Is it finally time for new motor/rebuild

#21

What should our compression numbers be for a fresh engine that makes 160ish HP on the dyno? I dynoed a new to me, but used, motor in my car yesterday and made very similar numbers to Smithk. Compression numbers on this engine were all 160ish. Of the three engines I’ve dynoed, they all make low 140s for both power and torque. Everything to include TPS appears to check out, but I’m clearly missing something that is keeping these engines from producing 160 HP like some are making. I have a lower mileage daily driver with compression above 180 psi on all cylinders; should this motor make 160 HP? What are the secrets that I’m missing?

#22

You can’t get wrapped up into compression #'s. Those are for comparing your cylinders to each other. A leakdown test is better for comparing to other engines, but you have to do the test very carefully so you are sure that you have really good #'s before you can draw any conclusions. All it takes is for your supply pressure to be 95psi instead of 100psi and suddenly your leakdown #'s are goofed a bit.

I went thru the same crap you did. Orient on the A/F curves in the dyno reports, that’s where my problems were. The motor is just a big dumb pump. The DME is taking inputs from sensors, making decisions and then providing spark timing and fuel. Don’t orient so much on the big dumb pump.

Take a look at all your dyno results. Run the dyno app so you can see the data. Massage the graph so you can see good detail in the A/F, hp, and torque curves. Look hard at how different DME’s, different AFM’s, different spark plugs, temp sensor, etc. are affecting the curves. And if you’ve not tried 8 DME’s, you’re not really trying.

Your goal is a A/F curve that spends a lot of time near 13. A secondary goal is minimizing the goatscrew that occurs between 4400 and 5200. Probably because the DME is pulling back timing. Keep swapping parts until you can see that some part combinations are a little better than others.

Lower your sights. My cherry .020 over motor made 159hp/156ftlbs on it’s best dyno run. And that was after two dyno days of swapping parts. And I still can’t pull ahead of Fred Switzer and he has a 150/150 motor. It makes me wonder if maybe all this effort and money chasing after a couple elusive ponies might be more interesting then useful.

#23

What Ranger said about leakdown is spot on. 1-3% is a good range for this class. The question you have to ask is where the problem is coming from. Probably going to need a head refresh anyway and get the valves to seat properly.

Sometimes with a leakdown gauge when you are running the test you can hear where the leak is coming from by listening in the intake or the exhaust. I said sometimes…not all the time.

#24

The engine is sitting on a stand and waiting for me to conduct the leakdown test tonight. From comparing my A/F ratio and dyno plots to others, it appears that my DME is in great working order. A/F ratio is good throughout the range and I have very little drop (power or A/F) near the 5000 rpm point where most cars stumble. From what I understand, your power is a function of three things:

  1. Your engines ability to pump air
  2. fuel delivery
  3. Ignition and timing.

If any of these three factors are off, you won’t make peak power. Correct me if I’m wrong, but your engine’s ability to pump air is largely a fucntion of compression. If numbers are low, it could be rings, valves, or the head gasket to name a few, but regardless low numbers yield lower power. Leakdown helps locate the problem but won’t you know if you have a problem just by checking compression? Fuel delivery is a function of your pumping system to include injectors. I have great fuel pressure, but haven’t gone beyond checking the resistance on my injectors. All appear to check out within Bentley specs. Ignition and timing could be DME, TPS, cap, rotor, plugs. This all really confuses me; please correct me if I’m off base with anything above.

A few other questions:

  1. Will vacuum leaks reduce power or just make your car run a bit rough at idle? Does vacuum interact with timing adavance?
  2. What about the AFM? The Bentley says resistance increase linearly through travel. Mine does, is there anything else to check?
  3. Bentley says compression should be 142-156psi; that seems a bit low…
  4. Where can we get injectors rebuilt and flow tested?
#25

I’ve only seen one motor above 160 on a dyno. Expect to get something in the 150 range with a rebuild. You can get your injectors cleaned here

http://www.cruzinperformance.com/fuelinj.html

Very good and reasonable prices.

