Will the real compression ratio please stand up?

[2332turboSF]

How do you guys build your engines? Do you base everything off static or account for the cam and use dynamic CR? Ive been doing a lot of reading and have come across this a lot recently. People around here throw around static CR like thats all that exists Either that or Im just not realizing when people mean one or the other.
For example, my new build has a static of 9.82:1 but because the FK44 duration my dynamic CR is 8.18:1 so I base all my tuning, fuel and boost levels based on 8.18:1 correct? Of course not taking into account my boost levels but just plain old CR before boost.
For example http://johnmaherracing.com/tech-talk/what-effect-does-compression-ratio-have-on-bhp/

[2332turboSF]

I need to add this also. Some of you may remember my disgust with how my previous combo ran off boost right? Well after doing the calculations based off the WAY TOO BIG cam that was in it before (320 advertised-287 @ .050, .400" lift at the cam (plus my 1.4 rockers put me at .560") on 110 LC. The static on that was about 7.5:1 and taking the cam into account I was running a WHOPPING...Get ready for it lol....4.4:1 Compression ratio before boost UN....BELIEVABLE! How the hell did it even run? LMFAO! No wonder I could boost 20lbs on pump gas haha.

[[email protected]]

you should base your "tuning" on what the engine wants, not some chart or calculation.

You can't "calculate" everything in advance. 9.8:1 will work well with the FK44, but on boost I'd drop down to 8.5-9:1.

People have the mistaken impression that "tuning" will be done in 2-3 hours. Try 2-3 MONTHS.
_________________
It's just advice, do whatever you want with it!

Please do NOT send me Private Messages through the Samba PM System (I will not see them). Send me an e-mail to john at aircooled dot net

"Like" our Facebook page at
http://www.facebook.com/vwpartsaircoolednet
and get a 5% off code for use on one order for VW Parts ON OUR PARTS STORE WEBSITE, vwparts.aircooled.net

[2332turboSF]

[[email protected]]

with turbo engines there are 2 approaches.

High CR and low boost, snappy and immediate throttle response. Lower peak power

Low CR and high boost, a dog throttle response but max power when it comes on.

You can't run high CR and high boost, you'll break stuff. If you have low CR and low boost you have a pig.

I think if you intend on turbocharging, you need to drop down to 8.5:1. NA 9.5:1 would work great with that cam, but that's not you.
_________________
It's just advice, do whatever you want with it!

Please do NOT send me Private Messages through the Samba PM System (I will not see them). Send me an e-mail to john at aircooled dot net

"Like" our Facebook page at
http://www.facebook.com/vwpartsaircoolednet
and get a 5% off code for use on one order for VW Parts ON OUR PARTS STORE WEBSITE, vwparts.aircooled.net

[2332turboSF]

I think youve missed my entire point completely John. The way I understand it Im already effectively running about 8:1 because the longer duration bleeds off the higher pressure of the 9.8:1. If I were running a very mild or stock cam then my static would closely match the dynamic ratio.
I know you have a LOT more experience than me so Im not trying to educate im just trying to find out if this is the way it actually works.
Take this for example. I ran my engine on pump gas at or above 20lbs of boost 32* advanced (before I invested in boost retard and other things) on my previous build with no det., now if the static 7.5:1 CR was what my engine saw prior to boost theres NO way I could of pulled that off. The only reason I didnt Det. on that is because the real compression was close to 4.5:1. ?Right? Or am I way off base here...

[2332turboSF]

Something else to drive my point home.. When I bought my rail the engine was originally built by VWP as a NA engine but the PO added shims and a massive deck to come up with 7.5:1 for the turbo he added. I could never figure out why in the world my heads were cut down to 35cc. Well after doing all kinds of research and probing Jason I figured it out. With 35cc chambers and a deck of .040 the static CR was over 14:1 but the dynamic ratio was almost 8:1 after the massive cam bled off the pressure. The engine ran fine on pump gas before in that configuration also. I know Jason would not build an engine @ 14:1 for a buggy intended to run on pump gas.

[[email protected]]

you are making the mistake of calculating the "bleed off" twice.
_________________
It's just advice, do whatever you want with it!

Please do NOT send me Private Messages through the Samba PM System (I will not see them). Send me an e-mail to john at aircooled dot net

"Like" our Facebook page at
http://www.facebook.com/vwpartsaircoolednet
and get a 5% off code for use on one order for VW Parts ON OUR PARTS STORE WEBSITE, vwparts.aircooled.net

[2332turboSF]

Im relying on this calculator found here http://www.jeepstrokers.com/calculator/

[Quokka42]

You are kind of right, but should heed John's warnings. The DCR gives us a guide to how much we can raise compression in a naturally aspirated engine. The thinking is that the piston can't start filling until the intake valve closed as the piston won't generate any pressure. In a turbo the mixture is being forced into the cylinder and is already at pressure, so this total is then compressed by the remaining stroke. A DCR of 8:1 is going to require you keep boost levels very low, unless you are running methanol, so a lower DCR and a bit more boost might give you a better driving and longer living motor.
_________________
There has only ever been one man who was perfect, and they nailed Him to a cross.

[2332turboSF]

Ok. Its already built so I don't plan on changing it. Ill be breaking in the cam tonight and maybe take a spin tomorrow. I do have water/meth injection so I'm sure that will help. I calculated a cr of 14:1 on 12#s of boost which is going to be all I run this time, I should he able to get away with that with some good fuel.

[[email protected]]

If you aren't going to change anything why did you even ask?
_________________
It's just advice, do whatever you want with it!

Please do NOT send me Private Messages through the Samba PM System (I will not see them). Send me an e-mail to john at aircooled dot net

"Like" our Facebook page at
http://www.facebook.com/vwpartsaircoolednet
and get a 5% off code for use on one order for VW Parts ON OUR PARTS STORE WEBSITE, vwparts.aircooled.net

[2332turboSF]

Mainly I wanted to have a discussion about it. That and to see if what I researched was correct.

[Quokka42]

I just want to see pictures of the results.
_________________
There has only ever been one man who was perfect, and they nailed Him to a cross.

[modok]

In reality you will find that engines can handle more boost at higher rpms, same way that "dynamic compression" (really cranking compression) works in NA engine
the higher the powerband, the more CR wants

So if it starts making boost at 4500 rpm, and you shift at 7500, then you can run a LOT more boost and/or compression than if it boosts at 3000 rpm and shift at 5000

[hoghead5150]

i think most people use the static compression ratio when they build a motor. reason is this is the "standard" way people talk about motors. now don't take the word "standard" as meaning correct.

what i've learned over the years is there are engine assemblers and engine builders. the assembler buys parts, and puts them together. this works, and sometimes it works good. but it's a hit and miss thing. usually the guys that have been "assembling" for years learn what works and what doesn't.

now engine builders tend to look at the engine as a whole system and they "build" the engine on paper, computer, mind, whatever you want way before they ever turn a wrench. they take into account how every piece will work with the next. these motors usually turn out good and make more power, last longer, and typically outdo and "assembled" engine in every aspect.

for years i didn't know what dynamic compression was. back then we built our motors strictly off the static compression and we had good success with that. it was alot of trial and error.
_________________
"i got a friend that knows a guy who's best friend has an uncle that used to have a bug just like yours!"

i measured once, cut it three times, and the damn thing is STILL to short!