BaddBird

iTrader: (6)

243s milled .020" with mls.051" gaskets 11:1 compression
243s milled .010" with .041" gaskets 11:1 compression
Am I missing something or is the end results the same other than the change to the DCR? Do people choose to mill more off the heads because its cheaper to mill then to mill and use thinner gaskets?

chrs1313

iTrader: (24)

that will not create the same compression...

milling heads more & a larger gasket will give a lower compression ratio vs

milling heads less and running a thinner gasket...

milling the head chamber is not the same area as the complete bore which a thinner gasket provides...

also a thinner gasket will give a better quench and will resist detonation better...i.e. running higher compression and still being able to run 93...

for reference I am at 11.9:1 and still get away with 93 and have 0 knock...I run just a splash of 100 to be safe but it would be okay if I didn't... .040 cometics here

vettenuts

iTrader: (4)

I think more people mill more because using the thinner Cometics have resulted in leaks and the OEM gaskets are more forgiving on surface preparation. I checked flatness and surface roughness to make sure it was within the requirements established by Cometic.

69LT1Bird

iTrader: (1)

....and the stock head gaskets cost half as much.

Check how far your piston is above the deck, that is critical in your selection and the final CR number and quench measurement.

chrs1313

iTrader: (24)

I never really understood the 100 dollar cheaper reason...most people that do heads and cam swap are at least 2k deep anyways what's another 100, to get a better quench and push compression safely higher & be less prone to knock...

69LT1Bird

iTrader: (1)

If you don't really need it why waste the money, that's the idea behind it.

thunderstruck507

iTrader: (18)

aren't the Felpro replacements .041 too? They are cheap...

EDIT: well looks like Felpro has thinner ones and the stock thickness replacements...guess which ones I have sitting on the shelf I was planning to use?

The_Rizzle

iTrader: (4)

A thinner head gasket is more comparable to milling the block, and will do more to bump compression since there is more volume removed from .010 (or whatever the net change would be) of a full bore dia, than 3/4ish compared to a chamber area.

This also brings the piston closer the to head, which is mostly a good thing, until you get too close lol.

BaddBird

iTrader: (6)

Is not optimizing quench going to be a matter of leaving power on the table with an 11:1 street car or will it be a matter of not being able to run 93 without detonation?

chrs1313

iTrader: (24)

the way I see it is you want to get the most compression you can for a given setup and running a thinner gasket will give you that vs milling cause you will need to mill more to get to an equal cr the thinner gasket provides...

how can anyone say you don't really need it...it allows you to run a higher compression safely...so if you can get away with 11.5:1 on a stock mls, why not run a .040 and get away with 11.9:1...I just see it as optimizing the setup...

11.9 is even less prone to knock vs the 11.5...

quote from Patrick g's 500rwhp thread

"Compression is your friend, especially when running a larger camshaft. Raising static compression will really pump up torque, but so will raising dynamic compression. Every 1 point increase in static compression ratio (SCR) will increase your torque 3-4%. There are limits to how much you can run on pump gas, but you can cheat this limit with tighter quench and better tuning.

For my quest to 500 rwhp, we chose to get static compression into the upper 11s and dynamic compression into the upper 8s. A 57cc chamber coupled with flat top pistons and 2cc valve relief made my compression fairly high for pump gas, but we got some additional help with a .041" head gasket (Fel-Pro 1041). The thin gasket raised our compression from 11.50:1 to 11.92:1 (compared to a stock .053" gasket), but the tighter quench made the motor less prone to detonate at 11.92 than it would at 11.50:1."

and even if you are running 11:1 compression and can only lose .010 clearance due to ptv...the added compression of running a .040 vs .010 mill is around .25 compression ratio higher...so .25 * 3-4% increase in power on a 450 rwhp car is 3-4.5rwhp...

for 100 and getting 3-4.5rwhp more isn't bad at all...

1000 fast setup would have to gain 30-45rwhp over an ls6 intake to have the same ratio...

just a different look at it, hope it makes some sense, I know not all agree

BaddBird

iTrader: (6)

Will the Fel-pro 1041s work on a stock bore ls1?

The_Rizzle

iTrader: (4)

11.9 with optimized quench is even less prone to knock vs the 11.5...

Just so people don't pull that sentence out of context, i fixed it for ya.

At 11.5 you wont get 3-4 rwhp from a .25% bump in compression. Maybe 1. You can't use the same percentage increase at higher comps vs lower comps. You will gain a slight increase in efficiency, but again, barely noticeable at that slight bump.
If quench has already been optimized to where it works with an aluminum block (expands more, so should be closer COLD) you wouldn't be able to romp on it until it is good and warm. A .040 thou gasket w/ ~.007 out piston gives you .033 cold. You could, It you did it incrementally, get that down to .025 cold and still be ok w/ an aluminum block. BUT you'd need your pistons to be clearanced the same way, otherwise you'd hit just from piston rock, and make sure they all protrude from the deck the same amount. That would be an optimized combo.

chrs1313

iTrader: (24)

thanks for the edit, also for the percent increase I was referring to people who didnt want to flycut and still want to mill a little, it would be better to get the .040 for the cr bump over the same mill distance...so most likely they are still in the 10.x:1 range...

that is true about deminishing returns with higher compression...but we are not talking 15:1 or 16:1...

just trying to help people think through the whole process...might seem expensive for gaskets but in the whole scene of the project it really isn't that much more...

BaddBird

iTrader: (6)

Well in my case I need a clean up mill of .007" so
Option 1 Mill heads .020" with mls .051" gaskets and let quench fall as it may roughly 11:1
Option 2 Mill heads .010" with .040" gaskets with a tighter quench and a little over 11:1
If I understand this right you subtract the above deck hieght of the piston from the gasket thickness.
Option 1 giving me .044" quench
Option 2 giving me .033" quench Based on .007" above deck (rough numbers have not measured yet)

If I got this right we are only talking maybe 2hp if that between the two. If I am incorrect please correct me.

chrs1313

iTrader: (24)

running the smaller gasket will give a higher compression vs milling the heads the same distance...as stated above...

depending on what numbers you use the cr with the thinner gasket in your second situation will be .2-.25 higher...some say stock is .051-.054 etc etc...given the low side of 3 % increase for every 1 point in compression you would be around 3hp gain and high side of 4% at around 4hp gain...even call it 2hp if you want...

so 100 more for 2 hp...sounds silly but we do it all the time...ud pulley, fast intake over ls6 intake, ewp, 2400 afr's or TFS usually add 45-50 hp so that is right in line...2.1hp/$100...

anyways just another way to look at it...

The_Rizzle

iTrader: (4)

you're also getting better quench with the cometic gaskets, on top of the ~2hp. This can help make the tuning window bigger, and make it less prone to knock around pk torque.

You also have to consider whether you have an aluminum block or cast. The .044 would be ok on a cast block, but an ally motor will want the .033 as it expands more (slightly)

Mighty Whitey

What kind of gains could be had going from an lq4's 9.4:1 CR to an 11:1 CR? Boltons, 23x/23x cam, etc??

BaddBird

iTrader: (6)

Since the ally expands would that not make the quench area tighter? From what I understand ideal quench for a street car is between .035" to .045".