I posted this on another forum tonight but thinking about it, its interesting to see peoples thoughts on here, maybe people like carl or anybody who has played with big turbos on rotarys can input as its nice to share info and not be all secret squirrel.
You will probably gather where i posted it
\'same target power, different comp to exhaust wheel ratio
Ok maybe mr howard coleman can input on this or someone with a long standing idea on rotarys.
say you have a predicted output 750bhp which is roughly 100lbs of air, or 1000bhp in piston terms
you have looked at 2 turbo chargers, both claiming 1000bhp, ill list the specs below
Borgwarner s475 with 96mm turbine wheel on 1.32 turbine housing
Borgwarner s478 with 83mm turbine wheel on 1.1 turbine housing
as can see, they are different in there approaches to making power,
one buy decreasing backpressure to increase VE, which in theory is great for a rotary to reduce overlap of exhaust gas in the intake stroke.
the other runs a smaller turbine wheel which in theory means more backpressure but makes its power by running a larger comp wheel to \'in theory\' make more lbs of air at the same boost.
now spool is effected by trim, comp and turbine wheel sizing but say you had to choose between the two, which would be your choice and why?
for example, which do you believe would spool sooner, make more power on a rotary and be better overal turbo.
to give you more reference
s475 with 83mm on 1.1 housing is claimed 950bhp(piston)
s478 with 96mm on 1.32 housing is claimed 1050bhp(piston)
as you can see there isnt alot in it.
my thoughts are the s478 due to the smaller wheel and turbine housing will allow it to spool enough to make it reasonable to someone with a 8.5-9krpm limit
where as the s475 with the big rear end are useful for guys running e85/meth or running a big bridge and a decently high rev limit
it would be really great for everyone to jump on board this conversation because ive never really seen a thread on here about what overall is best for a rotary and this place is where alot of people come to learn\'
well from what ive gleened looking into turbos over the last few months the basic rule is you pick the turbo for the power you want to achieve at a given psi with the tune/porting you have.
so if the smaller one can produce the peak power you want at a reasonable amount of psi that isnt overworking it in any way then it would be the one that would spin up quicker giving you more power sooner in the rev range.
if all you want is peak power and are going to tune everything to the turbo to achieve its max power then go for the bigger one but it will be at the expense of lag at lower revs as it will take a lot more to spin it up. i would expect to see a steeper power graph thats starts at higher revs.
it really does depend what you want.
having said all that until you get one or the other on the car you really arent going to know how they are going to be as the larger one may be overkill anyway.......
Iv been trying to think of intelligent answer to your questions TT.
But all I can think of is that comparing those 2 turbos without using the same hotside is a bit like comparing apples and stones.
Surely stating the hp figures on turbos with the same hotside but different size inducers is a better question. And the answer would be that they would surely produce the same peak hp but at different points. I would personally get the smaller inducer but preturbo WI to increase power producing capabilities to match the larger charger peak hp numbers. Giving a better range of useable power.
I\'m sure I have just confused myself.
Having 100 hp between the 2 turbos you have given is a huge amount, but it all comes down to the power being useable.
2 examples;
1, drag car
2, your 200 mph run?
Quote from: Grant Monkhouse;764037Iv been trying to think of intelligent answer to your questions TT.
But all I can think of is that comparing those 2 turbos without using the same hotside is a bit like comparing apples and stones.
Surely stating the hp figures on turbos with the same hotside but different size inducers is a better question. And the answer would be that they would surely produce the same peak hp but at different points. I would personally get the smaller inducer but preturbo WI to increase power producing capabilities to match the larger charger peak hp numbers. Giving a better range of useable power.
I\'m sure I have just confused myself.
Having 100 hp between the 2 turbos you have given is a huge amount, but it all comes down to the power being useable.
