Compression - How much is too much?

Joined: 17 Jan 2009 Posts: 8 Location: Melbourne, Australia

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 Smile

Adam

Posted: Sat Jul 04, 2009 3:49 am Post subject:

The factory ECU doesn't know what compression you are running, so
it doesn't care Smile

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 Smile

Cheers... jondee86

Posted: Sat Jul 04, 2009 4:49 am Post subject:

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.

Joined: 17 Jan 2009 Posts: 8 Location: Melbourne, Australia

Thanks for the replies guys. Smile

Dan, it was actually your signature which made my decision to post this thread. Puh

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. Stupid

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?

Cool

Posted: Sat Jul 04, 2009 8:43 am Post subject:

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

Posted: Sat Jul 04, 2009 9:21 am Post subject:

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/0311_phr_compression_ratio_tech/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

Takumi Joined: 20 Mar 2005 Posts: 263 Location: Hungary

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 Very Happy

I wouldn't try it, but it is working for a long time for him...maybe 99+ octane petrol, I don't know.

AE86 : Technical > Compression How much is too much

Joined: 17 Jan 2009 Posts: 8 Location: Melbourne, Australia

tolisspi: Thanks for your experiences. Smile

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 Puh

Takumi Joined: 20 Mar 2005 Posts: 263 Location: Hungary

Joined: 17 Jan 2009 Posts: 8 Location: Melbourne, Australia

Posted: Sat Jul 04, 2009 3:18 pm Post subject:

Takumi Joined: 20 Mar 2005 Posts: 263 Location: Hungary

Posted: Sat Jul 04, 2009 7:44 pm Post subject:

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...

Joined: 17 Jan 2009 Posts: 8 Location: Melbourne, Australia

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. Smile

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.

Thumbsup

Posted: Sun Jul 05, 2009 5:21 pm Post subject:

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.

AE86 : Technical > Compression How much is too much

Posted: Tue Jul 07, 2009 10:18 am Post subject:

Posted: Tue Jul 07, 2009 11:50 am Post subject:

Posted: Tue Jul 07, 2009 11:57 am Post subject:

Posted: Tue Jul 07, 2009 12:02 pm Post subject:

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/0311_phr_...o_tech/index.html

Posted: Tue Jul 07, 2009 12:15 pm Post subject:

Posted: Tue Jul 07, 2009 12:45 pm Post subject:

I understand what you said, but just disagree with you Smile

Higher compression ratio's, all other things left stock, do not consume more air or fuel. The ECU does measure the amount of air going in,but it stays exactly the same.
If you read the link and some other references online, you will find decent (better then I can give for sure) explanations of why higher combustion pressures lead to a more efficient burn and more power.

All this is valid for a stock engine, with increased CR. Change anything else (affecting Volumetric-efficiency) and the amount of air changes.

Posted: Tue Jul 07, 2009 12:51 pm Post subject:

Posted: Tue Jul 07, 2009 1:07 pm Post subject:

Ah, ok, for arguments sake:
From Wikipedia:

Posted: Tue Jul 07, 2009 1:09 pm Post subject:

Joined: 07 Jul 2009 Posts: 7 Location: San Mateo, CA USA

This is why I am switching over to 91 small port electronics. that way I have a block with a knock sensor and a computer designed to work with it. I think it will be a much better start for a hi-comp engine.

I think you'd be fine bumping the CR up by a full point with the stock ECU. If you want, I have a set of standard bore 10:3 hi-comp pistons and rods I was about to put on E-bay. You can e-mail me through my website if you'd like to buy just the pistons. If you can, get your hands on a small port intake and epoxy it up like this (assuming you're not in a RWD car - if so you also need to relocate the TB of course):

http://www.marcmedina.com/intake.html

It made a significant improvement. Also, the longer tube from a small port that goes between the TB and AFM (assuming you're not MAP equipped) was also a nice improvement. I didn't believe it either till I tried it.