Decking Heads / compression ratio
 

A Couple related questions:
What is the stock compression ratio on 1.6? (9:1 ish?)

On stock components...
How much clearance do I have on the heads before the valves and pistons become kissing cousins, (even if a timing belt snaps)?

And, how far do you have to machine the head to achieve a 9.5:1 and a 10:1 compression ratio respectively?

If anyone knows, or could point me in the right direction I'd appreciate it.

this is a turbo forum.

WOAAAAH!
:makeout:
:hustler:
:jerkit:
:fawk:
:2cents:
:laugh:
:facepalm:
:noob:

(none of them fit )



You lookin for an all motor build?

:hustler::hustler::hustler::hustler::hustler:

Yessssss!!!




(from a "compression" search on "search")

let me point you in the right direction:
low compression: lots of spark advance
high comp: lots of slow gayness and detonation

I'm making a substantially more torque on my 8.6:1 motor with a gt2860rs than about anyone on a 9.0:1 static motor. So much that I was surprised. Take a look through my build and dyno threads...the #'s speak for themselves. My car isn't sluggish off boost either, I wouldn't change a thing if I had to do it again.

Now, flame away and tell me about how my turbine-housing is too big, my compression is too low, and how I constantly pull the wool and how you've never seen a naked chick in your life, bro. Bubble hard in the BRG, flex the roll, with the top dropped, holler back...

So i can ask it a different way.
I would like to run 8 lbs of boost. I'm running a Greddy system (not the greatest) with a piggy-back (not the greatest^2) and don't feel like tuning a high boost car with just fuel pressure or getting a piggy-back fuel management system. High power is not my goal with this build. Nor am i trying to make more than 180 horse on this car.
Usually the recommendation would be to drop the compression, add thicker head gasket for instance, then add boost to make power. I personally don't want to tune above 10 lbs w/o a stand alone, nor am I going to buy one because of my financial situation.

I have my motor apart right now and am just going to use stock components putting it back together b/c I don't have any $$ for forged components or lots of machine work. In fact, I'm taking out a loan just to fix the car because I have to have it running. What I can do is deck my heads for free. My plan is to push up the compression a tad and run mid-to-low boost.

What is the optimal compression ratio on stock components without running into heat and detonation problems?

I would imagine like 9.5:1 or maybe 10:1; even though that's pushing it.

Next question is how to achieve that ratio with just decking the heads.

... closer to a 'turbo forum' question now or does 'head work with boost application' fall under a separate category? It sounded like an "engine building question" to me, but I could be mistaken.

It is interesting to correlate 'high compression' with 'slow gayness and detonation.' If i wanted to run 11:1... I'd say that's just an N/A build or a bit ridiculous if you wanted to add boost.

My question is; given 8 lbs of boost, what is the highest compression ratio you can safely run; and how to achieve it. I assumed I had a good idea what that ratio would be and wanted to ask a simple question.

It is also interesting to correlate intent with a technical question. I'm not here to compare my nuts to anyone. I just want to try and upgrade my car within the means of time and resources I have available to me and hoped someone would know.

I think y8s has been running boost on his 10:1 stock pistons for a while now so its clearly possible (I'm not sure if the 10:1 pistons still clear the valves in the event of a cambelt snap, anyone care to comment?)

You have the engine apart so the easiest way will be to measure your engine. I wouldn't feel safe machining bits based on information from someone else's motor.

Also there is no way in hell you are going to raise your compression from 8.8:1 to 10:1 by having your head skimmed. Only way is to use domed pistons with recesses cut for the valves if you are worried about cambelt snapage. Unfortunately you are out of luck for drop in replacements because all of the higher comp pistons are for 1.8's so its going to be an expensive experiment.

If you have a really good buddy who does shit hot welding you could try adding material to the combustion chambers between the valves, thought this will require you to do a lot of grinding and polishing so that all of the cylinders are "cc'd". But it's a good excuse to port the shit out of the head while you are there.

Some people I know have tried adding material to their pistons, they also then had some interesting crankcase ventilation so I wouldn't advise it.

Let us know how you get on.

HAH wow i never thought i could find someone soooooo gay, do you know anything about your engine at all. Or did you just buy everything you see that you want and slap it together. Let me guess you bought a kit or parts that someone said will work b/c they put a lot of time and energy researching and making custom so you can whore it out and say you are fast?

The head was finished being ported today. Also was free, along with dyno tuning, decking, valve job, laping, and porting the chambers.

