Scion tC Forums banner
1 - 20 of 25 Posts

·
Former '05er
Joined
·
12,467 Posts
Discussion Starter · #1 ·
Given the injector flow numbers from the Factory Service Manual (FSM), fuel pressure of 44 psi (minimum FSM spec), BSFC of .55 (typical supercharged brake specific fuel consumption), 6psi boost, and an 80% duty cycle, the maximum crankshaft horsepower these injectors support at minimum flow is 158. That's less than stock for a reason I explain below. Best case is 205hp if your base fuel pressure is 50 and your injectors are pumping out the maximum 368cc/min. The New Car Features book says fuel pressure is 47.0+/- 0.47 psi. That means 197 hp at 6 psi boost and 80% duty cycle, 247 if you want to flirt with lock up at 100% duty cycle.

With no boost, the stock injectors at all mid-range values should easily support 236 hp at 80% duty cycle.

So you guys thinking about forced induction should be expecting to at least replace the injectors.

There's another very problematic issue with the fuel system. There's no pressure increase (or decrease) with manifold pressure. Typical forced induction systems have the fuel pressure set to a base value, and the fuel pressure regulator is used to maintain that pressure above the absolute pressure in the manifold. This means at idle, fuel pressure is below the base setting, but on boost the fuel pressure rises above the base with the boost pressure. It keeps the pressure differential across the nozzle (or pintle) constant. This returnless system does not do that. It sets fuel pressure to a fixed value and keeps it there. So all the fuel maps are designed knowing the fuel pressure is fixed.

Not bad for the stock system, but when you add boost the pressure differential across the nozzle tip drops and your maximum flow actually decreases. So increasing boost appears to reduce the fuel available to the engine.

Adding boost to this engine is LOT more complicated than it first appears...Jotech is doing it the easy way, scrap everything from the stock system and start over. I wonder what TRD is struggling with to get their supercharger reliably producing power with this configuration. No wonder it's delayed.
 

·
Registered
Joined
·
11,881 Posts
I thought Jotech was using stock fuel injectors and rail with their current system, and are in the process of upgrading it now.
 

·
Registered
Joined
·
197 Posts
isn't TRD supposed to do a complete flash of the ECU when they install your supercharger? That would mean, to me, that they'd change atleast the computer level of the fuel system...The reason it might be delayed over and over is because they're trying to perfect the ECU code around what you've brought up, which are all very good concerns, so that it's A) reliable, B) fits the promise if not overcome it.
 

·
Ironhead
Joined
·
13,253 Posts
^^^ thats the same info i have. unless something has changed, which is always possible.
 

·
Former '05er
Joined
·
12,467 Posts
Discussion Starter · #7 ·
Toyco doesn't build flashable ECMs. [EDIT] This statement is wrong, the tC ECM is flashable, and we all know that now.[/EDIT] It's either piggybacks or daughterboards with EPROMs. They are very serious about not being able to download or reverse engineer their ECM code.

If Jotech is using the OEM injectors, they would have to use a rising rate FPR. There are a number of FPRs that increase pressure at a greater than 1:1 ratio as the conventional ones do. The trick with a rising rate FPR is to be sure all the support plumbing can withstand the higher pressures. I'm not confident this could happen with the Scion, all the fuel system plumbing is push-on and snap-together. Great for the assembly line, not so great for 100+ psi to provide adequate fuel at 30 psi of boost. I bet they just used bigger top feed injectors with a custom rail. I would.

An AFPR will help, but the system is returnless from the factory. Returnless systems are becoming the norm with OEMs because they reduce evaporative emissions. For an AFPR to work correctly, the fuel system will need to be completely replumbed with a return line for the FPR you will have to move to the engine bay from inside the fuel tank.

All this is a LOT of work. I'm pretty sure Jotech just entirely ditched the whole OEM fuel system, including injectors, throttlebody, and fuel pump. There's no way you could get the kind of power they are getting without starting all over again. If you notice, they take an open deck block and close it off with the new liners. They are not afraid of starting from scratch to achieve their goals...
 

·
Former '05er
Joined
·
12,467 Posts
Discussion Starter · #9 ·
No, the system was designed with a return from the FPR to put the FPR at the rail and provide most accurate pressure control without using some kind of expensive calibrated electronic device to control fuel pressure. They are doing this now to control evaporative emissions and reduce fuel termperature. The systems with return lines were known to heat the fuel and both reduce fuel density and create a lot of vapors in the tank. Fuel density needs to be predictable for open loop maps to work, poor thermal control of the fuel makes this hard to do.
 

·
Former '05er
Joined
·
12,467 Posts
Discussion Starter · #10 ·
Time to reopen this thread. It's been collecting dust too long.

Here's some base information I calculated today based on information in the Factory Service Manual (FSM). The FSM states the injectors are acceptable if the produce a minimum of 76cc in 15 seconds and a maximum of 92cc in 15 seconds while connected to the fuel system. The OEM fuel system allowable pressure is 44 to 50 psi. The New Car Features states 47 psi, so this range sounds plausible. Given these numbers, we know:

The injectors are rated at 304 to 368cc/minute at 47 psi +/-0.47 psi.

The FSM also indicates there is no variance in the pressure between test mode with the engine off and full battery voltage applied to the pump, and idle where a return style system drops about 8 or 9 psi from manifold vacuum. So it is reasonable to assume the fuel pressure is fixed at 44 to 50 psi.

