Low Compression after fresh rebuild

[Kash]

The compression readings across all six cylinders are exactly 120 psi (hot and cold), very consistent but about 30 psi too low in my estimate.

 

Per the available online engine calculators, my static compression is approximately 7.6. I checked and double checked the timing chain and it is spot on…cam sprocket notch is a tad to the right of the mark at TDC on the power stroke.

 

Quick overview of short block rebuild (somewhat unconventional):

 

LD28 stroker crank

OEM L28 Turbo pistons with crown machined down by 2.25mm (Pin height is 35.85 and new dish volume is approximately 3cc)

New ITM rings set off Ebay

New Timing Set (Chain, guides and tensioner)

L28 rods

Metal headgasket, 2.2mm

P99 virgin/uncut head

 

Bock was roughed up/deglazed by a rental hone form Auto Zone with a little extra attention at the upper and lower sections of the cylinders to remove old ring stops/edges to allow for the longer stroke (top/bottom). Car starts up and runs great.

 

Any ideas why my compression is so low? My original turbo engine (with an OEM compression ratio of 7.3) was in the upper 140’s.

Edited October 5, 2012 by Kash

[johnc]

How fresh is the rebuild? I would expect you'll need a few thousand miles for the valves and rings to settle in a seal correctly. Most new engines make their best power after about 20,000 miles of driving.

[Kash]

Only the short block was rebuilt after my 3.1L stroker block was destroyed.

[Kash]

I completed a follow-up compression test with oil (four squirts in each cylinder from a 4in oil can) and the readings are as follows: #1- 175psi, #2- 170, #3- 177, #4- 175, #5- 172, #6- 175

 

Can anyone elaborate on what the new numbers are tell me…is this much of an increase normal or expected for a properly sealing engine (from 120 to 170psi)?

[Tony D]

Sounds to me like you need to let your rings seat.

 

BMW's were tested in a long - term program (leave it to Ze Germans...) and were found to still be increasing in compression and horsepower production at the 30,000 mile point.

 

Meaning, technically, at 30,000 miles, they still were not completely "Broken In" yet!

[shri2222]

I agree with TonyD if your build is so fresh that you haven't yet hit 500miles your rings haven't yet seated.

[Tony D]

Mine, as well as John C is it's probably going to take 1000+ miles to get a 'good reading'....

 

If all the readings were taken from the same gauge is another consideration, and if that gauge reads and accurate number.

 

As mentioned above, that they all are very consistent tells me it just needs to wear in a bit more.

 

The wet test kind of confirms that. This may increase in pressure over time. It may not.

 

There may be a ring seating/bore finish issue.... what it comes down to then is leakdown test results to confirm it's passing rings...and an oil consumption test.

 

But oil consumption is done over 1000-3000 miles.

 

500 mile test would only be to see that they are all relatively equal IMO (which they are) and that no ring gap stacking has happened.

 

I would run this engine to 1000 and recheck, or even 3000.... and watch oil consumption.

 

This may take 10,000 miles to get to working pressure.

 

Or you can put it at peak torque on an engine dyno and run it there WOT for Two hours.... that should break it in sufficiently. (200 hours is equivalent in OEM testing standards to 100,000 street miles, so that dyno test is considered 1,000 miles equivalent... and is what some engine builders do now to allow customers to just 'get in and drive'! )

Edited October 5, 2012 by Tony D

[Sam280Z]

Check your valve lash too. Too tight will result in low readings by holding the valve open too long.

 

 

There are too many variables in a compression test to make the absolute values of the numbers worth much.

battery voltage, starter speed, all plugs out?, throttle open?, etc...

I prefer leakdown tests to compression tests for these reasons and the fact that if there is a problem, you know what it is.

 

Sam

[Kash]

Tony D, I have a lot of respect for your input so I will hold off on changing the headgasket and run it for at least 3000 miles.

 

Sam, I'm running a p90a head with hydraulic lifters.

 

Thanks to all for your inputs.

[Kash]

Quick follow-up, all compression readings now range from 140 to 150psi after 3000 miles...still not happy, was hoping for 160 or higher. 

