Dyno results, tuning help needed

[boogiedude]

So I took my car to get dyno'd for the first time. Was a fun experience, especially getting to see a 400+HP mark1 jetta and e36 m3, but that is for off topic

Specs for the motor:

1756CC
4mm dome pistons(~9.8:1CR)
Alquati 42-82 cams
Weber 38ADL
Lightened flywheel
Allisons automotive aluminum radiator
Millers Mule Adjustable cam pulleys
Early 4-2-1 exhaust manifold with straight pipes to ANSA muffler

Back on topic, the car has about 1500 miles on it now and has been running pretty well although I've been having trouble getting the mixture/tuning of the carb correct. The AFR is all over the place, and the idle is VERY lumpy up until the carb transfers from idle to main circuit(you can see part of it in the photo). It also appears to have a slight miss at idle. Once the car transfers to mains, the AFR drops way off the map. It appears to go below what the dyno sniffer was able to even pick up(flat line at ~3000 rpm) it then slowly climbs up after awhile. Does this indicate the need for larger main jets? smaller air jets? I'm a complete novice at carb tuning and dont really know how jetting theory works.


WP_000333 by boogiedude, on Flickr

Also notice the "roughness" of the power/torque curves. Is this normal? product of aggressive cams? detrimental to performance? tunable?

Video of the run(forgive the crappy audio, shot with my phone I may get footage off of a friends GoPro at a later point):

http://youtu.be/6Mc_NWmieB8

can't figure out how to embed?

Any tips/advice?

[baltobernie]

I'm afraid I have more questions than answers

1. Were the valves adjusted to the specs provided by the cam manufacturer, and are the cams timed correctly with the adjustable wheels?

2. What distributor are you using, and what is the initial and total advance?

Once the valves are opening and closing at the correct time, and the spark is arriving when needed, then you can work on carburetion. When you say that the A/F "drops way off the map", do you mean way too rich, or way too lean? I hope it's not "too lean", or those high RPMs may have caused some damage.

A note on A/F at idle: An O2 sensor in the header is required to get an accurate mixture reading. There's not enough velocity in the exhaust stream to accurately measure A/F with a tail pipe sniffer. These generally don't begin to record accurately until 2000 RPM.

You and I have similar motors and carb, and it is pretty tricky to get a big carb to work on these little engines. Weber suggests that you begin with the idle, get that settled, then move to the mains and airs. Redline and others have online guides to tuning for "best lean idle". I spent a lot of time & money proving these instructions worth following You must be able to idle the engine using the idle circuit alone - no fuel dribbling in from above, and zero ported vacuum. Keep working on this circuit until you get it right per the instructions. Look for vacuum leaks, bad spark, bad timing or advance ... all these things can have you chasing the wrong bunny.

It appeared to me that you were idling at 1200 RPM, which seems a bit high even with big cams. It may shake the rear view mirror loose, but you should be able to dial down the idle low enough to proceed with tuning.

[majicwrench]

More questions...I don't see AFR on the graphs?? How did you check AF?

[boogiedude]

AF is on the bottom of the graph in its own little graph.

The valves were gapped by the shop that did the machining. The cams were timed via guy crofts method with the head off the car.

I believe the distributor is a stock electronic marelli unit from 79+ with the vacuum advance unit removed and locked.

[sptcoupe]

What is the jetting in your carbs? Do you have a header? What manifols are you using?

[baltobernie]

AF is on the bottom of the graph in its own little graph.

Sorry, I should have seen that.

First, the HP and torque curves look pretty good; the engine makes power all the way to the top; torque is rather soft until 4k, but that's what autocross cams are all about!

You're running rather rich until 4k, and too close to overly-lean on the top. I would first try to get the idle sorted per my earlier post, because it affects the other jetting as well. I'd go one turn Out from snug on the "mixture", making sure that the throttle plate is truly closed (see above & Redline). If you're now somewhere between 3/4 and 1-1/2 out on the "mixture", try one size smaller idle jet. You're advised to buy a "jet pack" (no, not the kind you strap on your back ) with a couple of different sizes on either side of what you have in there now.

I'd also stay away from high RPMs until replacing the air correction jet at least two sizes smaller.

But I can't stress enough the importance of getting the idle sorted first.

[boogiedude]

What is the jetting in your carbs? Do you have a header? What manifols are you using?