#26

The Cruzin guy has cleaned a bunch of my injectors too.

A vac leak will make your motor run lean.

What were your compression #'s? Did you have a charger on your battery when you ran the starter? Did you run the starter for the same duration each time? What was the sequence of the test…cylinder 1 first and cyl 6 last? Do you think the batt was as strong for the last test as for the first? Was the engine the same temp for the first test as for the last?

Re. Bentley compression specs. There’s too many variables in compression tests to precisely spec #'s that apply to all engines. Your results could be around 150 and be ok if all cylinders are around 150. Or you could get 200 and be ok if all cylinders…

I vaguely recall seeing a dyno plot of yours, but I don’t recall your A/F. Lets get some more opinions on your A/F.

A/F in the 4400-5200 stumble zone. A/F is usually at it’s best here because the pulling back of the timing richens the mixture going out the tail pipe. So it’s not unusual to see your best A/F in that zone.

#27

Conducted leak down test on my recently removed motor last night and I’m a bit disappointed. Motor has 32 hours on it and leak down was 16 to 24 psi across the board. I pulled the #6 piston and rod and found that the top compression ring was broken into about 15 pieces and the ring groove was pounded out on one side. I guess that piston is ruined… Cylinder diameter measured at 84.46mm and piston at 84.25mm. From these measurements, it appears that I have single oversized pistons (+.25mm) in a motor that was bored for double oversize (+.50mm). Would it be safe to assume the excessive cylinder to piston clearance allowed the top compression ring to twist and ultimately break? I can’t think of another reason why the ring would shatter.

The good news is that the rod bearings look brand new and the cylinder head is straight, shiny, and appears to be in great shape. I’ll do a drip test to see if a head R&R is necessary.

I’m thinking my cheapest rebuild option would be to find a engine block that I can bore .25mm over and transplant my crank, pistons, etc… That is assuming my other 5 pistons are in good shape. That is unless anyone has some +.50mm pistons they would like to unload on the cheap…

#28

I looked up the specs and I’d agree with your conclusions re. pistons and cylinders.

It might be optimistic to hope that these problems are limited to #6.

Here’s some options.

  1. Cheapest and easiest solution is swap engines. Long range you can rebuild the problem child.

  2. You could rush a motor rebuild but it’s not going to be cheap, especially if you have to buy new pistons.

  3. I have a lot of engine parts, but I’m a long ways away. That includes 2 complete engines and a block. So I can help with hardware, but the distance is a problem.

  4. If you were especially motivated, you could show up at CMP Friday afternoon with your car. We’d swap one of my spares into your car in a couple hours and we’d be talking the second epic CMP engine swap for the next year. But if we’re going to do something like that, we’re going to need to create a plan tonight. Folks are already packing up. I have to be at the track at 0700 Friday morning so the clock is ticking.

That was good work you did pulling your bottom end apart, measuring things and figuring out what the problem was. I was pulling apart engine #5 before I could have done that.

#29

If one piston has obvious ring grove damage, I’ll bet on the other pistons having ring groove damage. Therefore I’d say that the pistons are now trash. If the bores mike out, the best plan would be to get a set of 50 over pistons and have the block cross hatch honed.

I do have a spare set of 50 over pistons that I might be talked into selling. I think I paid $630 for them. Come close to that and we’ll have a deal…

#30

I pulled all the remaining pistons last night and all six top compression rings were shattered. Four of the pistons are no good. It also appears that my block was a bit over bored (84.53mm on two cylinders) and is unusable. Jim, let’s talk about those pistons at CMP!

#31

FishMan wrote:

Okay…

#32

FishMan wrote:

These are the kinds of stories that make me want to just keep running BMW built junkyard motors. It seems you can never find quality machine shops unless you have a push rod V8.