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Old 10-13-2008, 10:56 AM   #1
jayjack
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turbo questions

i was looking for to learn a little more about turbo efficiency ratings and i stumbled onto a forum where someone asked a bunch of questions and no one replied but i thought most of them were really good questions and was wondering if anyone could answer any of them for me. it was...
i am not sure if i am allowed to take what i found and paste it into this forum and if i am not allowed i am sorry and you can just delete this post
"I have been reading up a little on FI and it occurred to me that there ought to be some difference in the quality of Turbo vs SC boost. I’m just going throw out some assumptions that make sense to me and would love to get any feedback rooted in real experience and practice. Sorry of some of the questions are obvious but it can be a little tricky to find detailed information on this on the net, at least for me.

1.Well first of all SC would have nothing have no turbine in the flow of exhaust so one would expect backpressure on engine with turbo to be higher proportional to the amount of boost.

a. Does that mean that turbo engine will have slightly more trouble with scavenging exhaust from the cylinders thus leading to higher heat buildup and intake charge dilution?

b. Can SC engine be further enhanced by tuning the exhaust help scavenging via exhaust pulse timing (ugh, can’t remember the name of the effect)? If so, is this done and how significant are the gains?

2. Twin screw SC as well as rotos blower, at low boosts, are as or more efficient, in terms of adding heat to the charge, as turbo. However turbo is hooked up directly to the exhaust plumbing. Does the turbo efficiency rating take into account whatever heat seep you would get that fact or is it purely impeller efficiency rating?

3. It seems like it wouldn’t take a lot of tq to drive the compressor/blower so I would expect the ratio of tq to hp be higher on sc engine vs turbo engine while turbo would naturally have higher hp pound for pound of boost.

4. Specifically in RedShift SC thread it was mentioned at some point that it would be possible to control the boost per gear if real tuning options were available. While this can be done easily in turbo via waste gate manipulations how can this be done with SC particularly one placed after TB?

5. What ARE FI cams exactly? Do they just have more valve overlap to use the boost for a little blow through? If so are injectors usually tuned to fire with a little more delay after valve open to prevent blowing threw fuel?

6. This is a little general but with higher tq and hp output per rpm do oil/water/steering pumps become linearly better at doing their work or does it make sense to under drive them to the power level that they would see on a stock engine?

Are there any other differences in “quality” of SC vs Turbo boost or tuning directions taken with them? Thanks for any comments that will bring further insight!"

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Old 10-13-2008, 01:24 PM   #2
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Re: turbo questions

Quote:
Originally Posted by jayjack
1.Well first of all SC would have nothing have no turbine in the flow of exhaust so one would expect backpressure on engine with turbo to be higher proportional to the amount of boost.

a. Does that mean that turbo engine will have slightly more trouble with scavenging exhaust from the cylinders thus leading to higher heat buildup and intake charge dilution?

b. Can SC engine be further enhanced by tuning the exhaust help scavenging via exhaust pulse timing (ugh, can’t remember the name of the effect)? If so, is this done and how significant are the gains?
1- yes. backpressure before the turbo is generally higher. You can alter the shape of the turbine and impeller and sometimes get more efficiency; that is to say that you can get the same turbo shaft force with less backpressure using more advanced designs
a- its not so much that it has trouble, its that it should be avoided. In an NA (naturally aspirated) engine, the scavenging helps pull fresh intake charge in using the inertia of the exiting exhaust. In a forced induction engine, the positive intake pressure does far more than scavenging could hope to do so its not necessary. It should be avoided because the positive pressure in the manifold will only serve to blow air and fuel directly out the exhaust valve during overlap. Not only does that reduce performance by (in effect) making a leak, it will raise EGT and cook turbos and converters.
b- I'll add my 2 cents, but I'm not really an expert on SC exhausts. With a turbo, you basically want no backpressure at all after the turbine. The more freely the exhaust flows after the turbo, the better. It has done all of its job and just needs to get out of the way. The same is true of a diesel -even without a turbo- because there is no overlap with almost all diesels. No overlap = no scavenging. With an SC, I would assume you don't really want a lot of overlap because (just like the turbo) you would be making a leak and blowing part of your intake charge directly out the exhaust.