Free is good......Gimme

.... so 8.8:1 is the stock compression ratio on a 1.6L...

that's all I really wanted.

Thanks

Intake ports mild port job
 

Attachment 206873

just a pic of the head i ported going on this car, exhaust pics to come. Still needed to be polished when this was taken

The thing you might not fully appreciate is that 0.5psi will give you more return on investment than going from 9:1 to 10:1 in your compression ratio. And your increased ratio will preclude further gains that can be achieved when you are in a better financial situation. It is a foolish move by any measure for an automobile that will be used on pump gas.

You and the raven fellow are obviously in need of a book or two in your private libraries regarding turbocharging gasoline engines. I would recommend "Maximum Boost" by Corky Bell as a point of embarkation.

Raven: Don't be so foolish as to mock The Hustler when he is offering wise advice. There is nothing unsound in his statement except the holler back part.

And he never said he wanted to go to 10:1 all he wanted was to know what the stock compression was, nothing more. He does not want to run more than 8 psi b/c he is not going to upgrade the fuel system, so he is going to pull a little more compression to give mabye only 3 more hp but 3 more is better than 8.5 psi and a blown engine. And i have built quite a few forced induction engines, last one put down 731 to the tire.

Should have used a limited slip diff.

:laugh:

then, at your gay-lower power goals, stick with a stock motor. Spark advance manifold pressure, and fuel ratio are fun variables to play with. Read my threads, read other's then see what you want to do.

The sad truth is that you're a fucking moron and don't understand that 8psi from a 2554 is different from 8psi with a 2871...so leave the motor in stock form, buy a 2554, and check out my tail lights while your girlfriend is rimming my sphincter on the way home from the chili-cook-off. She's a whore.

If you registered in 2009 and posted in this thread, you have absolutely no fucking clue what you're talking about.

This thread sucks, so does the idea. If there is nothing wrong with the stock engine, why open it to machine the head to raise the compression .5, which will only increase the chance for detonation and yield little to no gains. I machined my head when I rebuilt as well, and wish I could undo it. I'm probably at around 9.6-9.8:1

i saw the reported post. he's got some more bruises to take first.

I'm pretty sure that's EXACTLY what he did.

Why don't you read up a little...lots of info on this forum, but if you come into class thinking you're the professor, you won't learn shit.

Either your reading comprehension skills or your memory are failing you desperately. Smurf asked for several bits of info between his first two posts. Are you sure he didn't say he wanted to go 10 to 1?
And we were pointing out that doing something potentially counterproductive was a fool's venture. Further, it should not be undertaken by anyone needing a reliable street car that is already REQUIRING A LOAN to reassemble.
And I know a couple of Puerto Rican guys that build 9 and 10 second forced induction drag cars that don't have a clue why most of the things they do even work. They just copy the successes of others. Building a powerful engine isn't difficult at all. Explaining the intricacies of the relationships between the components and the variables involved can be somewhat more consuming.

True; so maybe I introduced my question all wrong. I probably should have introduced myself and my situation first instead of posing it like a performance mod-type question.

I was loosing coolant, about .5 gallons every month for like 3 months w/ no external leaks. I was letting my fiance' drive the car while I'm away at school and I didn’t have time to look at it. Compression test: the middle two cylinders had low compression, about half as #1 and 4; probably a head gasket ---> took off head and just planned on replacing gaskets. Looked at the pistons...

Pictures by Smurfn90 - Photobucket

Doesn’t look the greatest, and I didn’t know if there was ring involvement, and I seriously started to question what was done to the car before I got it. Take the motor the rest of the way down, and the first compression rings were carbon-caked onto the piston seemingly not holding compression, w/ blow-by to ring #2. Have a friend who knows my situation, said he’d do my head-work for free, including a port job (who would turn down a free port job), and take a couple thousands off to the head to make sure the surface is true, especially since it over-headed. What is the stock compression? If it’s 9.5:1; take off the most minimal amount possible. If it’s 9:1, there is some wiggle room, up to 9.5 because I’m not going to run high boost. I know everyone here would want me to run higher boost or help me make more power, but this will be my daily driver for the next 2 years and I’ll only have all of a parking lot and a socket set to fix the car in 2 months b/c I’m moving.

I was hoping someone who has machined enough miata heads would just say: 8.8:1 and don’t take it past 5 thousandths b/c you run into clearance problems or push the compression too high to reasonably run boost.

Let me know what you think.

