Scotts 1992 SC300

Q	How much boost can I run on stock compression?
A	A very complex issue. Its dependent on many factors: 1. engine condition, a worn engine can generally accept more boost than a new one due to less overall compression. 2. What modifications have you made to the car? 3. What octane fuel are you running? 4. What is your air/fuel ratio? A simplified answer is: On stock internals with stock cam, 93 octane fuel, an engine with good compression, at an 11:1 air fuel ratio can run 7-8psi of boost without detonation using moderate boost retard of around 5-8 degrees from stock. That said, I can tell you that you can run more boost in certain areas of the rpm band. The most sensitive area to detonation under boost is near the torque peak. It is at the torque peak that you can run only 8psi of boost. For instance, my torque peak is around 5000-5500rpm with upgraded cams, so I dial in less boost in this area. My boost curve starts around 12psi at 3000rpm, then dives down to 8psi around 5000rpm, then back up to around 10 psi near redline. This is on 94 octane. On a stock cam vehicle, you can dial in significantly more than 8psi in the 6500rpm range. Sure, you can't do much with this, because the fuel cut is near 6500rpms, but it is a gain up top from around 5500+. This of course is all dependent on purchasing a boost controller like an Apexi AVC-R that can vary boost at different rpm. There are also two different factors that determine the overall boost limit at a given rpm. The first factor discussed above, is simply the limit based on cylinder pressure caused by the boost itself. This limit is where the engine will detonate due to sheer compression ratio. This is an instant limit, as soon as you hit this limit, you get detonation. The second factor is EGT limiting. A stock compression vehicle can run significantly higher boost in some rpm areas, just not at the torque peak. Getting into these different areas can yield some gains, but over a few seconds you also reach another limit that is simply temperature related. Its up to you to determine how high of an EGT limit you're going to set for yourself, its useless to quote EGT figures unless you're mounting the probe in the exact same location on an identical manifold. I've got a mental note of a temperature limit of 1550 F at the extreme end of the scale. Granted, the EGT probe is mounted in a very hot place about 6" from the nearest exhaust port in the plenum portion of the manifold, where the runners meet before the turbo. The impact of EGT on boost limiting is TIME related. If you don't detonate at a given rpm, gradually turn up the boost at that rpm and look at the EGT for the secondary limit. Generally what this means is that you can run that higher boost, but only for a brief period of time i.e. 1st gear or 2nd gear. What this means is that in 4th or 5th gear if you drop the throttle and get 12psi for ten minutes, your engine will be toast. High EGTs will cause exhaust valve damage first resulting in burned valve seats which will cause ragged compression,, then will destroy other parts as well at higher levels. If you mount the EGT probe in a location downstream of the turbo, you are going to me measuring EGTs alot lower than we're talking about here, and at a reduced accuracy due to the fact that there is that much metal in front of the sensor absorbing the heat at different rates. You will also get alot more laggy readings if placed aft of the turbo. The way to make the most of this time related EGT limit is gear based boost, which also comes back to the AVC-R boost controller. This box allows you to reduce the boost across the board equally in all rpms based on what gear you are in. It will continue to follow the rpm based profile or curve that is set- i.e. 12psi@ 3000, 8psi@ 5000, 10psi@ 6500, but it will do so at a reduced or increased level. Generally speaking, you want less boost in 1st. (to limit wheelspin), maximum boost in 2nd and 3rd gears, then with the EGTs in the higher gears being higher, you want to taper the boost down by a certain percentage in 4th and 5th to limit peak EGTs to your acceptable level. Do not tune boost for EGT in 1st, 2nd or 3rd gear, as the time is not long enough to develop any significant EGT levels. Do find this limit, you've got to do 4th and 5th gear pulls. Overall boost in these gears will be set in the gear based boost setting on the AVC-R, and not on the boost/duty cycle setting. As a simple reference, my AVC-R start duty (gear based) is currently set at: 1st =-50%, 2nd =+5%, 3rd =0%, 4th =-5%, 5th =-10%. What this basically means is that the AVC-R in 1st gear will reference the original boost / duty map that you set up based on RPM and subtract 50% from those values. In 2nd or 3rd gear, is where I determine the detonation margins and actually dial in the correct boost pressure on the duty map. Of course, 4th and 5th are reduced to limit peak EGT to the levels I've set. Doing all of this modification to boost pressure is assuming that you have direct fuel control and can maintain a steady a/f ratio around 11.0:1, and have dialed in the .5 to 1 degree of retard per psi of boost via MSD 6BTM ignition box or other means. Another way of increasing overall boost is to use an Apexi ITC for rpm based ignition retard or advance. This box is NOT a boost retard box, so can't be used to retard your overall profile by the .5 to 1 degree per psi that you need, but it can be used to surgically take out a little timing in the area near the torque peak or to add in extra timing up top or down low. Obviously you couldn't advance timing and boost, simultaneously, but If you're aiming for a steady 10psi boost profile, the ITC could probably help you reach 10psi and still eek out the gains up top and down low through timing advance.