Minor Piston Damage

[xonix_digital]

Hey guys,

 

My auto mechanic friend says these are ok to be used but I would like a second opinion.

 

there is minor pitting on the top of 5 of the pistons. The worst of which are these two.

 

Piston #2:

 

 

 

and Piston #4:

 

 

 

Would you guys replace these?

In you opinion, does this look like detonation?

 

If I did replace them I would definitely go with a cast aluminum replacement. This motor will not see enough horsepower to justify buying forged.

 

 

In case it matters I'm building an N42 based L28 with an MN47 head, (obviously) dished pistons, and a 1mm Felpro Head Gasket. Stock everything else.

 

 

I am replacing the bearing and rings just because I have it apart. The center cap crank bearing was the only one that showed any wear at all.

[RB26powered74zcar]

Long as the det did not collapse a ring land (I doubt it from the looks of that hint of det) you should be fine using those for that mentioned re fresh....

[Tony D]

Penetrant Inspect the ring lands, that's detonation damage there.

 

If penetrant reveals no cracks, and all measurements are within specification doing a low-cost refresh would likely be O.K.

[BRAAP]

As Tony mentioned, dye penetrant the lands and dimension the pistons. Checking for collapsed skirts are an easy cheap way to check for damage. If the skirts are collapsed, no need to bother with the dye penetrant. If the skirts are good, then dye penetrant the lands.

 

 

...

In case it matters I'm building an N42 based L28 with an MN47 head, (obviously) dished pistons, and a 1mm Felpro Head Gasket. Stock everything else.

...

 

Dished pistons with the MN47? The one benefit the MN47 brings to an L6 is its squish pad which requires flat top pistons, or a piston with its dish formed in the shape of the open chamber so its flat area matches the flat area of the head, retaining squish. Using dished piston with the MN47 eliminates the squish, the only benefit the MN47 brings to the mix over the Z car N47 or N42 head, especially if you go through the trouble of installing the larger L28 Intake valves/seats, opening up the "D" intake port, etc. with the MN47

 

Unless the combination of a dished piston L28 and MN47 has another benefit I am unawares of, (Tony??? ) in my opinion it would be far less hassle to just install a standard N47 or N42 head and end up with less chance of detonation and if the MN47 is stock the N42/N47 will deliver more performance on pump gas due to being able to utilize the full ignition advance that the heads make best power with and the larger intake ports/valves.

[xonix_digital]

I purchased the motor with this head and the configuration listed.

 

The dished pistons allow the motor to run with a 9.2:1 compression ratio (with a 1mm gasket). If I were to run flat top pistons I would see a ratio in the high 10s.

 

Are you saying that I would see less of a chance of detonation with the cooling properties of the quench/squish event on the piston heads in a flat top configuration, even though its in the 10.8:1 comp ratio range?

[BLOZ UP]

The one time my boost controller lost control, I ended up cracking a head all the way down the middle, long ways (VG30E). Anyway, after I tore it apart, everything looked fine. So, I placed one of the pistons on the table. Once it tapped that table, one skirt fell off and most of the rings and ring lands fell apart.

 

So I rebuilt it and made a bazillion horsepower.

[BRAAP]

I have not ran the Comp Ratio calcs out, but if you had a 9.2:1 with no squish, vs 11.2-4ish ish with the squish, (what the MN47 and OE flat tops calculates out to), then 9.2 "should" be less sensitive to detonation. Though if this is the setup that you were running and you have visible signs of detonation, you can expect more detonation if no measures to combat it are taken.

 

The Aluminum Datsun L6 is hyper sensitive to detonation, ask anyone that has ran the N42 or N47 with flat tops at only 9.4:1 compression ratio. It rattles like a diesel at that compression ratio which older technology iron head engines can run all day with no detonation. To keep those from rattling you have to pull so much ignition timing that you are loosing more power from retarded ignition timing than you gained in compression ratio increase, 1 step forward, 3 steps backwards. I feel the reason for that is addressed in this thread;

http://forums.hybridz.org/index.php/topic/59029-head-cooling-on-cylinder-5-solutions/

 

If this were my engine, all to remain mostly stock or even mildly prepared, if using factory dished pistons I would use the N42 or N47 head. If flat tops are to be used, then the P79 or P90 to be used. Either combination will perform similarly. In my mind it just isn't worth trying to split hairs for some specific compression ratio. It is far more important for producing power, engine longevity, fuel mileage etc, to build in only enough compression ration to allow the engine to run its optimum ignition timing rather than have to retard the ignition timing because some "target" comp ratio in an engine known for being sensitive to detonation, i.e. ideal/safe compression ratios for other aluminum head engines don't carry over to the L6.

[xonix_digital]

Braap, thank you very much for your expertise and the link.

I will read that thread in full.

