Compression to high?

bdp55 · Dec 28, 2006

Hey Guys,

I have patriot stage 3 heads (I know you may not like them, but that isn't my question) A buddy of mine told me that if I use them it will raise my compression too much to be able to run a lot of boost. Is he right, or am I okay to use them. I building my motor this winter and once everything is back together and running I'm hoping for around 600 hp. Does anybody know if these heads are okay or will my compression ratio be too high?

Stan Weiss · Dec 28, 2006

It is a little hard to know what your CR will be. What will your bore and stroke be? What kind of pistons, how much dome volume? How many cc's are the combustion chambers? How thick a head gasket? What deck height will the piston be at?

mogs01gt · Dec 29, 2006

bdp55 said:
Hey Guys,

I have patriot stage 3 heads (I know you may not like them, but that isn't my question) A buddy of mine told me that if I use them it will raise my compression too much to be able to run a lot of boost. Is he right, or am I okay to use them. I building my motor this winter and once everything is back together and running I'm hoping for around 600 hp. Does anybody know if these heads are okay or will my compression ratio be too high?

If you are running the stock dished pistons, stock head gaskets and stock bore, your compression shouldnt change. Unless they did work on the combustion chamber, your compression ratio will not be affected. Now Im pretty sure they will use a valve that is flat, not dished in a little like the stock one. Those valves wont change a thing other than maybe .0# worth of compression.

Fast Eddie 41 · Dec 29, 2006

Like Stan said, it will depend on many factors...are you running the stock bottom end? In addition to the things that Stan Weiss mentioned....it will also depend on how much was removed from the combustion chamber.. and how much the deck of the heads were milled.

Gearbanger 101 · Dec 30, 2006

Long story short, the only thing that's going to change our compression ratio in this car, will be your pistons.....not your heads. If anything, the Stage III's might have the emissions mask ground off, which would in turn would actually lower your over all static compression by a tiny bit over stock PI castings.

If you're shooting for 600rwhp with boost and on pump gas, my suggestion would be to have pistons installed that will put you right around an 8.5:1 CR. The stock 9-9.2:1 would work as well, but with those kind of horsepower figures slightly less than stock compression is going to be easier to tune your spark and fuel curves.

LaserRed01GT · Dec 30, 2006

Gearbanger 101 said:
Long story short, the only thing that's going to change our compression ratio in this car, will be your pistons.....not your heads. If anything, the Stage III's might have the emissions mask ground off, which would in turn would actually lower your over all static compression by a tiny bit over stock PI castings.

Bad information. The deck of new cylinder heads is always milled to make the deck "true" as so the head gasket will seal as good as possible. How much they are milled will effect the compression height and consequently the SCR. Additionally "worked" heads will have some material removed from the combustion chamber. It all depends on how they go about the deshrouding and emission mask work. The piston is by far not the only thing that affects static compression ratio. When talking above the block deck, you also have the head gasket compressed (installed) thickness and head gasket bore. All of which will significantly affect SCR.

EDIT: We also still need to know if he is using the stock shortblock. Then we can calculate his SCR and DCR if he knows his cam specs.

Gearbanger 101 · Dec 30, 2006

LaserRed01GT said:
Bad information. The deck of new cylinder heads is always milled to make the deck "true" as so the head gasket will seal as good as possible. How much they are milled will effect the compression height and consequently the SCR.

If mine was bad, could we consider this statement worse?.....

You’re still thinking in terms of an OHV engine when you speak of milling the head affecting compression ratio. The old pushrod castings could be milled as far as the deck height on the head would allow. OHC heads when they are milled (if at all) are done so to the absolute bare minimum in order to clean up the deck surface.....no more than .005 to .010 is normally taken off, which has next to no effect on your static compression ratio since anything more can have a negative effect on cam timing (which a whole other ball of wax). As a matter of fact, I believe if the deck surface of the head needs to be milled more than about .012 (going from memory here), the head is either considered junk, or is no longer able to be utilize a standard head gaskets.

LaserRed01GT said:
Additionally "worked" heads will have some material removed from the combustion chamber. It all depends on how they go about the deshrouding and emission mask work. The piston is by far not the only thing that affects static compression ratio.

When I stated the piston was the “only” thing that affected static compression ratio, I meant to any significant amount. If you’ll note, I did mention that removing the emissions mask, which in turn deshrouds the intake valve or performing othe chamber work (polishing) would have some effect. Regardless.....removing a little material from the combustion chamber isn't going to "significantly" decrease it's volume. At least not enough to alter compression ratio to any great extent.

LaserRed01GT said:
When talking above the block deck, you also have the head gasket compressed (installed) thickness and head gasket bore. All of which will significantly affect SCR.

Since head gasket thickness has already been addressed above, why would you think his cylinder bore would affect things to any great extent? The increase in volume from stock to say a standard overbore of .020 is something like less than a single CC where these small bore engines are concerned, which is hardly a significant amount. That quantity could be made up with carbon deposits after a few miles.

As far as what his short block consists of.....I was under the impression I had already touched on that point by mentioning his piston (and accompanying rod selection). It's true, that cam timing will also play a significant effect, but I kept my example specific to static compression (which I failed to specify.....my bad ) with regards to stock vs. worked heads and piston selection. As stated before, cam timing is a whole other ball of wax, but it affects the dynamic ratio, not the static ratio.

