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Old 06-15-2008, 09:19 PM
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Very rich for more horsepower???

I just took my truck to the dyno the other day and have some disturbing results. First, it doesn't make as much power as most people feel it should. Another thing, which could ultimately lead me to the problem, is that it needs 11:1 AFR to make power at higher RPM's. The guy running the dyno had never seen it before, but says I should see my max horsepower/torque readings using enough fuel to stay in the 12.3-12.8 range. Everytime we took some fuel out of it, it lost power. He thought his wibeband O2 sensor might be bad, so he used a different sensor, with the same results.

The truck is a '91 Chevy 1500 with a new 350 crate engine, vortec heads, mild full-roller cam, short headers with 3" exhaust, bigger throttle body, K&N air intake, and a custom tuned chip.

Everything about the engine is new. All new stock ignition coil, GM distributor, various sensors, aeromotive fuel regulator, etc. EVERYTHING is new. It is normally aspirated with 9:1 compression.

I've been looking around for a few days, and no one seems to have an answer. The closest thing to an answer I've found is that a few people say I should dump the short headers for some long tubes and replace the Y-pipe, but I don't want to spend a ton of money on more new headers if it isn't going to fix the problem. Having said that, the O2 sensor is in the collector of the side with the Y-pipe, so maybe that has something to do with it??? Do you have to run more fuel if part of your exhaust is a bottleneck?

Any help is greatly appreciated. Thanks.

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Old 06-15-2008, 10:23 PM
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If everything is up to snuff, then here is my initial thought.

It could be fuel atomization problem from the injectors.
More fuel is necessary to get enough vaporization to burn for power, so more unburned fuel is left over= showing rich. Maybe a higher fuel pressure and shorter pulse time would enhance atomization.

I'll agree that it might be possible that insufficient scavenging at higher rpms might be affecting mixture burnability.



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Old 06-15-2008, 10:45 PM
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One or more cylinders are running lean. The EFI O2 sensor and dyno wide band/gas analyser read an average of all the cylinders. Running one or more of the cylinders lean may cause detonation in those cylinders requiring extra/ excessive fuel @ WOT to supress that detonation.
Inspect (bench test) all the fuel injectors for flow pattern etc and look for a intake manifold vacuum leak. Check the indivdual exhaust temps. The offending cylinders will be way off.
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Old 06-15-2008, 11:01 PM
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I concur with both above statements......but everybody thinks their engine puts out more HP than they usually do. You didn't mention the exact numbers your's dyno'd at but I've seen it before. A guy says he's got a 400 hp motor but then it really dyno's at 325 or so. The parasitic loss of a trans. and rear end can really suck up some power. Along with flowing the injectors you might want to check the elec. fuel pump, the in tank pump screen and any fuel filters.
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Old 06-16-2008, 08:11 AM
406cu.in. of tire smokin' fun
 

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The injector spray pattern of both injectors looks perfect. I also use a timing light to check them since it makes them easier to see. They are slightly larger and are running at a higher pressure than the stock setup. They have also been removed and professionally cleaned recently.

I'll check the header temps with an infrared gun and see what I find. The last time I pulled the plugs they all looked normal though.

I'll also re-check for an intake leak and get back to you guys.

The horsepower at the rear wheels was 220, which puts the crank horsepower somewhere around 250. It's low, no exaggeration there.
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Old 06-16-2008, 10:20 AM
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Crate engine assembly oops:

Retarded cam timing could do that.
Check and see if the cam is running one tooth retarded.
Yes they will run one tooth retarded without mechanical interferance.
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Old 06-16-2008, 10:05 PM
406cu.in. of tire smokin' fun
 

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I actually replaced the cam that came with it, so I know it's degree'd right.

The exhaust temps are all pretty close to the same and the plugs all still look the same. The injectors still spray nicely, but obviously I can't check them at higher RPMs. There doesn't seem to be any intake leaks either.

Since the engine computer has no idea what I'm feeding it to produce more power, I doubt the problem is electrical. When I was tuning it there was no O2 sensor feedback to the computer, so it's not running rich because there is a problem; it's running rich to produce power. I'm thinking there is something in the exhaust that's resticting the flow, possibly my Y-pipe. Maybe the exhaust just can't escape fast enough and the burned fuel is getting back into the cylinder and taking up space that fresh air should be? I thought about connecting a vacuum gauge to the O2 bung to read the pressure, but I don't know if it would survive the heat from a run up to 5500. I'm not exactly sure what it should be anyway.

