Hey guys, new here with my first question. I have been building a turbo set up for my 66 fury for the past year. It’s running good but anytime I go full throttle it loses power. I’m running 7 lbs of boost that comes on around 2200 rpm. It spools up and has good power at half throttle but you go full throttle and it’s a slouch. When it happens my AFR is at 11.8 which is what I set it to on the base map. It’s not tuned.

I’m not sure what info is needed but here is my setup

496 stroker big block (383 block)
Stock 906 cylinder heads
Cast exhaust manifolds
2.5” tubing into the turbo
3” exhaust out,single with hi flow muffler all the way to the back of the car
A single Cx racing turbo 76mm
1.15 a/r housing
Holley sniper efi
Msd ignition.I have my distributor locked out at 32° and the msd pulling 1° per pound of boost. (Getting a hyper spark set up today)
I have a two filter catch can attached to my valve covers
A Bosch 044 fuel pump
An aeromotive regulator set @ 58psi

that’s all I can think of at the moment. Please if you need any other info let me know. I would really appreciate any help. I’m pretty sure my wife is on the cusp of stabbing me in the throat if I spend anymore money. Thanks.

 

Is it possible that it's just not breathing enough on the exhaust side? Near 500 cubes through a single 3 inch pipe may be quite the restriction at higher rpms? Just a thought...I'm running full 3 inch on my bone stock 4.3 with an h1c turbo. Only half the displacement and it seems pretty happy. Any way to eliminate the full system and just a short pipe to get it away from the engine bay and try it?

Joel

 

3 inch down pipe is way to small for your cubes, I would run the biggest pipe I could make fit. My 360 has 3.5 inch down pipe into a 3 inch exhaust piping way down stream, just before the muffler, due to space requirements or it would be 3.5 all the way out.

 

I'm guessing that the turbine may be small too if it's making boost at 2200 rpm.

 

Yeah,from what ive been reading today im choking the engine. I can try larger exhaust or just a short down pipe for test purposes.But truth be told im unsure if my turbo is just to small. It has a 74 mm turbine wheel and a t4 1.15 A/R housing. Im wondering if i should of got a t4 1.25 or just went t6 with a 1.32 housing. Or a larger turbo in general like a 7875 with the larger turbine wheel. Any suggestions on that would be awesome.Also I appreciate the responses guys. If you have any recomendations on a properly sized turbo im all ears. Im looking to make around 650hp at the crank and just have a little more gusto for the streets, im not trying to set the world on fire. Thanks.

 

You need a s475 with the 96mm turbine wheel.

 

Awesome. Thank you Mnlx. I’ll look into it later today. If you don’t mind could you elaborate on the reason for your choice. I’m not to savvy obviously and would like to know the parameters you went by. Thanks again.

 

I agree with @Mnlx the big turbine s475 would be a decent match, with a 4 or 5" downpipe. Another option would be a second of what you have now. You are likely right on the verge of needing something even bigger than the single 96mm turbine, can you talk a little bit about your power goals and what you do with the car?

 

Im looking to make around 650 hp. I don’t have main caps,studs,or a girdle so nothing crazy like 800 to 1000hp. It’s pretty much a family cruiser honestly but I wanted more power to have fun with when my family isn’t in the car. It’s a sport fury so hardtop coupe. It looks the part so all the kids in their v6 camaro s try and race me every other day. But it’s quite heavy being a C-body car. So that’s where I was going with it.

 

S475 is just right then

 

The S475 is a good all around turbo for engines making 400ish hp na. I think it's a good fit with your stock cylinder heads, if you had an aftermarket head, i'd be looking for more turbo yet. The 96mm is light years ahead of the turbine you have now in terms of design, and is much bigger/ better suited to your application. You can get it in a T4, but I would personally stick with the T6 flange. It's typically cheaper, and again, better suited to what you're doing imo. Look for a S475 with the 96mm 1.32 a/r T6 turbine. There are genuine Borg Warner's, and China knock offs, both will do the job.

 

Sounds good to me. Thank you sir.

 

When you say 650hp do you mean Dynojet 650rwhp or 650bhp (530rwhp)

The S475 looks like it can flow for 900 horsepower, which is far more than you need according to what you said, making it a terrible choice.

