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EA81: Maximum Compression on pump gas?


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15 replies to this topic

#1 RavenTBK

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Posted 09 October 2006 - 01:22 PM

The topic says it all. What is the maximum compression that an EA81 can reliably run on pump gas? Mainly, what CR is the max for both ends of the pump gas spectrum (87 - 93)?


Re: http://www.ultimates...ead.php?t=65542

#2 NorthWet

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Posted 09 October 2006 - 06:32 PM

And the answer is...





...it depends.

Everything is a balancing act. If you are willing to run less ignition advance, you can run more compression. If you run the engine cooler, or run the engine faster, or don't use so wide of a throttle opening, or run better fuel mixture control... then you can run a higher compression ratio. If you run a wild/hot cam (one with more duration than stock), then the effective compression ratio will be reduced, so you want a higher static compression ratio.

I would guess that you could run 9.5:1 or 10:1 CR with proper mixture and timing control.

#3 GeneralDisorder

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Posted 09 October 2006 - 09:17 PM

9.5 would run nicely. The EA82 SPFI runs that comp already, so just using the SPFI pistons would work. Take .020" off the heads and you might get up close to 10:1. Should still work fine, but I would probably use the SPFI on it.

GD

#4 bgd73

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Posted 09 October 2006 - 10:05 PM

Go with what you want, then work with it. Seems as though, learning from this forum, the heads blow a leak at some C ratio,and with wrong stuff to go with it. I learned that from v8s. I have seen 11:1 dynamically stay good on pump fuel. I always thought it was a hoax by manufacturers to keep power down, by claiming "10:1 is as far as it can go". Just look at the 11:1 mercedes v8 for example.. doesn't need any special fuel, like I have encountered myself, with high compression. The worry with ea81/82 is heads physically handling demand.:)

#5 RavenTBK

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Posted 12 October 2006 - 10:49 PM

Those are pretty much the answers I was expecting to get, yet at the same time, has never been officially answered in the forum....least not in any of the search results I've read over the last couple weeks. :)

I knew EA82 SPFI motors ran fine at 9.5:1, but was rather curious how an EA81 would run at 9.5:1... I know head design and such plays into how much compression can easily be turned up on any given motor.. just didnt quite grasp it with the EA81.

I'm not wanting to have to do any crazy mods to keep it running.. just a simple swap and go.

So, pretty much leaving things more or less as they sit, only swapping to SPFI pistons, keeping the Weber and ND dizzy, 9.5:1 should run just fine? Sweet. Time to start on phase 2 of my evil plan.... :grin:

#6 NorthWet

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Posted 13 October 2006 - 02:31 AM

The basics of the the EA81 and EA82 engine designs are pretty similar. Same bore/stroke, cylinder layout, cooling system, etc. IIRC (too dark to check at the moment), the EA81 uses a "bathtub" combustion chamber and the EA82 uses a "wedge", so the EA82 should have better combustion characteristics. The EA82 SPFI uses a "hotter" cam (longer duration) so its effective CR will be less than the EA81 w/stock cam. The EA81 probably has a more conservative set of ignition timing curves, so it should be able to handle a higher CR with possibly just a little retarding of the static timing.

So, yeah, the EA81 should be able to run SPFI pistons with little more than maybe adjusting the ignition's static timing.

#7 daeron

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Posted 13 October 2006 - 02:41 AM

The basics of the the EA81 and EA82 engine designs are pretty similar. Same bore/stroke, cylinder layout, cooling system, etc. IIRC (too dark to check at the moment), the EA81 uses a "bathtub" combustion chamber and the EA82 uses a "wedge", so the EA82 should have better combustion characteristics. The EA82 SPFI uses a "hotter" cam (longer duration) so its effective CR will be less than the EA81 w/stock cam. The EA81 probably has a more conservative set of ignition timing curves, so it should be able to handle a higher CR with possibly just a little retarding of the static timing.

So, yeah, the EA81 should be able to run SPFI pistons with little more than maybe adjusting the ignition's static timing.


wasnt the ea81 a 1600 engine? or was that the ea71? have i been misshapen?

#8 RavenTBK

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Posted 13 October 2006 - 01:00 PM

wasnt the ea81 a 1600 engine? or was that the ea71? have i been misshapen?

misshapen? Out of shape.... tsk tsk. I want 100 more sit-ups out of you to lose that gut. ;)

According to all I've read, including the HTKYSA books, the EA71 is the 1600cc OHV motor, the EA81 is primarily the 1800cc OHV motor, the EA82 is the 1800cc OHC motor. Now I say primarily with the EA81 as in the book, it also states that in the early years (78-80), there were 1.6l "EA81" stamped motors.. so according to the texts, the EA81 came in both displacements, either by design or by mistake. However, in the context of USMB, the term EA81 always refers to the 1.8 OHV. :D

#9 RavenTBK

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Posted 13 October 2006 - 01:07 PM

The basics of the the EA81 and EA82 engine designs are pretty similar. Same bore/stroke, cylinder layout, cooling system, etc. IIRC (too dark to check at the moment), the EA81 uses a "bathtub" combustion chamber and the EA82 uses a "wedge", so the EA82 should have better combustion characteristics. The EA82 SPFI uses a "hotter" cam (longer duration) so its effective CR will be less than the EA81 w/stock cam. The EA81 probably has a more conservative set of ignition timing curves, so it should be able to handle a higher CR with possibly just a little retarding of the static timing.

