Technical Help w/compression ratio

ICONs have 17cc domes, I think that’s figured in their CR chart already.

 

Compression Ratio Calculator - Wallace Racing

 

Since I owned a few of these,are you sure of compression ratio or just going off what you think it was?

 

The piston is a 402 standard bore, so the 396 needs to be +0.030". The piston info listed on the link shows a compression height of 1.765", so at standard deck height and the listed stroke and rod length the piston is down 0.020". The crown is listed as minus 17cc. All built into their calculator, but needed for others.

 

Just to muddy the waters a bit more, quench (distance between the piston and chamber at TDC) and cam specs including centerline have more to do with tuning than just a mechanical compression ratio number. Try to measure everything you currently have and then find a program where you can enter all this stuff.

Oh, and squirrel has a LOT more experience than most here.

 

Minus 17 cc? I thought it he was saying it’s plus 17 ccs. If it”s minus 17 CCs , and .020 down, that thing ain’t have enough compression to run! I though it meant it displaced 17 CCs more than a flat top.










Bones

 

That's the way they express it in some calculators. The minus means it subtracts from the total cylinder volume. It confused me at first, too.

 

Then you need a 90 cc head or a bigger dome piston or a different cam or a blower and a different cam.

Stuck on 10.5 to 11:1 compression because of a cam? The heads will measure the Cc they actually are vs advertising if you measure them, solid info. Bore times stroke will give you swept volume, solid info. Next is the variables @ having some parts assembled to measure to finish your calculation.

I thought built a combo that theoretically would yield 10.7:1 based off published info and when it was actually built and actually measured it was actually just 10.38:1.

 

Why not shoot for a lower compression ratio to make the engine more street friendly and less likely to suffer from detonation/preignition? Gas is about 60 cents a gallon cheaper when you don't have to use high octane all the time. Shoot for about 9.5-10 and you don't have to worry if the block and heads were previously machined a little. When you shoot for 11, those deck and head remachinings can put you in a range you don't want to be in.
If its THE correct engine for the car, you might want to just put it in a crate and save it and bolt a different (454?) in it. Last thing you would want to do is damage an original documented engine for a car like that...........

 

With aluminum heads you can easily do 10.5:1 CR, but even 11:1 CR might have detonation with premium pump gas....you just wont hear it. But I agree with ekimneiro that I'd want a lower 10:1 CR if the car sees much street driving on today's crap gas.

 

Ok, ok. I’m not using a CR calculator, I’m doing the old school math way, you know pie are square! The same way I did it 55 years ago, but then it took me longer, as I did it with pencil and paper, with long math. Today with a calculator built into my IPad , it’s real easy!








Bones

 

I thought he mentioned, or was mentioned a+16/17cc dome?
Also quench is not looked at from piston to head, but from the flat area on the piston t the flat area on the head, right?

 

True, no matter how much math you do, and how precisely you measure, the outcome is usually somewhat different than expected. I built a .030 over 392 hemi several years ago and the math said 10.55:1. When it was all bolted together and the burette was filled with oil it measured 10.83:1. I imagine that wasn't exactly right either because some of the liquid would have seeped through the gap in the top ring, but the 2nd was gapless so the margin of error would be minimal.

 

I’m sorry, I’m really confused.
All this for a cam ?!?
In a documented engine in a(guessing) numbers matching kick ass car.
Why not run a different cam?
Not trying to be an ass or undermine you.
But seems like a lot of hassle and cost for a minuscule gain.
Most of this can and should be answered by your machine shop in assembly and when sourcing parts.
Unless your doing it all and the burden falls on you.
Good luck, sounds like a cool project.

 

I also took a slide rule class in college, 1969! About the time portable battery powered calculators came out! I couldn’t afford a Texas Instrument SR10 at a $150 in 1972. It could figure square root. Now I check out at the supermarket and see calculators for a dollar that can do it. I made a dollar every two minutes, when I retired, back then the SR10 was close to a half a months pay! Crazy!






Bones

 

I heave access to plenty of no corn 93, sometimes I mix in some leaded aviation gas and can run Race Gas Concentrate tho’ doubt I need it.

The cam requires at least 10.5 and I want 11:1. Very difficult to find small cc heads for 396/402.

The 396 is date code correct, but no numbers matching as they were cut off last time block was surfaced. We can get any #’s we want stamped on blocks these days.

It’s beginning to look like pistons might have to go back unless I can cut enough metal for close to 11:1.

Comments re: real world vs calculated are well taken. I’m trying to grease the engine builder’s path. Don’t think he will take great pains to “get it right”. If I can get him the parts for 11-11.25 CR, it’s easier to reduce compression than to add ?

What’s a safe amount to cut off heads and how much would the cut help ?

 

If your trying to get the maximum compression ratio for you existing parts,the only way to get there is to deck the block and mill the heads.Unfortunately the closed chamber has a small deep chamber vs the open chamber being more spread out and shallower,you'll probably reach the minimum thickness of the deck surface of the head before you reach the cc's you want,so that leaves you decking the block too,which you can do to the point of leaving some of the piston sticking out of the top of the bore just make sure to keep the top ring down in the bore,if you choose this route you have a lot of math ahead because you have to make sure that you don't have more piston sticking out than head gasket thickness plus .010-.015.This route will also involve a lot more machine work because you will have to mill the intake and china wall surfaces to accomodate the lower head placement.The additional machining costs will likely meet or exceed the cost of a different cam or pistons.Also the recommendation of minimum 10.5-11.00 to 1 CR by the cam manufacturer is just that a recommendation I've personally run a cam that had 10.5-11 to 1 CR recommendation in a 432 cubic inch engine at 9.5 to 1 CR and while its street manners were a little degraded I thought it was still livable,but I am pretty tolerant of a lopey idle.