The S364 or S366 can easily 650rwhp and more closely match your desired goal. And it will spool and react much faster than anything larger (the reason we use the smallest turbo possible is to max spool character)

heres an S364 making 530rwhp (650bhp~) and has more to give (650rwhp is no issue)
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Compare this graph to a S475

It can do this with 3" downpipe. Engine displacement has no bearing on downpipe size. When it comes to downpipe for turbine the flow rate in CFM or even mass flow (mass per unit time) is the deciding factor, not displacement. For example S366 on a 2.0L Engine still requires the same downpipe as a S366 on a 6L engine if they are making same power.

 

This statement doesn't mean squat unless you provide pressure ratio info. 900hp at what boost level? How much boost do you think it'll take to make his goal? Choosing a turbo based solely on max hp potential is terrible advice

 

I will help you understand how to think about this

Choosing the correct turbo is easy just follow these steps
Here is step1: Identify the power necessary and convert to flow rate
For example 500hp = roughly 50lb/min.
1000hp = 100lb/min
This step should be intuitive and easily accomplished
This has nothing to do with boost level because turbo flow rate is independent of boost for the most part. For example you can flow 500hp with 15psi or 7psi or 3psi of boost. The engine flow rate is independent and will dictate what boost is required to reach whatever flow rate. Since engines and compressor wheels gradually age and lose their respect sealing and hardware the flow rate changes every day slightly, insignificant on a daily basis but overall after 20 years or so is quite significant when considering new vs 20 year old hardware. Thus we have much to do beyond establishing the starting flow rates- we must also predict the future flow rates and worst case scenarios.

Explanation (you can skip if you want and just goto step 2
The Flow Rate of turbocharger compressor wheel (any centrifugal pump) is plotted on the compressor map and as a function of wheel speed, they just convert wheel speed to volumetric flow rate. So if they give you flow rate volume, they are giving you wheel speed. Performance applications which double as reliability apps require wheel speed sensors to ensure consistent turbo operation/behavior and whenever wheel speed rises above expected values the engine is shut down for diagnosis as a safety protocol. This is a standard low cost feature for modern turbochargers even my S364 has a wheel speed sensor port.

The wheel speed is a variable, it can be anything, 0, 500, infinity, for the sake of modelling 'on paper' such as using matlab which is how the turbo is designed from an engineering perspective before production and empirical testing a variable can and does yield a family of curves for an infinite number of potential outcomes.

On the other hand, some static data, also known as CONSTANT is what we (as potential purchasers) are concerned with since once you buy the turbo the CONSTANT cannot be changed. It is engrained.

The constant in this case is wheel mass, kg of wheel mass per unit volume flow rate. In engineering terms, we are looking at wheel mass vs volumetric flow rate capacity.

The goal of this exercise is to understand one simple aspect about flow rate involving wheel speed which will be constant through a series of similar turbocharger designs:
Lower mass compressor wheel = lower flow rate

The reason we don't put a 2000hp turbo on a 500hp engine is because the compressor wheel will be massive, and take too long to spool. It is common sense at that point. For the same reason you wouldn't put a 1900hp turbo on a 600hp engine. It should make sense, to keep the wheel mass minimal as possible. At some point the numbers should 'make sense' but where is that?

Step2: Determine flow rate modifiers
Example 1: When somebody said 500hp but meant 500rwhp so really need 630bhp (brake horsepower) to make 500rwhp, which means really need a 63lb/min turbo compressor wheel, right? 50lb/min is only going to get to maybe 400rwhp. Practice converting quickly in your head and remember setups with heavier drivetrain parts (4l80e will rob more power than anything I can think of) absorb more power to get them moving (invested momentum/kinetic energy)
Example 2: The plumbing has a bunch of tight 90* angles or an oversized intercooler. Intercoolers induce friction and cause turbulence, they disorganize the airflow and take some of it's kinetic energy, reducing flow rate. Correct size intercoolers with wide sweeping angles may take 1 to 2lb/min of airflow (we are just trying to overestimate slightly to get just a bit more compressor than we need) but poorly oversized setups or tight 90* angles may take 3lb/min or 5lb/min instead.
Example 3: Air filtration and pre-compressor kinetic energy considerations matter. Longer intake tubes on naturally aspirated engines are bad for the same reason they are on a compressor wheel inlet and everywhere else in the system- they cause friction to moving air which is based partly on length of the system. Furthermore air filtration presents an energetic cost to the incoming kinetic energy which must be paid somehow. A recirculation style of bypass valve which can hang open widely at idle is ideal and optimal for performance because kinetic energy can be somewhat conserved and this will maintain wheel speed during off-throttle situations, maximizing spool character to the fullest possible potential.
These minor effects can add up to a couple lb/min.
Example 4: Leaks and aging and wear and tear matters. The compressor map assumes 0% leaking and brand new, and in reality no plumbing can truly be sealed perfectly, and nothing can stay like new for long. Molecules are very small ~250picometer or whatever, they will leak around the sides of the blades, the debris over time in air will trash the microscopic edge of the wheel, bearing & journal wear, balance, temperature influence wheel to housing clearance during operations, and so forth. Give yourself a 1lb/min or 2lb/min extra to account for leaking, and age related wear.

