High Compression Ratio ----> Detonation

[apardal]

Hi!

I'm experiencing some detonation on my 4AGE MAP Blue top after I've done some modifications on it.

- Wiseco Pistons 81.5mm
- Head decked 0.6mm
- Block decked 0.1mm
- TRD Metal head gasket 0.8mm
- (Calculated Compression Ratio 12:1)
- HKS Camshafts 264/264 8.1mm lift
- Fidanza cam gears
- Complete 2.25" SS exhaust with Tsudo Headers
- Stock intake without TVIS
- Mild head work (smoothed combustion chamber edges, cutted bronze valve guides, polished exhaust...)
- Beru Silverstone Spark Plugs and TRD spark plug wires
- OEM ECU with new HC Crystal, changing rev limit to 8500 rpm
- Apex'i SAFC1 Piggy back
- NGK Powerdex AFR
- Running Unleaded 98.

The problem I'm dealing with is Detonation under normal Ignition Timing (10º, measured as the manual recommends). I've installed a home made stetoscope (copper pipe, bolt and canister) to hear detonation while driving.

Using this gadget, I started lowering the Ignition Timing until I reached 0º, and I still had detonation. If I get it around -5º I'm able to stop engine knock. This knocking sound is heard around 6000rpm under load and the more I advance the ignition timing, the sooner (lower rpm) I'll hear knocking.

My camshafts are dialed in the exact same way stock ones are, and I think I have the exhaust cam 2º retarded. HKS states 110º BTDC and ATDC for both cams, I might have to re-check that.

What do you guys think my problem is? Incorrect mapping? Incorrect cam timing? Unapropriate spark plugs? What?

Thanks in advance

[dikolotour]

Apardal,

without being sure i'm just saying my opinion. I think there is too much compresion. There is a peak were fuel can resist knocking. After that peak it knocks. Try use some fuel additive and let your advance at 10o. Maybe more octane stops knocking.

I hope my thought helps...

[NoHachi]

I'd try the simple things first..aka sparkplugs of a colder grade..12:1 compression with such mild cams will generate a hell of a lot of heat.

[apardal]

How colder ? I'm looking for NGK in the heat range of 7.

I also have the possibility of exchanging my cams for a pair of TODA's 288 7.9mm lift, but I think I'll have the same detonation problem at high rpm's.

Best regards

[Eircamae86]

Down to fuel id say, you will need extra octane feul mix or aditives, the head wont take that sort of abuse for long,

[NoHachi]

One grade colder as a maximum..anything more and the car will bog and splutter at low revs and idle. Rally cars running very high compression also have this problem..they need two sets of plugs to race..one set to warm the engine up at lower rpms and one set for full load high rpm race duty.
More overlap will reduce the engines effective dynamic compression ratio. You could play around with the effects by advancing the intake cam by 1 or 2 degrees and see if it helps..if it does, the TODA cams might be worth a try.

Did you try running a pig-rich mixture at full load(lambda 0.8 or so) to cool the intake charge? Might also be an option, certainly around the peak cylinder pressure point (max torque..guessing yours would be around 5000-6000rpm).

[Ivan141]

Pig-rich mixtures arent the best way to go, that could cause accelerated cilinder bore wear due to washing the oil off. If fuel and sparkplug measures dont help, you're probably better off just fitting a thicker headgasket. What's the point in having such high compression if you need to carry around additives and keep you're ears focused all the time..it's not a full-on race car right.

[apardal]

Well, thank you very much for your educated replies.

Talking about spark plugs... I would buy any other colder one, if I knew the heat range of mine (Beru Silverstone).

The compression ratio will stay the same, no matter what. I'm not disassembling the head again unless something falls apart. Knock on wood.

What I want to do is to take the fullest advantage of my compression ratio, adding what is needed. Camshafts, spark plugs, ECU, trying octane boosters... whatever. 12:1 is not impossible to cope with, I'm sure.

Spark plugs and cam timing are on my "to do" list.

Thank you again.

Best regards

[oldeskewltoy]

12 to 1 compression on a stock ECU = detonation.

BTW I think you may have MORE than 12 to 1!!! Your head is shaved .6, block .1, headgasket difference .4 just these calculations equal 1.1 on the CR for a 4AG... Wiseco has a BROAD range they set for 4AG pistons... it could be anywhere from 10.2 - 11.8!!! Add in the 1.1 and in worst case scenerio you are running CLOSE to 13 to 1!!!!! Have you done ANY combustion chamber mods? If not figure closer to their higher figure


WHOOPS... didn't see your last post. Ok, keeping the compression... can you at least do a static compression test for us??? We can back calculate your CR(approximately) from the engines actual compression test. If it isn't TOO high... a good engine management system(prices are ALL over for these....) should be able to adjust timing enough to use a high compression set up. If you are at 12 to 1+... You need a lot more duration cams then the 264 offer.... say a minimum of 280, but maybe closer to 288.

[NoHachi]

Backcalculating static compression ratio's from dynamic compression ratio's is a no-no. Certainly with headwork, different cams, pistons etc.