7.7:1 plus how much boost = NA 9.2:1

steptoe · Post by **steptoe** » Sat Feb 13, 2010 9:58 am

Just wondering for you calculating ones out there who have done this before.

I think someone has expressed 7.7:1 EA82T compression plus factory boost of 6psi as being roughly equal to 12:1 compression ratio.

Is there a publicly available/known formula for working out what equivalents are?

It is all in the working out a safe initial timing for twilights EA82 NA mpfi of likely 9.2:1 comp ratio engine WITH the additon of his EA82T gear

I guess twilight will also need to look at limiting advance and ensuring his boost retard is enough to suit his explorative tinkering

twilightprotege · Post by **twilightprotege** » Sat Feb 13, 2010 10:10 am

isnt the NA mpfi 9.7:1?

Morcs · Post by **Morcs** » Sat Feb 13, 2010 12:01 pm

Till it goes bang then back it off a little...

Outback bloke · Post by **Outback bloke** » Sat Feb 13, 2010 12:18 pm

There is a simple way to work it out. It is not 100% correct due to inefficiencies of manifolds, turbos, heads etc and the atmospheric pressure being not exact. It is very close to being right though.

Atmo pressure about 14psi
Your boost 6 psi
So divide 6psi by the 14 gives you 0.42
Now at 0.42% to your static ratio. 7.7x1.42 = 10.93:1 compression

9.2:1 x 6psi is 9.2 x 1.42 = 13.06:1 compression

get the idea?

twilightprotege · Post by **twilightprotege** » Sat Feb 13, 2010 1:01 pm

found this:

http://www.gtsparkplugs.com/EffectiveCo ... nCalc.html

7.7:1 with 6psi = 10.84:1
9.2:1 with 6psi = 12.96:1
9.7:1 with 6psi = 13.66:1

so 9.7:1 with 6psi = 7.7:1 with 11.4psi

but you *should* make more power with 7.7:1 and 11psi than 9.7:1 with 6psi

Gannon · Post by **Gannon** » Sat Feb 13, 2010 2:06 pm

twilightprotege wrote:but you *should* make more power with 7.7:1 and 11psi than 9.7:1 with 6psi

So why not run with 9.7:1 with 7psi?

Lower boost pressure from the turbo means less heat generated which puts less load on your intercooler and means you can run more timing.

Why do factory turbo motors have low compression ratios and high boost, why not the other way around?

Is it because the transition to boost isnt as noticeable with low boost and it takes the fun away?

twilightprotege · Post by **twilightprotege** » Sat Feb 13, 2010 2:22 pm

not sure if i wrote that correctly.

in terms of power, 7.7:1 with 11psi *should* make more power vs an engine with 9.7:1 and 6psi.

however, 9.7:1 with 6psi is a better every day setup. certainly better for offroading imo

steptoe · Post by **steptoe** » Sun Feb 14, 2010 12:30 am

And regards to timing based on these figures?

As twilight has changed a few things, factory timing for either engine is not expected to be correct, so set low and see, wait to see how it goes with some light boost. 6 degrees BTDC?

Always thought it so inefficient to have 7.7:1 like get less fuel efficiency off boost then throw fuel in under boost yet it works gives same economy as my EA81's and a bucket load more squirt

twilightprotege · Post by **twilightprotege** » Sun Feb 14, 2010 6:05 am

7.7:1 and 6psi is nasty in anyone's mind. can easily run the same compression ratio on the mitsi 4g63 (2L, early rancer evo and galant) and heaps and heaps of boost and get around 400hp out of the engine before you need to start upgrading things

Gannon · Post by **Gannon** » Sun Feb 14, 2010 7:45 am

The EA82 wasnt designed as a power-plant, it was designed to be economical and light.
Subaru were pushing the envelope when they threw the turbocharger on it, thus why they limited it to 115hp. Any more than this and longevity started to suffer.

steptoe · Post by **steptoe** » Sun Feb 14, 2010 9:10 am

seen longevity issue with the 4G62/3 too. It was called a balance shaft belt that, tired or lazy mechanics, did not see or replace when doing timing belts. Once simply parked a Cordia one day, its BSB got caught up as engine stopped turning. Never turned again. It screamed minutes earlier. I wanted one up until then, and when its MT4 played up...NEVER. Still enjoyed the drive.

steptoe · Post by **steptoe** » Thu Jun 28, 2012 11:29 am

Found this thread on google, thinking I'd better look before asking on the board - again it would seem

. Actually thread link flashed before my eyes through googles search proces and had to delete bits and search again to find it return.

