Too much boost?

LightEMup

Registered User
I just did an engine rebuild not to long ago on my 89 SC....I had a supercharger sittin on the shelf and i didn't know it at the time, but it was a later model one...so now that the car is on the road i'm gettin 15 lbs of boost pending on the temperature, on a FREEZING cold day itl pin on 15 at like 4 grand, and other days itl take 5500 to pin it on 15....anyways my question is....i know 15 lbs is bad for stock exhaust, due to HG's.....but, if i were to get an OD pully and get a free flowing exhaust system, and run the same ammount of boost (15lbs or so) with free flowing, would it be just as bad for the HG's running 15 lbs with free flowing as it would running 15 lbs with stock exhaust?
 
LightEMup said:
I just did an engine rebuild not to long ago on my 89 SC....I had a supercharger sittin on the shelf and i didn't know it at the time, but it was a later model one...so now that the car is on the road i'm gettin 15 lbs of boost pending on the temperature, on a FREEZING cold day itl pin on 15 at like 4 grand, and other days itl take 5500 to pin it on 15....anyways my question is....i know 15 lbs is bad for stock exhaust, due to HG's.....but, if i were to get an OD pully and get a free flowing exhaust system, and run the same ammount of boost (15lbs or so) with free flowing, would it be just as bad for the HG's running 15 lbs with free flowing as it would running 15 lbs with stock exhaust?

There is no reason to expect 15psi to blow headgaskets regardless of your exhaust system. What will blow headgaskets is extreme engine temps and detonation from lean mixtures. Those are your real concerns.

Oh, and your engine will always make more boost when it is cold out.
 
Boost vs. Oxygen

XR7 Dave said:
There is no reason to expect 15psi to blow headgaskets regardless of your exhaust system. What will blow headgaskets is extreme engine temps and detonation from lean mixtures. Those are your real concerns.

Oh, and your engine will always make more boost when it is cold out.
Dave; Isn't it a matter of how much Oxygen goes into the engine, & NOT boost pressure? Boost pressure is an easy number to "fixate" on, but it doesn't tell the the whole story. High pressure from an M90 = High Heat. Ergo the need for an IC.

Won't a High concentration of cool oxygen, at a lower pressure, make more power than low density - hi pressure (hot) oxygen?

It just seems to me that it shouldn't be all about boost pressure. It should be about cool oxygen.

What thinks you?

68COUGAR
 
well the cooler the oxygen the more the molecules condense hense putting more air into the combustion chamber.
 
895speed said:
does'nt the boost drop a couple of numbers when you upgrade to a free flowin exhaust? :confused:

Yes.. less backpressure = less boost in the motor.

While keeping an eye on the amount of boost you push into the motor is important, it's detonation and pre-ignition that are your biggest enemies. Those two (especially working together) can do some serious damage. I found an interesting write-up regarding the two HERE.

Basically, you don't want to overdrive your supercharger on a stock SC exhaust, because yes, then you're asking for a blown head gasket. I've also been told that the 10% overdrive pulley is a waste because you're pushing the M90 past it's maximum efficiency. So a 5% overdrive on a modified exhaust would be ideal, along with 22* copper plugs and modifications to the whole s/c setup itself, such as larger tubes and a bigger intercooler. Yes, this will re-introduce you to your wallet, but it sure beats shelling out for a rebuild because you blew a hole through one or more of your pistons. In fact, when I ordered my forged Wisecos I had them cut down the compression ratio a couple numbers from stock just to be on the safe side.

More info HERE from Hastings.
 
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Mark, you are right, which is why I said boost levels and exhaust systems are not your biggest concern with blowing headgaskets. There are many myths surrounding the headgaskets on these cars. Here are some facts.

1) All OE gaskets are defective (or at least a faulty design). Therefore if you haven't blown the gaskets yet, you will soon. Nearly all forced induction motors require periodic changing of the headgaskets. Ours require more than most.

2) More power no matter how you look at it (more air into the engine = more cylinder pressure) will put added stress on the already inferior gaskets. These engines have a known tendancy to blow headgaskets and it's NOT just the supercharged ones.

3) While a restrictive exhaust will have a marginal effect on the cylinder temperatures and pressures, AF mixture is a far great contributor to gasket problems. Anything you do to the motor to increase it's VE will result in a lean condition unless programming changes in the EEC are made to compensate. This includes freeing up the exhaust, porting heads, OD on the supercharger, etc.

4) Normal combustion temperatures under boost are about 1200-1300 deg. A lean mixture will raise that to 1500-1600 deg real quick which will melt pistons eventually. Detonation on the other hand, hits over 2500deg. Think about that for a moment. Every time the motor "pings", that is what just happened.

5) Our motors can handle in excess of 400rwhp on standard composite gaskets. This is not taking into account boost levels, although on a very efficient motor we have seen as high as 18psi on standard felpro composite gaskets with no problems. Others have run as high as 19psi AND 100HP nitrous without blowing an MLS gasket. Point is that tuning is the key, not the parts themselves or the boost levels achieved (or not achieved) as the case may be.

The bottom line is that air that goes through the motor will get burned. Air that gets burned will create heat and pressure. However, keeping boost levels in line does have a big effect on the tendence of the motor to detonate. This is where your concern should lie. Keeping it under 15psi with a stock IC is important due to the temperature rise in the IC system. With a big intercooler there really isn't a limit to the boost you can run other than the output capacity of the blower and the availability of good fuel.
 
I have read the 10% OD on the SC creates so much heated air it is a wash.
(horsepower used to OD vs added HP from 10%)

However I have a MP FMIC, I wonder if the 10% is still a wash?

Or it it will cause my stock fuel system to be to lean?
 
ok...i don't know what brand head gaskets i got when i did the rebuild, i got the most expencive ones i could get at car quest...this is b4 i knew about felpro, it could be that but i'm not sure. How would i tune my EEC?
 
However I have a MP FMIC, I wonder if the 10% is still a wash?

No it's not a wash..with the larger intercooler you can safely run more boost and make more power. I would suggest 10% at a minimum.

Then again I overdrive mine 23% daily and sometimes as high as 26%, so 10% just doesn't seem like very much to me.

David
 
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