Finally, example 5: Engine operating range specificity.
This is just like choosing a centrifugal fluid pump for your pool when you know the established rote fluid rate necessary when the system is turned "on". For example in drag racing the engine will quickly race to a peak number and hold near that range of flow rate for most of the time. This is called steady flow.
Here is an example of steady flow from my application:
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This baby street drag-racing type of setup intended to reach peak flow and hold there for the duration.
Notice the power climbs to some number, then just sits there. In this example of steady flow, the turbocharger flowing approx 65lb/min steady rate, it just sits on one spot of the compressor map more or less.

This is where application comes into play. For drag racing, and situations of steady flow, you will need to weight the benefits of steady flow near the center island of a compressor map vs having to deal with the extra mass of the wheel.
In some cases a trade off (running steady NEAR the center but not exactly perfectly on the best island) is necessary.
It depends how the turbo is being spooled. Nitrous / 2-step for example can spool a very large wheel while the vehicle is sitting at 0mph ready to go, making the wheel mass unimportant. In those cases you will want a turbo that is going to sit steady flow near a center island because all of the spooling is being controlled with the 2-step and nitrous every single time the turbo is used.
On the other hand, if you are daily driving the vehicle, there may not always be any 2-step or nitrous available to spool a large turbo.
In that case, you will want to give up as much wheel mass as possible. Still, focus on the application. If the vehicle will regularly race from 65mph rolls on the highway it will have a fully warmed up turbine and a higher rpm starting point than something which mostly stop-light races from 0mph with a cold turbine after just being started in the morning. Pre-warming the turbine and high rpm starting allows more wheel mass because it is easier to get a wheel moving when everything is fully warmed up and moving on the highway.
Cold, low rpm operation are the type of turbocharger wheel mass ranges that OEM manufacturers like Nissan & Toyota use, turbos which function well which the vehicle is still cold and just warming up in the ice temperatures.

This is how I selected the S364 for my application.
#ad


The pressure ratio is on the y-axis, and keep in mind that is not exactly a boost pressure even though I will be calling it that. I believe a pressure ratio of 2.0 means the pressure is double of the inlet pressure, so if there is 1.5" Hg post air filter pressure drop your 2.0 pressure ratio is not going to be a fully 14.7psi of atmospheric pressure, probably more like 14.2psi. Likewise when not at sea level the ratio is going to be based on atmospheric starting pressure, not sea level (2.0 on the graph isn't always 14.7psi of boost)

lets notice a few things on the picture,
1. the compressor map kind of slants to the right (not all of them do but this turbo does) so as boost pressure is increased and flow rate is increased the map begins to extend itself to the right. This allows you to select turbos which conform to the displacement of the engine more specifically, as in this case a 5.3L flows just enough extra per PSI Of boost increase to warrant the rate the graph is being extended. A 2 or 3.5L on the other hand will not move nearly as much to the right as PSI of boost is increased, the map can be narrower.
2. The final red dot should be overestimated to ensure staying clear of the right edge of the map. In true operation it should not actually reach your final red dot on the right hand side if you've done everything to this point properly.

