Effects of boost
Turbo Mini Forums

I've been thinking about the direct effect of adding boost to a Naturally aspirated engine. For example I had 100hp from my new 1380 7 port engine running without turbo at 100kpa. Therefore now with the turbo fitted at same compression ratio (9.8-1) and 140kpa (6psi boost) I should expect 140bhp?

However, I would also expect to lose some of this gained power due to turbo back pressure, turbo efficiency, additional heat, pressure drop through hoses, intercooler and as a result of retarding the ignition? My thoughts are possibly loses of about 15% of overall gain, increasing at higher boost levels, therefore giving me approx 134bhp

Does this sound like logical thinking?

What confuses me is certain 5 port engines claim to be running 150bhp at 10 psi, with mild road cams. They would need to running 105-115bhp in naturally aspirated form to achieve this. Also seen people claim 200bhp from 14.7 (1 bar), this again equates to 110-120bhp with loses

This can't be right, can it?

My 1293 5 port turbo outputs 150bhp at 14.7psi. Therefore 75bhp at 100kpa (0psi) plus 15% of uplift to give 86bhp in n/a form.

Also Standard MG turbo engine is 94bhp @ 7psi. Therefore at a pressure ratio of 1.5. So divided by 1.5, would give 62bhp in standard form. With extra 15% of uplift would so 67bhp. Based on this the losses may be slightly less, but somewhere close.

Edited by alaskanow0 on 4th Aug, 2013.

Class C 3rd Place Avon 2011 14.18 @101mph

If something is worth doing, it's worth doing half of.

That's a good point John.

I'm a missing something here or do boosted engines perform exponentially better under boost conditions or does pressure differential effect air consumption in a linear way. (Given all things same ie charge temp)

I would expect higher charge temps to reduce oxygen content and therefore reduce power. Back pressure would also consume power, especially using smaller turbos at high boost levels.

Edited by alaskanow0 on 4th Aug, 2013.

Class C 3rd Place Avon 2011 14.18 @101mph

I don,t think that you can straight line extrapolate the figures as you have suggested.

As John suggests, most figures are not strictly comparable when taken from different RR,s etc which is why the outputs are often referred to as bananas on here, not claiming them as accurate bhp figures.

One RR operator told me that he never expects to see more than a genuine 100bhp from a turbo A series !!

Yes i moved to the darkside

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I totally agree there are so many variables to take into account. Just trying to understand the theory really.

Edited by alaskanow0 on 4th Aug, 2013.

Class C 3rd Place Avon 2011 14.18 @101mph

i dont see why is so important to make X nanas or curllys if you feel its slow turn it up and build it better/stronger
altho the mine makes X vs your what ever makes things interesting

One thing I'll chuck into this,

Manifolds... If the motor that makes 150hp at 10 psi is the one I think it is, it runs an LCB cut up to make a manifold, I think this is a good cortibutor to power,

Also Mike races Side mount seemed to produce a decent hike in power IIRC

Yep! Mike made great power on little boost with his side mount.

If something is worth doing, it's worth doing half of.

That's a good point, my 100bhp base line is taken with a lcb fitted. If the log style manifold effectively reduces power under boost then more losses need to be taken into account.

Class C 3rd Place Avon 2011 14.18 @101mph

Also although one would expect similar. or perhaps lower VE when correctoed for boost on a turbo motor, I suppose there can be situations when a port/ manifold/carb etc is more efficient when being blown down rather than sucked,

15psi boost at >7000rpm = 200hp+

Remember that power is a measure of work done and is a calculation of rpm and torque. Make the same torque higher up the rpm range and you make more hp.

Boost is only one factor in a complex machine. There are many many other things affecting how that boost affects the final power output.

As with any of these engines, getting the best results means optimising all the different aspects. some things there can be some comprimise but others it just means a waste of effort.

I understood that you could expect 4bhp from each psi you increased the boost by so you would be looking at 124bhp

Own the day

Now that is a made up figure.

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

compression ratio

Getting the compression ratio right is important to be able to use more of the energy of the burn in the cylinder early on in the cycle.

without getting into the technicalities of the dynamic compression ratio wich is dependant on valve timing, an NA engine with a comression ratio around 14:1 will turn as much of the energy in the burn into something that can become useful. Over 14:1 the losses tend to outweigh the gains. The cylinder will only ever consume the charge, in an ideal situation at 14.5psi absolute at full throttle, maybe a bit less.

Add a turbocharger and things change a little with regard to the compression ratio due to detonation limitation. That is to say if we were to use the exact same engine we used in the example above for NA with a compression ratio of 14:1 but boosted the engine by another 14.5psi to give an absolute pressure of 29psi, you would reach the point of spontanious combustion (detonation) way before the spark even occurs (known as detonation limitation).

As we all know, you need to lower the compression ratio on a boosted engine to allow the cylinder charge to be compressed in the cylinder without detonating.

The fact that we reduced the compression ratio and then added another cylinder full of charge actually brings up the theoretical compression ratio to that of the NA engine but most cases we can excede the theoretical compression ratio that made the NA engine impractical to go higher than. We still have to respect detonation limitation.

Obviously we have just been talking about pressure here, but in the bigger picture temperature plays a big part too since air is less dense the warmer it becomes. The warmer it becomes the lower the detonation limit becomes, the less energy that can be usefully extracted.

Going back to what I said before, making torque higher up the rpm range will net more power and one reason why the 16v turbo motors tend to make more power on less boost. The other factor there is the 16v engines are more detonation resistant so can run higher compression ratios (or more boost if you are that way inclined) to begin with.

Edited by Sprocket on 4th Aug, 2013.

Ok, so considering all things equal, ie same rpm, fueling, air temp, transmission loses etc, the formula looks ok in principle.

Also my understanding is a turbo engine with 9.5-CR over the same engine with 8.5-1 will only produce about 3% more maxium power, but will be more efficent and perform better in off boost situations.

Next question, why does a bigger turbo produce more power at the same psi?

Ie a GT1752 over a T2, as in the twins turbo case.

Edited by alaskanow0 on 4th Aug, 2013.

Class C 3rd Place Avon 2011 14.18 @101mph

not as simple a question as you think.

This has a lot to do with how efficient the compressor is, as well as how efficient the turbine is.

Less efficient compressor means a hotter discharge temperature.

Less efficient turbine means greater back pressure. The back pressure dilutes the inlet charge reducing the amount of cylinder filling

as above,

in some intances its possible to achive lower turbine pressure than boost pressure,

I think that the first thing you have to consider is that it's not boost that makes power but mass flow. You can have a shitload of boost and not much flow ( think about blowing through a tiny straw vs a big straw, first one will have a much higher boost pressure but a lot lower mass flow). when you put in a more efficient turbo you reduce the bottle necks on both ends of the chain ( they are physically coupled as well so any restriction in the chain will affect both sides ) and it will be able to move a bit more mass into the cylinders . Put on a better head, better intercooler : more mass flow more power.

That sir, is not rust, it is the progressive mass reduction system

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