model timing chain and gear sets that are designed for emission-controlled engines. These timing sets are machined in a retarded position and are not recommended for this camshaft installation. Edelbrock Timing Sets feature three keyways for specified timing selection.

Footnote 1: Adjustable pushrods or rocker arms required.
Footnote 2: Do not use high pressure racing valve springs. Use only recommended Edelbrock Sure Seat Valve Springs #5712.
Footnote 3: Use stock ratio rocker arms only
------------------------------------------------------------------------------------------
CompCams recommended cam:

Part Number: 42-210-3
Grind Number: 275DEH
Lifter Type: Hydraulic
Engine Family: oldsmobile 260-455
Description: Hydraulic- hi-performance street
Cam Family: DUAL energy hydraulic flat tappet
Specifications
Intake Exhaust RPM Range: 1500-5800
Valve Lash:
Valve Timing: 0.006
Duration: Intake 275 exhaust 282
Lobe Separation: 110
Duration @ .050" Lift: Intake 215 Exhaust 233
Intake Centerline: 106
Valve Lift: Intake .0476 exhaust .0508
Lobe Lift: Intake .0298 exhaust .0318
Valve Timing @ 0.006 Lift:

Exhaust
Closes 27 ATDC
Opens 75 BBDC
Intake
Opens. 32 BTDC
Closes 63 ABDC
------------------------------------------------------------------------------------------
My worries on each are that the edelbrock cam requires use of factory ratio rocker arms. And I doubt 1.6:1 is factory for olds, but I'm not sure.
The other think I see a pretty big difference in is the valve lift. This is the amount the valve opens right?
The other huge variable I see differences in is where the valves open at. Both seem very far off from each other?!

 

[Kurtm333]

Close

Seams pretty close to me? After the heads and pistons ar together measure it? See how much room you really have or have a machine shop measure for you.

They would be able to tell you if the 1.6 - 1 rocker arms will work also. You might have to relief the heads or make sure the valve springs don't bind at full lift, probably lots of options just need to find the least expensive one.

Good luck
Kurt

 

[gold03]

My small block Chevy liked about 9.6:1 compression, premium gas, stock tires on a 1970 Malibu, 3.08 gear. When I moved it into a 1951 Dodge M-37, 5.83 gears and 39 inch tall tires it would run OK on regular gas.

My ramble means this. your compression ratio choice is determined by vehicle weight, gear ratio and tire size. Higher compression is more forgiving if the car is real light and the engine can get RPM up fast with minimal load. If you are going to power a 1979 Cordoba with 2.73 gears... lower your compression. If its a model T you can get away with more compression. As long as you keep it cool. Premium is pricy for a daily driver. If its a weekend warrior, premium is no big deal.

You have to know what you are going to do with the car, be realistic and then make your decision.

gold03

 

[jmlcolorado]

My small block Chevy liked about 9.6:1 compression, premium gas, stock tires on a 1970 Malibu, 3.08 gear. When I moved it into a 1951 Dodge M-37, 5.83 gears and 39 inch tall tires it would run OK on regular gas.

My ramble means this. your compression ratio choice is determined by vehicle weight, gear ratio and tire size. Higher compression is more forgiving if the car is real light and the engine can get RPM up fast with minimal load. If you are going to power a 1979 Cordoba with 2.73 gears... lower your compression. If its a model T you can get away with more compression. As long as you keep it cool. Premium is pricy for a daily driver. If its a weekend warrior, premium is no big deal.

You have to know what you are going to do with the car, be realistic and then make your decision.

gold03

Thank for your input Gold! All good advice!
The truck will not be a daily driver, just on the weekends. And with the family, it might not even make it out every weekend. So premium fuel isn't a show stopper.
The truck is geared 3.07 right now and will have a 700r4 to gain overdrive as the miles it'll see will be a solid mix of side streets and highway.
I certainly want a bit of kick to the truck, and the current parts, as well as the remaining parts I have lined out, set this engine up for good power.

I'd rather to pay a bit more to really enjoy the truck, then pay a little less and kinda enjoy the truck

 

[jmlcolorado]

Check THIS out!
Same block, same heads, I could use the same pistons and work up to the same CR. this engine, which is damn close to mine swung almost 400 hp and a touch over 410 ft/lbs!
I'm not saying that the two engines wild be identical, and even if they were, they wouldn't make the same power, but what if I build mine with the parts in this one as a baseline?
They used flat top pistons, which tells me I should be able to make them work in mine.