Compression ratio calculation

[satchel]

Does anyone know how to calculate the change in compression with a certain change in gasket thickness. I believe I have read the stock gasket is around .052in thick. If I have .01in milled off my head and .008in decked off the block, and I use a oem gasket, is there a way to calc the new ratio? Oh and what is stock anyway. I've read 8.7 and also 9.

As a side note, I'm planning on the new turbo and already have stock bolts and head gasket and would prefer to use those unless that seems like a big no no to the turbo wizards. No plans to go to more boost than what this initial kit offers.

 

[scottryana]

Your compression will change about 0.2

You will be fine and the OEM gasket and OEM bolts are fine.

 

[satchel]

Perfect thanks. Actually one more question I just thought of. Assuming my bump up in piston size .1 over bore to a 4.6 along with the changes above still not a problem but that is in play as well. No idea how much extra compression that adds to the mix.

Your compression will change about 0.2

You will be fine and the OEM gasket and OEM bolts are fine.

 

[satchel]

Using the summit compression ratio calc, I'm thinking including the .1 overbore I'm at close to 9.6??? Anyone way smarter than me (that's everyone) feel free to chime in.

Stock, I think..

.1 overbore

If the bore alone changes the compression by about .4, and sounds like the head milling/engine deck netted me approx .2 additional compression, am I correct to assume I'm around 9.6?

If so, I'm assuming that number is too high for 8psi, but would love to be told I'm good as is, or that I'm really bad at math.

 

[Heckraiser]

I'm not sure on the compression numbers, but here are a some thoughts to consider:

If you're talking about the wits' end turbo kit, it is designed to work well with a stock engine and stock fuel management. If you increase the displacement of your motor, you are going to spool your turbo faster, which could potentially put you in boost before your computer has switched over to closed loop and started dumping fuel, contributing to a lean condition (this is where things get hot and potentially break). If your cylinder walls are in good shape, I would not bother with the oversized pistons. I also believe the wits' end kit is setup to run approximately 6psi, not 8. I frankly don't see the harm in boosting an engine with compression in the ~9.5:1 or lower range, but ultimately your air/fuel gauge will be the judge.

Or if you're planning on stand-alone fuel managment, just build it however you like and tune to suit. Do you have CAC testing in Erie? Are you in Weld Co or Boulder Co?

 

[satchel]

Yeah, I'm already at a 4.6 now so no turning back on that. I do have CAC here, in Weld county. I'll admit this stuff is way over my head, but yeah, was hoping to run the wits end kit. Not really wanting to design my own.

I'm not sure on the compression numbers, but here are a some thoughts to consider:

If you're talking about the wits' end turbo kit, it is designed to work well with a stock engine and stock fuel management. If you increase the displacement of your motor, you are going to spool your turbo faster, which could potentially put you in boost before your computer has switched over to closed loop and started dumping fuel, contributing to a lean condition (this is where things get hot and potentially break). If your cylinder walls are in good shape, I would not bother with the oversized pistons. I also believe the wits' end kit is setup to run approximately 6psi, not 8. I frankly don't see the harm in boosting an engine with compression in the ~9.5:1 or lower range, but ultimately your air/fuel gauge will be the judge.

Or if you're planning on stand-alone fuel managment, just build it however you like and tune to suit. Do you have CAC testing in Erie? Are you in Weld Co or Boulder Co?

 

[Heckraiser]

Well in that case, go for it If you spool too quickly, just put a restrictor (dent) in your downpipe or add an extra cat or something

 

[satchel]

Haha, ok. Interesting stuff for me to noodle on. I guess my main questions at the moment are:

1) is my math above correctish, that if I use a stock HG I would be around 9.6
2) if 9.6ish is correct, is that ok with this kit
3) would it be better for me to use a thicker gasket to reduce compression and what would be recommended?

Sorry if I'm coming off a bit dense.

 

[scottryana]

People often quote the 4.6L but it's less than a 2% change in displacement going from 100mm to 101mm.

 

[satchel]

Ahhhhhhh, I think I see my issue. I need to just CC this thing when I get it back, but I was assuming I needed to add .1 inches to the calc for the overbore instead of 1mm. That sounds much more reasonable and nets a .15 ratio increase and not .4. Thanks!

People often quote the 4.6L but it's less than a 2% change in displacement going from 100mm to 101mm.

 

[coax]

@satchel how is your setup working out? I’m in a similar boat; bored +1 mm. 6/1000taken off the block and expected 10/1000 off the head. Considered going w cometic gasket but would prefer to just use the oem gasket and bolts I already purchased. Will probably run no turbo for a while but I did install oil bung for turbo sometime

 

[satchel]

@satchel how is your setup working out? I’m in a similar boat; bored +1 mm. 6/1000taken off the block and expected 10/1000 off the head. Considered going w cometic gasket but would prefer to just use the oem gasket and bolts I already purchased. Will probably run no turbo for a while but I did install oil bung for turbo sometime

Going pretty well. Have about 2500 miles on it at this point. I'm not sure I can tell a difference on power but I may just be used to it, hard to say. I've been using mid grade but I may try to go with low grade and see how it sounds. I was the same with the gasket. Found differing views on the reliability of the cometic and most people mentioned that with <7* of boost the stock gasket is fine so that's what I did.

I think I may also dyno mine soon to see if they can dial in the timing any and see what my afr is. I'm at stock timing but curious with the difference in compression if I should go forward or back a tad.

And after CCing my engine I believe I am somewhere around this compression.

 

[coax]

Going pretty well. Have about 2500 miles on it at this point. I'm not sure I can tell a difference on power but I may just be used to it, hard to say. I've been using mid grade but I may try to go with low grade and see how it sounds. I was the same with the gasket. Found differing views on the reliability of the cometic and most people mentioned that with <7* of boost the stock gasket is fine so that's what I did.

I think I may also dyno mine soon to see if they can dial in the timing any and see what my afr is. I'm at stock timing but curious with the difference in compression if I should go forward or back a tad.

And after CCing my engine I believe I am somewhere around this compression.

View attachment 2349821

Awesome thanks! Good to hear it all went together and is running well. I wasn't sure how much to make of the slightly lower stack height of the decked block and head...so this thread makes me feel better about just running what I got. I guess in grand scheme of things in terms of both compression numbers and distance from timing chain gear on crank to head...its such a very small amount its not worth worrying about. So once I get my head back I'm just gonna toss it on with the OEM gasket/Bolts and run it Thanks!

 

[satchel]

Awesome thanks! Good to hear it all went together and is running well. I wasn't sure how much to make of the slightly lower stack height of the decked block and head...so this thread makes me feel better about just running what I got. I guess in grand scheme of things in terms of both compression numbers and distance from timing chain gear on crank to head...its such a very small amount its not worth worrying about. So once I get my head back I'm just gonna toss it on with the OEM gasket/Bolts and run it Thanks!

Yeah I believe the minuscule amount of slack it created in the timing chain is made up easily by the timing chain tensioner.

Follow back up once you get yours running and let me know how it feels to you. Mine is certainly smoother than it was, but I'm curious to see how you feel about it power wise. I don't think I'm getting the power that @powderpig does on his 4.6 builds even though the shop did similar work on my head to what he does (same machine shop, port and polish and all that) but I am certainly not the magician he is either.

 

[Saito]

Had these guys’ compression ratio calculator since 2002. Way good. Awesome company.

http://www.performancetrends.com/