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How size matters. Compressor map illiterate? By Zero R

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How size matters. Compressor map illiterate? By Zero R

Old 09-03-13, 04:25 AM
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Why is this thread a sticky, when it is just a link to another thread that is locked for some inexplicable reason?
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Old 07-06-14, 11:26 AM
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Good Question
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Old 08-18-14, 10:36 PM
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why is the linked thread locked?

The info is expired so it is not available to regular Members.(old stuff gets lost or the site gets revamped and some info gets lost)
I was able to FIND IT!!..yay!
It is posted Below (ZERO R is the OP.)

Last edited by misterstyx69; 08-19-14 at 08:44 AM.
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Old 08-19-14, 08:39 AM
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This is the missing thread info!!!(original poster is ZERO R)

Most of this knowledge is common knowledge but it needs to be here too many times these questions get asked.

People on this forum for years have asked "What size turbo should I go with?" The first question to ask is "What is you intended use for the vehicle?" Than ask youself what your budget will allow. Just because you want a street turbo doesn't mean you can afford the baddest setup around. Meaning you may need a more involved setup with one turbo than with another(ie. fuel, intercooler). So with that said. On to sizing.

First there are two ways to do this, you can go with what is proven to perform in the HP range you are looking for. This can make things much easier with less headache involved. (I will have a chart down below.) Or second you can actually try and size your turbo according to your needs. This can be time consuming and you may feel like you are running around in circles at times but it can pay off big. I wouldn't recommend this for anyone but the most hardcore builders. Most turbo distributors can help some if you give them the proper information. However, they are only as good as the information you provide.

The key is to balance good power, with good efficiency, while maintaining decent response. You may get great efficiency at 15lbs with a huge turbo but if your waiting through two thirds of your RPM range to get it, how fun is that? Ever notice how most factory boosted cars hit right around a third of the way in their RPM range?Manufacturers know most driving is done on the street, from one light to the next, or in traffic, or in a parking lot. They want you to feel like you have a good midrange and more useable torque. Not peak HP. This is where the compressor map comes into play.

Compressor Maps

Compressor maps are read like a topographical map. I think they should come colored for the comp. map impared. (pic below, MAP.gif) They read from the centre out. Generally being around 75+% and going down from there. The trick is to stay in the most efficient part of the map for as much of your driving/ rev-range as possible. If you go to far to the left of the map you will surge, this is when the compressor is pumping out air faster then the motor can ingest it. If you go to far to the right of the map you will choke this is when the compressor cannot keep up with what the motor requires. It simply reaches it's maximum flow.

**Disclaimer** These numbers are for estimating or getting you close to some idea of where you'll be. Reason being things like VE change with RPM and are dependent upon CFM which can and does change due to altitude, temperature, humidity etc. So instead of going into depth about the Ideal Gas law and using it to convert whatever "X" measured flow is to STP(Standard Temperature and Pressure) and make this explanation way too overboard. We estimate

So to figure out how to do it you need to figure out these few things.

1) Pressure ratio, This is simple 1bar+boost/1bar = X or 14.7+boost/14.7 = X

EXAMPLE: 15lbs 14.7+15lbs = 29.7/14.7 = 2.02
So 15lbs equals 2.0 roughly on your comp map.

2)VE(volumetric efficiency) This can vary on opinion(some say more some say less) but I run 90% for a few reasons.

3)Displacement (again opinions very) but plug in the number 2.6 and it all works good. Plus you will be using a rotary equivalency factor later in your math.

Once you know those things you can plot a point on the compressor map and see where you land simply by adding RPM into the equation.

Displacement x RPM x VE x PR / 5660 = CFM

So 2.6L x 8500RPM x 90%VE x 2.0PR /5660 = 702.82 CFM or 49.25 lb/min

Plot that on a 35R comp. map and you see that your in a mid 70% efficiency at 8500 rpm. Not too bad.

