Dyno Run: Things don't seem right

#1

After not getting the chance to get on the dyno at nationals, I finally got on a dyno locally today. Unfortunately it was a Mustang dyno so I can’t directly compare the numbers to those done at the tracks. But it’s better than nothing. By the way has anyone run on a mustang dyno? I seem to think someone in Cali has, I’m not sure why that is in my head though.

On to the info, the AFR was definitely not constant, fluctuating the whole pull. HP drops around 5300rpm then comes back up to hit its’ peak at 6000 then falls away instantly.
Max Power: 114 @6000 rpm
max torque: 123.5 @4500 rpm

The dyno guy said on average the mustang will read about 18% lower than a dynojet, not sure how accurate that is but that would give me around 134.5 HP on a dynojet, not too good. I guess what are people’s thoughts and/or suggestions on what could be the issue(s). I’ll check compression this weekend, was going to do it last weekend, but couldn’t get to it.

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#2

A mustand definetly reads lower than a dynojet but not that low. Those #s are very poor:(

#3

The fluctuating AFR might be from wear in the resistance path for the AFM. You can check for that by seeing how smoothly the resistance changes as you move the vane.

#4

OK, so after doing some tests this weekend, I haven’t found anything so far out of whack. Tested the AFM, TPS, and coolant temp sensor, all seem fine. With carb cleaner in hand I didn’t find any obvious vacuum leaks. Checked compression and that is good too, actually kind of high. The head was rebuilt before I bought so I’m guessing they milled the head a bit and maybe there’s some carbon build up adding to it also.
#1- 180psi
#2- 170psi
#3- 185psi
#4- 180psi
#5- 170psi
#6- 175psi
all the plugs looked like everything was burning properly also.

Was going to check fuel pressure but didn’t get the chance.

Power is smooth, there is never any hesitation or anything like that.

A friend brought up a possible timing issue. If the timing is off by a tooth on the M20, how does it behave? Is it something very noticable or would it be something more subtle?

If anyone has any advice on other things to check it would be greatly appreciated.
thanks

#5

I would suggest doing leak down test and see what that tells you (test is more deterministic then compression test)

#6

I saw 170 peak hp at one dyno, and had 159 peak hp at Mid Ohio, and the same at a dyno here in Raleigh.

Before you do anything drastic, get it on a Dynojet somewhere.

Carter

#7

So I finally got to check out some more things with the car (you wouldn’t believe the amount of stupid things that kept happening causing delays) and I think I found the problem. It seems that whoever put the timing belt on last, previous owner, didn’t get it quite right. Looks to be off by one tooth. I took a photo of the marks on the crank pulley, am I looking at the right marks? The red arrow is the TDC mark on the pulley and the green arrow is what I think is the mark on the lower cover that it is supposed to line up with. When the photo was taken, the TDC mark on the cam gear was lined up with the mark on the head.
http://img.photobucket.com/albums/v93/secu256/TIMING.jpg
When the marks on the crank pulley lined up, the cam gear was off the mark by exactly one tooth.

Am I correct about all this, or do I have more exploring to do?

#8

I checked my belt thinking it was off a tooth cause it was acting weird and still is for that matter. But it was nowhere near that far off. So yes I would say you are off a tooth. I dont know how it would act I imagine it would depend on whether it was advanced a tooth or retarded a tooth. but remember to change that belt if you release the tension on it. if the tensioner is fairly new you can reuse it but not the belt from what i have heard. hope that takes care of your problem. I still need to figure out whats up with mine, I was in hope it would be the belt but it fells really strong just feels like its missing at idle but pulling each plug wire individually all makes it run a little worse so it cant be 1 cylinder not getting spark. Let us know how it turns out for you.

#9

It looks like it’s off to me. If PO did not put timing belt properly it could happen. Take a look at photos on this e30 timing belt replacement procedure for reference.

#10

Yep, that sure looks to be off. When you go to do the replacement, before you remove the main crankshaft pulley, go ahead and get that lower mark lined up correctly, because after you complete the disassembly, the crank pulley, and the plastic cover that holds the reference mark, will have been removed. You’ll want to rotate the camshaft the 1-tooth adjustment, as opposed to the crank. This will make your life easier.

If you have some Torx sockets, use one on the cam sprocket retaining bolt to give you leverage for rotating the camshaft.

A critical step, that the PO or his mechanic apparently skipped, is once the new belt is on, do a full 2-rotation test turn of the engine by hand (clockwise) using a 22mm on the crank nut. Do this after reinstalling the lower plastic cover and the crank pulley so you can recheck the alignment. Sometimes it will look good on initial assembly and it’s not until you turn it a rotation that you find you were actually off a tooth.

#11

Thanks guys. I was pretty sure I was looking at the correct mark on the timing cover, but it never hurts to double check before taking things off. At this point I’m just glad that my lack of power seems to be a simple fix, hopefully this gets things working better.

#12

After you get everything else off of the front of the motor there are two more marks to use on the bottom end. One is on the crank hub itself and one on the block.

JP