I believe you are a bit off on this. The centrifugal SC (which looks like half a turbo marketed by Vortesh, Paxton, etc.) does make boost in a linear fashion, but the Roots/Lysholm SC's make a substantial amount of boost right off idle. Those are the ones marketed by firms like Jackson Racing, Blitz, Comptech (except the S2000's). This might be a good alternative for the RX8. The turbos usually generate their boost at around 1/3 of the revs (in this case I would expect it to have boost around the 3500 rpm & up range). Usually turbos do develop more hp than SC's because they are driven off spent exhaust gases vs. belt driven off the engine (SC's take a bit of the hp generated to drive the belt to turn the SC), but they are usually a bit more complicated to install, and can be more expensive (though from this Blitz kit...YIKES !), and needs more tuning.

If 60 hp is about the most that the Renesis/RX8 can generate safely it might be a better fit for a Roots, as the kit can probably generate that quite easily, if there is no upside (turbo can turn up the boost). The Celica Blitz unit is priced at $5K on Hop Up racing, but if the RX8 unit really is $7K I would seriously doubt if they would sell many of them. At $5K (and if they could generate 60 hp, not 50hp that they are currently claiming) I would consider it, but at $7K, dont think so.

 

Still too expensive at 5k for 60 hp.

Need more like 100hp

We need a better ratio of return on investment.

Hell for that price you can port and polish and get 40 hp for less than half the price.

 

where does it say its 7k? Superchargers tend to be more costly than turbo's but you should be able to save on time and labor.

 

Just the retail price on many of Blitz's SC kits. They seem to charge a big premium.

 

I assume that you are referring to my comment about the centrifugal SC having "rpm lag". I agree that a Roots SC does not suffer from "rpm lag", but you still have to set it up so that it only spins so fast at the rpm limit. And like the centrifugal SC, the farther down the rpm range, the slower it spins, and the less low end boost you will have on a Hi-po Renesis with a Roots SC than you would with a Roots SC on say a Miata (or Lo-po Renesis even). So you will still be compromising the benefits of a Roots and Lysholm SC on a higher rpm engine.

My point is that you can't make an assumption like that. The point at which a turbo starts producing positive boost isn't measured at a fraction of the rev limit like an SC. A turbo could be producing positive boost around 2500 rpm on a Renesis just as easily as on a 6k rpm limited 4 banger. Mind you, this all depends on the size and type of turbo you select and other things, but you have more flexibility in designing that with a turbo.

I agree. How much you want to spend in time, money, and effort is a major consideration in making your choice. But there have been other situations where a SC kit is difficult or awkward enough to design that it will start getting expensive (or even impractical in some cases, like the MR2 Spyder).

It's not just more power that the turbo has over the SC. At 25% increase of power, the SCs will have to work hard enough that they will show their limitations of either rpm lag or having to set up the SC to spin only so fast at certain rpms. That amount of airflow also means that you only have to make the turbo so big, and if you size it right, you can have it spool up rather quickly, and still have a broader power band than an SC (an exception might be a Lysholm, but that all depends on alot of things).

And turbos also have a side benefit for those of us at altitude like Denver. Like a normally aspirated engine, an SC will only draw in a certain volume of air at a certain throttle/rpm combination, so it will draw in thinner air at higher altitudes. But as long as your turbo is big enough, it will draw in a larger volume of air at higher altitudes to end up at the same boost levels. Turbo lag does increase (and you have to watch octane levels), but other than that, you basically get the same power. For example, if you are high enough in altitude that a NA engine is only making 80% of power in the thinner air, a 250 HP engine will be making appox. 200 HP. A bolt-on SC kit that increases power by 25% at sea level will give that same engine appox. 250 HP at altitude. A bolt on turbo kit (with a big enough turbo) that increases power by 25% at sea level should pretty much give that engine appox. 310 HP at altitude. That's a very general calculation, but you get the idea.

---jps

 

You're wrong in many places but the one that really interests me is your altitude compensating turbo. I think you need to move up to some higher math. It is published on this forum more then once. Try searching.

Not only would your system interest me I can name a couple of aero engine mfg that want to hire you.

 

the blitz supercharger looks the same as the toyota design used back in the 80's (?)
it also looks like the smaller version used on the 1.6L and there was a longer higher displacment on for the 2L.
these SC had straght lobes not helical. they did have a little lag and were said to come on boost 1000rpm from idle. I have heard of people using the 2L SC on 3.6L motors with good results with low boost but if blitz are using the smaller version (which displaces about 1.4L per revolution from memory) I don't see how it can keep up with the rotary's need for air.
one thing they have a clutch on the front of this SC and can turn it on and off. if they over drive it for low rpm then they can turn it off to stop it spinning to fast and damaging it's self.

blitz would have considered all these and i'm sure the kit is half decent.

 

Well, I've always heard that turbos tend to suffer less from "thin air syndrome" then superchargers or normally-aspirated cars. I thought it had something to do with the thinner air causing less resistance to the spinning turbo impeller however.

jds

 

And it's thinner coming out also. On top of that it is colder outside and therefore there is less in the way of volume to spin the turbine. No free lunch here.
A variable stator might help but still never gonna get anything for free.

How come they only used SC in WWll on the aircraft. There was one turbo by the ***** but unsuccesfull compared to SC engines. They did use nitrous though on the fighter planes for hi alt dog fighting. But they couldn't carry enough to use it except in emergency.
Also of note should be that they had an inert gas to pressurize the nitrous to a given press. Not reliying on just the natural press which varies with amount left and temp.

Not aware of any rootsblowers ever used on these engines. There were some tests by Allison for turbos but I don't know what came of it. There were some multi stage centrifugals as on the Griffin. 2222 cu in V-12 Rolls Royce used in some transport planes.

 

Are we comparing airplanes now?

(sigh).

 

The thing about a turbo system that makes it appear to work better at higher altitudes isn't the turbo itself but rather what boost level is regulated at. If the wastegate is set to open at 8 psi, it is irrelevant how hard the turbo is working to et there as long as it does. At a lower altitude where the air is thicker, the turbo doesn't have to work as hard to get that 8 psi. As the altitude rises, it becomes harder to get that pressure since there is less of it. The turbo just works harder since the wastegate doesn't work as soon. The wastegate does allow the turbo to still hit 8 psi so it appears that turbos do in fact compensate for altitude. However this is only true to a point. As the turbo works harder to get the same boost it would at a lower altitude, it also heats the air up more so there is a drawback. The same basic mentality is used as an excuse by many people to keep their air filters in the hot engine compartment. They assume that the turbo will just spin faster to make up for the less dense air which is true but it is still much hotter. From this standpoint I will agree that a turbo does have an altitude advantage over a geared supercharger. A supercharger that is geared for 8 psi at sea level will not make 8 psi in Denver. A turbo will but it is also working harder and producing less power than the same 8 psi at sea level. It would still have an advantage but by no means is it a be all end all altitude compensation. There is always a tradeoff.

For street use this is probably one of the only areas that I will rule in favor of the turbo. When it comes to other things such as reliability, heat, etc, I won't.

 

RG, tell me is the waste gate working in gauge pressure or absolute. It seems to me that they work on gauge pressure which means that if it makes x psi over the ambiant it still does at higher alt.
Thus it is only x higher then the now lower absolute pressure which is what the SC does. +/- that is. Clear as my usual explaination, right.


Prey tell why would you not look to the aerospace industry for advanced engineering. They have alot more money and stricter rules. require more testing and if something fails they cannot get out and push.

 

You don't know what you're talking about, I read it in Makes and Models so I know its true :D

jds

 

The wastegate is nothing more than a mechanical spring that is set for a certain amount of pressure attached to a trap door. It isn't affected by altitude. It would still take the same amount of pressure to compress it in space or on the moon too. A boost controller, if so equipped, is calibrated to sea level pressure.

While a supercharger will give you say 8 psi over ambient pressure, the wastegate on a turbo will allow you to get 8 psi over sea level pressure.

 

That's why I asked YOU. Now I concur if it's working off absolut pressure then it will until actual density keeps it from continuing approch it's spring pressure. It's still going to have to work harder though.

Who is Makes and Models?

 

That was the point I was trying to make.
I agree with that too. There is always a tradeoff. I was being VERY general in my math, just to show the effect. The turbo will have to be large enough to draw in the larger volume of air required, and it will have to spin faster than at sea level, increasing turbo lag, and increasing intake charge temperatures from both friction and compression. It won't make up for all of the altitude losses, but it will make up for alot.
If I'm not mistaken, they did use turbos in some planes like the B-24. In those days, and still in some countries, people would/will call turbochargers "exhaust-driven superchargers", or "superchargers" for anything that is boosted. So, you will read about a some bombers being supercharged, when they are actually referring to exhaust-driven superchargers.

---jps

 

If you look at a "supercharger" on a B-17, it is nothing more than an exhaust driven turbocharger with a very large pilot controlled bypass valve (wastegate). This is how they could just turn on the supercharger at higher altitudes. The pilot also had to manually increase the richness of the mixture. Thank god for modern computers!