What is the key factor that allows an Evo engine to sustain 35 psi without damage? In other words, why is a stock Evo (no mods) only able to safely handle 21 psi and a highly modified Evo able to safely handle so much more? What is/are the modification(s) that allow the engine to do this? Is it cams alone?

 

there are a lot of factors and i dont know all of them. for one even big turbo evos are only running the 21 psi on pump gas. they cant crank up the boost until they get some race gas or meth in the picture. thats when they are able to turn up the boost and you will see a few stock turbo evos running 28 psi with meth or race gas. i think that thats where the stock turbo runs out of steam and a bigger turbo can run more efficiently and put out the higher boost levels without becoming ineficcent and heating up the air too much making it so it can run higher boost levels without too much knock on race gas or meth. someone correct me if i am wrong

 

big turbo, race gas, and a solid tune(probrably aftermarket engine management in most cases).

 

I'm fairly ignorant when it comes to these things, but I like learning new things. So here's a question - Why, specifically, would it ruin a 100% stock Evo's engine if you set the boost to, say, 28 psi? What would technically happen that would cause engine failure? And how would the above mentioned items remedy that? Thanks...

 

Increasing the boost pressure increases the pressure inside the cylinder. The higher the pressure in the cylinder, the higher the temps. High temp and pressure leads to knock/pre-ignition. Knock causes extremely high pressure spikes in the cylinder which causes things like rings, rods, and pistons to break.

Therefore, you want to prevent knock. The knock limit is highly dependent on the fuel octane level. The higher the octane number, the more resistant to knock the fuel will be.

You can have a fully built motor, but if you run 87 octane in it and even try to boost 21psi, there's a good chance you'll cause a lot of damage.

That's the overly simplistic version of it.

 

spdracerut is exactly correct. The problem is heat and pressure leading to detonation. Once detonation at power starts occuring, you start to get scoring, carbon buildup's, etc. - then pre-ignition can enter the picture.

Detonation is pretty hard on an engine at power, but in some circumstances a single pre-ignition event can send the engine in for replacement or overhaul.

-jjf

 

also the extra heat from the extra boost causes the rods to fail as they are not a forged metal. this was a corner that cut in order to keep prices down. if the rods were forged, it would be possible for an evo to run higher boost but keep in mind that the stock boost setting is in the optimum range for the stock turbo. the motor can run higher boost but the longevity of the motor is significantly reduced. this is why the FQ-400 requires maintenance every 5000miles...

 

+1

 

Thanks for all of the replies. Makes sense now...

 

Hate to second guess you , but i thought all con rods are forged , its just that race moters run con rods forged out of better steel .ie 4130 , titanium, ect , please correct me if i am wrong all , thanks

 

pump gas at high boost will allow the engine to experience detonation. Alky, meth, and race gas prevent this. It is the same reason high compression naturally aspirate race engine MUST run race gas. I could eleborate on the technicalities behind this but in a nutshell that is why you can run high boost with meth and cant with straight pump gas.

 

Eh? Service intervals on all Evos is 4500 miles. The FQ400 is no different to any other FQ variant, or any other Evo for that matter.

MB