Technical Help w/compression ratio

Lots of details if you want an accurate compression ratio number.

In addition to knowing the volume of the head and the (protrusion) and (dish or pop up) volume of the piston, you need the compressed head gasket thickness, fire ring diameter, piston top land diameter, top ring height from the piston crown, and bore diameter, as well as an accurate crankshaft stroke number.

We used to have an old, mimeographed worksheet at work for the calculations back in the latter part of the last century when we would do the calculations by hand back in the engine lab.

 

At this point, the best thing you can do is go get actual measurements of the rod length, and the distance from the deck to the Piston pin. Till you get some actual measurements, you can't make a logical decision.
It would actually be best if you could use the same rod/pin at each corner of the engine so you know the deck is somewhere near level and what it's actual deck height is. We had a previous thread on here and when the fellow got his engine back from the machine shop, his deck varied .010 from front to rear. That's enough to ruin your day ( and engine) if you build it with minimal deck/head/Piston clearance. You can't deal with it till you get good dimensional information by actually measuring.

 

If you think the advice given here is over your builder's head, find a better builder before you do anything else.

 

Ever get the idea that maybe the OP is the builder? Anyone that is an actual engine builder would know how to calculate compression and that is done BEFORE a camshaft is ordered. That way you know what you have going in and what to expect. Using off the shelf parts takes compromise in some things. I always use my simulator to "build" an engine before the first part is bought.

 

I’m in a rural area. Many “engine builders” don’t pull engines. Others don’t have space to store car in dry. Some don’t have a working relationship with machine shop. Still others insist on replacing every moving part. Some allow no customer input as to part selection. Some suggest an 8:0-1 454 truck motor.

In a perfect world, I’d move to SoCal long enough to get engine built, then move back.

Otherwise, I try to make the best decisions given choices I have.

 

We all started out knowing little or nothing. Its very easy to make a bad choice on what you are doing and when a problem happens......all those people deny that they are responsible and point the finger at you or someone else involved in the process. The guy who built the engine isn't going to remove it from your car, and the guy who put it in the car isn't going to remove it for free.
What we are trying to tell you is to take the time to ask questions here and learn to do some basic things yourself. Once you do that, things begin to make sense. You are trying to start out with a combination that is fraught with more difficulty than many engines because of the unique valve angles of a big block engine and piston crown necessary to make the compression where you want it.Its a hard/expensive lesson if you make the wrong decision. I would think seriously about putting the 396 aside for the present time and buying something that is running and easy to put in the vehicle. I would stick with a big block. If you have the money, you might want to look at a GM crate engine because you get a warranty with it. Warranties from machine shops often don't turn out well. Get the car running and enjoy it. Then buy some of the popular books on the market and read them. You will become more knowledgeable and you can progress on the 396 in a slower learning curve. Right now, I think you are trying to do the right thing but just don't have quite enough experience to do it.




These books will help you a lot and in the end, save you more money than they cost you. If you have difficulty measuring your engines components, check Ebay and Facebook Marketplace and buy some reasonably priced used measuring tools. Its really not difficult. Just read the books and then ask if you are unsure how to check something. We all want you to succeed in building an engine, but you have to take the initiative to learn more about what you want to do. Right now, the choices you offered up look like more bad choices than good ones. You also might want to join a local car club and some of the guys will usually tell you the best choices and shops. Good Luck!

Also, what "rural area" are you located in? Maybe you are nearer to someone on the Hamb or they know someone near you.

 

A GMPP, seriously ?

“Right now, the choices you offered up look like more bad choices than good ones. You also might want to join a local car club “

This comment begs the ?,
IYO. What bad choices are you referring to ?

Most at local car club members I’ve met can’t change out spark plug wires and think Edlebrock carbs rule. Out of 50 or so members, not a one has or knows anyone that has a Holley jet tray. They are well versed on feather dusters and car wax, tho’

 

Might be that the easiest, cost effective solution may be to tame the cam down some?
Was the cam choice based on RPM range advertised?

 

It’s high school math. You need to do a lot of measuring, and some basic geometry. It shouldn’t be over your head. It damn sure shouldn’t be over an engine builder’s head. If it is, you need a new builder.

