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Evo 3 knock threshold

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Hi, first of all I need to thank Adrew and the HP team for the platform, spent years trying to learn and this is fantastic and I'm grateful .

I know there are a lot of great builders in here, hoping to get a bit of knowledge from your experience.

Got a 3, on UK pump fuel (shell 99), and anything more than 1bar on a bastard 20g and it starts knocking, no matter how much I retard timing, on conservative 11.5afr, although only 1 degree no more than 4, no different if at 1bar or 1.3.

If I am correct, the 3 has the highest compression ratio from factory of the lot.

The more research and learning, the more I realise I don't know, so my question is, if I build a forged short block, will knock threshold be the same if I keep the CR the same at 9:1 and the same setup?

Will it matter if is a 2 or 2.3L?

I realise that static compression is only one piece of the puzzle here, and there are a lot of other elements in the equation, but from your experience would you recommend going to a lower CR 8:5?, no E85 here.

Dynamic compression will be dictated by camshafts choice, correct? not much I can do here apart from trying to make an educated decision.

I'm on old HKS 264 at the minute.

Quench area,

Piston crown design (designed by manufacturer, so nothing I can do here)

Head gasket (heard really good things about HKS on the earlies)

Head bowl, castings are not great, was thinking on a clean up, and make them all as close as possible.

So once I can measure and try and calculate dynamic compression, what would be a good ballpark for a mostly street engine?

Can I use head gasket thickness to fine tune, are thicker gaskets prone to induce knock?

I got no interest in dyno numbers, I would like to build an engine to have fun with, probably a 2.3 and keep the turbo as it spools really quick and holds till red line.

Dan,

This sounds like a fun project. Without knowing the root cause of the knock, I can only guess at the solution. I will say hybrid turbos like what you're describing, tend to have oversized wheels for the housing size, resulting in low efficiency, high heat, which is a recipe for knock on pump gas soon as you flow enough air for the issue to get severe. With another turbo you may be able to run quite a bit more airflow without knock on the same fuel. Intercooling the car as best you can, and reducing turbo inlet depression and exhaust restriction will all help as well.

If the turbo is the root cause, I wouldn't expect a compression ratio change to significantly alter the behavior, assuming you run enough boost to match your current airflow.

In terms of displacement, all else equal, larger bores tend to be more prone to detonation, but the benefits of the displacement often outweigh the potential need to dial back ignition advance a bit.

I've never been a fan of using a head gasket to alter compression due to the negative impact on quench area. I suggest achieving your target ratio via piston design.

I'm also not clear how you're determining the engine is knocking, but those engines do not have good knock sensing in stock form. Audio review with a Bosch sensor is suggested.

Unrelated, I suggest trying less boost up top, more timing to see if another combination might net you better results on the dyno. If the turbo is simply well out of its efficient range, that strategy tends to help.

Best of luck with the project!

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