Hi all

Ther has been quite a bit of discussion on bigger turbos running lower boost being better for engine logevity. Now I can understand that a bigger turbo wont need to run as high a boost to feed the engine the equivilant amount of air. But what I dont understand is how this is any easier on the engine.

Ie Stock turbo running 13 psi will put X amount of air into the cylinders lets say y atmosphers, or whatever it is measued in.

But a bigger turbo will only need 9 psi to x amount of air into the cylinders for y atmosphers.

I am sure there is so much more to this, but with my level of understanding, I cannot understand how this works.

Scotty

Running a higher boost is like running with a higher compression ratio, and puts more stress on pistons and rods.

You need to look at the base/static compression ratio plus boost to derive the effective compression.

Historically, if you want to run high boost cars you should lower the compression ratio. For example, my rebuilt motor uses 8.3 to 1 instead of 8.7 to 1 standard compression. If you want to run very high boost you'd typically lower the compression to around 8 to 1.

If you run a higher rev limit as well as higher boost you are more likely to cause engine failure.

Note that apparently Ford only tried a few turbos before settling on the 3540. It simply met the requirements. No doubt there may be better combinations available if you want to run higher boost, but the 3540 is there to provide good throttle response and the required power level for a large capacity engine using low boost.

Brian

OK I understand that a turbo will increase the compression ratio of a engine.

Here is what I dont understand.

Stock turbo running y boost will shove x amount of air into the cylinders to make z to 1 compression.

Larger turbo at j boost will shove x amount of air into the cylinders to make z to 1 compression.

How is the later any different on the strain to the engine.

Scotty

Heres what I think, keeping in mind im no expert either,

If all things remain constant, that is manifold pressure and temp, rpm and static compression ratio and of course the physical manifold and intake ports, then power should be exactly the same. At around 14 psi of boost the tubo will have to pump approx. 8 litres of air for every two revolutions, I say approx. because I you would need to account for volumetric efficiency. A bigger turbo will have less restriction in its compressor housing making it "lose" less pressure in the housing itself. Also less restrictive intakes, such as APS or nizpro intakes mean less pressure drop and as such the turbo doesnt have to make as much pressure in the first place and then a bigger turbo will have to work itself even less hard to make that initial pressure. A bigger turbo will also have more flow up its sleeve for big top end gains as we've all seen on here.

As far as longevity, the same size bang, the same wear. Quality components are the key to big power and longevity.

I'm sure ill be shouted down if im wrong, as I said im no expert

Caspertt

Well higher boost produces more force (pounds per square inch)

Larger turbo gives you more volume of air.....

what cause boost pressure though? the volume of air the turbo produces or the intake system. how much boost would a turbo produce if the volume of air coming out of it had no restriction placed on it?

EDIT: a larger turbo provides the same "volume" of air as a smaller turbo. The air is denser so the mass of the air is higher....because the larger turbo should be operating inside its effciency range and not outside it like a smaller turbo pushing higher boost to produce the same air mass..

EDIT VER.02 The above comment is made in reference to the above post about all things being equal only the turbos being different...

I have been thinking about this. We all know that cars run better in th cooler hours of the night, due to the fact that the air is densor an contains more units of oxygen per cc for better bang.

Now turbos to my understanding do not cool air only force air into the system and increasing atmospheres. The intercooler to my knowledge cools the air.

So bigger turbo more efficient ability to make more power, but I still fail to se how it can cause less stress on the engine, unless somehow it makes the air more dense and that allows more oxygeb units per cc to enter the cylinders.

Scotty

the air is more dense coming out of the bigger turbo because it has to compress the air volume less to get the air mass required to make "X" kw's, the smaller turbo has to compress the air to a much hirer pressure to cramp the same mass into the same volume...whenever you squeeze a gas it gets hots...the less you squeeze it the less it warms up..bigger turbos also heat the air up just not as much as the smaller one to create the same mass of air to create the same "X" kw's as the smaller one (all other things being equal..)

does that make sense...

the bigger turbo is more effcient if you want to make the same power as the smaller one..once you start stretching the limits of the bigger turbo the same thing happens all over again

EDIT: your comment about turbos increasing atmoshpere isnt really what they do. the increased atmoshperes(pressure) is only a way to measure volume of gas in an area not mass. the mass of O2 plus the fuel(plus all the other engine factors,compression,heat,spark etc) is what makes the power. 14.7 to 1 afr's is 14.7parts fuel to 1parts fuel is a measure of mass not volume..,.but if you can get twice the mass of air you can use twice the mass of fuel and make (theoretically speaking.general terms here)twice the power..

sorry about the long winded post...

couldn't be bothered reading aeverything but don;t bigger turbos take longet to spool up meaning the engine would not be on boost more = less stress on engine?

The size of turbo has nothing to do with the volume of air making it into the engine, only PSI has any impact on this.

A turbo is a centrifugal compressor, and not a positive displacement pump, and thus the size is irrelevant (provided it is suitable for the engine).

When the inlet valve opens to allow air to enter the engine, the difference between a turbo charged engine and a normally aspirated engine, is that the n/a engine depends on the atmospheric pressure (1 atm) to get air into the engine, where a turbo charged engine gets force fed with air from the turbo, and the higher the pressure, the more air makes it into the engine before the valve closes.

Air is denser when cold (which is why a hot air balloon rises), so the colder the air, the more powerful the combustion becomes, almost like having 1/2 - 1 psi more boost.

Conversely, if you drive up a mountain, the engine will gradually become less powerful, as the drop in atmospheric pressure, results in less air density, despite the lower temperatures.

Hope this makes sense