Reason came to mind again was just pulling up long haul up hill in fifth in the GLTM5 EA82T LPG, 80 kph and only 3 or 4 psi boost if that. Was all very quiet, clean, smooth travelling and was thinking EA81 never pulled up this hill in 5th so quiet and effortlessly. Was wondering what the comp ratio of 7.7:1 and three psi would equal in roughly NA comp ratio.
Maybe lower compression ratio engines pull sweeter and crisper up hills ??

tambox · Post by **tambox** » Thu Jun 28, 2012 6:23 pm

Some notes about EA82 boost pressures
When I did my first EA82 turbo conversion (jap import 1/2 cut), as soon as I replaced the cat, the boost changed.
Intially it would pull about 8 pounds boost all through the boosted rev range, with the cat.
Without the cat it was very different, up to 4000 rpm, 8 pounds boost.
On hot days it would boost to 9 pounds above 4000 rpm, but on cold misty nights, it would boost up to 17 pounds.
I did nothing to it except remove the cat, no pinging, it worked it all out by itself.
I asked people for an explanation, I gave up after the stories I was told.
Then the oil pressure dropped slowly to almost nothing, after about 3 years, due to big end bearings becoming bigger end bearings.
Had it bored to first OS, new bearings, then it ran 8-9 pounds booost all the time.
It went good until it was "fixed".

steptoe · Post by **steptoe** » Thu Jun 28, 2012 11:12 pm

Wow! Did it go like 17 psi too, or do you think gauge got confused ?

steptoe · Post by **steptoe** » Thu Jun 28, 2012 11:19 pm

Just paid a visit to the calculator - tis still there

reckons 3 psi boost on 7.7 gives it 9.27:1 so maybe it pulls like an NA, at 3 psi ? . Guess really need to see torque figures

4psi boost on 7.7 gives 9.78?:1

and 5psi up to 10.32:1

might scare myself if I put in 10 or 12 psi

12.94:1 and 13.99:1

As said before these EA82T's aren't race engines just nice torquey little tourers compared to EA82 NA

tambox · Post by **tambox** » Fri Jun 29, 2012 8:03 am

Went very well @17lbs boost. Lucky it was constant 4wd.

H-top · Post by **H-top** » Fri Jun 29, 2012 7:48 pm

same, i was rather impressed at 1.1bar for a 20something year old 1.8l

discopotato03 · Post by **discopotato03** » Sat Jun 30, 2012 6:13 am

4G63s have a pent roof four valve chamber and like most similar things they resist detonation a lot better than a chamber that looks like a bath tub and the same with the piston crown . The static CR in the later Lancer versions was 8.75 compared to an EA82Ts 7.7 .
Yes they have balance shafts and so does just about every engine in their 4G family . The golden rule is change all the belts at the same time , and if smart the water pump too .
EA82Ts could make more torque/Hp but the ports are really sized to make low rev torque . What helps overall was than a RX L Series is pretty light at 1070 Kg . The middle 80s was reasonably early in the days of production turbocharging and even F1 hadn't reached its performance peak until about 1987-88 so development was ongoing .
Hp wise EA82s are limited because they lack breathing at revs and generally things that make higher Hp numbers make power at higher revs .

With compression ratios don't forget that static and dynamic CRs can be very different and if you could the one that would be preferable to measure is the dynamic one . Any loss anywhere means you don't get that 7.7 pre turbo boost compression ratio . Normally the greatest restriction is the throttle because it doesn't spend much time fully open .

discopotato03 · Post by **discopotato03** » Sat Jun 30, 2012 2:04 pm

What forced induction does for a given head/engine is get more air in than the atmosphere can push .
Urban myth is that engines "suck in air" , reality is that engines create an area of low pressure on the induction stroke and the atmosphere does its best to restore the cylinder to atmospheric pressure . If the atmosphere could do this you would say the engine has 100% volumetric efficiency (filling ability) .
The reality is that ducting/filters/manifolds/ports/valves all have some resistance to air reaching the cylinders so you won't even get close to 100VE on an early production engine .
EA engines have small ports to promote reasonably high gas speeds at lowish engine revs , the aim would have been to make good low rev torque in a reasonably small engine (1.

in light cars .
Back to forced induction , having positive pressure (ie above atmospheric which isn't zero anyway) means the engine acts as if atmospheric pressure just increased so greater pressure to fill the cylinders with . Forced induction can make some pretty agricultural heads , guess which , pass a lot more air than NA but if the ports are small it will limit total flow . This is why if you want an EA82 to have a good spread of power the turbo needs to be reasonably small . It has to work within the airflow constraints of these engines meaning it has to boost from pretty much where the std turbo does and maybe extend a little higher .
Strength wise EA82Ts are fine , if you can keep the charge temps down all temps actually and not let them detonate they would be reasonable grunters . Possibly the main weakness in the bottom (center LOL) end is the three main bearing crank which isn't as well support as you typical 4 cyl five main bearing crankshaft .
I'd say the reason people think these engines are weak is that its now 2012 not 1986 and most cars have multi cam multi valve engines which work better and feel stronger . They are more complex and whilst flat engines are too the early ones hve the complexities without the efficiencies .
Later engines of similar capacity are hauling around heavier cars and because they can run leaner and make better use of their heat they use less fuel doing it .
My Lancer is a 99 GSR and it makes 138 Hp/Litre , if the L makes 135 total thats 76 Hp/Litre . The major differences in std form is 15 years 220ccs 290 Kg and 151 Hp .

A .

steptoe · Post by **steptoe** » Sat Jun 30, 2012 7:13 pm

Mmmm...the way I look at the old EA82T in its L series and a modern vehicle - I use a Toyoda Kluger with an engine power that is double, its weight is double yet gets about same fuel economy. Not recall eng cc ....

All things considered I am still happy with my old EA82T - cost me less than a power up cat back exhaust on a WRX, fun to drive, bit slow off some marks, somehow loses traction through the front wheels

and been a bit of a workhorse lately putting into use a towbar - na,na Gannon where's ya towbar

? It certainly improved things tightening up the ducting for the 10 psi