305 "416" heads on 305 .40 over...What would the comp ratio be? - Hot Rod Forum : Hotrodders Bulletin Board
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Old 03-23-2011, 09:04 PM
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305 "416" heads on 305 .40 over...What would the comp ratio be?

Hey all. I am budget building a 81 malibu station wagon. I just bought some used 17" Panther PCW wheels for 220 dollars delivered. I am looking to replace the motor and transmission, as the motor that came in the car was choclate milk, there for, probally cracked block/heads or bad gaskets.


From what I have seen, the intake and head gaskets were good. the intake gasket had a suspect area around the passenger rear water port, but who knows.

Anyway, I am crossing my fingers that the 416 heads are good, as they are built, and have 1.94 valves!! that means someone put 350 valves into them, so alot of work was done to them, they are clean, and look like new.

----> My good friend is selling me a 305 longblock for 50 bucks(CDN) that is .30 over and ran good up until he sold the heads off of it. I ovbiously have all the other parts, and IF, my 416 heads check out good, then I will have

58cc 416 heads on a flat top 305.

WHAT WILL MY COMPRESSION RATIO BE, see attached old skool compression chart. This does not help me as my 305, is .30 over.

How thick of head gasket can I get for my 305, to lower the compression ratio. I am building a daily driver engine/car.

As far as I remember( its been about 5 years since I was bulding small blocks) 9:8 will run on 89 octane(which is our lowest)

Can I get away with 89 octane on this motor??????????????????????????


Thanks all! 25 and still learing! Hell, my 81 Malibu is older then I am by 4 years
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Old 03-23-2011, 09:38 PM
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You can calculate it here
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Old 03-23-2011, 09:46 PM
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The heads are no longer stock so the chamber volume is now different.
Usually larger once 1.94 valves are fitted.

If you want an accurate answer you need accurate input.

CC your heads and measure the piston deck clearance at TDC.

http://www.wheelspin.net/calc/calc2.html
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Old 03-23-2011, 10:21 PM
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Hmmm, I agree with your response, and all though I wish I had an accurate awnser of what the combo would be. Could you lay out a guess.

Also, could you awnser my question? How high of compression ratio ,works on 89 octane? In other words, how high can I go before pre detonation or "pinging" as others call it.

I have no problems with running a thicker head gasket, as this engine WILL NOT see high RPMS. This combo will be for a daily driver, and I know for a fact, the higher compression, with a properly tuned carb and timing setup, will be great on gas... Also, I know the gear ratio, and synthetic fluid options will ovbiously add to the economy, but I will be modifying those once I have it on the road..

I am going to put the engine combo together even if I don't have a mathimatical awnser to its compression ratio... But I sure would like to know before hand if someone has experience in this area of mechanics...
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Old 03-23-2011, 11:21 PM
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Quote:
Originally Posted by GMMAN
... I don't have a mathimatical awnser to its compression ratio... But I sure would like to know before hand if someone has experience in this area of mechanics...
You got something against using the calculator(s) linked to above?? The one at wheelspin FB88 linked to is the one I use. With the other calculator, I do not know if a dished piston volume is entered as a negative number or not.

Your bore is 4.04
Stroke is 3.48
Head gasket bore is around 4.1
Head gasket thickness is from .015 thick to .041 thick. Enter the thickness you plan to use.
The combustion chamber needs to be measured, you could guesstimate it at 60cc for a rough estimate.
If you have 4 relief flat top pistons, estimate the dish volume as "-6". The stock, round dish 350 SBC pistons are around -12 to -14cc.
Deck clearance is around .025 stock, this needs to be measured, too.

For the best economy, performance and to use the least octane, the distance from the top of the piston to the head should be as close (but not less than) 0.040" as possible. If the piston measures 0.025" in the hole (like many stock engines are), you'll want to use the thinnest gasket possible.

If you want to use a steel shim gasket, this needs a good smooth, flat surface to seal correctly. A thicker composite gasket will seal better to a surface that isn't freshly milled- as long as it's relatively flat.

So as you can see, there's no way to just tell you what you want to know, re the CR of this engine.

Last edited by cobalt327; 03-23-2011 at 11:27 PM.
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Old 03-24-2011, 12:35 AM
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Quote:
Originally Posted by cobalt327
You got something against using the calculator(s) linked to above?? The one at wheelspin FB88 linked to is the one I use. With the other calculator, I do not know if a dished piston volume is entered as a negative number or not.

Your bore is 4.04
Stroke is 3.48
Head gasket bore is around 4.1
Head gasket thickness is from .015 thick to .041 thick. Enter the thickness you plan to use.
The combustion chamber needs to be measured, you could guesstimate it at 60cc for a rough estimate.
If you have 4 relief flat top pistons, estimate the dish volume as "-6". The stock, round dish 350 SBC pistons are around -12 to -14cc.
Deck clearance is around .025 stock, this needs to be measured, too.

For the best economy, performance and to use the least octane, the distance from the top of the piston to the head should be as close (but not less than) 0.040" as possible. If the piston measures 0.025" in the hole (like many stock engines are), you'll want to use the thinnest gasket possible.

If you want to use a steel shim gasket, this needs a good smooth, flat surface to seal correctly. A thicker composite gasket will seal better to a surface that isn't freshly milled- as long as it's relatively flat.

So as you can see, there's no way to just tell you what you want to know, re the CR of this engine.

I appricate your detailed response tonight. I just learned many indepth details I never knew, or cared about ,before tonight that is.

After much measuring, tonight I relized that the valves are infact stock size 1.840 and 1.40. The head gaskert surface has definatly been decked, but I did notice at the ends of each head, a wee bit of taper on my straight edge.

Also I was searching, and if the intake surface was not decked as well, that could pose as bit of a gasket problem, and maybe that is where the engine was getting water into the oil and not oil into the water.

looks like I am going to get the heads checked out, before I chuck them onto my flat top 305.

Heres hoping they are good.
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Old 03-24-2011, 04:53 AM
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stock size exhaust valve is 1.50
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Old 03-24-2011, 11:41 AM
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Seems my eye was off a bit last night then. It is indeed 1.5

I couldn't see any cracks, but the castings sure are rough in the exhaust ports, I might just polish them up a little bit.
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