If you wanna get a guess as to WHP, way back in the day I was told 1lb/min = 10hp It seems somewhat true, but basically you are slightly more than that at torque peak and slightly lower than that at HP peak. So we take the middle ground. So all you need to do is move the decimal over one place 492.5whp (that would be piston motor) We don't have any of those.

To figure it out for a Rotary you would need to do it a little different. 1lb/min equals a little over 7.5hp.
I use 1lb/min= 7.69hp and it seems about right. (If you want to be a little conservative just use 7.5)

This is a ESTIMATED number that I have seen to be pretty constant over the years of comparing cars I have worked on and owned. Others who have put their math up on here only confirmed this for me to some degree.

So 2.6 x 8500 x 90 x 2.0/5660 =702.82 or 49.25 lb/min x 7.69hp = [email protected]

Another formula borrowed from another Forum member goes about this differently. I'm not sure how he got his numbers but look how close they work.

2.6 x 8500 x 90 x 2.0/5660 = 702.82 or 49.25lb/min x10 = 492.5whp/ 1.3 = [email protected]

If we use 7.5

2.6 x 8500 x 90 x 2.0/5660 = 702.82 or 49.25lb/min x 7.5hp = 369.37

I can tell you these are pretty close as I have seen numerous 35R's on the dyno and stock port cars run right around those numbers. Some higher, some lower but they all are within reason of these numbers. This of coarse is assuming all else is good on the vehicle.

For ported motors On "AVERAGE" if you then add 15% for a street ported motor you'll be pretty close again. 378.73 + 15% = [email protected] AGAIN ESTIMATED, but the dyno's are showing pretty close to those numbers.

Hope that helps,the thread will continue below and be updated.

Quick sizing examples for REW

GTR series
35R 360-450
40R 400-500+
42R 500-700+

TD06 400-450+ (25g)
T78 500-600
T88 500-700


TO4R 400-500+
T51R Kai 500-700+

Last edited by misterstyx69; 08-19-14 at 08:45 AM.
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Old 08-19-14, 08:41 AM
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Continued thread.

How to increase spool/response


It gets asked what can I do to increase spool? I want " FULL BOOST" by xxxxrpm.
First "full boost" is a relative term. 12lbs? 17lbs? 25lbs? Which is it? Even "When does my turbo spool?" Is kinda tricky.
Your turbo spools when it hits positive pressure. How long it takes to get from .1 to say 15lbs is not LAG it is your boost threshold. So by asking "When does my turbo spool?" If you were running a 35R I could say it spools by 2300rpm. If you say your running 15lbs, I could tell you It reaches it's "full boost" by 3400rpm. So your boost threshold is 1100rpm to go from 0lbs to 15lbs. Your lag is the time it takes to get from no pressure to positive pressure. Not how long it takes for you to step on the gas and then really feel a rush. That's usually lag+boost threshold To put it simply boost threshold is what RPM you reach the desired pressure.([email protected]) and lag is the time it takes to step on the gas and see boost. Hope that confused someone!! That was the way it was explained to me years ago.

Now that that is out of the way there are a few simple things you can do to increase response. The easiest is to eliminate backpressure. Any backpressure after the turbine will have a NEGATIVE effect on the turbines response. It's complete garbage that some backpressure helps. If someone starts spouting that, you might as well not listen to another thing they say afterwards .With that said, run the largest exhaust you, your car, your neighbors, cops etc. will, can tolerate. With the smoothest bends possible.

Next would be turbine A/r. A smaller A/r would equal quicker response but choke the top end and have higher back pressure before the turbine. A larger A/r would be less responsive and have less back pressure. Ideally you would want the smallest A/r you could run and reach the HP goals you are looking for. My preference is big turbine wheel in a smaller turbine housing. Others may have their preference. But having a larger turbine wheel with a smaller A/r and a larger exducer bore seems to give me the results I like. There are good examples of similar setups with guys running over 500whp on smallish(for rotary) .8X housings and guys running over 700whp on 1.00 housings. The idea of running a larger A/r seems like your wasting more exhaust energy at the expense of reducing backpressure before the turbine. Not a bad thing at all in most cases, but there are limits. How much benefit is it if you aren't getting good power until 5k? The whole idea is to balance power with response. You will have to give over here to get over there. No power till 5K on a drag car may be fun, but it is not fun if you want to actually take your car to work, enjoy it, and are stuck in traffic.