 

As far as "Measures to combat", I will not really know where I stand until I get it back together and start tuning since I am tossing the EFI setup it came with in the bush and installing my triple Webers.

 

If I have to start retarding the timing on this too much, I will take your advice and look toward a different head, but not before getting some solid dyno time in .

[Tony D]

I was going to mention the quench issue, but since it was in another current thread, left it out. I tend to agree, I'd save the head for another application where it can be best taken advantage of all aroudn.

[z-ya]

The Aluminum Datsun L6 is hyper sensitive to detonation, ask anyone that has ran the N42 or N47 with flat tops at only 9.4:1 compression ratio. It rattles like a diesel at that compression ratio which older technology iron head engines can run all day with no detonation. To keep those from rattling you have to pull so much ignition timing that you are loosing more power from retarded ignition timing than you gained in compression ratio increase, 1 step forward, 3 steps backwards. I feel the reason for that is addressed in this thread;

http://forums.hybridz.org/index.php/topic/59029-head-cooling-on-cylinder-5-solutions/

 

Paul, you know (from previous discussions) that I don't completely agree with you here. Yes, you have to run less timing with an N4x/flat top combo, but there are still significant power gains with increasing CR over just head work. In fact, it is a much more cost effective way to build power for NA applications (sorry, I know you build heads for people). For turbo, P-series only. Now if you combine the proper head work (this is where your craftsmanship comes into play), cam, N-series, flat tops, and 10:1 CR you can run 34+ degrees advance on 93 octane pump gas all day. I know someone with that exact configuration, and it's seen many races.

 

I'll take a higher CR with choice of cam over head work with choice of cam any day.

[josh817]

I always went about with the logic of above 10:1, start running 93 or more octane.... AKA I wouldn't go about more than 10:1 on the street... In fact that is what I should be at now with a .6mm HG, KA24 dished pistons and a P90 head, doing 35º of advance on regular pump (83 octane or something?). Haven't heard any marbles yet.

[BRAAP]

Paul, you know (from previous discussions) that I don't completely agree with you here. Yes, you have to run less timing with an N4x/flat top combo, but there are still significant power gains with increasing CR over just head work. In fact, it is a much more cost effective way to build power for NA applications (sorry, I know you build heads for people). For turbo, P-series only. Now if you combine the proper head work (this is where your craftsmanship comes into play), cam, N-series, flat tops, and 10:1 CR you can run 34+ degrees advance on 93 octane pump gas all day. I know someone with that exact configuration, and it's seen many races.

 

I'll take a higher CR with choice of cam over head work with choice of cam any day.

 

 

Pete,

I agree with you to a point. That point is when compression is raised to a point that ignition timing has to be backed off due to ANY preignition/detonation, it is from that compression up that you are loosing more power from retarding ignition timing than you are gaining from the compression ratio increase.

 

Typical L6 Datsun, that threshold is MUCH lower than most typical aluminum head 2 valve designs, again, I believe that is due to the discussion in the thread I linked. The L-6 "should" be able to run as much as 11:1 compression ratio without having detonation issues, but that isn't the case, it rattles terribly at a mere 9.5:1 comp ratio on pump gas. Yes, there are few out there that seem to be getting away with running more compression ratio and "almost" full ignition timing, but that is more than odd than the norm. Someone running a P90 at only 34 degrees full advance N/A is giving up power if that same engine can run more ign timing with higher octane fuel or less compression ratio. By winning races, assuming that is autocross or road racing, is not necessarily an indication he is producing any where near max performance that engine could produce, that is more an indicator the driver is savvy at the task of driving. I've seen on numerous occasions cars that are far less capable spank other cars that are far more capable. Know of daily driver Yugo that was stomping many C5 corvettes, and even Porsche on one track, witnessed first hand a bone stock '85 Chevrolet Celebrity OE junk type tires with a tired 2.8 V6,, (I think I still have that VHS around the house) annihilate all the C5 Vettes including trailered Z06 race car on race tires, a couple FD RX7, S30 Z cars, L6 and V8 powered, etc. Driver was the factor there, not the car definitely not the engine.

 

The norm being that gains of approx. 3% per full point of Compression ratio bump, all else being equal. If you bump the comp ratio up on the Datsun L6 by 2 full points you should expect to see 6% power gain, optimistically, maybe 7 or even 7.5%. On an L6 that is producing 150 HP, even an optimistic 7.5 % is only 11.25 HP increase from raising 2 full points, say from 8.3:1 up to 10.3:1. Due to the L6 propensity to detonate at elevated compression ratios, you would end up having to back the ignition timing off so much you would be throwing away more than 15HP i.e. 11 HP gained from C/R increase only to remove 15+ HP from retarded ignition timing. Realistically it would be more like 20+HP loss from the retarded ignition timing.