Not sure where you got that number. What Gompers tossed your way started at the '94 (1.8) model year. Not sure about yours, but my '90 1.6 came with 9.4:1 pistons.
Reading comprehension is your friend on this site, Smurf'n. Your initial guess of 9-ish:1 was pretty good, though. You might want to check the Power Mods section at miata.net for more answers on head milling as related to compression ratio.

Leave the compression also, mill head only if nessessary (if the surface is warped). Spend your time balanceing the stock rods and piston. Clean up the head (ports, bowls for casting inperfections. Then wait till you get the funds to up-grade. cause there will be no turning back once you take off .030-.050 from the deck :(, but replacing the head.

get a real job too, so you don't have to get a ghetto loan.

you dumb shit. have you ever heard of a thing called dynamic compression? whether you raise the boost level or the compression ratio you are effectively doing the same thing to cylinder pressure. soooooo either way; fucktard, you have to do something in the way of more fuel / less timing. so if you don't have the means to run more boost, you don't have the means to run more ccompression.

hustlers advice is sound lower compression means larger volume. so in turn for a given dynamic compression ratio, a lower static compression ratio will allways yeild more power because there is more room to cram in air and fuel. think about it like this; if somebody were to kick fuck your anus with a pound of C4 on their boot and light it off there would be a big explosion. now if that same guy were to put two pounds of the stuff on his boot and proceed to stomp a mudhole in your rectum there would be a bigger bang. he ain't gunna to put it up there with any more pressure than before because his leg is only so strong. there will just be more of it under the same pressure. the only reason one should mill the head is to make it have a flat surface again.

So this thread is not completely worthless...

If anyone actually wants to know how much decking their head will effect their compression ratio, I just did the research myself, broke out a spreadsheet and answered my own question.


Stock compression on a 1.6 is 9.5:1
Stock gasket thickness is 1mm (or .040 in)
Bore/stroke 78mm x 83.6mm

Calculate the stroke volume and combustion chamber volume. (and i'm going to assume that decking the head would give comparable combustion chamber volume changes as changing the head gasket thickness).

I used HKS's info on their head gaskets to make sure the compression ratio and thickness of the head gasket change would be a linear relationship to make the math easier... didn't see any reason for it not to, but its good to have at least 3 points on a curve.

their gaskets: 1.2 mm = 9.37:1 CR
2.0 mm = 8.72:1 CR

With the spreadsheet. I made CR a function of head gasket thickness (and/or change in combustion chamber volume) using the stock parameters. I plugged HKS's info in and got:
1.2 mm = 9.33:1
2.0 mm = 8.73:1
So a tad off for the 1.2mm measurement, but good enough for government work.

Now, all I had to do was plug in various height changes that could be made after a head is machined.

So from my original question before going crazy-off topic...

How far to machine to 9.5:1?
Retarded question knowing now that that is the stock compression (which is why it was my first question). I think, then, it would be really, really good to know how far you can deck the heads before you hit 10:1.

According to my calculations; I figured it to be around 0.55mm or (0.0217 in).

Now you now you loose/gain about 0.5:1 compression ratio per .55mm or .0217 inches assuming the stroke volume isn't changed from stock 1.6. That may help if you have your heads decked and add a different size head gasket and want to know where your compression ratio will land after both are done.

The heads were decked today, and it took 0.012in to achieve a true surface. All things being equal, that puts my compression ratio to about 9.77:1.

If you want to discuss boost vs. compression physics, you already have a thread that's great. Use the search option and find "Compression Ratios and Forced Induction."

If anything, someone should at least recognize I've gained about a quarter point in compression; according to the rule of 4%, that's at least a 1% net gain (rough estimate I know) and the heads will flow better being ported, both increasing my engine efficiency. No one else cares but me, but if the option is to put the engine back together stock, or get free horsepower (although small) I'll take the free horsepower.

No measureable gain, just a bigger war against Pump gas vs. more boost = Detonation. It's your car so do what you like, although .012 is more then the machine shop usually takes off to true the head. Some times you have to do what you have to do to get it right. Well good luck with that rebuild, and hurry up summer is almost here.

Thanks LowBoostn. I was hoping someone would recognize that 12 thousands is a bit much to take off, or caution me about valve clearance and let me know how much clearance is in the 1.6. The engine did overhead the first time the coolant was low and after pulling the head off, the head gasket didn't look that bad, provoking me to look further for the significant compression loss. I thought I may have to get close to the tolerances if the head was warped or misshaped and wanted to know how far the shop could go (original intent of the thread) and if that would significantly affect compression.

It should be together next week and we'll see how it looks after the break-in and tune.