Recently one of my firend's prostock_bigblock made his dyno run and made some crazy power on low boost. he was called a lier etc. well lets analyze why he made the higher then average power:

1-turbine diameter: matching the turbine major diameter with the major compressor diameter . can mean decreased lag. and can flow more and restrict the exhaust less. making an engine that is also working as a pump work more effciently

2- turbine a/r: ask your self what are my intentions ?? do i wanna have a responsive turbo while losing power on top end? or vise versa ? or may be even some where in between? or may be even concentrating more on usuable power between shifts, like for example if you shift at 6000 rpm and your rpm falls at 5000 rpm . may be you wanna concentrate on that??
remember lag in most of the cases is something that you will see once and after that is what really matters. you can go around spool/lag by using torque convertors , trans brake .etc but you can't go around the exhaust restiction created by smaller a/r and turbines. how much restriction depends on your setup. and turbocharger.

3-bigger turbine have more turbine torque?? well what is turbine torque??
the bigger the diameter the the further the blade tips are from the center?
thus have more torque on the turbine/compressor. if you can have reasonable torque on the turbine then use it at your advantage by not reducing the a/r ratio and reducing back pressure or pressure on the exhaust side.


i think you can minimize the losses and increse the gains? for example
if you get a turbo that has a small turbine and a big compressor side, well you would think yeah i will be able to spool that high flowing compressor and have less lag. well not really you will have a sluggish turbo that also acts as a restiction in higher rpms or boost levels. having a nicely matched turbine and compressor can minimize your losses and gives a more effcient setup.

other things to note are:

1- bigger turbines and a/r ratios give you higher overlap threshhold (you can run more overlap to a certain extent due to reduced back pressure). the idea isnt overlap by it self. its the idea that you you can have effcient camshaft cenetrlines and still be able to runaway with the overlap that comes with it. in my point of view you can have a more effcient camshaft with less duration. and since the engine enters boost it does require the same valve events required by a naturally aspirated car. the late intake valve closing required by some intake manifolds or to take use of the ram effect created by them . is no longer required or less important in a boosted car. and the earlier intake valve closing = higher dynamic compression ratio , which is important expecially on cars that allready run low compression.

same thing goes for the exhaust lobe/ valve events. opening the exhaust valve late can make your car responsive and build boost quicker. combine that with some of the effcient heads that flow on the exhaust side like new tfs heads and you got one hell of a combo.

well at least thats what me and prostock_bigblock are thinking of doing. and so far so good.i just thought i should clear things up,because there are a lot of people that dont hesitate to bash just because the numbers we achieved are higher then what you would normally see for similar setups on ls1tech.

please share your thoughts and ideas on this issue. after all this my point of view it does not necessarily mean they are universal.

Thanx,

 

ttttt

 

I think your rationale is completely logical....

its a game of matching flow characteristics of the turbine (to include induced backpressure) and desired spool characteristics

 

i need feedback on this issue. and in my point of view it an issue worth addressing.

 

Good subject. Most people get so worked up in how hard the turbo hits and then wonder why they cannot make big power. Most people will drive a turbo car that ends up with 4:1 p/r's and like the way it acts on the street. On the other hand they don't like the way the big hp car drives on the street because of the 1:1 p/r's and a turbo that does not feel so responsive every where. When I race cars my rpms are never below 4500 in a ls motor. So why does every one get cought up in miss matched turbo combos? They are not building all out race cars and they are happy with 500-600 to the tires.

 

completely agree with this thread. I beleive most people are so obsessed with a huge compressor and fail to adress the hot side sufficiently. The reality is that people need to choose a turbo for their application based on the hot side first, then the compressor is just XXmm for given amount of airflow. A simple fact, that you stated it the fact you only see lag one time, and with a stall converter even that is a non issue. I beleive there is more power in the hot side than the cold side to some extent. Aside from price, this is the main reason i went with the standard BW s400. I liked the huge turbine and large A/R of the housing. I could have payed twice as much for the ST or SX, but in reality a little bit of lag is easily made up for with top end breathing.
As a side note, in the rotary world, there are many reports of the standard s400 outspooling a GT42, this is from a number of people BTW not just one case.
I think some of this has to do with an extreme decrease in backpressure, like headers on a N/A car. fact is my car makes ALOT more torque at low rpm, and very little boost than alot of others ive compared to. I too attribute that to turbine tq, as i have thought of that on a few different occasions.
Gary

 

The way I look at it, the turbine size doesnt matter much once you get into boost because the wastegate will always vent the exess exhaust pressure. The turbine housing size is more of a "when will boost start" issue than a horsepower capacity issue. Meaning there shouldnt be a difference in back pressure between a .68 housing and a .96 housing under boost as long as the wastegate is properly venting the exhaust.