 

The Icon minus 41cc pistons will get you where you want to be.

 

Those might get past where he wants to be!





Bones

 

This is the info you need to know, this is my 355 when I was figuring out what cc head to get a certain compression.

 

Whether block is decked enough to get close to a zero deck assembly or the piston is still far enough down at TDC to allow the use of a steel shim...with the 101cc chamber volume you are floating around near the 9.6:1 compression(give or take .15 depending on final actual spec and gasket used) area with that 17cc dome.

You can't just throw steel shim gaskets into a zero decked assembly, or you will bounce the piston off the head and junk it all.....minimum assembled piston-to-head clearance is about .035".....so a .018-.020" steel shim and a zero deck piston will not go. Rod stretch, pin flex, metal expansion due to heat, piston rock, crank flex, block flex are all small factors that can add up, eating .010" of cold measured clearance when hot and running at high rpm.

You either need a smaller CC combustion chamber, by at least 15cc if you want 11:1 compression, .....or the same amount of additional CC added to the 17cc piston dome, for a 30cc dome volume or better with the current heads.
Whether there is a 396/402 shelf stock piston out there at near 30cc I have no idea....it might be custom only at this point....in the race of Big Block cubic inches, the 396/402 has pretty much been thrown on the scrap pile by most piston companies....not enough sales volume anymore to warrant carrying a bunch of different dome volumes.

At around 400" engine size, every 1cc pretty closely equals one tenth points (0.1)of compression

You can't mill the head enough, deck the block enough, or use a thin enough gasket to get you where you want to be....mechanical and dimensional conditions will not allow it to be run that way.

If you've got to have that magical 11:1 compression figure, your gonna need a different set of pistons....since a good low cc chamber volume head is non-existent.
If you have to go custom, look into RaceTec/AutoTec pistons, owner is the guy who started up JE Pistons years ago.

If you go the big dome route, you may want to overshoot on dome volume a 2-3 cc amount, since it is likely with a modern piston dome design you are going to have to clay up the entire dome and check the entire chamber for dome-to-chamber interference, or just being too close to run safe even if it clears on the engine stand.....you'll very likely have to clearance either the chamber or the dome in a couple of spots each cylinder.

 

The Icon 9949 hi dome piston according to the pictures is clearly an open chamber design dome shape and will require some clearancing on the sparkplug side of the dome to clear the closed chamber.A closed chamber design hi dome will basically look like the 9948 piston dome only much taller.One thing to keep in mind with the hi domes was that they were so tall that they could sometimes interfere with flame travel so make sure it has a flame slot such as the one shown on the pic of the 9949 piston.Even the old TRW hi domes years ago had the slots so I would think newer versions would too,just something to verify.In your original post you talk about wanting to confirm CR once the engine is built but this is something you will need to address before the build,once the engine is assembled you have what you have and the only way to adjust is to mill the heads and run a thinner gasket and that will only adjust a limited amount.

 

The Icon IC9949 minus 41cc piston is described as a closed chamber design. Listing shows 10.6 CR with standard deck and 119cc heads. So again, until the actual deck height of the block is a known this isn't going anywhere. The OP said his target CR is a minimum of 10.5:1 and really wants 11:1. But yes, the minus 41cc piston may need some slight milling to get back down to 11:1, depending on a bunch of part details we don't know. But looks like perhaps a better starting point than the minus 17cc pistons based on the amount of work to get to his stated goal. But I would not buy them until some of the actual details get fleshed out.

 

But, he has 101 CC heads, the 41 CC dome will take the CR way high.






Bones

 

Do you have the factory 396 pistons, rods, and crank? If so stick one in and see how far it is down in the hole at TDC and compare to the factory spec. The amount it's higher than spec is what the block was decked. But if it was decked and they stuck rebuilder pistons in you are back to square one without chasing part numbers. If you can nail down the deck height you can play with the calculator to work the rest of the parts out to get your desired CR.

 

Some good comments here, and somewhat over my head (and I’m afraid over builder’s head)

Assuming 17cc pistons and a block previously milled .01. Would it be reasonable to cut deck an additional .020 ?

Tech at ICON calculated 17 cc pistons, 101 cc heads and .20 steel shin head gasket might yield 10.25 CR, at O deck height.

IF I could cut additional.020 off deck, I might be at a CR I can use ???

(Engine has Hyperutectic pistons from prior re-build currently)

 

The Icon spec sheet calculates out to the piston being 0.020" down with the standard deck. So if the block has been previously cut and you cut another 0.020" the piston will be above the deck . So no, that won't work. You could zero deck it, but until you get some info on current deck height you won't know what it will get you. Or just do it and live with whatever it turns out. Or zero deck it and take some off the heads and call it good.

 

Thanks. !

(What a mess!)

 

Actually, not a mess. It just a process of shorting out the parts recipe before buying the parts. You should understand it if you are going to be building an engine. If you feel at the moment it's above your head there are a number of books that can get you up to speed. It truly not rocket science.