Max Boost
 

I am running a m-4 procharged 509 cubic inch BBC. The motor started out life as a Mercruiser 502. On the last rebuild JE blower piston were added, Arp rod bolts and the Tops of the pistons and the exhaust valves were ceramic coated. Cylinder heads are Brodix BB2's and a single Nickerson carb. The previous owner claimed he was running 7.5 psi. I am only getting 4 psi. I am at a much higher altitude although I do not know for sure if that would cause a lower boost level? I am at 4500' feet compared to sea level where he was running it. My question is what do you think a safe boost level would be with this bottom end? I was thinking of adding a blow off valve and methanol injection along with a MSD boost timing master. I am thinking detonation should be under control with this set-up just wondering at what point I should build a better bottom end. With the Procharger blow off valve I should be able to set the boost where ever I want with a slightly smaller pulley. What do you think? Leave it at this level until I build a bullet proof bottom end or can I get away with bumping it up a little?

Thanks blower engine gurus!

I'll bet it lost boost because of the heads. The stock heads (I assume on the 502) are considerably smaller than the bb2's. As for the bottom end.... I wouldnt be scared to put 10+ psi to it. The problem you are gonna find is having enough meth for the injection when running WOT. For short bursts this would work well, but then propping it for the top speed with the meth injection is gonna make it a pig in other spots.

Just my two cents here.... I am no guru.

Sorry, I knew I didn't get all the info in there. The motor has not been changed from previous owner to me. He claimed 7.5, but I never rode in it to verify it. I buy it, drag it to 4500' and I am getting 4 lbs. I am not as concerned about that as what I can get away with for boost. Also I was planning on running a progressive meth injection kit so hopefully it will not get any lag. If I could get away with 10 psi that would really wake it up!

Also, I am running an intercooler.

at 4500ft air density is approx 85% what it is at sea level.
thus you should be seeing approx 6lbs

We need more info-when you say the previous owner added blower pistons-were they the j/e's with reverse domes (turbo/blower dishes or "cups") or just flat tops? I used to run a 502,I blueprinted it and got rid of the factory j/e's with the mini domes and went to the flat tops,I ran 8-9 psi of boost with no problems on that motor at 900 ft above sea level. If you have true blower psitons (which is what I have now in my 540) your compression assuming you have typical 118 cc heads is around 8-1,with the aw504 innercooler or the even bigger one (don't know swhich one you have) I wouldn't hesitate to run 10 psi of boost on 93 octane,I'm running 12-13 psi on 93 but I have ceramic coated piston crowns,aluminum heads but I'm also running head studs,mls gaskets,extra hold down studs in the valley,dart big m 3 block. No signs of detonation,no knock events on the computer and it makes a TON of hp. I would go down 2 pulley sizes if it were me,example-your running a 4.25"-go to a 3.85". Make sure your belt is tight too first. Air density-at 4500 feet elevation your going to loose a ton of measured boost due to the thin air,Smitty

As far as the meth injection,I wouldn't worry about it unless you can't get 93 octane or you really want to push the boost up (this is If you have dished blower pistons). Some guys have posted about how great the meth injection is working for them but keep in mind this stuff is very corrosive to engine parts,if some dribbles into your engine when its shut off or whatever it will attack your carb plus can start some nasty pitting or galling on your valves,Smitty

Just a note regarding pulley diameter......A larger diameter pulley (both drive and driven) has an improved angle change ratio. The radius is a larger arc with more surface area and tooth count. Therefore,belt slap and required tension may be less of a concern with larger diameter pulleys. Friction,heat and belt life may also be improved.

The rule of thumb for density altitude is for every 1,000 ft. of altitude climbed subtract 1" of pressure.

Density altitude is comprised of three components; Known altitude,outside air temperature and barometric pressure. The supercharger tricks an engine into thinking it is at some lesser altitude than actual. However,as density altitude increases more manifold pressure is required to cause the engine to think it is still at beta.

Go with a smaller pulley. You can handle 10 lbs of boost as long as you do not detonate. As steated before you lost your boost due to the elevation. Also make sure as stated before that your belt is tight. It is easy to loose boost with a loose belt. Been there.