My EA82T Compression Ratio findings .

discopotato03 · Post by **discopotato03** » Wed Apr 22, 2009 3:03 am

Well this didn't go as I planned it but that may not be for the worse .

As some of you would remember reading the plan was to have my spider turbo EA82T engine rebuilt and along the way mod a few things inc a raised compression ratio - 7.7 to 1 sounds a bit pathetic by today's standards .
When the engine was striped it was found that the pistons were proud of the deck faces and my engine builder told me that this engine had been opened up and freshened up before .
It was all in good condition but you (I do anyway) fit new rings and bearings and hone the bores .
I asked for ~ 8.5 - 9.0 CR and when measured I was told it already was about 8.5 to 1 - because of a typo in the calcs .
The piston crowns were ceramic coated to make up for the EA82T's lack of piston oil squirters and it was assembled late last year .
At the time it may have been suggested to me that in std form the pistons in these engines don't reach the tops of the bores so it was easy to assume that someone had milled a fair amount off the cases of this engine pushing the static compression ratio up to 8.5 to 1 .

Now recently people here came back and said no the pistons don't sit down the bores on a std EA82T so what are you on about ?
Thinking this didn't ring true I asked my engine builder for the various measured volumes so I could do the calcs myself .
Well all looked about right until I noticed that the volume in the head gasket above the piston didn't tally with the CR number given . What I didn't know at that point was the compressed thickness of an EA82T head gasket so I asked again and mentioned the odd looking gasket volume .
I really had to know that gasket thickness to be certain that the pistons weren't to close to the heads deck faces .

Anyway these are the volumes given today and as I feared the CR will be just over 8 to 1 .

Chamber 38.2cc , Piston dish/valve reliefs 20cc , Head Gasket 6.647 cc , minus 1.66 cc for pistons being 10 thou proud of deck faces .

So CR = Swept Volume + Clearance Volume/ Clearance volume or 445.39 + 63.187/63.187 = 8.049 meaning near enough to 8.05 to 1 .

My first though was can it be raised reasonably easily but I don't think is doable in practise without custom made pistons . At a guess you'd be struggling to make a worthwhile difference by machining the heads and if you remove too much the deck rigidity can suffer and not clamp up reliably on the head gaskets . Also the inlet/coolant ports creep closer together which means mods to the inlet manifold so its ports line up with those in the heads .
Even the cases wouldn't stand having more than 5 removed before the piston/head clearance got to the 25 thou minimum . No cost effective solution .

The only good thing to come out of this was that Stu Wilkins , who rallied an 86 RX Turbo in the mid late 80's , said you don't want high CRs in these engines anyway . In those days the rally people tried raising the CR as well and found that the only way to stop them detonating was to retard the ignition timing which killed their power no matter how much boost they ran .

I'm not certain but I think the regs of the day forced the teams to run the std header pipe and of course no intercooling and I reckon both would have a major say in what you could screw out of these engines power wise before detonation became the limiting factor .
I guess they were running leaded Avgas and the homologated turbo and camshafts but you can only push so hard on such a basic engine design head and manifold wise before the grim reaper starts knocking on the pistons and head gaskets .

So Scoobidoo in you unique fashion you gave me reason to question what I though was fact and unbeknownst to me at the time I was wrong .
I suppose I should be thankful for it being up 0.3 of a ratio and I'm going to have to think a little harder about finding ways to pull the bottom end up - without resorting to small turbos .
I have asked an SA contact to rat up a pair of 87 only hotwire NA EFI cams which should be a quite mild upgrade over the std turbo cams .
The IHI VF8 and VF10 turbos arrived from Victoria today so I have something larger than the std one to think about .
The immediate choices would be to either use the EJ flanged turbine housing off the VF10 because it uses the same turbine as the EA82's turbo and is slightly larger (15R vs 12R) and effectively recreates the highest performance std turbo used on any EA82T - but with EJ mounting and dump pipe flanges . Or use the VF10 complete .
I managed to find a Vortex owner in Europe (through USMB) and he confirmed that the 87 (I think) version in some parts of Europe had the Spider manifold/best std factory turbo cams/rare EA flanged 15R turbine housing and made a std 136 ps vs the usual L EA82T's 120 ? Torque is supposed to be up 10 ft/lbs as well .

Not huge numbers but with better header/exhaust/intercooling maybe adequate .

