Crappy Dyno Results on 3.4L Rebuild

[PFM]

Jim,

Get your part number and check hereStan Weiss' - Electronic Fuel Injector (EFI) Flow Data Table to see what pound per hour and at what fuel pressure your injectors are rated. Porsche tends to push towards 95% duty cycle there is a column for HP per injector at rated pressure.

Just a guide but worth a look.

[jungspike]

Before I discuss the limit of the fuel system, I should state that a new fuel pump was installed as part of the build and my injectors were inspected and cleaned.
PFM brought up a good question, what is the limit of the stock fuel system? Assuming my injectors are the stock Bosch 0-280-150-158 (I think that is the right model number?) and the fuel pressure is 36 psi (from the Bentley manual), then according to Stan Weiss’s website, they should output Fuel Flow = 21.8 lbs per hour per injector. To convert this to maximum rear wheel Horse Power:

RWHP_max = FF * BSFC * DutyCycle * (rwHP/BHP) * #Cylinders

Where FF is fuel flow in lbs/hr, BSFC is Brake Specific Fuel Consumption or the fuel flow required to produce one HP, in units (lbs/hr)/HP. Not sure what this value should be? The higher the BSFC, the LESS efficient the engine. Stan’s website suggests a “Modern Engine or Light Modified Engine” is 0.52 BSFC and a racing engine is 0.47—so I assume that my engine falls in the Light Modified Engine category (0.52 BSFC). Next, (rwHP/BHP) is the fraction of power that passes thru the drivetrain, which I assume is 85%. For Duty Cycle I assume 0.95. Does anyone know the maximum? Can they reach 100%? For these numbers, maximum Rear Wheel HP is only 203 HP (233 BHP), ...which is close to my max 196 rwHP.

I find this hard to believe because everyone doing moderate modifications would run into this limit. It would be a bigger problem at lower altitudes. If I use BSFC = 0.47 and 100% Duty Cycle, then my fuel system can support 237 rwHP and it isn’t a problem. Does anyone have better numbers for BSFC and Duty Cycle? Also, Could I increase the fuel pressure to 43.5 psi via the fuel pressure regulator? Then, I could support 213 rwHP, but can the fuel pump support this? If I put a pressure gauge on the fuel system, what should I see at high loads? Bentley provides 29 psi for “Engine running”, but I assume this is not under load. I guess I’ll start researching fuel system on Pelican and see what I can learn.

Tim

[safe]

The 964 injectors are smaller than the 3.2, but operate at a higher preasure, 3.8 bar I think, and they support up to 300 FWHP.

[Steve J.]

I don't think the problem is your system's ability to flow enough fuel because your peak power fuel is fine, which is at the highest flow rate. The 4 to 5 K rev band is your problem area, which has a lower demand for fuel compared to peak. I wouldn't change the chip before you recheck after retiming the cams. Then do another dyno run. Be sure to get the fuel readings in table form for Steve W.

[PFM]

Steve,

Since the motor is normaly aspirated peak fuel usage normaly happens at or near peak TQ. So I do not agree. I would still be looking at fuel. An easy test is turn the pressure up a few pounds and see what the air fuel ratio does. I see Porsche commonly runs 90% plus duty cycle so a bit more cam a bit more displacement running out of fuel is no great shock.

[jungspike]

OK, I have done some calculations of fuel flow. I found a graph of BSFC for a 911 SC from this link:

The BSFC chart thread (post 'em if you got 'em) - Page 20 - Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com

which I reproduce here here:

[IMG][/IMG]

I then converted the BSFC to lbs/hr/HP (Americans can think in metric units):

[IMG][/IMG]

I do not know the quality of the data, but it is the only BSFC curve I could find for 911s, but all BSFC curves I saw for other vehicles had similar shapes. This graph confirms Stan Weiss's BSFC = 0.52 for a moderately modified engine. However, above 4,000 RPMs, since BSFC rises, the engine is getting less efficient (takes more fuel flow to produce 1 HP)--and is approx 0.60 at 7,000 RPMs. Now, to figure out the demand on each fuel injector, I multiply BSFC by HP and divide by six cylinders, which yields this graph:

[IMG][/IMG]

So, the max fuel flow occurs at maximum RPM (not maximum torque and not max HP). I also used the BSFC plot to calculate the fuel flow for a theoretical stock 3.2L and a 3.4L with 240 BHP using the same BSFC curve. (Yes, the Carrera and SC have different fuel injection, but the BSFC shouldn't vary much). I also drew a horizontal line showing the limit of the fuel injectors (21.8 lbs/hr). Thus, a 240 HP 3.4L will probably start to lean-out above 6,300 RPMs. Does that check with 3.4L owners? but, back to my "crappy dyno run," since I was only producing approx 200 HP, my fuel injectors were probably not the problem.

