The Math behind the High Compression EA82T

Gannon · Post by **Gannon** » Tue Aug 01, 2006 5:19 pm

I think this is a good basic way to figure out what happens when you run turbo stuff on a NA block.

The car is running 1.8 liters at normal atmospheric pressure which is 14.7psi. So, normally aspirated car that 14.7 PSI gets compressed 9.5 times to a pressure of 140psi. Now, normally in a turbo car the compression ration is only 7.7:1. So, when you take the 14.7 and add that to the factory 7psi boost to get 21.7psi compressed 7.7 times for a pressure of 167psi. So, in doing the math of 21.7psi with your 9.5:1 pistons you get 206psi. "reverse" the math and that is like running 12psi of boost. 206psi(your cylinder pressure)/7.7(turbo pistons)=26.7psi. And 26.7psi-14.7(Atmospheric)= 12psi.
Originally posted on the USMB by Rally Keith

12 psi,... that should be ok intercooled;)

Gannon

BlackMale · Post by **BlackMale** » Tue Aug 01, 2006 5:41 pm

[quote="Suparoo"]
12 psi,... that should be ok intercooled] are you asking us or telling us?

PeeJay · Post by **PeeJay** » Tue Aug 01, 2006 5:55 pm

My EA82T used to run fine on 12psi with no intercooler

MUDRAT · Post by **MUDRAT** » Tue Aug 01, 2006 6:05 pm

Whoa . . . that is sooooo wrong on so many levels. I think Bernoulli just rolled over in his grave.

Just to start with, Volumetric Efficiency has been completely overlooked. Does the Compression test measure psi(a) or psi(g)? They're just small things. The major thing is air is elastic, it isn't just a simple block or parcel that you can take from x and add to y, or just "reverse the math" as Mr Rally Keith says.

To do this properly you'd have to write a logarithm that was a function of bore, stroke, VE and boost. I could do it but I've really got much better things to do with my time.

To pluck a number out of my head, I'd say that if the two charges were at the same temperature the "adjusted" charge pressure would be closer to 10 psi (gauge pressure)

Gannon · Post by **Gannon** » Tue Aug 01, 2006 6:06 pm

Jaffa wrote:are you asking us or telling us?

Just opening it up for coments

MUDRAT wrote:Whoa . . . that is sooooo wrong on so many levels.

Thanks Muddy, i was hoping somebody would contradict it, i thought that 12 seemed a little high. But this formula is the best explanation i have found. And if the formula works out higher than in reality, that only means i have less chance of breaking stuff.

I will hopefully be getting new pistons for my block and i am seriously considering pistons with higher comp than stock 7.7:1

9:1 pistons will give equivilent to 10.6 psi
8.5: 1 pistons will give 9.3psi.

And a stock turbo engine (7.7:1 pistons) needs 3.5 psi of boost to make the equivilent cylinder pressures of the NA engine (9.5:1 pistons)

Im facinated by it:p

MUDRAT · Post by **MUDRAT** » Tue Aug 01, 2006 6:13 pm

Did a quick Google search for "Compression Ratio Calculators" and found this one:

http://www.turbofast.com.au/TFcompB.html

BTW EA82 Bore x Stroke is 92 x 67 mm.

Geez I was close!!! (assuming a VE of 70%)

subelover · Post by **subelover** » Wed Aug 02, 2006 8:26 am

good work i like the math thing im very interested in that sorta stuff aswell, makes sense the na higher comp pistons and the atmospheric pressure to give the higher psi reading and makes even more sense to do it and add a cooler....hmm

good work hopefully everyone has there 2c worth and someone actually does it.... cheers Ben

Gannon · Post by **Gannon** » Wed Aug 02, 2006 6:46 pm

Yeah i think that Muddy may have hit it on the head.

Still open for coments;)

mAJORD · Post by **mAJORD** » Sun Aug 06, 2006 8:14 pm

yeah unfortunatly it just doesn't work like that.

I'm running 12-13psi on a 7.7:1 turbo engine, but because of the overall setup, and power being made it's pretty much all I'll get out of it reliably.

Like I've said before, people have gone down the turbo'd NA enigne path before on USMB - they made shit all power, and blew engines up.

= waste of time if you're chasing power

The engines just don't have the combustion chamber efficiency to cope with high compression + turbocharing at any reasonable level. And tbh it really isn't needed. Even with a bigger turbo mine is perfectly driveable, dad drives it every day.

It has at least as much torque from 2000 to 3000 as my 95 liberty as it starts to make a bit of boost, then after that it's a whole other ball game.

now if if you wanyt OODLES of low down torque and don't care about outright power then fine, go the turbo NA route, but keep the boost LOW ( well below 10 psi) and keep the small turbo.