Stan Weiss · Dec 30, 2006

The problem is in a 281 CI engine a couple of cc's here and there does make a difference. An engine with a 9.0:1 CR will increase to 9.5978:1 with just 5 cc's removed from the total volume.

bdp55 · Dec 30, 2006

Here is what I got so far. I have a forged cobra crank, 19cc Diamond Racing Pistons, Manley forged rods, patriot stage 3 heads, crower stage 2 or 3 blower cams (I haven't decided which to go with yet), and then the other mods mentioned in my sig. Do you think this will be a good combination for 600 rwhp? All the above mention parts, crank, rods, and pistons are good for atleast 800. I just don't want to get it all put together and then find out my c/r is to high to run a good amount of boost. My block will be bored over .20 or .30 (again haven't gotten that far so I haven't made that decision yet). Thanks for all you help so far

Stan Weiss · Dec 30, 2006

You do not have the information needed to calculate your CR. When a new engine is assembled from new parts the first time is to just check clearances. When that is done you will have the information needed to calculate your CR. If it is to high Diamond Racing Pistons can tell you how much can be milled from the piston. Remember you want to do this first assembly BEFORE you have the engine balanced.

LaserRed01GT · Dec 31, 2006

Let's see if gearbanger can understand this. Let's just assume for a minute that we have a stock motor for comparison’s sake. According to my personal measurements on a '02 motor I just experimented with, here are my findings:

3.552" bore
3.543" stroke
8.934" deck height
1.219" compression height
-15.78cc piston volume
42.44cc combustion chamber total volume (including valve dish to compensate for the lost volume of the emissions mask or swirl dam)
.0153" piston to deck clearance (deck clearance)
.030" stock head gasket compressed (installed) thickness
3.625" head gasket bore (fire ring bore)

I documented much more info, but the rest is not relative to use for compression ratio calculations.

According to the above findings, I calculate the SCR of this motor to be 9.746:1

Gearbanger 101 said:
Since head gasket thickness has already been addressed above, why would you think his cylinder bore would affect things to any great extent? The increase in volume from stock to say a standard overbore of .020 is something like less than a single CC where these small bore engines are concerned, which is hardly a significant amount.

I said "head gasket bore" not cylinder bore. Two very different things. I realize now that you don't know this, but the head gasket bore (fire ring bore) is always larger than that of the cylinder bore. Combined with the compressed thickness of the head gasket, this equates to head gasket volume which will obviously affect SCR.

Now that we've cleared that up and back to the SCR of this stock motor that I experimented with, let's assume just for a minute that we will keep my shortblock in question otherwise un-modified. Now let's assume we will install a set of Stage III Patriot heads and use a gasket with a stock .030" x 3.625" head gasket.

Most of the Stage III 2V modular heads that I have worked with have right around a 46cc combustion chamber. The set I have in the shop right now measure exactly 46.20cc.

That change alone in combustion chamber volume takes the stock compression I measured of 9.746:1 down to 9.273:1....or basically half a point. On a 281" motor, like Stan mentioned, is definitely significant and will change the way the motor is tuned and the amount of boost it will take.

Now with the last information that bdp55 posted, this of course will change the SCR as compared to a stock shortblock. Like Stan eluded to, without knowing the specifics of your final build yet, it is impossible to finitely calculate your SCR. But, will just make a educated guess.

bdp55, assuming going .020” over, having the block square decked, using the Patriot Stage III heads with a 46.2cc CC, assuming the piston is .010” in the hole (fairly common on your type of build using a Cobra 4340 3.543" crank and 5.933” rods), and assuming you use a .030 gasket (Cometic or stock), and the 19cc pistons, I “estimate” that you will be around 9.09:1 SCR….. this should be relatively close, BUT, just remember this just an estimate since we don’t know your final build specs.

Gearbanger 101 · Dec 31, 2006

I have nothing more to add. I don't have the hard numbers to work with (and most certainly not the experience) that you do by comparison and by the sounds of it, this type of works seems to be part of your lively hood and you seem to have a better grasp of the specifics on the subject than most of us.

Good Info.

bdp55 · Dec 31, 2006

bdp55, assuming going .020” over, having the block square decked, using the Patriot Stage III heads with a 46.2cc CC, assuming the piston is .010” in the hole (fairly common on your type of build using a Cobra 4340 3.543" crank and 5.933” rods), and assuming you use a .030 gasket (Cometic or stock), and the 19cc pistons, I “estimate” that you will be around 9.09:1 SCR….. this should be relatively close, BUT, just remember this just an estimate since we don’t know your final build specs.[/QUOTE]

Thanks for all your help, this atleast gives me a good idea to work around.

fastfive_ogt · Jan 1, 2007

Holy smokes people settle down, I believe I went through a mess of schooling also in automotive, not to mention have been a Ford tech for 2 years and other makes for a total of 10 years. You guys sound like a text book from school. Good information though.

LaserRed01GT · Jan 2, 2007

bdp55 said:
bdp55, assuming going .020” over, having the block square decked, using the Patriot Stage III heads with a 46.2cc CC, assuming the piston is .010” in the hole (fairly common on your type of build using a Cobra 4340 3.543" crank and 5.933” rods), and assuming you use a .030 gasket (Cometic or stock), and the 19cc pistons, I “estimate” that you will be around 9.09:1 SCR….. this should be relatively close, BUT, just remember this just an estimate since we don’t know your final build specs.

bdp55 said:
Thanks for all your help, this atleast gives me a good idea to work around.

Just let us know all the specs once the motor goes together and we can calculate your compression ratios fairly close. There are lots of little tricks to lower the SCR even more if should so choose. Once you know your cam specs, we can then talk about your dynamic compression ratio (DCR) and how it will affect everything. DCR will actually be the controlling factor in tuning and your boost delivery.

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