Tim

[PFM]

I sit corrected... Teach me to reply from work without looking at my data....

[304065]

The spec I used for my 2 liter was 225 g/hpH

That is 225 grams per horsepower per hour or .496 pounds
That is the old factory carburetor spec for best economy I think it is in the fwm

[jungspike]

Status Update: Got a call from the builder today:

Spark plugs were very white, as expected. He found the fuel pressure 3 psi low. Cam timing was perfect. So, hopefully, the lean mixture was only caused by the fuel pressure and custom chip.

THE BAD NEWS: He heard a noise from the right bank--so some damage was done. He is going to drop the engine and take a look. He is going to fix it under warranty--thank goodness. Hopefully, it is something in the heads and not in the cylinders/rods. FYI, compression tests were good. Even though he is going to take care of it, I am still very nervous about what he will find.

So, no Drivers Ed for me, but I'll go autocross with my 16 year old son in his Impreza.

[AdamFritsch]

Some important questions before you can jump to conclusions
What deck hight are you running?
Do you know the compression ratio of the engine after the build?
Is it a single plug still or dual?
Also i would not be the least bit concerned about a compression test, because it dose not tell you Anything about what is happening inside the engine~ i would be asking for a leak down test so you can see whats really going on~ are all the valves sealing? did the rings seat properly? all that and more can be determined from a leak down.
If you can get the answers to those questions id have a better shot at helping you out.

[jungspike]

The Mahle pistons were supposed to give 9.8:1 compression, but I do not think the actual ratio was ever measured. I don't know the deck height. It is single plugged.

[PFM]

Jim,

So do you have an update on this?

[jungspike]

Thanks for asking. No significant updates. Builder said he was going to pull the engine and fix it at his cost. However, he said he is busy with the racing season, so I told him that he could get to it when he had time. Maybe I was too nice? Oh well.

[AdamFritsch]

I would ask him for the answers to the questions I asked you and he is right it is race season and we are all slammed, in reality it takes much more time then any one ever gets charged on builds to finish well built, thought out engines that will be reliable and make good power. So you being to nice wouldn't be what I'd be concerned with
Remember the the old saying "you can't have all three cheep perfect fast, you have to pick two
keep us posted

[safe]

Originally Posted by jungspike

However, he said he is busy with the racing season, so I told him that he could get to it when he had time. Maybe I was too nice? Oh well.

Not a good answer, you have to set a deadline, otherwise you'll get it back next year. I guess you wan't to use the car this summer.

[toddu]

Originally Posted by jungspike

but I do not think the actual ratio was ever measured. I don't know the deck height. It is single plugged.

Isn't this a little unusual?

Todd

[jungspike]

You guys are right. I'll stop by the shop early next week.

[Lapkritis]

Just let him know you want it for the season... not a back burner project.

[jungspike]

Stopped by the builder today. He has my engine apart. The cylinders (which were bored and plated for the build) had some minor marks on them. Couldn't feel them by running a figure over them, but could see them with a flashlight. The marks are from high temperatures caused by the lean mixture causing binding-I almost ceased the engine! Luckily, nothing is out of spec. He still did not find the source of the engine noise, so he is going to remove the connecting rods and check the bearings. The exhaust port on #5 was black (not white as you would expect from lean). He thinks that the injector wasn't spraying right, so he is replacing that. No damage to the heads, valves, etc. So after checking the rod bearings, he will re-assemble and re-tune the Motronic with a flapper mass flow sensor.

[lindy 911]

Unfortunately, there is no such thing as "minor" marks on the cylinder. If the piston tried to seize due to an overheat and left a mark on the cylinder wall, rings are most likely toast. Also, if it got hot enough to partially seize it's very likely it was detonating which pounds the crap out of the rod bearing. Could end up being the source of the noise from the right bank.