Compression - How much is too much?

[adam]

Hi guys and girls,

I've been a long time reader, but first time poster due to never having anything good enough to contribute, but I've got a question I can't seem to find an answer to and it's doing my head in.

First of all, I've got a 4AGE big port motor with 9.4:1 compression ratio. Everything is standard apart from TRD copy extractors and a pod filter.

I've recently blown my head gasket and while the head is off, I'm thinking of adding a thinner head gasket/shave a bit of the head for higher compression and doing some basic port and polishing.

What I'm stuck on is, how much can I shave off before it is too much for the standard ECU?

Installing a 0.5mm TRD head gasket to my current setup will give me roughly 10.1:1 compression ratio, will my motor run okay? How about going all out and shaving 60thou off the head and use the OEM head gasket? This will give me a compression ratio of about 11:1.

I'm planning on purchasing some 256 and 264 degree cams with about 8.35mm lift as well as a standalone ECU, but I'm just thinking, 'while the head is off, why not get myself some more compression?'

I will be running the car on 98 octane fuel at all times and am aware that I will need to purchase some adjustable cam gears to readjust the timing.

Sorry for the long post, but my question in short is, how high can I take the compression ratio before my standard ECU starts playing up on me?

Cheers
Adam

[jondee86]

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

[oldeskewltoy]

I'm running 10.3 to 1 high comp pistons under a TVIS head and intake, and management system.

Toyota had a 4AGE (Singapore and some areas in Europe) without a catalyst, and so those engines had 10 to 1 originally from the assembly line

Your 10.1 to 1 will be a nice mover... will work with stock or mild cams. You might need a bit more fuel when you run a bit more cam. I recommend a 4.0 to 4.5 kilo flywheel to go along with that

Cutting the head, and or thinner gasket will require timing pulleys to correct cam timing.

[adam]

Thanks for the replies guys.

Dan, it was actually your signature which made my decision to post this thread. And with your first sentence, you mentioned that you're running a TVIS head and intake, and management. Is that TVIS head's standard management system or an aftermarket? I think it's just the way I'm reading it, I'm not sure if you meant aftermarket or standard.

With the Singapore and some Europe countries running a 10:1 compression ratios, they would be running a different tuned ECU as well wouldn't they? Would it be possible for me to purchase one of their ECUs and run it here in Australia (as a cheaper temporary option)?

Also, I've read of a few people running their small port (10.3:1 comp ratio) motors with the 9.4:1 loom and ECU. They got it running fine, but I am unsure as to whether they were running it with any detonation problems, would anyone have any experience with this?

[tolisspi]

im running a 4ag(z)e with high compression piston, head blue top, adjustable cam gears with stock cams(i was running 272 10,5 but its another story). The ecu and harness is AT160 (JDM) with O2 and i have remove the catalyst. CR 11,5:1 (pistons info). Also i replace the fuel pump with bosch 044 and the whole system.
The car was running very good up to 5500rpm. Upper 5500rpm had problems with "ping" and the car wasnt running very good, lost power. But unfortunately i had electrical problem and i dont know if the ping was from the poor fuel (stock injector) or the electrical problems.

BAD BAD ENGLISH

[jondee86]

Have a read here for a pretty lucid explanation of the factors involved
in getting the most out of increased compression...
http://www.popularhotrodding.com/tech/03...index.html

It's a little bit of a juggling act, but providing you don't go crazy, the
stock ECU should be fine. Racing engines run a lot closer to the edge.
Stock engines have a lot of latitude built-in to handle normal rebuilds
and carbon buildup etc.

Cheers... jondee86

[ZDoman]

I used to do my engines around CR 11:1 if I removed the "joggles" inside the combustion chamber. Running on 98octane petrol. Without any problems by this time.
If you don't remove the joggles I think around 10,3-10,5 is the safe.

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) because 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. He 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.

[adam]

tolisspi: Thanks for your experiences. And your english is fine, I've seen much worse coming from those who have english as their first language.

jondee86: Cheers for the link. Definitely a good read, but will have to give it a few more reads for some of that information to sink in. :wink:

ZDoman: With your setup of 11:1 compression ratio, were you running this with the standard big port (9.4:1 CR) ECU?

I've begun work on de-shrouding the combustion chamber, still currently trying to get each chamber equal before polishing though.

12:1 CR running standard ECU is insane. hahaha

Edit: I forgot to mention that after a few hours of searching, I've come to the conclusion that big port ECUs powering a 10.3:1 CR motor is fine. Only bad thing people got was the motor running rich because small port motors have larger injectors.

Also, I will only be shaving/milling the head now due to modifications to the combustion chamber in the head. I just want my CR to be as close to a full number as possible. ie. 10.5:1 or 10.7:1, not 10.3348:1

[ZDoman]

adam Wrote:ZDoman: With your setup of 11:1 compression ratio, were you running this with the standard big port (9.4:1 CR) ECU?

No, some of my engines were bigport EU-DM MAP and the others are smallport AFM and MAP aswell.
No one was bigport AFM (9,4:1) But I don't think this matters that much.

[adam]

ZDoman Wrote:No, some of my engines were bigport EU-DM MAP and the others are smallport AFM and MAP aswell.
No one was bigport AFM (9,4:1) But I don't think this matters that much.

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: