fc3s4utnv

Yes i do, I was saying one it does get spinning the turbo will spin faster than what it was designed for. If you got a Holset HC1 or something like that would work better, and also a turbo you get off of a "big rig" will have the wrong pitch, and completely wrong impeller design. I would guess that a rx7 could spool one but it would not be fast, you would have to have an external gate and not use the internal bc it would be way to much for the engine. The wastegate only regulates how much "boost" gets to the motor not how much "flow" the turbo sees.



And yes I do realize that desiel trucks have a differnt powerband, they are more oriented on pulling power than anything.

defrag010

20,000 rpm's? Try more like 130,000-150,000 rpm. It's not Flow that spins a turbine, it's Heat with flow being way less important. Diesel trucks run about 1200F EGT, which is cold compared to a F/I gas engine. That's why they need Huge efficient turbines. A rotary running 1700F+ can easily overspin a turbine of the same size. Depending on how big the turbine is, you can run into turbine rpm limitations. Bigger turbines will be balanced to a lower rpm, so with a small A/R, high EGT, and a large turbine, you could easily overspin it if you lost control of your EGT's. More exhaust heat will yield a quicker spool, so it doesn't matter if you are running 3 or 30 psi, the turbine is still spinning the same speed. That's how turbos end up with shaft play! - rotational vibrations from spinning the turbine too fast!

psyclo

OK defrag wanted to get picky. Above flow is exhaust manifold pressure, which is a function of flow, and temperature and the like. So true he has a point that the higher the temp the more pressure you have but I dont think your right in assuming that heat does all the work. I could head up a trubo and exhaust manifold in an oven but is that going to spin the turbine?

teknics

i just like how he said its for diesel trucks, its meant for high rpms.



any diesel truck ive ever been in doesnt rev too high.



kevin.

fc3s4utnv

[quote name='fc3s4utnv' date='May 9 2005, 04:01 PM'] Deseil turbos are not ment for high rpms.


[/quote]

Does that help?

defrag010

Right, except you're taking it a little out of context. Heat does not do all of the work, it just does most of it. Without any heat, boost would be non-existant.



Think about it this way. The air coming into your engine is going to be the same amount that comes out the exhaust, right? The only difference being the heat from the energy of each combustion. Let's say, for the sake of explanation, that your exhaust was to be the same ambient temp as the intake air. Even though the flow through the exhaust manifold ( pressure is irrelevant to spool charachteristics, rather manifold restriction) is spinning the turbine, it will only spin the compressor just as fast which is the same as the amount of air coming into the engine. It's this logic that confirms why you can not build boost while revving in neutral. Your engine is pumping out xxx cfm @ xxx rpm, no matter what load, but the only difference is the exhaust temperature. This is also why displacement is somewhat irrelevant to turbo spool charachteristics.

What does hot air do? It expands. A sealed ziploc baggie full of hot air with the outside air at ambient temp will inflate the bag to the point where it will burst. Now, while keeping this principle in mind, picture the hot air coming through your turbine housing. It is still flowing the same xxxcfm @ xxx rpm, but now we have Hot air coming through. Following the ziploc baggie principle, the hot air flowing past the turbine at xxx cfm has more stored energy (heat), so the hot air means the turbine will spin faster than it would at a lower temperature at xxxcfm. This is why you actually build boost at 5k rpm in 4th gear vs. 5k rpm in neutral. Your engine has a larger load, and is creating more exhaust heat, which spins the turbine faster than it would just by blowing air past it.

At ambient air temp, the air moving past the turbine will only spin the turbine at a certain rpm (value based on turbine efficiency and airflow in cfm). This means that the turbine is only spinning as fast as the air is flowing by it, which is the same amount of air that is coming into the engine and past the compressor wheel. To build positive pressure (boost), you are going to need the compressor wheel to spin faster than it would spin with the amount of ambient air flowing past it. To achieve a faster rotational speed, the compressor relies on the turbine to spin faster than the formula for turbine rpm at a certain efficiency and airflow in CFM. So to build boost, the turbine needs to spin faster than the flow of air that is going through the manifold. This is how heat plays the most important role in building boost, because the stored up energy from the exhaust gasses is released on the turbine, spinning it faste and faster than it would ever spin with just ambient air temp alone.



Again, using this logic, it makes sense that the "hotter than the rest" exhaust gases of a 1.3L rotary engine can spool larger turbos than larger displacement piston engine counterparts. My 2.0L 4g63 talon, which has quite a lower average EGT, could spool a large turbine at let's say, 4000 rpm. How is it that a 1.3L rotary can spool the same turbine at a substantially lower rpm? Heat is the answer. Following this, it will become more obvious that Displacement ( a.k.a Airflow) does not play nearly as an important role as heat does in spooling up a turbo. In conjunction, a 6Litre engine with a 1000F EGT temp will have similar turbo spool charachteristics to a 1.3L rotary engine blowing out 1600-1700F exhaust temps. Does it make sense now?

RONIN FC

I think your both right, The heat creates x amount of expanded air that needs to flow through an x amount of space. Without either of those in the right proportions, you will not have the ratio of air volume between exhaust and intake to make boost.



Which means the "big rig" turbo is prolly too big.

defrag010

yep. Airflow spins the turbine, and the heat spins it faster to create boost.

psyclo

^^ thats what I was getting at.

fc3sboy1

wholy ****, that was technical. lets just say send the turbo out to a turbo shop tell them to make the nessesary changes so that it will work ok, i belive the turbo im runnin came from a small mack truck and works great now that it has been worked over from majestic