[ktm]

Thanks for the follow-up, but if you read John C's and Tony's posts again your engine is still breaking in.  Are you driving it like you stole it (I hope so) or are you babying the car (a no no)?

Edited September 1, 2013 by ktm

[Tony D]

Thanks for the follow-up, but if you read John C's and Tony's posts again your engine is still breaking in.  Are you driving it like you stole it (I hope so) or are you babying the car (a no no)?

 

Argh.... EXACTLY! And have you had your compression gauge CHECKED AND CALIBRATED? What IS "150" on your gauge? Is it really 150? Is it actually 175? What is it? They're even across the board and within range of a proper service. Dude, what you WANT means nothing. Mechanical realities are mechanical realities. WANT in one hand POOP in the other, and see which has the better odds of becoming filled first!

 

And EXACTLY how is 150 NOW "low" when in your OP you stated "120, which I think is 30 too low in my estimate".... Now you are SPOT ON your original stated desires and saying you think 160 is the point you should be at??? WTF??? Because you did a wet test at 175? That's the difference between TOTAL RING SEAL AND EVOLVING RING SEAL I thought this was made perfectly clear, it sounds like you will be unhappy no matter what....it's going to take TIME to seat the rings. You were TOLD what to do if you wanted FINAL compression readings: Put it on a dyno at PEAK TORQUE for TWO HOURS W.O.T. and then see what it does... That's a broken-in engine. Yours is not. And it won't be for another 17-27,000 miles. 

 

Where's your leakdown readings? WIthout it, you are giving more than worthless data, and you are doing the worst possble thing imaginable: becoming disenchanted with mechanical work based on incomplete testing and some misguided sense of 'what should be' without a clue of the realities involved. 

 

Chill out, run the damn engine HARD now that you have some ring seat and see where compression as well as LEAKDOWN go.

 

But since you didn't check leakdown, you won't really know from start, will you?

 

Some people obsess on book numbers, others realize the world isn't like the internet. You better learn the different between reality and theory and live with normal "RANGE" of readings or find another line of relaxation. There is no story here. There is no reason for being unhappy. Get over it, move on, drive the damn car and quit with the OCD Incomplete Diagnostics.

 

Edited September 1, 2013 by Tony D

[Kash]

Thanks for the follow-up, but if you read John C's and Tony's posts again your engine is still breaking in.  Are you driving it like you stole it (I hope so) or are you babying the car (a no no)?

No babying after the first 800 miles oil change...have been running it pretty hard. The compresion readings I am getting now is what I was expecting after the rebuild...I was hoping for mid to high sixties once broken in but it looks like I still have a few more miles before it is fully broken in but I really don't see it increacing much more.  Still feels a little too lazy drinig off boost, think I'm going to rebuild my 3.1.

[Tony D]

Drive in the proper gear for speed and you won't have that issue, 109% guaranteed!

 

Compression increasing won't appreciably change what you feel off boost.

 

Would you continually drive a crammed N/A car at 1,500~2,500 (with a cam that "came on" at 3,500) and complain of mushy performance off cam?

 

Why do people expect their turbos to turn the car into a lovey V8 capable of performance at 1,500 rpms? Just like a car with a cam, drive it in the proper gear and you don't have those "mushy response" issues.

 

Has to be THE most common complaint I hear from Z-People, especially in "The Heartland" who cut their teeth on big engines.

 

These ain't Chevys....don't try to drive them like one!

[Gollum]

Fact: Compression readings â‰  Horsepower...

 

My NA motor has compression readings WAY higher than yours, but I'd be SHOCKED to see it put out 100hp to the wheels...

 

Unless you have EVERY factor accounted for, which you obviously aren't even close to, then the compression readings are really just a sign or a possible indicator to the health/sealing of the chamber, which says NOTHING as to how much air is getting in, and how well it can burn fuel.

 

If you're unhappy with your off boost performance, I'd have a million other questions to ask, which had NOTHING to do with your original post, but nether did your complain of power.

 

1. Do you have a wideband on the car, and what do your logs look like? Give us a WOT in 5th gear from 1000RPM to 3000RPM. Then also give us a log showing coasting to WOT bellow 2,000RPM in 4th or 5th gear, so we can look at transient fueling.