Custom blueprinted engine builds to exact specs aren’t cheap. There’s a reason for that.



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Not surprising in today's world, the ICON tech guy is also clueless.
Can't run a zero deck with a .020" thick shim gasket....that isn't enough piston to head clearance to be safe....one missed shift and rpm buzz over the redline and you just blew it up bouncing the pistons against the head...
So the fact that comes up close 10.25:1 on a zero decked block is just worthless info, since you can't run it that way.

If the block was only decked .010" and you've confirmed that, the .020" steel shim would put you at 10.0:1 comp, but at a risky .030" total piston-to-head clearance. Up to you if you wanted to roll the dice and gamble on that....auto trans is more likely to never have a problem with that low clearance....but a 4-speed and potential missed shift flare of rpm well beyond redline could be fatal.

A more reasonable solution is a regular composition gasket, then take the available #9949 41cc dome and mill some of the dome off it, get it down to a mid-to-low 30's dome cc and set the compression ratio right where you want it that way.
It is listed as a solid dome, so it could be milled clear down to a flat top if someone wanted to, no worries there.

In best practice, with no previous competition build and usage evidence to guide you, going below .035" total piston-to-head clearance is not advisable.....comp engine builders who do creep up on it, pushing the clearance limit down until they have a collision problem, them add a little more back into the next build and call it good for their race program....
A "only build it once situation" like you have is not the place to push the limit, since you don't want to have to test and rebuild to find your limit you have to stray to the safe side. You need to stay in the .035" or more zone if you want to be safe.

Jim - Jmountainjr is n the same page as I am.

 

Don't give up on this, but do read up. One final engine spec only casually mentioned is quench. It's the clearance between the flat part of the piston and the flat part of the head at TDC. On a BBC the typical street spec is 0.035" and 0.045". It becomes of extra concern if you zero deck the block. The info your Icon "tech" guy gave you is flawed from a total build perspective. With a zero decked block the compressed thickness of the head gasket controls quench distance. So it should be obvious that a 0.020" steel shim head gasket is not the answer. Anyone accepting money to build an engine should know all of this. But you should understand all of this as well.

 

I think it’s more likely the “builder” is a guy that reassembles stock components with a little machine work to clean up surfaces and maybe a slight overbore. There’s enough safety factor in most stock engines to get away without precision measurements and calculations.

Asking this builder for a high performance blueprinted build is like expecting a medium rare wagu steak from the local McDonalds.


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Can't run a zero deck with a .020" thick shim gasket....that isn't enough piston to head clearance to be safe....one missed shift and rpm buzz over the redline and you just blew it up bouncing the pistons against the head...

How does overrevving cause piston to hit head ?

 

My BBC is called out as 10.25:1 Cr, with as I recall 120 CC heads. 1970 engine Factory) never verified it or had a need to, but with smaller bore and head cc with a 396, is it because pistons are not available? Or is it just too much of living on the edge to the the CR that the OP wants?

 

Budget36,
There’s a low compression and a very high compression piston (that could be cut down) . Your 402 (?) and my 396 are becoming dinosaurs. Our small bores inhibit big valve heads used on high cube motors.

Current trend is bigger CI engines that are low performing. Much easier to build a 400 horse 454 than a 400 horse 265.

 

Over-revving causes the rods to stretch (non-permanently) a few hundredths of an inch, allowing the pistons to rise up enough to kiss the cylinder head flat area.

Loads increase exponentially with rpm, so at very high rpms the piston and rod assembly can act as if it weighs a couple tons.

 

Yes, rod stretch. bolt stretch, rod bore ovalization, pin flex, piston flex at pin bore, crank flex, block flex.

Bet you didn't realize at full song a typical street/strip build stock SBC or BBC block will twist more than 1/16" from one end to the other.
It was eye-opening for me when I saw David Vizard show that info.

It's amazing the forces that manage to be contained in so little cast iron thickness, really.

 

Ericnova72,
No fan of Vizard, here. I do know Bill Jenkins (and others) turned down the all aluminum 427, possibly 430 all aluminum BBC CanAm motor as it flexed too much. This speaks volumes as the weight savings were monumental. Aluminum blocks of those days were also too unstable temp wise.

