The Cost of Higher Boost

[Moai]

I am often surprised by how resistant people are to upgrading their cams. Few mods yield as much bang for the buck when you put a mid-sized turbo on your car as larger cams do. In fact, it is quite possible that cams will actually end up saving you money. To make much beyond a certain power level on most cars with the stock cams, you need to start pushing higher boost, which we will argue is 20psi or higher.
In fact, a while back somebody was bragging about hitting 465whp on a 4g63 with stock cams, a GT30R and a very expensive intake tract that included a massive FMIC and an aftermarket intake manifold all at 30psi!

The high cost of high boost goes as follows:

1.) Higher boost requires more attention to fueling and timing, particularly on pump gas. This means you either have to spend more as you go for race fuel or put yourself at much higher risk for detonation and pre-ignition damage.

2.) Higher boost requires the turbo to work much harder. The blades must spin faster to provide higher boost. The turbo lasts fewer miles and the probability of catastrophic failure which can damage the engine through the ingestion of blade material increases substantially.

3.) Higher boost always results in higher outlet temperatures (you cannot break the laws of thermodynamics). Higher outlet temperatures means that you have to spend more on intercooling to get the same intake temps and to prevent premature heat soaking.

4.) Higher boost requires a stronger spark to light. The stock ignition system is good to a certain psi and then it becomes weak and unreliable. It has nothing to do with how much power you are making, it's all about how much pressure exists in the cylinder at the time the spark goes off. To run higher boost, you will have to spend money on upgrading the ignition system.

5.) Higher boost puts more stress on the head gasket. This raises the potential for HG failure, particularly on engines with aggressive timing curves or open decks.

6.) Higher boost takes longer to build, which means that your spool suffers and your torque curve will take longer to reach its peak. Higher boost is also harder to maintain at higher RPMs, so your torque curve drops faster at the top end.

7.) Higher boost puts more stress on your turbo hoses and clamps. Those puppies are expensive to replace.

8.) Higher boost pushes against the intake valves harder, raising the possibility of valve float at higher RPMs. On vehicles with interference engines, valve float can become a lethal mishap. For example, as i've learned - some Subaru engines are a double-interference setup. Not only do the piston and valve share the same space, but the intake and exhaust valves ALSO share space! A floating intake valve may not only collide with the piston - it may also collide with the exhaust valve!
Accurate valve timing is CRITICAL to engine survival, as well as power production.

9.) Higher boost requires your fuel pump to push against more pressure. Flow drops steeply relative to pressure so you get a lot less out of the same fuel pump at higher boost pressures. As you raise intake pressure, the pressure regulator on your fuel rail will proportionally raise fuel pressure to compensate. For example, my base fuel pressure is set at 40psi. At that level, my pump has been measured to flow 305 lb/hr, which is roughly good for 440hp at the crank, or 375hp at the wheels. [assuming a BSFC of 0.55].
For argument's sake, let's assume a 1:1 rising rate in terms of boost pressure vs. fuel pressure. So if i run 20psi of boost, and therefore make my fuel pressure 60psi, my fuel pump can now only deliver 243 lb/hr - roughly good for 350 at the wheels, assuming a pump in PERFECT health. Higher pressures mean lower flow and if you're running a power level close to that limit, any kind of boost spike will spell certain death for the motor.
If you want to see how flow drops relative to pressure increase, just take a look at the following test chart for fuel pumps:
http://www.wolfkatz.com/Articles/MR2...g%20Report.pdf

You must consider how fast your pump flow drops when you raise pressure, especially if you are contemplating raising fuel pressure in order to make smaller injectors flow more.

In conclusion, high boost and daily drivers don't mix too well. Just some things to consider as you plan your modifications.

[whitejdmse-r]

very good post

[mikecentola]

great post again bro!!!

[mikecentola]

stuck for ya

[Toneman14]

This is great. Im learning all this stuff BEFORE i turbo my car. Thanks!!

[Celtic1982citleC]

30psi evo on meth with no worries

[1.8tV-Dub]

wow thats alot of good info

[Shawn96VR-4]

Good post. Moai is against high boost applications so he is bias.

With that said, a lot of good posts but some people don't want to install anything but boltons and CAMS, IMHO, are NOT boltons. CAMS also change the way the motor sounds and modifies the powerband. Those are some negatives.

For me, and a lot of other companies, we can run 20-30PSI and make 600AWHP or so w/o issues. Yes, probably would net gains with the CAMS but I want complete stock internals.

If I didn't care about that, I'd rock cams in a second.

[Wayne64SS]

I'd like to hear more on this cam thing. I personally love the sound of a nice lopey idle.

[1.8tV-Dub]

wow all this info is helping me build my car with a bigger turbo, i'll remember all that