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post #1 of 10 Old 04-11-2002, 11:52 AM Thread Starter
 
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Boost (MAP) discussion, how it relates to part failures>>>

In another thread there is a discussion about boost breaking parts. There are many misconceptions, let me see if I can clear up a few:

#1 Boost is ONLY a measure of the engine's restriction to air flow. Let's call it by the correct term, manifold absolute pressure (MAP).

#2 MAP is different from engine to engine. Meaning you can have two identical blowers pullied alike and see different MAP on each engine. For this discussion we'll use engine "A" and "B". Both engine have exactly the same bottom end with the same compression ratio. Engine "A" uses stock heads and engine "B" has ported heads. Both engine use the same blower with the same pulley setup. At any given rpm engine "A" will see more MAP than engine "B". This is because engine "B" is capable of flowing more air because of the ported heads, thus it see less of a restrictive to flow (MAP). Because engine "B" is flowing more air it will develop more power at any given rpm than engine "A", even though it sees less MAP (Boost). Given this example you can see how MAP (Boost) is relevant when comparing two IDENTICAL engines. There are several other factors that play a role in MAP; such as timing, a/f, cam overlap, etc. But that's going deeper than required for this discussion.

#3 MAP (Boost) does NOT break parts. Never has, never will. MAP can blow out intake gaskets, hang open intake valves, etc but it does not break parts. Two factors break parts:

1) Detonation - AS MAP increases so does the IAT. As IAT & MAP increases so do the cylinder temperatures & pressures. Compensation must be made via timing/fuel to counteract the increase in cylinder temperatures and pressures. This is why the "tune-up" is critical on a forced induction engine. Dentenation is caused for the gas in the cylinder reaching an extreme temperature at which point it ignites without aid of spark. Under normal conditions the cylinder sees pressures between 1000-1800psi (example only, many factors determine cylinder pressure). However, when detonation occurs the cylinder pressure can spike to 4000-5000psi. When this occurs it 'hammers' the piston back downward against the stoke and the result can be severe. Detonation can be subtle or severe depending on the tune-up. Subtle signs are 'peppering' of the spark plugs. Which in reality are pieces of melted piston sticking to the plug. Severe detonation can break/distort pistons, bend/squash rods, flatten bearings, blow head gaskets, etc. Because of these factors, engine "B", in the above example, will be easier to tune, make more power, and be less likely to detonate.

2) Weak parts - Parts that are not strong enough or that are fatigued will break. The parts used must be matched to the desired power level to be achieved. There is various level of parts durability and they are rated for certain power levels. Keeping the engine within the capability of the parts used will help to ensure longevity. This assumes a 'correct' tune-up. However, part breakage can still occur even given all the correct matched parts, this is inevitable to performance engines. The key to maintaining a performance engine is to evaluate each failure logically and determine the cause of the failure. Once the cause has been determined steps can be taken to rectify the problem. Examples:
1) A rod break in the center - this is NOT caused by detention but rather by an inadequate rod for the application. 2) A rod bends - under normal circumstances this is ONLY caused from detonation. So evaluate each failure, make no A$$umptions, and choose parts carefully.

3) Longevity - Given two engines developing the same power. Once is forced indutcion and the other is naturally aspirated. Which will last longer? Answer: neither, both will have the same duribility/longevity assuming proper parts are used and proper maintenance. Why? Because power is developed from cylinder pressure. Both engine must develop the same cylinder pressure to obtain the same power levels


This is a long in-depth discussion and I have only covered the basics. But, I hope this helps clarify BOOOOOOOST (MAP) and how it relates to an engine.

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post #2 of 10 Old 04-11-2002, 12:06 PM
 
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Joe,

Excellent info. One question, though. If cylinder pressures of 4000, 5000 psi pre-ignites the fuel charge, how does detonation occur on a completely stock motor? Some of us experience severe pinging on stock motor with stock tune. Hell, sometimes on a real hot day, I get it running 93 octane.

Thanks

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post #3 of 10 Old 04-11-2002, 12:09 PM
 
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Thumbs up Booooooost!!!

Nice job Joe!!!




-Tony
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post #4 of 10 Old 04-11-2002, 12:13 PM Thread Starter
 
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Quote:
Originally posted by 96GTCONV
Joe,

Excellent info. One question, though. If cylinder pressures of 4000, 5000 psi pre-ignites the fuel charge, how does detonation occur on a completely stock motor? Some of us experience severe pinging on stock motor with stock tune. Hell, sometimes on a real hot day, I get it running 93 octane.

Thanks
Think you misunderstood, those extreme pressures are the result of detonation.
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post #5 of 10 Old 04-11-2002, 12:15 PM
 
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Yep. Reread it. Gotcha, thanks. Hi, my name is Jim and I am a boost article dislexic.
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post #6 of 10 Old 04-11-2002, 12:20 PM
 
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Quote:
Originally posted by 96GTCONV
Joe,

Excellent info. One question, though. If cylinder pressures of 4000, 5000 psi pre-ignites the fuel charge, how does detonation occur on a completely stock motor? Some of us experience severe pinging on stock motor with stock tune. Hell, sometimes on a real hot day, I get it running 93 octane.

Thanks
You have it backwards.

pre-ignition can cause 4000-5000 psi of cylinder pressure

Not,
cylinder pressures of 4000-5000 psi can cause pre-ignition

EDIT:
Joe, you beat me to it
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post #7 of 10 Old 04-11-2002, 12:23 PM
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Nice simple write-up Joe. I completely agree with everything you said and it jives with what I've learned over the past few years. Many people don't understand the basic principles of engine dynamics and therefore it is hard for them to understand more complex systems, such as nitrous or boost.

Don't fear the boost.....

Eric

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post #8 of 10 Old 04-11-2002, 12:41 PM
 
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Excelent post. That is what I was trying to get at in the other thread.

Robert
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post #9 of 10 Old 04-11-2002, 12:50 PM
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Appreciate your time, Joe. Thanks. Wasn't really anything I didn't know already, but it's still great info. for those looking to learn about blowers, etc.

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post #10 of 10 Old 04-11-2002, 01:48 PM
 
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Let's go a little deeper here. Many people expect to see much lower boost levels when they swap heads, but it doesn't happen. Why is that? Well, the intake/engine/exhaust path is not open all the time. It closed by the valves. So boost only drops in proportion to the amount of extra air filling the cylinders.

Let's say engine B makes 100 HP more than A, with the same blower/pulley combo. The boost only drops by the amount of air needed to created 100 HP. Just don't execpt hugh drops in boost levels with new heads

David

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