Quote:
2. Twin screw SC as well as rotos blower, at low boosts, are as or more efficient, in terms of adding heat to the charge, as turbo. However turbo is hooked up directly to the exhaust plumbing. Does the turbo efficiency rating take into account whatever heat seep you would get that fact or is it purely impeller efficiency rating?
No, its not taken into consideration, but the air spends so little time in the compressor housing that the added heat isn't even really measurable.

Quote:
3. It seems like it wouldn’t take a lot of tq to drive the compressor/blower so I would expect the ratio of tq to hp be higher on sc engine vs turbo engine while turbo would naturally have higher hp pound for pound of boost.
Other way around. The exhaust is wasted heat. It represents a huge amount of energy that the engine wasn't able to extract as HP. A turbo captures that energy and puts it back in. It does require that the pistons use a little extra energy to push the exhaust through the turbine, but its almost a free (or at least very low cost) power adder. The SC borrows hp directly from the crankshaft to spin, and a blower with all of its bearings, weight, and rotating parts costs more HP to run than a turbo. Its not a huge difference, and if you look at it from a cost perspective, many SCs are so much cheaper than a turbo that most buyers choose the SC and put up with that little engineering snafu.

As far as where it makes tq and hp is all in the tune. You can tune a turbo or SC to make all of its boost by 2000 rpms, or tune it so that it makes almost none until 6000.

Quote:
4. Specifically in RedShift SC thread it was mentioned at some point that it would be possible to control the boost per gear if real tuning options were available. While this can be done easily in turbo via waste gate manipulations how can this be done with SC particularly one placed after TB?
If the blower is after the TB (like a roots), I would say its difficult to control boost with electronics. First of all, a roots blower is a positive displacement blower. I had originally thought of something like a variable diameter pulley that grows in size and reduces blower RPM. The problem is that if you slow down the blower significantly, the engine will fight for air. If its a carbureted application, you also can't just vent raw gas with something like a blow off valve. If its EFI, I suppose you could inject fuel downstream of the rotors, but there are pretty big benefits to injecting fuel up top. So, for practical purposes I would say that any SC after the TB its just not practical. With a turbo, of course, that's all built in under your right foot.

An SC before the TB could be boost controlled with an electronically controlled blow off, but it would be pretty pointless to use crankshaft energy to make boost only to vent it off. It would be a waste of energy that would negate any benefits. This is one of the conundrums that has faced SC owners; a turbo is totally self-regulating once installed. More foot equals more boost. An SC is belt-driven, so it provides full potential at all times based on RPM.

Quote:
5. What ARE FI cams exactly? Do they just have more valve overlap to use the boost for a little blow through? If so are injectors usually tuned to fire with a little more delay after valve open to prevent blowing threw fuel?
EFI cams are built with larger LSAs to prevent overlap. One of the more critical sensors is the MAP. lots of overlap means intake vacuum fluctuations that can confuse the MAP sensor and lead to computer mayhem.

Injector timing is not quite as much of a dark art as people think it is. When the injector fires has been shown to have very little effect on engine operation. Many port-EFI systems including GMs successful TPI used batch firing. There were only two injector events; the right bank would fire all four, then the left would fire all four. Sequential fire is a bit more complicated but has benefits in emissions. But for the DIY tuner, batch fire is perfectly viable. It basically injects fuel whenever, and it gets sucked in when the valve opens. The effective scavenging on most mildly cammed engines is minimal. It helps get things flowing, but the actual amount of raw fuel that gets sucked out the exhaust valve is neglegible. I've pulled apart enough batch-fire EFI heads to see that the residues on the valves and in the ports is very even which indicates that its all good despite injector firing at times that are a bit random.

Modern engines use all kinds of tricks. Starting in the early 90s, most EFI was on its way to being sequential firing. In the early 2000's some started getting really sophisticated with multiple firings per stroke, injector pulsewidth modifications based on ignition feedback... they are so sophisticated now that they can read weak or strong cylinders and adjust fuel trim for each cylinder. BUT, most of that is for the sake of emissions and reliability. You might notice a tiny difference in drivability, but for the DIYer there is no reason to incorporate such lofty goals. Batch fire and be done

Quote:
6. This is a little general but with higher tq and hp output per rpm do oil/water/steering pumps become linearly better at doing their work or does it make sense to under drive them to the power level that they would see on a stock engine?
All of those accessories work based on RPM, not power. For instance, if you have a 100 hp engine driving a power steering pump, and you turn the wheel, the pump takes 5 hp. It extracts that hp from the moving belt. If you put that same power steering pump on a 300hp engine, it still takes 5 hp to turn the wheel. You're not providing the pump with an excess of HP. All it knows is that when you turn the wheel it asks for 5 hp. If the belt can supply it, its all good. Think of it this way; a rich guy and a poor guy go into a store and each buy a soda. It doesn't matter to the cashier how much money is in their wallets, as long as they both have $1, they can each have their soda.

