I'm currently working on a turbo build for a ford 4.9l I6 engine and was hoping to pick someone's brain for a little bit of advice. The first turbo I bought was a cheap ebay one and I think it ended up being too small, but I don't know how I'd know that for sure without special flow metering sensors. These cheap ones also don't usually come with a compressor map. I went with the cheap one because I was very new to this.

Engine specs:
Ford 4.9L 300ci
Offenhauser DP intake
Edelbrock 500cfm carb
Stock 240ci head
Ford HD exhaust manifold w/ the 2.5" outlet
Stock valves/cam/pistons
8.8:1 compression

Here's the specs of turbo #1:
Compressor:
Trim: 44
Inducer Diameter:50.5 mm
Exducer Diameter:76 mm
A/R:0.5

Turbine:
Trim:73
Inducer Diameter:65.5 mm
Exducer Diameter:55.9 mm
A/R:0.63

The truck seemed to run fine with this, but I wasn't overly impressed with the performance gain considering all the work this took. I was hoping for like 300-320whp. After doing loads of research, I believe this to be too small to reach that goal, so I bought another ebay turbo.

Here's the specs of turbo #2:
GT35

Compressor：
Compressor Trim：57
Inducer Diameter：61.5 mm
Exducer Diameter：81.5 mm
Compressor A/R：0.7

Turbine：
Turbine Trim：84
Inducer Diameter：68 mm
Exducer Diameter：62.4 mm
Turbine A/R：0.63

Is this better? How do I know that this would reach my goal? I'll be running 8psi boost pressure. Otherwise, is there a better suggestion you could give me for a turbo size? Any guidance would be much appreciated.

 

Some of it is just experience, sometimes you can find a turbo map for something similar, most of the eBay turbos are just a knock off of something else. The easiest way to source a turbo is base it off the NA power of the engine, and the final power goal.

A 50mm turbo is small, it is in the realm of 100hp 4 cylinders. I think you were choking the engine on both sides. The gt35 is good to 4-500hp depending on which one you choose, it may still be on the small side for your combo, you have quite a bit of displacement but it might work ok. You should really be looking at t4 stuff, something in the 62-65mm compressor range (minor diameter), turbine in the 75-80mm range (major diameter)

Also, eBay ads are notorious for posting incorrect turbo specs

 

The GT35 is definitely a step up, but I agree with Ben, i'd go with something a 60-65 mm compressor, with a P trim turbine (74.5/64.5), or a S360 if you're willing to buy a non China turbo. Whatever you choose, it should be a T4 imo. Also, you may not make your hp goal at 8psi, 300 at 8 psi will require 200-225 na.

 

Thanks for the responses guys!

Mnlx- my question to you is how you’d know I’d need 200-225 NA to reach 300 at 8psi? Is that just from your tribal knowledge or is there a formula or chart that got you there?

 

Just a generic density comparison. You theoretically double your hp at 14.7 psi. You typically need more though, due to efficiency, and charge air cooling, or lack of.

 

Ok now we're getting to the root of my disconnect. How does the size of the turbo play into that? Surely 15psi out of a 50mm turbo is going to be different than 15psi from a 72mm turbo. Is that where compressor maps come in?

 

Exactly. A compressor map will tell air mass at a particular boost level, and how efficiently its compressing the air. There are quite a few good write-ups around explaining how to read one. Garrett had one on their site, and there was one here as well. Very valuable info to know, even if you only get the basics.

 

You can do a little math to figure out your power gains vs boost. Atmospheric pressure at sea level is 14.7 psi. For every 14.7 psi you add you get a 100% increase in power. So 8 psi is 8/14.7 or 54%, then just multiply your HP by 54% and add it back to the HP (or just add 154%).

If you know your base power, and the power you want to make you can use the same math. Say I have a 180 hp engine, and I want to make 300 hp.
300/180=1.67 or 1.67x the stock power level
1.67 - 1 (for the 1 atmosphere you already have) =0.67 or a 67% power gain requirement.
0.67*14.7 = 9.85 psi boost pressure
Just plug in your own numbers and run the math.

This same calculation is useful to figure out how much boost you can add to an engine, or how much power loss you have if you are above sea level. For instance, I am at about 5000 ft. So my atmospheric pressure is only around 12.2 psi. That means I am 2.5psi down, use the same math as above to figure out the power loss is about 17%. Also lets me know I can run 2.5 psi more at this altitude safely, and that is a huge amount. If my base engine makes 450hp thats a 77hp gain.

 

Good Job on working with a 4.9L Turbo Set-up.
Many many years ago before turbos were accepted by the masses, Belt Driven Roots Superchargers being the old Standby some Ford Engine Engineer guys and I built a Twin Turbo engine with EFI. 3 runners feeding each turbo.
At that time the turbos were typically smaller units vs the stuff we have today.

I believe the turbos we used were Garrett GT25 turbos but two of them.
We built the engine and turbo system to "smack down" the naturally aspirated 5.0L and 5.8L truckers. Then we went to the Ford Test Track for race day on the straight-away.

Cutting to the chase, we destroyed the V-8 boys. Nino Novelli was the boss over the 4.9L calibrators, Gino Novelli, (his brother) was the lead mechanic/calibrator. The engine was inter-cooled with a Garrett Air to Air Inter-cooler and the engine was computer controlled and EFI injected. Cast iron head.

Some years later when the aluminum head was available from Australia we tried to build a 2nd gen turbo but it was shot down. Point being we were working on the 3.5L ECOBOOST stuff at the time and everyone said we need less cubic inches, not more.

Big Chief with his 482 Pontiac engine runs one of the smallest engines on the show vs the 600-
800+ cid engines on every show.

Tom Vaught Ford Research Senior Engineer for Turbo Programs.

 

Tom, when will we get a 5.0L ECOBOOST?

 

Cool info Tom. This being my first build I thought a 300 would be a good one to start with. Tough to wreck them! Haha

 

Never, probably. Ford is now anti-displacement in anything less than a 3/4 ton. Anti cylinder as well, its a miracle you can get a coyote in the trucks at all. Small ecoboost v8 would be nice but I think its a pipe dream. The predator is pretty good though