Now the compressor is a different story- Getting your combo's power to be in the most efficient island of the map is the trick to making high power with low boost.

 

that is not correct at all.
First off, you have to look at turbine tq. On a large compressor/small turbine turbo, it takes x amount of force to turn the mass of the unit. this force is applied to the turbine blades as pressure by the gas trying to flow past them.
now on a small wheel, short radius blade, it will have to apply pressure x across the length of the blade to turn it, and with the area of the blade being small, the pressure must be large.
go to a larger wheel, effectively adding area to the blade, and mechanical advantage with the increase in radius, the pressure can be less to equal the same force. Hence, pressures are GREATLY different between different sized turbines, and even different blade designs and # of blades.
So, as you can see, even though the wastegate is releiving pressure, it has no effect on the drastic differences inside the system realative to one another, these differences seem to be rather constant from what ive seen. Now housing A/R, i really dont know how that changes the aspects so i wont try to guess
Gary

Also i would like to add, that on the compressor side, these new wheels are so efficient that getting a perfectly accurate placement on the island is not that big of an issue. As long as your on it, past the surge line at a decent level and have some overhead to work with, 2-4% efficiency that you might lose is really a negligible amount when you consider the exhaust side which can have a pressure difference of 10-40% in extreme cases between 2 setups.

 

Say you have two turbos with identical compressors- but one turbo has a O-trim wheel and the other has a Q-trim wheel; both have the same A/R housings(not really important for this comparison). If both compressors are moving 50lbs/minute of air they will be both spinning at the exact same RPM. Meaning torque applied to the wheels will be different because the O-trim wheel will require less flow to maintain the desired RPM- but total exhaust flow will be the same because the O-trim's wastegate will be flowing more. The only way I can see where turbine torque would matter would be in an application where a wastegate would not be used.

There was a guy at turbomustangs that did a backpressure comparison on a big block Ford (460?) with an S480 turbo with a 1.2 T6 housing. He then built a plate to block off one scroll of the turbine. The backpressure was much higher with the blocked off turbine up until the point where the wastegate opened- at this point the exhaust pressure between the two tests was equal because his JGS 60mm wastegate was able to vent off all of the exhause that the turbine didnt need to keep the wheel spinning.


I am not saying I am right and you are wrong or anything, I am just not convinced yet

 

Thats fine till you get to the point where you need to max out the turbo by keeping the gate shut. Then you will see some mega high pressure ratios. Also the time spent at a high pressure ratio is not good either. This is were high EGT's and reversion come into play. This limits power potential because exhaust gas does not burn so well. Then you will need to pull timing. This is why one guy can make 900whp on pump and the other guy struggles to make 700whp on the same setup minus a different exhaust wheel/housing.

Yes the big high flowing gate will help to a point though.

 

what are your thought about the cam/valve events i described in the begining of this thread??

lets engage in the technical aspect of things

 

so im lost trying to figure out what size turbine to use. im trying to make something close to a gt35r, without spending all that damn money. ive figured a t04e 60 trim is a good compressor. one speed kit ive seen used a t3 turbine, while atleast t3 flanges. those fancy after market housings probably flow well enough that theyre suitable because of space restriction. i dont have any space restriction, so a regular t4 would work for me, but would that be too big? so how do i figure out what i need? how do you read a turbine map? what is correct gas turbine flow?

 

are you talking about a twin turbo setup??

 

yes i am

 

If spool up isn't ur main concern , depending on the boost and power production ur turbine should increase as the amount of boost increases, also if ur goals is runnin low boost, then the turbine will not be a big a restirction and u can gain spool up by runnin small turbine.

if ur planning on runnin highr boost, u should free the restriction in the exhaust by runnin a bigger turbine/ housing.

its better to run a bigger turbine with a smaller A/R.

in conclusion, the less back press u have on the exhaust side will lead u to less exhaust reversion and heat build up in the exhaust., the more heat u create in the exhaust means a bigger exhaust gas expansion, also making the engine less efficent pump. ............ keep that in mind.
although the wastegate might be open and releeving some of the exhaust back press, its still doesn't solve the problem with higher EGT and back press.

I am not a thermodynamic engineer but i know that the higer the EGT the less pressure relief.
a restrictive exhaust, remains restrictive even though bleeded through a wastegate.