Short answer - Boost doesn't really matter.
What is a safe amount of boost for a 05 gt (stock internals)...i hear 6 then i hear 8....anybody know what that equates to in horsepower?
Long answer below:
I don’t think “boost” has been advertised or explained correctly by the industry. And then the common “base knowledge” that we have is lacking in scope or detail.
A blower is only an air pump. When you pick a pulley size you are picking how much air it will pump through out the RPM range. At the end of the day it takes “X” amount of air (plus a little fuel of course) to make “X” amount of HP. Of course engine size, compression ratio, engine efficiency, etc have a factor in this but on a basic level if you jam 1000cfm of air into a 4.6L V-8 motor it will make almost the same HP as if you were to jam 1000cfm into a 2.5L 4cyl. Granted the 4cyl with have more boost (see next paragraph).
Boost is only a representation of the restriction that the motor represents. For example, if you put heads/cams on a blown car and don’t change the pulley it will “loose boost” because it has less of a restriction. This doesn’t mean the blower is not operating efficiently or there is something wrong. It just means it’s easier for the air to get through the motor. In the above example it will make more HP and have less boost.
Boost is not the only factor when the term “safe” comes up (in regards to the combustion chamber). Boost is coupled directly to timing, air/fuel (A/F) and intake air temps (IAT’s) when it comes to keeping things safe. They all work together. For example, you can run a high boost numbers and not lean on the timing real hard (were going to assume good IAT’s and A/F) and have a perfectly safe combustion chamber. On the other end of the spectrum you can run low boost with a lot of timing and put holes in pistons. A/F has a lot to do with what is safe in the combustion chamber also. If you run leanish numbers (12.0-12.5:1) it doesn’t matter what your boost or timing is, you will start hurting parts. And lastly if you have high IAT’s they will hurt the combustion chamber almost in the exact same way running lean does. The good thing about our ECU’s is that they automatically start pulling timing as the IAT’s go up as a “safety net” in case the car is heat soaked or the IC pump stops working. So by itself boost does not hurt parts, only boost in combination with aggressive timing, high IAT’s or lean A/F ratios hurts stuff.
Boost, when thought of as cylinder pressure, or cylinder pressure increase is a non issue. 15psi of boost only adds about 8% to the cylinder pressure or “rod loading”. Which is very, very little. For comparisons sake if you change a motors redline from 6,200rpm to 6,450rpm (which people do all the time) you are increasing the rod loading by the same 8% You are safer with 15lbs of boost than revving your motor to 6,500rpm (which people do all the time).
An example of “Shaky Science” and a contributor to some of the disinformation out there is Kenne Bell (I used to have one on my car…..junk). They will tell you that their kit will make “X” hp at lets say 8.5psi. If you read carefully though you will see that they always run all of their “tests” at 11.5:1 A/F ratio with……..wait for it……25deg of timing! That’s a LOT of timing and they are only getting away with that by using 100+ octane fuel. They say for consistence sake. So if you back the timing down to even “aggressive” (16-18deg) you are looking at a considerable HP loss. When I was at 450hp-ish the base KB kit was supposed to make 450+ at 8.5psi. I put the supplied pulley on my car and had it tuned it made 438rwhp at 10.2psi. Yeah, their “8.5psi” pulley made a touch over 10psi on a stock motor and it didn’t make as much power as it should have with less boost. We ended up running 16deg of timing to keep things nice and safe. And to poke another hole in KB’s “Science”/claims you could only get 1-2 runs before their crappy IC system heat soaked and it pulled enough timing to chop 50-60hp out of it. All of their published numbers are with a constant IAT that is not pulling timing. That is not at all representative of real life unless you are making a pass, cooling the car down for an hour, making another pass, etc. Just driving around town has the IAT’s up near the point where it will start chopping out timing.
Now for the other “safe”. The safe threshold for the stock motor is 450-500rwhp. Assuming everything that is going on in the combustion chamber is okey dokey the safe HP is based solely on the mechanical strength of the motor (Rods, pistons, crank, block, bolts, etc). In actuality it’s the rods that give up first. It doesn’t matter how you are making HP, be it with supercharger boost, turbo boost, nitrous, or naturally aspirated (as long as it is not being spun to the moon) the lower end will only take so much force before it breaks. It doesn’t care how that force is being made. Be it 5psi, 25psi, a bucket of nitrous, etc. the bottom end does not know where the power is “coming from”.
So at the end of the day to keep the motor safe you want to exert only 450-500rwhp on the mechanicals of the motor. And you want what is going on in the combustion chamber to be safe. For the first tune on our test car we went REAL SAFE with the tune. We could have easily thrown more timing at it and leaned it out slightly and made close to 500rwhp without changing the boost. So there is your sliding scale. Do you want 450rwhp with 12psi and super safe timing or 450wrhp with 10psi and timing with a little attitude? We went with the safe route. We could probably run 89 octane with this tune and not hurt a thing. We won’t be doing that, but we could.
Additionally we have found it is VERY common for boost gauges to read incorrectly. Even the ones that are “attached” to the dyno’s out there. Every boost gauge should be considered suspect until proven otherwise.