Compression ratio? - Impala Tech
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post #1 of 13 (permalink) Old 10-24-2019, 12:19 AM Thread Starter
 
 
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Compression ratio?

Hey guys, need some engine guru help. Trying to learn the math on building an engine but its making me dizzy. Heres what i have:3970010 350 block bored .030 over. Has 345NP .030 pistons that have 1.540 compression height. Camel hump heads 3782461 2.02 valves(64cc?). Im trying to figure out compression ratio to decide what cam to get and what head gasket to get. I measured my piston at TDC from deck and got .045. Am i over thinking this to build any decent power numbers? Im hoping to get some advice from people who have done a set up like this before with good results given the parts i am working with. Thank you in advance!
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post #2 of 13 (permalink) Old 10-24-2019, 02:34 AM
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Couple of things:

One if you have not decked the block and heads you can not seal it with a stamped steel stock shim type gasket. this removes material and raises the static compression ratio. So you will need a 0/037 fiber core stainless steel ring style head gasket to to seal the motor. So you can remove 0.020" from the cylinder that you use to calculate your staic compression or just go with what ever piston compression the book calls for.

Two any job worth doing is worth doing right. So I recommend reading one of these two books:

https://books.google.com/books/about...d=du-5bBg2M4EC

https://www.amazon.com/Vizards-Max-P.../dp/1932494847

Big Dave
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post #3 of 13 (permalink) Old 10-24-2019, 11:14 AM
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I use summit calculator to calculate compression ratio. Summit compression ratio calculator

Assuming that the heads are 64cc (461 casting i have measured were anywhere between 62cc to 66cc) and head gasket is regular fel-pro with compressed thickness .039, piston .045 in the hole, your compression will be 9.91:1.

The problem here is quench that needs to be taken into consideration. if your piston is .045 in the hole and you use .039 this gasket, your quench is .084. That might be too much for a iron headed engine with close to 10:1 compression. It might be prone to detonation. you could get better quench by using thinner head gasket, something like .020, however that wold increase your compression ratio to 10.37:1 which will also cause pinging on pump gas.

Here is a good article on quench.
https://www.hotrod.com/articles/ideal-quench-height/
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post #4 of 13 (permalink) Old 10-24-2019, 12:14 PM Thread Starter
 
 
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I measured the gasket that was on it and measured roughly .040 at the ring, so that put me at about .085 quench?would it matter if those camel humps have been ported at all?
When i got this car it had a edelbrock torquer single plane with a 750 carb, way too much for the streets around me, did this previous owner know what he was doing or was it like the rest of the car? Lol you dont even wanna see the driveshaft he "made"
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post #5 of 13 (permalink) Old 10-24-2019, 01:09 PM
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Quote:
I measured the gasket that was on it and measured roughly .040 at the ring, so that put me at about .085 quench?would it matter if those camel humps have been ported at all?
First problem with old heads is they are not original. By the end of my days as an engine builder (before I got really sick) I couldn't find a set of untouched fuelie heads. They had all been cut to increase compression and some where also angle cut so that there were no steam holes by the spark plug (a sure way to get into detonation as the plug will act like a diesel glow plug long before it ignites the fuel as a spark plug). So you have to CC the heads to determine the combustion volume (hopefully they both measure the same).

Porting the heads has no affect on the compression ratio but a valve job certainly does. Every time you do a valve job you remove metal from the floor of the combustion chamber. With old heads they may have had many previous valve jobs, to the point where the valve seats have sunk below the floor of the head. This sinking means you are loosing lift. You could have to subtract 0.030" to 0.070" of an inch from your gross valve lift no mater what cam you choose.

The other issue with old heads is valve guide wear. If the guides are worn then the valve can not be held concentric to the seat. Therefore the valve will not seal as well as it should. Additionally with worn guides you get oil flowing down the valve stem into the chamber. This is a not only embarrassing in having a smoker; but oil in the combustion chamber lowers your octane rating on what ever fuel you put into the engine (why racers such as myself use a Moroso oil/air separator to prevent the oil from blow-by from getting into the intake and causing detonation).

Finally depending upon the year of the head the valves differed in size which means the valve spring diameter changed with size. There were two sizes of valve spring diameter on the SBC (three if you include the BBC springs used on aftermarket heads). The valve spring pocket may have been cut sometime in the past to accept a stiffer spring or a taller spring that could put you in a canoe (to quote Monty Python in their Socrates sketch) which is "F**king close to water".

This is also a problem with amateur head porting jobs. They don't have sonic checkers or years of experience in porting heads and some of the metal that has been removed could also turn your head into a canoe. A consideration a street head that sees a lot of heat cycles.

