I've been searching to see what other owners consider to be a 'safe' level of boost on a stock motor. I'm puzzled that so often boost level comments are synonymous with octane comments.

My question is, why is 25psi on 91 considered more risky than 25psi on 94, for example?

I realize that the motor on 91 will not make as much power as an identical setup on 94, as the motor running 94 can run more timing and a leaner AFR. However, assuming a good tune on both cars, it seems to me that 25psi is 25psi regardless of the octane. In fact, the pressures inside the motor would be greater in the car running 94 as it would be making more HP and more TRQ.

So, assuming a knock free tune, what is the max boost on a stock motor? And does it really matter what octane fuel is used?

 

octane matters all, ive found out that 21psi is the max boost for 91oct and this is with a tune of course and without a tune 20psi is the most you can run on 91oct.

 

the only reason the 94 makes more HP and TQ is because with the higher octane it is able to safely be tuned higher without knock than 91.

 

+1 @ Blizz

The octane that you run will determine how much boost you will be able to run, there by defining the fuel and timing requirements of the engine. I have personally seen a stock motor EVO 8 @28psi with a few mods running 11.45 tuned with a SAFC.

 

There is no answer to your question, because there is no special psi # that the engine can handle. It has more to do with other factors - not just straight boost level. Octane allows you to run more boost safely. Yes, you can run 25psi on 91oct with the stock turbo, but you're going to have to give it low AFRs and weak timing, which will likely result in high EGTs (also dangerous).

 

good question there Jim...
I often wondered the same thing...
how does low AFRs and retarded timing give high EGTs?

 

ya how does a rich condition raise EGT's. From what I know
With a very rich mixture, exhaust gas temperatures will be low. This is down to a number of factors, but the absence of sufficient air for complete combustion is the main factor. The flame front simply dies out. Also, the excess fuel has a high specific heat capacity, and is able to 'mop up' further heat. Also CO rather than
CO2 is produced due to the partial oxidation of carbon in an oxygen deficient environment. This CO is then further oxidised to CO2 later on in the combustion process (say, when both valves are open, and there's a sudden influx of fresh oxygen). This resulting oxidation of CO to CO2 requires energy input, so temperatures are again reduced.

 

So, to reiterate...

Why is a knock free-tune at 25psi on 91 considered more risky than a knock-free tune at 25psi on 94, for example? (I'm assuming the turbo is still well within its efficiency range in either case.)

 

The octane rating of gasoline is simply its resistance to burn or "knock." The higher the boost, the more knock you are likely to encoutner, ceteris paribus (all other things remaining the same.) The leaner the AFR's (Air / Fuel Ratio's), the more knock you are likely to encoutner. And the more advanced ignition timing, the more knock you likely to encounter. You have three variables as the components to knock and horsepower / torque. Adding the octane level into the mix, you now have four. The leaner the AFR, the higher the boost, and the more advanced the timing, the more power you are capable to make. 91 will knock at a less powerful (HP) level than 93 would knock. So... To get to the most powerful (HP) level at let's say 25 psi on 91 octane, we would not be able to run as lean AFR's and advanced ignition timing as we would on 93 octane. In other words, 93 allows for the power-adding leaner AFR's and advanced ignition timing. That should answer your original question. Understand now? I think I explained that fairly well.

Keep in mind that all of these variables (Boost, timing, AFR's) need to be balanced out to maximize safety, reliability, and power. IE : A good PSI for 93 is not a good PSI for 91 because you must adjust the AFR's and Ign timing to compensate for the lower knock threshold of 91. Also, just straight octane will not increase HP alone. If your car is tuned for 91 and you add 93 to the tank, you will not be adding horsepower unless you change boost, AFR's, or timing. (Unless your car is seriously knocking on 91, but that is a different story.) In fact, boosting octane on a well tuned car with no knock, can actually make your car slower (without changing the other variables.

 

on 93 octane i ran up to 28 psi on stock turbo and block only thing i had was head studs holding them down at around 90ftpounds with zero knock on a reflash

 

can you run 30psi on stock turbo on 100 octane safely

 

Yes. Its all relative to tune, if its crazy then its no good. If its the Black Evo IX we tuned when we were down there it was knock free on the dyno. It also has the knocklight if something were to go weird. Any car should be tuned with that.

Aaron