Since injector ratings are always at a given pressure, what happens when the pressure changes? Let's add 7 psi of boost. See RC Engineering's site for the formulas.

Flow New Flow
304 284
368 344
440 424

When boost is applied, the pressure differential across the injector get smaller, effectively lowering the pressure in the fuel rail. So the flow decreases, which means the maximum power the engine's fuel system will support also decreases. How much does it decrease?

With everything stock and no forced induction, the fuel system will supply enough fuel to support minimum 207 and maximum 250 hp at the wheels assuming a 16% driveline loss and 85% injector duty cycle. Both of these are industry standards.

With 7 psi of boost (regardless of the source) the numbers drop to 127 to 154. This is why the injectors get changed for the supercharger.

With 440cc injectors at 7 psi, 85% duty cycle, and 0.60 BSFC; we get 229 at the crank and 192.4 at the wheels. If we play a few games, and make some less than stellar assumptions (leave less room for error), we 269.4 at the crank and 226 at the wheels. That assumes we can run these injectors at 100% duty cycle without having them lock up.

If we get all excited and buy that pulley from NST or ZPI for 8.5 psi of boost, what happens? Available fuel drops hp to 225 crank and 189 wheel with 85% duty cycle. At 100%, we get 264 and 222 respectively.

How about 10 psi? 259 crank and 217 wheel at 100% duty cycle with NO room for error. So what's the solution? Bigger injectors? Maybe. Maybe the fuel system really needs to get fixed so the pressure rises with boost the way it does on all other boosted engines.

The other issue with lower fuel pressure is atomization gets worse, which makes the droplets larger, and BSFC also gets worse, so all our calculations anticipating a respectable 0.60 BSFC might be optimistic at these lower pressures.

Just for grins, how much power do you suspect you'll get from 440cc injectors at 17 psi, even with everything perfect? Would you believe 233.3 crank and 196 WHP at 0.60 BSFC and 100% injector duty cycle? That's what I get from the spreadsheet.

Oh yeah, almost forgot, to get 300+ at the crank with 17 psi of boost, you're going to need 720cc injectors. They'll get you almost 325 crank and 273 WHP at 85% duty cycle.
 

·
Registered
Joined
·
742 Posts
Part of why I am coverting to a return system now to go with the 550CC injectors and FPR sitting on the shelf in the other room.

Since our car is not a daily driver, in fact will see very few miles on public roads, I have no problem making such mods as not out stinking up the world, not compared to all the old unsmoged nastalgia cars around;)

Rick
 

·
Registered
Joined
·
11,881 Posts
Actually, Lance. If Bosch ECM's are any indication, fuel pressure can be tuned and modified from the maps (albeit a 2D map instead of a 3D map). If this is true it demonstrates how the supercharger is able to handle more.

To whomever owns a supercharger: Would you guys be willing to run some tests in order to see how much fuel pressure you are running?
 

·
Former '05er
Joined
·
12,467 Posts
Discussion Starter · #14 ·
Safe in what respect? Safe for a long engine life or safe for driver and passengers?
 

·
Registered
Joined
·
1,357 Posts
Preferably both, I would presume.

This is very interesting, to say the least. I have some pondering I'm going to do tonight about all of this... not that I think I'll be able to come up with anything useful but this is important information to consider. How bad is the return style fuel system in terms of emissions and predictability?

I always knew there was a counterintuitiveness about returnless systems where, when more fuel is needed, less can get through. However, this is a little alarming.

But without knowing very much about fuel injectors, I conjecture that 550s will be able to handle 10 psi. I have a new research project...
 

·
Registered
Joined
·
192 Posts
both of course, I'm trying to get more horsepower out of my engine but with a reliability close to stock. So i guess something that won't blow or have to get work done to it every weekend and be safe for the passengers. I'm making a showcar so custom sub box and tv screens dynomat airbag system all added weight so i'm trying to get my car about low 14's high 13's if i can. I was aiming for 11's but now i've decided what i want done to my interior and it's too much weight for 11's. I kinda don't want it but i'm thinking of going all toyota S/C, intake, and exhaust, way later a zpi pully new flywheel shortshifter and clutch. so i should get about 250 maybe 260 at the wheels i think.
 

·
Ironhead
Joined
·
13,253 Posts
you are expecting 260 at the wheels with a supercharger, intake, exhaust, and a pulley? no way.
 

·
Former '05er
Joined
·
12,467 Posts
Discussion Starter · #18 ·
Originally posted by Obike@Mar 28 2006, 08:54 PM
Actually, Lance. If Bosch ECM's are any indication, fuel pressure can be tuned and modified from the maps (albeit a 2D map instead of a 3D map). If this is true it demonstrates how the supercharger is able to handle more.

To whomever owns a supercharger: Would you guys be willing to run some tests in order to see how much fuel pressure you are running?
How? There's no pressure transducer of any kind in the system and the regulator lives in the tank with only an in, out, and bypass connection. There are no electrical connections to the FPR.

I bet a LOT of money the pressure is completely fixed.
 

·
Registered
Joined
·
192 Posts
QUOTE
you are expecting 260 at the wheels with a supercharger, intake, exhaust, and a pulley? no way.[/b]
isn't the WHP of a S/C with a zpi pully 220 + the intake and full exhaust with a new flywheel you don't think 260 at the wheels is within reach??? I don't know much about engines so that's just a uneducated guess.
 

·
Registered
Joined
·
5,918 Posts
I don't think intake + full exhaust would give you 40 HP. Of course I could be wrong.
 
1 - 20 of 25 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top