**Split pulse or divided turbines can be the exception alowing you to run a size or two up and still keep the same response.**

Get good tuning!!! You would be surprised at the difference getting good tuning can make. It can help both in response and power delivery. If you can't afford a good tune in the budget you should readjust your build to include the tune or wait till you can afford it with the build you want.

There are things you can do to your IC set up as well but for the most part unless your making your own you are kinda stuck with what you bought. Otherwise keep you bends as smooth as possible and limit the length of plumbing as well. Having a huge amount of piping with a oversized core will do nothing but create a lot of room to re-pressurize every time you snap the throttle shut. People out there putting these oversized truck IC on their cars are just creating a poor system which doesn't actually cool all that well and adds response time. We can get into IC's later. But there is one other thing to mention on the induction side. A POV(pop off valve) not to be mistaken with a BOV(blow off valve). This can help with response greatly. By running a smaller wastegate(or even no wastegate in some) and using a POV you can get better crisper response. One of the reasons is a wastegate opens slowly bleeding off exhaust. Diverting any exhaust energy will obviously slow down how quickly the turbine wheel reaches it's desired maximum. Combine this with a constant on/off of the throttle and the wastegate fluctuates creating a rotating assembly that is speeding up and slowing down. Smoothing out those fluctuations will smooth out your response and power delivery. The wastegate then kinda works as a reliever of backpressure before the turbine, which in turn helps with reversion. This is nothing new. There are a lot of teams/builders out there doing this, and it was done back in F1's turbo days as well.

Where you reference your wastegates boost source can also play into it. If you have your wastegate directly sourced off your turbo it will give you a good safety margin because the engine will actually see less after passing through all the plumbing. But it will also start to bleed off exhaust energy sooner. So it will take longer for the compressor to push air through all that plumbing. How you have your wastegate routed can make a big difference as well. If it is a reroute it should be at least 18" off the turbine if possible. It should also have a smooth angle back into the main exhaust.

Wheel Sizing can have a adverse effect real quickly. Two thing to consider. Wheel trim and wheel ratio.

Wheel trim is best described as the difference between the exducer and the inducer.

The larger inducer you have on the compressor the more air you will flow but it will take slightly longer to respond. Not as bad as just slapping on a larger compressor wheel though. You have to think of it like a lever in a way. If you have too small a lever trying to turn too large a wheel, it will take more effort/exhaust energy to turn the wheel. This is where wheel ratio comes in.

Wheel ratio is the difference between the turbine and the compressor. Measure
the compressor wheel exducer diameter,and divide it by the turbine inducer diameter. This gives a wheel diameter ratio. A good wheel diameter ratio is between 1.1 and 1.20 (good boost response and low backpressure). Average is 1.20 to 1.25 (decent boost response and moderate backpressure). Anything
above 1.3 is horrible (bad boost response and very high backpressure). Anything above 1.4 is garbage (doorstop material). This rule of thumb is invaluable for turbo sizing. USE IT!!! As with any rule though, there are exceptions. For example F1 hondas were using wheel ratios around 1.3. Keep in mind that was years ago though, things have improved since.

Turbo selection as well plays into it and you should size it accordingly. (see previous post) I am a big fan of BB turbo's, some are not. That is there choice. Nissan and Honda both proved over 20% improvement in response time in testing, and as far as durability they have been used in F1 and Cart. It is the transitional response where they shine. Allowing for the turbine to keep spinning at a higher rate when you step off the gas and almost stall the exhaust energy.

The whole idea should be to ideally set up each individual subsystem to be at it's best to benefit the system as a whole. If you can get most of it right you will see a much bigger turbo's responding much quicker than you thought. The people who say Lag is overrated I tend to think only show they are happy with half the problem solved.
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