Compression - How much is too much?

[tolisspi]

ZDoman Wrote:I always used to resurface the cyl head and the block top. Especially the block top used to have some small "rust" points, it used to be -0,1-0,2mm off the top to get it OK. With this I wasn't able to use thin metal headgasket on any of my engine (i tried 0,8mm) 1because the piston was knocking on the cyl head at the first engine start. So be careful!

I know an AW11 guy who has 12:1 CR with stock everything and he rallycross race the car. 2He told its not revving over 7000 but has a great torque I wouldn't try it, but it is working for a long time for him...maybe 99+ octane petrol, I don't know.

1. How is the sound exactly? Every time you start the engine knocking? For 1-2 sec?
2.- Why not revving more that 7000rpm? Which is the problem?

About CR maybe is depending from the country and the car for example
Official Greek brochure for AE86 CR 9,4:1
Official French brochure for AE82 GT CR 10:1
and
Haynes service book(English edition) for AE82 GT 10:1
All the cars not cat.

[ZDoman]

adam Wrote:Sorry, but with your bigport EU-DM MAP motor, did this originally have the 9.4:1 CR or the 10:1? I'm not sure if Hungary run catalysts or not.

By the way, my motor is the MAP big port. :wink:

They were 10:1 originally.

[oldeskewltoy]

my engine in my black AE86 is a TVIS head and TVIS management. I'm using a 20V silvertop block, crank and rods, with 16V high comp pistons(10.3).

In essense this is a high comp short block with TVIS head and TVIS management. I am running larger injectors, Web's mildest cam upgrade, and AEM adjustable timing pulleys

I've been running this for about 6 years now...

[adam]

Sweet, thanks for that guys.

I think I will shave my head to get about a 10.3:1 CR after I find the final volume in the combustion chamber.

Once I get a standalone system and cams installed, I might try to bump that compression up a little more with a thinner head gasket, but that won't be for a while.

[oldeskewltoy]

I'm doing an extensive build on another engine right now.....

I found OEM chamber volume to be between 36 and 36.5 cc. When I was finished with the "joggle removal" (term used in TRD 4AG engine manual found in TRD Bible), the chambers were 37.5cc. Once the head was cut down the chambers were 33.5 cc. 33.5cc chambers, with the Euro/Singapore pistons(10 to 1), AND a STOCK head gasket(1.2mm), provide a finished compression ratio of 10.75 to 1.

[XJS]

jondee86 Wrote:The factory ECU doesn't know what compression you are running, so
it doesn't care Problem is that the fuel and igntion maps are not
optimised for the higher compression, so you can't really take advantage
of the potential. That will have to wait until you get your cams and new
management installed.

I believe that you need to be careful to avoid detonation at some high
load low rpm situations ?? Maybe someone who has done this can give
you an idea if it is a problem or not ?? I have a fully worked bigport
head sitting in the shed, and have been wondering if it would be worth
putting it on a bog stock bigport engine

Cheers... jondee86

The factory ECU does care. The main input for the ECU is some form of air measurement (pressure (MAP) or flow volume(AFM) ). If you pump up your compression ratio, the engine will consume more air, so the ECU will inject more petrol, simple is that. Of course there are limitations, but with moderate lift and timing there won't be any problem.

[NoHachi]

XJS Wrote:If you pump up your compression ratio, the engine will consume more air, so the ECU will inject more petrol, simple is that.

Ehm.. it doesn't consume more air at higher compression ratio's..its the exact same amount of air, compressed to a higher pressure.

Fueling stays exactly the same. Any problems will be on the ignition side of things and they will be difficult to detect without a decent knock-sensor.

[XJS]

NoHachi Wrote:

XJS Wrote:If you pump up your compression ratio, the engine will consume more air, so the ECU will inject more petrol, simple is that.

Ehm.. it doesn't consume more air at higher compression ratio's..its the exact same amount of air, compressed to a higher pressure.

Fueling stays exactly the same. Any problems will be on the ignition side of things and they will be difficult to detect without a decent knock-sensor.

Higher pressure itself doesn't give any more power, more air (and fuel) will. Hence the question: why the higher compression engines has more power, if they burn exactly the same amount of energy? Why all stronger engines has high compression ratio, some enabling the use of high-lift big overlap cams, if they will suck the same amount of air?

[NoHachi]

Higher pressure allows for better efficiency. You get more energy from the same cylinder charge.

Search around on google, there will be plenty explanations. One example:
http://www.popularhotrodding.com/tech/03...index.html

[XJS]

NoHachi Wrote:Higher pressure allows for better efficiency. You get more energy from the same cylinder charge.

Search around on google, there will be plenty explanations. One example:
http://www.popularhotrodding.com/tech/03...index.html

I don't want to deny any positive effects of the higher compression. I just said, higher compression will consume more air, and the ECU will compensate for it. Measure the air volume either case, measure the AFR accordingly, you will see the ecu trying to keep up.
But have a look from the other side: use slightly bigger cams, no shaving, no thin gasket, nothing. How the hell the ecu can compensate (up to a limit, of course) for the different than stock lift and duration, if it can't measure the air? Hence the question, what do you think what is the correlation of the static and dynamic compression?