I'm just thinking out loud, but everything else looks good.
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Old 06-16-2008, 10:25 PM
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I think you are correct with your thoughts of the exhaust not escaping fast enough. 3" exhaust is "huge" for a SBC, and the exhaust does not exit fast enough, for a good A/F Ratio test.
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Old 06-16-2008, 10:29 PM
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Quote:
Maybe the exhaust just can't escape fast enough and the burned fuel is getting back into the cylinder and taking up space that fresh air should be
What cam did you install, specs etc? Could very well be a reversion issue. Also, running the shorty headers and dumping into 3" could play into this. What about the TB or the custom tuned chip?
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Old 06-16-2008, 11:39 PM
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Thanks for this thread, and your self input. It has helped me figure out a running problem on one of my customers cars. She also has 3" exhaust on her SBC in a 1989 Camaro. I had a lengthy discussion with her today about drive-ability issues. It also want's to run too lean under load. I was thinking it still had the throttle body from the 305 it came stock with. (as I told her today).

This Camaro has the factory intake, and exhaust manifolds. I have had to replace the "back pressure" EGR valve 3 times, since she has been a customer of mine. This Camaro "eats" the EGR valves up almost yearly. She runs premium fuel and it still wants to "ping" from being too lean hot, under load. (I found this out today)

The timing, fuel pressure, spark plugs and gap, distributor cap/rotor, and plug wires, are correct, and good.

The lambada, (A/F Ratio) on my 5 gas analyzer shows it being just a touch into the lean side, out of the tail pipe. Under load it shows leaner, leading me to think it has the smaller 305 throttle body.

The problems with this car passing our smog tests, has been the NOX(which relates to cylinder temps) being too high(too hot, too lean). So I check the EGR valve with propane for leakage, find it bad, replaced it again last month, and send her back to the smog shop. (it passed)

So this makes sense to me. Too lean More Fuel. What is causing it? The exhaust not escaping fast enough, causing back pressure, creating heat in the exhaust leaning it out, and burning up the "back pressure" EGR valves.

Who says you can't teach an old dog new tricks?

Thanks again;

Stephen
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Old 06-17-2008, 12:12 PM
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Quote:
Originally Posted by Pre-Tuner
I just took my truck to the dyno the other day and have some disturbing results. First, it doesn't make as much power as most people feel it should. Another thing, which could ultimately lead me to the problem, is that it needs 11:1 AFR to make power at higher RPM's. The guy running the dyno had never seen it before, but says I should see my max horsepower/torque readings using enough fuel to stay in the 12.3-12.8 range. Everytime we took some fuel out of it, it lost power. He thought his wibeband O2 sensor might be bad, so he used a different sensor, with the same results.

The truck is a '91 Chevy 1500 with a new 350 crate engine, vortec heads, mild full-roller cam, short headers with 3" exhaust, bigger throttle body, K&N air intake, and a custom tuned chip.

Everything about the engine is new. All new stock ignition coil, GM distributor, various sensors, aeromotive fuel regulator, etc. EVERYTHING is new. It is normally aspirated with 9:1 compression.

I've been looking around for a few days, and no one seems to have an answer. The closest thing to an answer I've found is that a few people say I should dump the short headers for some long tubes and replace the Y-pipe, but I don't want to spend a ton of money on more new headers if it isn't going to fix the problem. Having said that, the O2 sensor is in the collector of the side with the Y-pipe, so maybe that has something to do with it??? Do you have to run more fuel if part of your exhaust is a bottleneck?

Any help is greatly appreciated. Thanks.
What is the injection system?

The computer has most of the control, a little can be fudged with fuel pressure using a variable regulator. If the engine is kept around 160 degrees operating temp the O2 sensor remains out of the loop and the cold start enrichment remains switched on. However, this will not pass emissions and fuel mileage will be low.

What is the custom chip? There are a lot of over the counter power chips out there, these are not custom chips. It is better to blow a custom chip than to fiddle with tricking the chip with a low temp thermostat.

If the chip you have doesn't ignore the O2 sensor at WOT, the O2 sensor will always pull the mixture toward stoichiometric which is good for cruise but will be too lean at WOT.

The exhaust system is fine, the issue is in the injection systems computer.

Keep in mind that most dyno's lie.

Bogie
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Old 06-17-2008, 12:35 PM
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The cam specs are 212/218 @ .050 (264/269 advertized) .488in/.495ex with 112 lobe separation.

The 3" exhaust seems to be the norm for my setup. I think my bottle neck could be the 1 7/8" Y-pipe. I was reading last night and found a guy that JUST changed his Y-pipe to 2.25" and gained 40HP. He also had the dyno slips to prove it.

Carsavvycook: My problem isn't going lean under load, it's the engine liking a ton of fuel (way more than it should).