Turning from head issues to the block. Chevy usually put the piston deck down in the hole about 0.025" plus or minus 0.010". So there is a wide range in deck height. Without measuring your motor with a caliper you have no idea where your pistons are sitting. There is also a problem of the block warping just like the head does from heat cycling. So there can be (0.015 to

Older engines had piston domes if they were high compression engines that required "high test" back in the sixties. High octane gas is no longer available at the gas pump any more. Today's best is 93 octane, but most states have a best of only 91 octane. Considering that back in 1966 through 1972 regular gas was 89 to 92 octane and high test was 100 to 104 octane. So if you have a high compression engine (11.0:1 or more) then the factory expected you to burn 100 octane gas. Today that is aviation fuel (why it still exists at the gas pump at the airport flight line is because all of the engines built back in the sixties are still running in old Piper, Cessna, Beechcraft's general aviation aircraft still exists.

Today we don't use domes if it can be avoided as the dome obstructs the flame front's path. This results in incomplete combustion, or detonation if the dome prevents the spark from seeing the back side of the dome. Today pistons are flat topped or dished, and we use a smaller combustion chamber to increase static compression.

Big Dave
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post #6 of 13 (permalink) Old 10-24-2019, 02:14 PM Thread Starter
 
 
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Thanks big dave, your knowledge makes me want to read more and more about how engines have advanced over the years!
My heads have had been dug into a time or two, valve guides have been replaced with thick bronze. Am really thinking about bringing the block back and deck cut, luckily i only have mains and crank in, oh cam bearings too. I did put number 1 piston and rod with new bearing to check deck height, so its not like i have the whole rotating assembly in. Cutting the deck sounds cheaper than buying new pistons that bring it further up in the hole. This budget build might take a lil longer than anticipated
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post #7 of 13 (permalink) Old 10-24-2019, 02:32 PM
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Quote:
This budget build might take a lil longer than anticipated
They always do. Usually more expensive than anticipated as well.

I'm ancient and have been playing with cars since 1959. I am a mechanical engineer that owned my own ten bay garage just to support my racing habit. Used to have employees working at the garage during the day and I would come in later at night and build race cars or motors for people after getting off my day job.

My best friend worked at Chevrolet as a line technician; and I used to hang with him every day before going to dinner then the garage. I became so well known at the dealership there that the local dealership sponsored my race car with parts purchased at dealer cost. I also became familiar with how Chevy thought about cars in terms of dollars and cents not performance or safety. Warranty claims drove design changes and made the car cheaper to produce maximizing GM's profits.

Big Dave
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post #8 of 13 (permalink) Old 10-24-2019, 05:38 PM
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Before you decide to do anything cc the heads so you know exactly what you have. Keep in mind decking the block will increase the compression. If you deck the block .020 you will be at10.4:1.
Another thing, do your 461 heads have accessory holes drilled?
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post #9 of 13 (permalink) Old 10-24-2019, 07:13 PM Thread Starter
 
 
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Thats kinda neat how you were so close to chevrolet like that big dave! Also, my 461 DO NOT have acc holes. I was going to cc the chambers, what kind of fluid can i use? Washer fluid or plain water? Can dished pistons take my CR down?
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post #10 of 13 (permalink) Old 10-24-2019, 09:10 PM
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Originally Posted by jmiller1119 View Post
Thats kinda neat how you were so close to chevrolet like that big dave! Also, my 461 DO NOT have acc holes. I was going to cc the chambers, what kind of fluid can i use? Washer fluid or plain water? Can dished pistons take my CR down?
Quote:
Also, my 461 DO NOT have acc holes
That means they were cast before 1968 when accessories holes first appeared. Fuelie heads (aka camel humps) first appeared in 1957 on the Rochester mechanical fuel injected 283 engine.

Quote:
I was going to cc the chambers, what kind of fluid can i use? Washer fluid or plain water?
You can use either. I use rubbing alcohol with some blue food coloring in it sop I can see the level. I'm lazy and alcohol evaporates without me having to dry anything. You are lucky as SBC heads are all under 100 cc's. So you can use a cheap measuring pipette. With a BBC you need to buy a rare 250 cc or a 500 cc graduated cylinder that costs hundreds more.

Quote:
Can dished pistons take my CR down?
Will drop your compression like a rock.

Big Dave
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post #11 of 13 (permalink) Old 10-24-2019, 09:41 PM Thread Starter
 
 
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My heads are sept of 64 by the casting off the top of my head.
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post #12 of 13 (permalink) Old 10-25-2019, 11:12 AM
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I would either go with dish pistons or different heads.
Not having accessory holes on the heads can be an issue.
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post #13 of 13 (permalink) Old 10-26-2019, 11:02 PM Thread Starter
 
 
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Just got a set of 062s for dirt cheap, came off a running 99 with spun bearing. No visual cracking but will have them fluxed this week, now gotta dig into getting the right pistons
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