So, yeah, the EA81 should be able to run SPFI pistons with little more than maybe adjusting the ignition's static timing.

So then, if I understand correctly, with the EA82 using a hotter cam, with less effective CR, the 9.5:1 pistons, in the EA81, with the stock cam, should give an even higher effective CR than 9.5:1? Hehe.. 9.55+:1? All this while still running regular 87 octane unleaded fuel with no detonation? Should be good for at least 95HP or so... :D

Or am I reading too far into it? I probably should stop thinking now while I'm ahead right?

This is turning into a very good discussion.. should help future search results greatly.

#10 NorthWet

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Posted 13 October 2006 - 01:27 PM

So then, if I understand correctly, with the EA82 using a hotter cam, with less effective CR, the 9.5:1 pistons, in the EA81, with the stock cam, should give an even higher effective CR than 9.5:1? Hehe.. 9.55+:1? All this while still running regular 87 octane unleaded fuel with no detonation?...

Detonation can be quelled/forestalled by reducing ignition advance. (Ignition advance does NOT create power; it is actually undesirable to need it, as it saps power. But there is an optimum advance setting for any particular set of conditions.) In the CR rnage that we are talking about, a 1.0 increase in CR is usually good for about a 10% increase in thermal efficiency. (i.e. - Either power or fuel economy, or around 10% increase in the combo.) The milder cam could amount to as much as .25- .50 increase in effective CR (I don't remember the respective specs).

#11 82bratavenger

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Posted 04 November 2006 - 12:47 AM

I am planning the piston swap to the ea82 spfi ones and am also planning on using an ea82 carb intake with a performance ground cam by Delta cams in Seattle area (only $75). I currently have the carter/webber and will upgrade to the webber 32/26 hence the ea82 intake. Any oppinions on the cam grind? Is it really going to make alot of difference? I am looking for 100hp to 115hp if I can.:burnout:

#12 GeneralDisorder

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Posted 04 November 2006 - 01:19 AM

I am planning the piston swap to the ea82 spfi ones and am also planning on using an ea82 carb intake with a performance ground cam by Delta cams in Seattle area (only $75). I currently have the carter/webber and will upgrade to the webber 32/26 hence the ea82 intake. Any oppinions on the cam grind? Is it really going to make alot of difference? I am looking for 100hp to 115hp if I can.:burnout:


If you are running the original 82 engine, pitch the heads in a dumpster. The intake valves are 2mm smaller than 83+ heads.

And SPFI is the way to go. I may be making wireing harnesses for it soon as well. I just made my second harness, and it was noticeably easier this time around.

GD

#13 Ross

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Posted 04 November 2006 - 05:17 PM

Just noticed in my owners manual for an '84 that all European/Austrailian/New Zealand ea81s had a compression ratio of 9.2:1, which i believe is higher than the US ones.....
The ea71 twin carb had a compression ratio of 9.5:1 in that year too.....

Whether we got different heads or pistons, who knows.....

Bt that certainly indicates that you'd be fine increasing your CR to at least 9.5:1

#14 82bratavenger

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Posted 14 November 2006 - 01:46 AM

I have an extra 1983 long block that I plan to build it out of so the head info is helpful. what are the advantages of the SPFI over the weber as far as adjusting it for proper flow and mixture? Do I have to recurve the Disty if I go with SPFI? I printed out the SPFI conversion info and am still not sure.

#15 GeneralDisorder

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Posted 14 November 2006 - 01:48 AM

I have an extra 1983 long block that I plan to build it out of so the head info is helpful. what are the advantages of the SPFI over the weber as far as adjusting it for proper flow and mixture? Do I have to recurve the Disty if I go with SPFI? I printed out the SPFI conversion info and am still not sure.


There's nothing to "recurve" in an SPFI disty. That's all done by the ECU.

http://home.comcast..../EA81_SPFI.html

GD

#16 82bratavenger

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Posted 17 November 2006 - 01:24 AM

sounds good to me. How much would you charge to make a harness for an 82 Gl Brat? I can get all the other SPFI gear from the pull and save. I just don't really want to tear into the wiring with splices right now. I guess I got to do what I got to do though right.




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