The custom chip is one that I have been working on for a while now. I have complete control over the computer and I'm using a wideband O2 sensor to aid in tuning. I tried the over-the-counter chips and that's why I'm burning my own. I currently have the computer running open loop all of the time because I'm not using the factory O2 sensor for adjustments. If I were, it would constantly try to undo what I've been working on by way of the BLM's (fuel trims).
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Old 06-17-2008, 01:16 PM
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A single 3" exhaust isn't ideal, but I don't think it's too bad for your setup. This is a relatively mild sbc so the 3" pipe should handle it ok. The 1 7/8 Y pipe could be an issue though. Whether it's causing your problem or not I don't know, but it's probably worth upgrading anyway.

As for the power, 220 at the wheels is 286 at the flywheel if you assume 30% power loss. My estimate for your engine would be around 325 at the flywheel, so you've got a bit to go but it isn't super far off. What are your power expectations? A lot of people would like to think they're making 350 or 375, but all lot of these people either don't know or exaggerate.
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Old 06-17-2008, 01:32 PM
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Quote:
Originally Posted by Pre-Tuner
The cam specs are 212/218 @ .050 (264/269 advertized) .488in/.495ex with 112 lobe separation.

The 3" exhaust seems to be the norm for my setup. I think my bottle neck could be the 1 7/8" Y-pipe. I was reading last night and found a guy that JUST changed his Y-pipe to 2.25" and gained 40HP. He also had the dyno slips to prove it.

Carsavvycook: My problem isn't going lean under load, it's the engine liking a ton of fuel (way more than it should).

The custom chip is one that I have been working on for a while now. I have complete control over the computer and I'm using a wideband O2 sensor to aid in tuning. I tried the over-the-counter chips and that's why I'm burning my own. I currently have the computer running open loop all of the time because I'm not using the factory O2 sensor for adjustments. If I were, it would constantly try to undo what I've been working on by way of the BLM's (fuel trims).
I run a smog legal (this being Seattle) injected 350 in my S10 with duals. When originally built it ran a 3 inch single with a 1 7/8s cross over from shorty headers into a large converter and Walker muffler. It now connects the shorties to 2.5 inch head pipes to a pair of NAPA generic cats with a 2 inch crossover just ahead of the cats where the heated O2 sensor is installed. After the cats it reduces to 2.25 inches into a pair of Thrush 30 inch glass packs then the 2.25 pipes continue to outlet under the valance panel at either side of the license plate.

This system significantly improved power and gas mileage everywhere in the powerband and reduced emissions, but I certainly doubt that the improvement is anywhere near 40 hp.

I still don't know what EFI you're running TBI or TPI, factory or aftermarket.

You comment that the engine wants more fuel is prima-fascia evidence that it's running lean. Lean isn't always a condition that is measurable with an O2 sensor. The engine can be overfueled and still be lean, a classic is cold start but similar conditions can persist where due to temperature or mixture distribution problems. At WOT power will continue to climb with more fuel, then it will flop over when it goes too rich this point is real close to 12 to 1. If you're still gaining power at 11 to 1, I'd first suggest the test equipment is reading poorly. Next on my list would be poor fuel atomization or distribution, this is suspect with TBI, but I wouldn't expect this with TPI. It could indicate insufficient compression or too cold operating temp. A heavy reversion wave resulting in fuel stand off, though this is more of a carb or TBI problem. Headers can have an effect here where the waves are passing thru the valves at overlap and lifting mixture from behind the intake back out the carb or throttle body.

With your cam and heads I'd expect this engine to put out around 370-380 hp.

Bogie
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Old 06-17-2008, 05:58 PM
406cu.in. of tire smokin' fun
 

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Oldbogie, I'm just South of you in Tacoma.

When I started this project, I was only hoping for 100 more HP than what I started with, which would put me at 300 at the crank. Of course I hope for more, but I didn't want to be completely disappointed when my 400 HP dreams were squashed by 300 reality.

I am running a 454 TBI with small 454 injectors. The 350 injectors were rated at 68#; the ones I have are the smallest 454 injectors at 75#. 80# and 90# are also available. The actual throttle body is a Holley, but the injectors are GM since the Holley ones are aweful. I'm running 18psi to these injectors at WOT and the spray looks good when I'm parked and I rev it up.

Everyone I've talked to seems to think that this engine combo should make it's peak power at 12.3-12.8:1 AFR, which is part of the reason that I think that it being 11:1 is a problem. It should have fallen over way before getting that rich.

I'm not sure what you're saying about it's possible to be overfueled and still be lean, because the way I see it "lean" by definition is underfueled. If I'm wrong, please explain it to me.

The engine only has about 3500 miles on it and I have checked the compression about 1000 miles ago. Everything seemed great then, and it doesn't run any different now.

The O2 system the dyno place had they said was calibrated. They were also surprised by the fuel requirements so they changed the sensor with no change in results. My WB also reports rich.

I'm running a 170* thermostat and this was on a 69* day.

I haven't decided yet if I'm going to replace just the Y-pipe or the headers also. We'll see where that leaves me.
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