Quote:
Are there any other differences in “quality” of SC vs Turbo boost or tuning directions taken with them? Thanks for any comments that will bring further insight!"
IMHO, turbo is almost always the way to go. SC makes nice power and only adds a minimum of maintenance, but its always there at full potential. Turbos take a lot of custom engineering and fabrication (or lots of money to buy a kit that someone else engineered) but to me they are the absolute best. They only provide boost when you're foot is in it and all other times they are transparent. There are turbos out there (aerodyne? I think?) that have become all but self-contained. They have their own oil sumps, have optional stainless steel housings, ball bearings, and all the goodies. If you want to take it one step further, VVT turbos are just about as good as it gets. The vanes in the turbine are variable pitch. They close down at low RPMs and act like a small turbo. They reach peak boost at very low RPMs, then as RPMs climb, the vanes start to open up to match. Most turbos have an operational range and you shoot for the middle. Anything below your target and you don't make peak boost. Anything above the target and the wastegate opens to control boost. With a VVT, the turbine vanes ARE the boost controller. A VVT is a great way to get boost really soon, and then maintain peak boost all the way up to redline.
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Old 10-13-2008, 10:06 PM   #3
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Re: turbo questions

Wow, props to you for that great response. It was way more than I expected. Thanks. But I have one more question, I have heard of a really good book on turbocharging but I can’t remember the name… I remember it had the word boost in the name, any idea what the name of it is?
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Old 10-13-2008, 11:04 PM   #4
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Re: turbo questions

Maximum Boost by Corky Bell
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Old 10-14-2008, 10:28 AM   #5
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Re: turbo questions

thanks again!
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Old 10-14-2008, 03:42 PM   #6
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Re: turbo questions

ok another question. turbo surge, can someone explain what it is and how its bad? i mean does it decrease boost alot and put the turbo way out of its most efficent range? or dammage the turbo? or both?
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Old 10-16-2008, 03:31 AM   #7
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Re: turbo questions

Quote:
Originally Posted by jayjack
ok another question. turbo surge, can someone explain what it is and how its bad? i mean does it decrease boost alot and put the turbo way out of its most efficent range? or dammage the turbo? or both?
Turbo surge is when the turbo is trying to produce more boost than it can at that airflow. It only happens when a turbo isn't matched to the right engine or you don't have a pressure relief valve to help with rapid throttle closures.

The results are the boost builds up then blows back through the turbo and keeps doing it until the conditions change.
It can cause serious damage to the turbo compressor blades, when the turbo blades have had enough they sometimes end up inside your engine. As you can imagine it's a situation you want to avoid.

If you pick your turbo size properly then it's rarely an issue you'll come across.
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Old 11-08-2008, 01:23 PM   #8
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Re: turbo questions

Hi everyone, it's my first post here

What an interesting thread and +1 on the great answer from curtis73.
I may add a few interesting things

On 1-b, Yes you can improve engine efficiency with a larger tuned header system, but it will change the engine cam timing requirements, reducing even further the overlap. Low pressure wave shall be present at the valve opening and no pressure wave should return until the valve closes. But it will not tune all the way trough like a NA system like Curtis pointed.

+1 on batch firing, but IMO, too much overlap may lead to erratic fuel distribution witch is compensated in the VE tables.
The fuel sits on the intake valve until it opens and is probably the first thing blown trough the exhaust during overlap witch add to the importance of selecting the right cam for the application.
When WOT, the duty cycle of the injectors is longer than the time that the intake valve is opened, so it make little or no difference, where it could help is while crusing around town, squirting fuel while the exhaust is closed may help emissions and fuel consumption

I have seen well tuned 5-6 psi systems using just RRFPR as fuel enrichment and a wideband O2 sensor as a mean of tune, 8 psi with boost retard, more with full standalone and higher octane fuel to about 15-19 PSI then fully built engines running insane boost levels that I would not drive

I hope it make sense
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