Cheers A .

steptoe · Post by **steptoe** » Wed Apr 22, 2009 10:33 am

Thanks for sharing the info with us. Interesting to read of the pistons going proud of the block after it has been tinkered with. The dish in the piston is more for compression reduction and not valve recess are they. In saying that I figure the difference between turbo and non turbo pistons is just the dish and no other dimension,think the NA pistons have a big shallow dish rather than flat tops ?? so the difference in pistons would be measured from top edge of piston not just reducing volume of turbo piston dish??

Your info from Stu starts to put a damper on my plan to screw turbo heads to na block complete with its na pistons and lesser crank vents, it will be a suck it and see event when it does happen for any of us doing someting different

Not sure Adrian why you did not simply spec NA pistons in the first place to up your comp ratio??

discopotato03 · Post by **discopotato03** » Wed Apr 22, 2009 12:28 pm

The reason was that factory turbo pistons are usually better suited to turbo apps than the NA ones , reduced CR is only part of it .
I have compared OEM turbo and NA pistons from other engines and its not unusual for the turbo version to have different ring spacing , wider ring lands . Other things they sometimes have is drilled oil return holes and thicker section piston crowns .
Most of it is aimed at coping with higher combustion temperatures and pressures and withstanding detonation a bit better than NA ones .

I looked into EA82 NA pistons of which there appears to be two versions and CR's , 9.0 to 1 carbed type and 9.5 to 1 MPFI NA type .
I was also told that I already had 8.5 and it would be more reliable with factory turbo pistons than 9.0 with factory carby grade ones .

The thing I probably won't ever know is how much the detonation characteristics are reduced with better headers and intercooling on the higher static CR . Maybe lots and maybe little , Russian roulette with dollars and I cant justify the cost .

If you had the ability to run permanently on a detonation resistant fuel like E85 it wouldn't be so much of a problem but getting that in regularly available supplies is a way off yet if ever .

No as I said I'm just going to have to do the best I can in other areas .
I think a fresh engine should make quite a difference over a worn 304K one so its not like I'm going backwards .

Cheers A .

steptoe · Post by **steptoe** » Wed Apr 22, 2009 12:58 pm

Dunno about you Adrian, but I am getting a bit impatient to see how all these years of your tinkering with these Subie bits comes together in your RX

I thought I was slow, although Cheap grief is sort of finished but the intention was (three years ago now ) to somehow turbo my Brumby - so project ain't entirely finished on my side of the fence either

discopotato03 · Post by **discopotato03** » Wed Apr 22, 2009 5:34 pm

Well for the record I think I drove it home early in April 07 so that makes it around two years and two weeks , but who's counting .

It took a lot of research to find out what the best bits are for these cars meaning mainly manifolds transmissions and suspension mods .
Some of my finds had to be freighted from Adelaide to Sydney , some from Victoria , some from Canberra and even some brake bits from Perth and Tasmania . Nothing from QLD or NT - so far .

Lots of seemingly insignificant but essential bits had to be chased up if they didn't come fitted to complete assemblies , for example the L AWD exhaust hanger and shift mechanisms steady bar bracket . Then there's the Vortex spider type snubber bracket and bar etc etc etc .

Along the way were diversions from the engine project like replacing the rear wheel bearings , getting a replaceable uni tail shaft made up , better fuel pump , one off anti roll bars made and fitted with custom rear U brackets , chasing back and forth with the Aust Koni agent in Victoria to sort out rear dampers (MX5) , sort Eibach small diameter springs and seats , get rear strut extension made and then shortened , get a turbo back exhaust made from scratch (all I supplied was the cat and turbo flange plate) , replace turbo pedal box and cable with non turbo ones , rip all the heavy bulky AC rubbish out , PS pump rebuilt , replaced front drive shafts with good seconds and later remanufactured ones , replaced heater fan motor resistor etc .

Things in the wings are the 5 stud kit and MY00 Rex wheels - have to sort front struts and uprights before that can go ahead . I have MY00 Rex front brakes and Kartboy rear caliper extensions to go with the aluminium rear twin piston calipers be they stupidly priced Rex ones or Skyline ones .

Probably more going on behind the scenes than you know about .

Dinner time , cheers A .

Suby Wan Kenobi · Post by **Suby Wan Kenobi** » Wed Apr 22, 2009 9:25 pm

The 8.5:1 compression pistons where in the 86 models as this was the introduction of ULP compulsory engine running but my guess is that it wasnt a problem and the CR went up in the following years motor. This 86 only is based on the fact that 3 L's i know of with original engines all had the 8.5:1 pistons in them when swapped for fresher engines. One thing i did note was the crown thickness was almost a mm thinner that the turbo pistons i had to compare with.

