How much boost is too much boost?

Hi, I’m a little bit newer on the car scene in the sense that my current car, a 1994 Mitsubishi Eclipse GS, is my first project car to really modify. I’ve been in and around cars and modifications for years, but no one’s ever really explained the process of figuring out how much/to boost a (for all intents and purposes, stock) engine.

How do you know where to start with numbers on pressure and horsepower increases? How do I know whether or not while my crank and connecting rods might be fine, maybe the engine spools too much and runs at much too high RPMs?

edit: the motor in my Eclipse is a NA 2.0L I4 16V DOHC, if that gives any helpful context

Find other folks that have done it. VERY expensive to be the one determining max boost.


It all depends on how well the engine was designed in the 1st place, not to mention how good/bad a shape it is today… You can only boost a stock engine so much before building up to much heat and the ring ends touch and bad things happen at that point… The best Boosted engine, is the best N/A engine before boost is added, meaning head flow, cam duration/lift, forged or cast pistons, rods and crank etc etc, the more boost the more piston ring end gap needs to be, to a point… …
A well built stock engine should be able to handle about a 30-35% increase in forced induction, in boost that is roughly about 5 psi, not to mention, that more boost = more air flow and with more air flow you need more fuel and plus the timing has to be retarded as the boosted cylinder pressures are way higher ( higher with higher boost) and you will have preignition preignition at high boost = engine parts that now easily fit in a 5 gallon bucket… Plus you will have to upgrade your Map sensor to compensate for the boost level…

So how do you know for sure how much boost is too much boost, parts f your internal engine parts will be on the outside of the engine…

On a Boost HP Calculator, 5 psi of boost gives you 34% added power… 7.35 psi gives you about 50% boost gain…Those are NOT guaranteed %

So now if you want to have 200 HP out of your N/A stock 135 HP engine you will need about 7.1 psi of boost, plus upgraded fuel supply, (pump, injectors, regulator, fuel management system as well as a timing retard controller and much more… And hop your bottom end can handle the added power…

You might even have to run race gas or convert everything over to E85 if not already able…

Your best bet is to find guys running your combo and talk to them as well as buy a bunch of S-A Design type performance books and talk to some engine builders…

If you want to play you have to pay$$$$…

And as mentioned, this forum is NOT the place to find the answers you are looking for…


Have you googled ‘Mitsubishi Eclipse forums’?

I know your question isn’t about max boost, but this thread covers a lot as it applies to DSM cars.

This guy knows boost… lol

I’ve looked through a few other forums, and I found a guy that did exactly what I’m trying to do: convert a stock Eclipse GS into a GST with a minimum of overhauling. From what I can gather I can get the job done with a turbo ECU, new MAF, a knock sensor, new oil pan, and an upgrade to the fueling system. Several people upheld that the stock 4G63 motor could handle a 10psi boost fine without much worry of detonation, so long as I was sure to get the necessary upgrades that go with installing a turbo. A couple people were arguing about whether a fuel pressure regulator was necessary or not, and I’m not totally sure about it.

I guess my main question out of this is how to know what’s reasonable to look at in taking a stock engine and adding boost to it without having to do pistons, cams, all the really dirty work, etc.

Sounds to me like you’re trying to do “power” on the cheap. And on a nearly 30 year old engine, too.

You can either pay now to do it “right”…or pay later when your “easy” upgrades blow the engine.

I’d think real hard about this. Best of luck.


The factory turbo setup works with 10 psi, that seems clear. If you compare the GS engine to the GST engine you might find part of your answer… If the GST pistons are forged, for example, that tells you the factory determined the stock cast pistons could not handle 10 psi. Same for connecting rods… are they a different part number? Or the crankshaft… is the GS cast and the GST forged? Researching those answers tells you a lot. Cams are probably OK, the valve springs may need to be upgraded.

Another thought about the “dirty work”… if you have a 90,000 mile engine, you need the dirty work, no question. Boosting a tired engine that is no longer cheap and easily available greatly increases the chances you’ll blow it up. So plan for the rebuild. Since it is already apart for new rings, bearings, ect… Why not put parts in that will accept the power the boost creates?

This is a rabbit hole every performance modder finds…


Years ago I had someone working for me that had that same car. He ran boost up to 11psi and had no issues. When he hit 13-14psi boost I found him in the shop scraping parts of the piston off of the cylinder wall.

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The N/A 4G63 has different pistons that the turbo engines, the turbo got lower compression pistons. I’m not an expert on the 4G63, but from what I’ve read the rods are the same between the N/A and turbo versions, but they don’t interchange between the 6 bolt and 7 bolt versions. Anyway the turbos made about 12 psi stock, but that’s with around 7.8:1 compression. The N/A engine has 9:1 compression. I wouldn’t try anything more than 7 or 8 psi on the stock internals.

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My advise, buy a 87-89 Fox body stang and build the heck out of it… There is probable more aftermarket support for those then just about anything else out there, other than a Honda… 90 was the 1st year for the air bag… They even have swap kits for the LS engine, they are a dime a dozen and can make a ton of power on the short block, might have to open the ring gap up a little, but they make power on the cheap compared… And know I am not a big Ford or Chevy guy, but those cars and engines have my respect…
And keep your daily driver stock and dependable…

If the OP wants to try to boost his old engine without destroying it, the first thing that he needs to do is to lower its compression. The second thing on his list should be the replacement of the rod bearings and possibly the main bearings. Then, after adding in the cost of the turbo unit, he can probably accomplish his goal–albeit at a cost equal to that of a newer car which has more power and stronger brakes to handle that power.