 

Step 3: Combine step 1 and 2
In step 1 I decided I wanted at least 750bhp = 75lb/min starting value
In step2 I estimated my modifiers and consulted application:
rotating losses would be near 16 to 18% due to 4l80e and high mileage engine 750hp * .84 = ~630rwhp potential
Then the myriad losses, you can get 'safer' here by overestimating and including more headroom than necessary.
-plumbing losses would be minimal because we used the right size intercooler and design the pipes well +2lb/min
-air filtering will be excessive +1lb/min
-account for 10 years of wear +1lb/min
-extra engine flow rate added for safety factory +1lb/min
My application mostly stoplight action 0mph with never any 2-step or transbrake, so I want to get as close to the right edge of the map as possible to minimize turbo wheel mass and maximize spool character while meeting the demand for flow established above so it can also highway and dyno pull on an efficient island (better than say 66%)

The demand for flow is 75lb/min + 2 + 1 + 1 + 1 = 80lb/min
The flow rate for 5.3L is approaching the edge of the map no matter what the boost pressure is set to because the slant maintains the efficiency island as boost is increasing at 5.3L in this example for this specific turbo combo

Result is a turbo which can quickly spool, and has enough map left on the right hand side that the IAT remains near ambient even during back to back steady state dyno pulls using gasoline on the hottest day in Florida weather possible. It also tries to run towards the edge quickly no matter where boost is set, minimizing wheel mass. And it doesn't care to what # the boost is set, it always runs down the same efficiency ranges at any boost pressure my setup will manage.

Short clips of the spool character I guess is warranted


That is through a full exhaust very quiet (you can hear birds chirping) with two mufflers. With the cutout open the spool is too violent and sudden so the full length exhaust is actually better suited to street use, not just in noise quality but driving experience.

 

Lets do another quick one to show how fast and easy now.

We want 1400bhp (1200rwhp?) , strictly drag car, 2-step, nitrous, t-brake only slicks.
140lb/min to the edge of the map, but we can be in the center since wheel mass is not a factor anymore thanks to nitrous and 2-step
-> Add enough map to the right to push a steady state to the center, perhaps 165 to 175lb/min on the right edge would land us 140lb/min near the center.

Now you find a turbo which can flow 170lb/min maximum and has near 140lb/min at whatever displacement and engine flow rate (Modifiers to the engine flow rate to find your place on the compressor map) that will land near a center island.


we should always calculate the flow rate of an engine in best and worst case scenarios and then dial up the pressure as needed to fulfill a desired output within that range, accounting for all the little inefficiencys and being aware of the difference between bhp and rwhp.
Once you dynojet the vehicle you can find where the turbo 'gives up' and it should be fairly close to estimated values if you've done everything right including pressure testing the system which is a critical component of matching the compressor map to the actual setup.

If the engine will be used at varying boost levels (I turn mine down to 3psi or high as 22psi sometimes) the compressor map should suite those needs, as discussed and noted the slant can assist with that

 

The math is cool. But an s366 would be instant boost and done by 4500 rpm (maybe sooner) feeding a 496 cid.

 

Your 76mm is too small.

496ci at 6000rpms is 861cfm, if it's running at 90% volumetric efficiency this drops to 775cfm.

Now, your 7psi, with assuming 1psi pressure drop through the air filter, is a 1.5pr. 1.5pr at 60% efficiency has air leaving the compressor at 165°F. Going through a 70% efficient intercooler drops it to 91°, giving it a density ratio of 1.41. 775cfm x 1.41 is 1,092cfm. Corrected to lbs/min is 84lbs/min. This isn't even on the map of a borg s476sx-e.

Now, assuming 1500° egt at the turbine inlet, the corrected exhaust flow is 109lbs/min, and required turbine expansion ratio will be 1.32. Not even garretts gt6041 with a 2.0 a/r housing will flow this. So this means you're going to have high drive pressure almost no matter what, in order to squeeze the exhaust thru the turbine and housing.

If your running the cx racing gt45, the gt45 with 1.15a/r housing, at 1.32 expansion ratio, only flows ~32lbs/min.

If you go borg, you need to be in the 88mm area if your s400 based. Large turbine wheel and 1.45 or 1.58a/r housing.
Other option is to get a second cx gt45, this will split the airflow load in half and they'll both be happy then.

 

I don't know all the fancy~pants math, but I don't think a factory iron headed 496 needs a GT6041, or will need 84lb/min at 7 psi imo. I believe your numbers are off.