 

2. What size, brand, and model injectors are you using?

 

3. What type of fuel are you using? Not just octane, verify state location for us, and that the fuel doesn't sit in the car for months on end...

 

4. What plugs are you using?

 

5. What's your spark arrangement? Are you able to log timing? That should be in the wideband log if possible.

 

6. What diff ratio are you running? What trans are you using?

 

7. What's been done to the head? Stock chambers? Stock Valves? Stock Cam? Stock Ports?

 

8. Has your intake been modified in any way? Which intake are you running?

 

9. When was the last time you pulled your turbo? How's the condition? Is the wastegate sealing/closing completely?

 

10. Have you done a leak test of the intake system (ie: with a smoke machine to do a REAL visual check to see where leaks are coming from)?

 

11. Have you verified installed cam angle?

 

12. Checked your driveline lately for excess friction?

 

13. When was the last time you pulled your plugs to read them directly after a WOT pull? What did you see?

 

14. Has there been a dramatic change in performance over a short period, or has this been a long standing complaint you've had and just now getting around to wanting to deal with?

 

 

 

I could go on and on, but that's a good place to start. My L28ET when running the stock EFI and stock T3 was almost impossible to keep out of boost. I'd almost have to FORCE it to not build boost if I wanted to be hard on the throttle, but keep it civil around town. The commute I had for a while had a nice long uphill stretch on the freeway. Anyone who stays light enough on the throttle that their automatic trans stays in top gear will find themselves doing 40-50mph by the top of this 1/4 mile uphill grade. Usually if I'm doing 80mph I still have a hard time not dipping bellow 65mph unless I'm really aggressive (depending on the car I'm driving of course). My goal with this hill for a good 4 months was to keep my L28ET out of boost all the way up. I'd get going up to near triple digits, and watch my boost gauge trying my best to keep it right at atmospheric, and I'd either end up slowing to a crawl (like 30mph or under) or building boost. I simply COULDN'T AVOID it. That's my point. I could drive around town doing 25mph in a school zone, in 4th gear, then stab the throttle when the speed limit lifted and suddenly I'm in boost with ease.

 

But that's the way the factory designed it. It's a small turbo, only adding about 15% of extra power. You're running a T3/T04E according to your signature, and that thing should build boost just as well... unless something is wrong....

 

And usually off-boost performance isn't the bottom end, unless something is SERIOUSLY wrong. Even if your rings were leaking, they wouldn't have THAT much of an impact on performance. You'll notice HEAVY oil consumption LONG before a horsepower reduction...

Edited September 1, 2013 by Gollum

[Tony D]

To summarize:

"Your off-boost driveability issue is directly related to driving in the wrong gear for the speed you are travelling."

[Kash]

To summarize:

"Your off-boost driveability issue is directly related to driving in the wrong gear for the speed you are travelling."

I have only been drive forced induction cars for over 30 years from Porsche to the 300zx twin turbo.  As in life, most things are relative, the poor of boost drivability I was experiencing is being compared to the lively 9.3 to 1 compression 3.1 stroker turbo that I overboosted and broke the block sometime last year.  

 

Yes, I said was experiencing...pulled the engine a few weeks ago and had it bored out and I'm in the process of rebuilding the 3.1, this time I'm lowering the CR to 8.7 from 9.3 (1 mm HG to 1.75mm). 

[Tony D]

How does anything you just posted refute or clarify that which I posted?

 

Let me restate what I think you just said:

 

"My 2.9 Turbo is less powerful off boost than my 3.1 Turbo off boost."

 

Uh....and?

[JMortensen]

I didn't read through this really closely, but I wonder how much valve lift you lose with hydraulic lifters when doing a compression test. If the valve lift and timing is altered because the lifters aren't pumped up, I would imagine that could change your numbers considerably. 

[Kash]

Yeah, I figure I must be losing a few psi due to the hy-lifters. However, I was hitting so much higher pressures with the 3.1 (all was above 180 psi, which made driving off boost much more pleasurable without driving around at high RPMs/ one gear up - that's my point). 

 

Just got done assembling the new 3.1, hope to have it installed and back on the road before the Giants game on Sunday.  New CR should be about 8.7...hoping to hit at least 175 psi across all.