With the better quality fuel available now it wouldnt be a bad idea to run a high static CR and with intercooling and low boost would be a good torque giver, i know you are trying to run ethanol blend 85, but to get the best out of it you would want a mappable engine ignition and fuel management. Basically a new ECU so it would amount to an expensive mission.

I will be aiming for a 8.5:1 ish static when doing my EA82T and adapting EJ engine management to the EA to make it more finetunable hopefully.

discopotato03 · Post by **discopotato03** » Wed Apr 22, 2009 10:19 pm

Actually thanks for the heads up on those early pistons , I had forgotten about them and I don't have the 85/86 factory WSM here to refer to .

I don't suppose you have pics of the tops of the 8.5 to 1 pistons do you ?

I wonder if they are still available or kicking around in good nick second hand . Do you have part numbers for them ?

Cheers A .

Suby Wan Kenobi · Post by **Suby Wan Kenobi** » Wed Apr 29, 2009 5:16 pm

I dont but i have a motor here that i believe is an 8.5er once i have got the lift kit orders sorted ill be pulling it down and ill post the results

discopotato03 · Post by **discopotato03** » Wed Apr 29, 2009 9:33 pm

Much appreciated SWK , one of the members recently posted me cam and compression ratio details from a factory 86 WSM .
He mentioned that one of the NA EA82's had 8.7 to 1 static CR though not sure if it was carb or MPFI .

Sometimes the tables in these manuals cover EA81 EA82 EA82 spider and don't differentiate .
I'm just not sure if the pistons in this 8.5 to 8.7 range are EA81 or not .

Really I think the only to tell with EA82 pistons is to CC the dish area of the piston crown if they don't look like the 9/9.5/7.7 ones .

I don't suppose its possible to remove one head and post a pic of the piston crown ?

Also lastly some OEM cast pistons have part numbers cast in the under side of them , worth a look cheers A .

Gannon · Post by **Gannon** » Wed Apr 29, 2009 9:45 pm

I have the pistons from my 88 turbo block laying around somewhere, i'll check them for ya.

shuffbag · Post by **shuffbag** » Wed Apr 29, 2009 10:28 pm

hey little Q? are ea82t in standard form a non-interferance motor like standard ea82

steptoe · Post by **steptoe** » Thu Apr 30, 2009 1:18 am

there was no 8.5:1 in the manual just the 8.7:1 and was in with the turbo stuff so assume it was all EA82 figures there Adrian.

Suby Wan Kenobi · Post by **Suby Wan Kenobi** » Thu Apr 30, 2009 8:25 pm

Yeah they are non interferance engines.

steptoe · Post by **steptoe** » Fri May 01, 2009 12:44 am

or free spinners as some may call it

shuffbag · Post by **shuffbag** » Fri May 01, 2009 9:45 am

haha now it makes sense, free spinners i had no idea!

SCOOBIDOO · Post by **SCOOBIDOO** » Sun May 03, 2009 6:00 pm

also known as a clearance engine..as opposed to interferance

discopotato03 · Post by **discopotato03** » Sun May 03, 2009 7:05 pm

I'm beginning to think the only way to get around this CR issue , without custom pistons , is to use a modern ball bearing turbo .
I think if it was sized appropriately it would do the trick by pulling the bottom end torque up a little . Maybe not off idle but these EA81/82's are usually not too short of low down torque if in good order .

Turbochargers are a little beyond the scope of this thread so I'll start another one later .

For a taste search GT2854R , cheers A .

Gannon · Post by **Gannon** » Sun May 03, 2009 7:29 pm

Whats wrong with custom pistons?

If custom pistons are too hard to come by,.. why not just use NA pistons and take some meat out of the head

Would there be any advantage of flattening out the compression chamber and reducing that 'bathtub' shape?

discopotato03 · Post by **discopotato03** » Sun May 03, 2009 11:34 pm

No I don't think I'd bother , the best combustion chambers are compact ones so making the bath tub larger is not the right way to go with detonation in mind .
By custom pistons I meant ones made specifically for the job not modified std ones .

A .

phillatdarwin · Post by **phillatdarwin** » Tue Jun 16, 2009 9:47 pm

just put a wolf 3d on it or some thing