2 examples;
1, drag car
2, your 200 mph run?
if you reread my post, what im talking about is two different turbos, one with a larger turbine wheel, the other with a larger compressor wheel.
they both make 1000bhp, but in different ways, the two turbos with 100bhp between are the bigger version of each of the turbos im talking about so can be ignored for the moment.
im asking on a rotary which do people believe is best...this would be at higher boost level anyway(25-30psi)
what im getting at is there are 2 ways of making power in a turbocharged environment
One, running on the premise that you want to minimise all exhaust back pressure at the compromise of everything else so you make maximum use of the pounds of air the compressor wheel produces.
ie the comp wheel makes 100lbs of air, you actually get 99lbs into the engine and make 990bhp
or the other way around
you run a smaller turbine wheel or a/r to help spool the turbo at the compromise of back pressure, increase the comp wheel size due to the allowance of earlier spool.
ie the comp wheel makes 110lb of air, you actually get 99lbs into the engine due to exhaust back pressure and make 990bhp
now the benefit of each are these in my opinion.
the first is that because of the lower backpressure you get a more effecient engine(VE) you get less exhaust gas getting back into the compression stroke to weaken the mixture, risk more chance of auto-ignition and inturn potentially cause the engine due to run at a higher temperature due to finding it harder to get the hot gases out.
the bad side of this is that by having less gas flow per cubic cm blowing on each turbine wheel cubic cm means that it takes a higher rpm for it to come into the boost threshold of the turbo.....this restricts low down torque of the engine(but potentially its known at a high knock point anyway) which in turn can narrow your powerband down to being slower on the road and unless you have the right gear ratios actually make your drag car slower.
now the smaller turbine wheel and a/r can be beneficial for the rotary for it allows the engine to make its lower down torque that it really needs to compete with the lower compression piston engines......obviously torque is a byproduct of boost so im still talking racegas/wi/meth to get over auto-ignition issues. this can make you faster on track/hillclimbs/street or anywhere thats requires you to use the lower rpm of your range.
so next thing we comes to is larger vs smaller wheels, now in both exhaust and compressor wheel, anytime you increase there size, it takes more gas to spin them up, this is what you mention grant.
if you increase the compressor wheel but keep the hot side the same you will increase boost threshold/lag but then make more pounds of air per psi of boost which in turn gives more power so you get a bit of pleasure for a bit of pain....thats a standard given.
but doing it the other way around as i mentioned above will give again the same increase in overall power and increase boost threshold/lag but through better VE instead of just more pounds of air.
So the idea is, whats best.....force the air in to make power, or let the gas out easier to make power.
This will apply to all size turbos, so from little gt35s up to massive gt47 sized turbos
whats your choice?
p.s sorry if its all a bit random but im just typing whats coming into my head while half watching tv and getting nagged by my girlfriend:Talking1
I understand now.
Well I think personally it comes down to porting and engine rev capabilities.
I read an engine/drag car build on ausrotary where they had a pp13b that was still making power at 11,000 rpm! i.e the hp numbers were still climbing at that point.
So the turbo with the larger AR would be definately be better there?? No point in trying to choke the engine.
However using the larger AR on a pp engine will still be good from the word go in my opinion due to the overlap those bad boys have. Yes exhaust gas goes into the intake charge, but fuel and air mixture goes down the exhaust essentially helping spool the turbo, a primitive form of antilag if you like?
This is probably the limit to my internet experience, I\'d better shut up before some accuses me of knowing nothing, and I only know from reading on the Internet. Lol
I would say this. I know that you are keeping track with rx7club and surely with Enzo´s work. And he even on stock port engine used BW S475 T6 frame 1.32 A/R housing. By trap speed we surely can say that engine power is over 650 HP. And we can´t say that it has bad powerband - stock transmition can cope with it, granted you know where to shift.
There are even more things to consider - good tubular header and downpipe with almost no restriction and good tune can provide much better spool than mucking around with turbine sizing.
Turblown even reported, that he has seen larger A/R hotside on same turbine and same compressor to have boost threshold earlier but target boost later, which isn´t necessarily bad thing - torque increase is smooth and kinder on drivetrain.
One thing that has huge impact on spool and boost response is target boost and wastegate spring on its own. Aim high and you will be surprised how boost can build when all exhaust gas is directed through turbine and not escaping by slightly opening wastegate.
Have you seen the thread on rx7 club :
http://www.rx7club.com/showthread.php?t=903611
Maybe once Howard starts the dyno runs it will give you more answers. As far as I\'m aware it can be down to the application. There isn\'t a cut and dried answer to the question because a turbo will be used in lots of different ways.
Shaun