KB 2.1 on '03 GT


New Member
Oct 2, 2017
Hey guys. I need some advice about adding a supercharger to my car. I have read and searched a lot and have read a ton of opinions, ect. My engine is fresh rebuild with stock internals, custom ground cams and a crap load of head work. My friend owns a machine shop and has been doing this stuff for 40yrs. All of this work has cost me zero dollars so I haven't pressed him about everything he did. I have a cam spec sheet, but I don't know how to read it. It's the one that shows where cams were checked on the analyser, not the card that comes in a box bought cam. He swears that this engine is between 400-450 flywheel HP, but that sounds high. He knows his stuff and has built many of these over the years. He also says that I can turbo or supercharge it with no problems as long as it's a safe tune. My question is that if I have stock internals in the bottom end, although balanced, ect. How can I supercharge it and use it as a daily driver and still get 100,000 miles out of it. I am old enough now that I can drive a strong car without keeping my foot in it, but I do want it there when I want it. Such as when passing or getting on highway. My desire is for 500 RWHP and from what he says I can do this easily and still only run 6-10psi boost with what he's done with the engine. When I needed the engine rebuilt due to a small crack in the block at the back that really wasnt an issue, but would gather oil and make a dime size spot after a few days, from an accident, I hadn't planned on doing all this so I didn't go with forged internals. Plus he doesn't charge me usually for what he does because we help each other out, but I can't ask him to rebuild an engine that he just worked because I was stupid and broke it, ya know? My neighbor came over last night and was saying I needed a bunch of extra things as the cars sits now and that a KB would destroy my engine pretty fast, even driving like I have some sense. He even said I should put on a Saleen SC instead because the KB is too much. He also said I need a Diablo Predator instead of the SCT tuner. He's had several SN95 Mustangs and says he's the "go to Mustang guy". I've read articles where these magazines have gotten 399HP out of a stock bottom end 4.6L 2 valve with cams and head work and TB, so I guess what he is saying is possible. If I am at that HP without a SC then it seems that adding a SC would put me well over 500 RWHP. I don't quite understand what it is that would actually kill the engine as far as boost vs total HP. I read see that many of you guys say 450-500 is max safe HP on stock internals, but is that at flywheel or RW? Is it the excessive HP the breaks the stock internals or running too much boost? Obviously boost increases HP, but if a NA engine makes 400-450 HP and is considered safe then could I add a SC and run lower boost with a safe tune to make 500 HP? All of the opinions and advice are confusing sometimes. What many of us need is someone who actually knows and can give advice from experience, not what they read or information they got somewhere else. I know I can pretty much install any part I want, but I'd like to avoid causing damage to this fresh engine. I'd like to get 100,000 plus miles out of it. Also, can anyone read this cam spec sheet and tell me what I have? I tried to figure it out, but I don't see how to figure it since I don't see the stock rocker ratio listed.


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Stangnet Facilities Maint Tech... Er... Janitor
Admin Dude
Jan 4, 1985
My neighbor came over last night and was saying I needed a bunch of extra things as the cars sits now and that a KB would destroy my engine pretty fast, even driving like I have some sense.

I read about this far before stopping. Respectfully, your friend has no earthly idea how a supercharger works.

No supercharger is going to 'destroy my engine' if it's correctly installed and tuned. Why, I could install the largest supercharger available on the market, onto a stock block car and tune it so that it doesn't go boom. [GRANTED], this would highly inefficient.

Positive displacement superchargers like the Kenne Bell Twin Screw are one of [the] most fun street superchargers available on the market. My suggestion would be to stop chasing dyno numbers. They are [irrelevant]! Instead, do your homework. Find the system that's best suited to your combo, install it, and get it tuned on a dyno, by a professional!

Perhaps it will produce 500HP in a safe boost range. Maybe it only supports 400HP in a save boost range. None of that matters really. Your torque with the KB will go through the roof and put you back into the seat [reliably], over and over again. It's at this point that you'll begin looking at the next weakest weak in the drive-train. It ain't the stock motor. :O_o:


New Member
Oct 2, 2017
Thank you for your reply. I pretty much felt the same way about what my neighbor has said. I agree with "stop chasing dyno numbers" because what I really want is to feel the torque when I mash the gas. Torque is what moves you. I know this isn't realistic, but if I only had 200HP, but had 800LBs of torque, a car would feel like it had much more HP. The main thing is I just want to play it as safe as I can with the engine as far as longevity and reliability goes and still find a compromise on things. My engine builder friend did say that he installed flat top pistons instead of the stock dished ones. I'm not sure how much this increased the compression. I really am at a disadvantage since I don't have a build sheet for the engine. I do trust my machine shop friend and he is confident that this engine can be boosted and safely tuned for longevity. I'm not a top end, high RPM guy. I want my power and torque from low to mid range to get moving quickly. I really don't race anymore and even when I did I wasn't a track guy, just street run type stuff. As part of my research, that is why I stay on this forum alot. Feedback and advice on here is invaluable to me. Thanks again and if anyone else has input, please, let's hear it.


10 Year Member
Apr 27, 2008
Stock displacement cars with Full bolton, trickflow heads, cams, and high compression usually make 330ish up to 340ish rwhp. Flat top pistons could be giving you 11:1 or 12:1 compression depending on heads / depending on pistons.

It is your motor. If he has his own machine shop he should be able to easily give you a run down of the measurements and parts he used. He is doing you a favor but it is good for you to know. Very few NA 4.6L make high 300rwhp. The setups that do have aggressive heads/cams, very high compression, and have 7k + redlines. Not calling the guy a liar on the 400rwhp claim. It is just highly unlikely and is probably a flywheel hp.

If you keep your current motor, add 6lbs of boost - you’ll probably have low 400rwhp.

Stock pistons and rod lifespan depends on many things. Higher RPM, tune, and higher loads all are factors. Anecdotes have made 400rwhp a commonly accepted “safe” threshold. Not many stock pistons and stock rods are lasting in the 450rwhp+ range. Higher RPM, especially above the stock redline will add considerable additional wear at higher power levels. Nothing wrong with stock rods. 400rwhp is a blast. If you want more though, going to need a stronger setup.
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