I'll have to pull out the jets later when i get a chance, but as far as i know its using the stock jetting that it used on the lancia gamma 2400,

I'm using my stock single plane manifold(car is a 78) and i have a '74 4-2-1 exhaust manifold and downpipe that has straight 1 3/4 tubing to an ANSA muffler in the back. No cat or resonator.

You're running rather rich until 4k, and too close to overly-lean on the top. I would first try to get the idle sorted per my earlier post, because it affects the other jetting as well. I'd go one turn Out from snug on the "mixture", making sure that the throttle plate is truly closed (see above & Redline). If you're now somewhere between 3/4 and 1-1/2 out on the "mixture", try one size smaller idle jet. You're advised to buy a "jet pack" (no, not the kind you strap on your back ) with a couple of different sizes on either side of what you have in there now.

I'd also stay away from high RPMs until replacing the air correction jet at least two sizes smaller.

But I can't stress enough the importance of getting the idle sorted first.

I've never used a dyno before this and havent had much experience reading the data sheets, but my assumption was that the lower the number in the A/F mixture, the leaner it was running? Your comments imply the opposite?

I'll get my jetting posted up here and see if you guys see anything blatantly wrong

[majicwrench]

Air Fuel ratio is usually a ration, air to fuel, like 14:1, so if they talk about "12:1" that would be richer, less air, same fuel.

So how are they checking this?? With a sniffer in the pipe??

[So Cal Mark]

well, I just can't imagine setting adjustable gears with the head off the car. Cams are set up in relation to the crankshaft. Also, 1 3/4 exhaust is too small for the motor you've assembled and I would think thats one reason for the severe torque drop as the rpms go up. With the cams you have, the motor should still be making power above 6500rpm. Your mixture does look way too rich until midrange then heading toward the lean side

[boogiedude]

Air Fuel ratio is usually a ration, air to fuel, like 14:1, so if they talk about "12:1" that would be richer, less air, same fuel.

So how are they checking this?? With a sniffer in the pipe??

That makes a lot more sense than what i had come up with in my head

also yes, sniffer in the pipe

well, I just can't imagine setting adjustable gears with the head off the car. Cams are set up in relation to the crankshaft. Also, 1 3/4 exhaust is too small for the motor you've assembled and I would think thats one reason for the severe torque drop as the rpms go up. With the cams you have, the motor should still be making power above 6500rpm. Your mixture does look way too rich until midrange then heading toward the lean side

Your header will be going on this car+at least 2 1/4 piping once i have the funding available, for now I've gotta live with my 1 3/4.


Quick question, on the printout, this is designated as being SAE smoothing 3 for the measurement. Is this wheel horse? or adjusted to predict bhp?

[fiatfactory]

.

[timinator]

Hi Boogiedude,

SAE 3 smoothing is a description of how the HP and Torque curves are buffered. You asked why the curves where so "rough". They are only smooth when buffered to look that way, or smoothed. Since you are looking for the general shape of the curve, the individual data points are not of much concern. It also looks less hectic when the data points are averaged or smoothed.

Tim

[timinator]

Hi Boogiedude,

Before you start changing things please put a vacuum gauge on your engine. I'm guessing that type of cam should be 14-18 in.hg. at idle. Higher would be even better. The torque curve looks like the intake cam is retarded. That should put idle vacuum down to 10in. hg. or lower. That would drive your low rpm afr rich. The top end afr looks ok at 12-13:1. Usually a carb that works on a similar engine should be close enough in jetting. Your engine should probably be making 110-115hp and 103-108lb.ft. I do agree 1 3/4 exhaust isn't helping. But your torque curve should start to climb sooner and steeper.

Tim

[boogiedude]

Thanks for all the advice, but I just blew my head gasket

So the tuning is obviously now put on hold

[baltobernie]

It may be a coincidence, but I blew my first head gasket after rebuild, too. I had the cams timed incorrectly (even though they were aligned to the timing mark on the housing). I share your enthusiasm for Guy Croft, but as Mark says, the cams must open and close per the manufacturers specs in relation to actual TDC. There's probably variation between different timing belts from the same belt manufacturer, let alone the variables introduced by milling and machining during the engine rebuild. So get a degree wheel, pointer and dial indicator, and learn to do this yourself, or find a mechanic to perform these adjustments before you fire up the motor. Idle vacuum, idle quality. compression ratio, carb jetting ... all these things are affected by cam timing. Get this right, and ignition timing right, and the rest will be easy. I wish you perseverance and persistence The results will be worth it!