I do know I’d one prominent HP engine builder today that decks out every SBC at “0”. Every BBC piston is at .003-.005 in the hole. I have one of his SBC’s.

Racers have been turning SBC’s to 9000 since the mid-60’s.

Back on topic....it appears my worries might have been for naught. Just noticed the same comments appear in cam catalogue for my old cam (which ran fine) that appear for new cam.

“ Mild Race Cams - Rough Idle, Low Vacuum. Not for Towing. Engine Modifications & Stall Converter Required. Compression 10.5:1 or 11:1. Spring Pressures - 120 lbs. On Seat - 300 lbs. Open”

This being the case, I’ll go for all the compression I can get w/parts I have
and be comfortable knowing outcome will be a little quicker than before.

Thanks for the constructive comments !

 

I'm glad you got it settled. As a retired Autoshop teacher I have helped many young guys (and a couple of girls) build their first engines. The ones that were happiest with the results were those that built proven combinations.

The guys here put forth a lot of good information and tried to help you get where you wanted to go. The problem is that you kinda did it bass akwards. Not trying to be a jerk, but the right way is to take all the given parameters and then figure out what is needed to make it work. The last thing you should spec is the cam, based on all the dimensions you have to to work with, bore, stroke, deck height, combustion chamber volume, piston availability etc.

As others have pointed out, if your engine builder can't or doesn't want to tell you exactly how he will satisfy your requirements, do it yourself. The one thing that you have to understand is that it will take way more time than you realize to do it right. The books sited will get you on the right track.
My method is to start with the engine teardown and measure and make notes on what you actually have as you go. Then you can see exactly what needs to be done to rebuild it to your blueprint specs.

I'm sure we would all like to see the progress on your build. And, I'm also sure that you will get plenty more advice if you do post it up on here.

 

Cam I chose (with tech support help)
is consistent
with cam currently in car,
which runs very sporty.

Only a change in LSA.

 

Are you aware that changing lobe separation angle will change piston to valve clearances even though lift wasn't changed? Also, advancing or retarding a cam will change that relationship, and some cam manufacturers incorporate advance into their cams. Last, verifying clearance between the pistons and valves should also be done 10 degrees before and after TDC. Piston to valve clearance is not just about the pistons and valves............

 

Yes.

 

I like the iron heads better myself. For a street cruiser, I like 454, iron 781 heads with 9:1 compression.. This engine prefers good 87 octane.. I like to run 22 initial a slow 14 mechanical in by 3000..

For the street, I prefer big cubes with low compression over low cubes with high compression..

 

I prefer overachievers.

Few know that before Chevrolet pulled out of racing winter of ‘63, most every SS driver drove a 409. Ronnie Sox had a car just like your pic.

 

I like iron heads also, but I changed out the iron 781 heads on my .040" over 454 and put Edelbrock oval port aluminum heads on, and the difference in HP was amazing. They really woke my build up, turned my gasser into an extremely quick street car, from a pretty quick street car.
And I agree about big cubic inches, and not too high compression. I have a high compression SBC in my other gasser in my avatar, and it's a PITA to run premium plus some race fuel or octane booster to make it run great and not get detonation with just premium gas.

 

I like overachievers.

 

Spend some money on your heads.
More power to be gained or lost in heads than any other parts.
If your married to the heads then get pistons made that get your compression up.
Not being a jerk, but having a negative opinion on Vizzard while this compression and quench clearance is admittedly over your head should lead you directly to a crate engine. I’d suggest blueprint engines thru summit.
Reading one of the books will help you anyway you decide to go.

 

Look here: gofastmath.com/Compression_Ratio_Calculators/Static_And_Dynamic_Compression_Ratio_Calculator

 

I believe this is the answer, sporty cams need more compression.

Looked up my old cam and it has the same “10.5-11:00” compression requirement as new cam. Since engine ran plenty quick w/old cam, I’m thinking any compression improvement will only help a good thing !

 

If you intend to run pump gas, you will want to keep dynamic compression around 8:1. A big cam/w lots of duration will reduce the dynamic compression ratio. However, a big cam is less than ideal for a street car.