 

From the SDS (Simple Digital Systems) web site tech page;

http://www.sdsefi.com/techmods.htm

Increasing the compression ratio is another way to increase power. It also increases fuel mileage. Unfortunately, the pump fuel available in most areas limits the compression ratio useable on the street to under 10.5 to 1 on most engines. The difference in power is minimal going from say 9 to 10.5 to 1 and it is a lot of work to shave the head or install new pistons. Again, if you get stupid and try to run an 12 to 1 CR on 92 octane fuel, you will suffer with lots of pinging and eventual failure. Many high compression street engines must have their timing severely retarded to avoid detonation which reduces the power right back to stock levels. Don't raise the compression ratio too high!

 

 

 

I maintain that it is far more advantageous to run only as much compression ratio as a particular power-plant can manage on the fuel to be used that does NOT engage in having to retarding the ignition timing. That means if you run higher octane fuel and can advance the ignition timing or lower the compression ratio and can run more ignition timing, then you weren't running as much ignition advance as the engine prefers in the first place, i.e. knock limited, and you were sacrificing more power from that than you were gaining from any compression ratio increase.

[z-ya]

Paul,

 

I agree with a lot of what you are saying, but I'm an Engineer, and I like to see actual measurements. Seat of the pants, guestimates, and rules of thumb mean nothing.

 

Show me actual HP measurements that compare the following:

 

L28, dished, N Series head (set timing just below detonation threshold)

L28, flat tops, N Series head (set timing just below detonation threshold)

 

Our club has done a lot of dyno days. Here is an example of a 19% increase (same day, same dyno):

 

Stock N42 engine (fresh rebuild), headers, EFI, 2.5" exhaust

 

Flat tops, N47, headers, EFI, 2/5" exhaust

 

I've not seen a stock N42 block/head combo that made more than 140WHP. Have you?

 

Pete

Edited November 10, 2010 by z-ya

[NewZed]

I too am a big fan of real numbers. Do you have the timing values for those two motors and dyno runs? That would be very interesting as far as showing that the higher CR needed reduced advance. Octane rating of fuel would be key also, since high octane could negate the need to reduce timing.

 

I'm not arguing or defending either side. But I think those two items, plus coolant temperature at dyno time, would add a lot to the example, and make things much more clear.

[Tony D]

I've not seen a stock N42 block/head combo that made more than 140WHP. Have you?

 

The one in my 76 Fairlady Z 2+2, bone stock EFI and 225,000 miles on it. Taken from a 1980 280ZX 2+2 that was purchased at police auction for $100 (abandoned and towed at the port of L.A.)

 

Spins the Dynojet to 147rwhp.

This was very close to what I got with the G-Tech.

The car with my 255# frame in it scaled at 2695# at SanAntonio Dragway, and turned 26 1/4 mile passes that day in a range from 15.50 to 15.60 at around 89mph.

 

I thought the scale said 2965 as time passed, but the horsepower to run that time, at that weight, and that trap speed is no where near possible...

 

I have replicated these numbers in several other vehicles. This is the car that had triple Webers, a header, and Crush-Bent 2.5" exhaust on it when I got it (and dynoed at 88 RWHP). Pulled off all that stuff, put a 1976 CA Emissions Spec ECU, with a Federal non-EGR EFI Intake and stock cast iron (unported) exhaust manifold on and went to the dyno...then to the track.

 

It was so consistent at the San Antonio Convention (with 186,500 miles on the clock then...) the guy with a nice shiny silver 280Z having a big throat and headers actually SWORE at me claiming it had a cam, it HAD to have a cam, since he ran consistently 16.47... a full second slower than me, no matter what he tried!

 

So much for the 2+2 hater myths of being slow!

 

He actually started SWEARING at me! That still amazes me to this day.

[BRAAP]

Pete,

I too like hard numbers.

 

Those are two different engines with two different tunes, (AFR's reflect such). Unless the entire exhaust systems were the same from the header to the tail pipe, and the inductions is the same, from air filter to manifold, injectors/carbs, cams the same, head work, if any, the same, etc, those two dyno sheets are apple and oranges. Even if those were identical twins in every respect, it would show a trend, but again, apples and oranges.

 

How about this. We know that compression ratio increase is typically worth 3% power gain per full point of increase, all else being equal. So next time you guys have a dyno day/session, using a known stock compression Z car with quality pump gas in the tank, after the base line run has been established, retard the ignition timing 5 degrees, run it again, then retard the timing another 5 degrees, run it again. Note how much power was lost for 5 and 10 degrees of ignition retard.

From all the L6's I have built and tuned myself, I have found when not knock limited, the open chamber E88, N42 and N47 heads with dished pistons will produce best torque when the full mechanical ignition advance is between 38-40 degrees BTDC, again when not knock limited due to bad fuel.

 

 

As an aside, many variables will affect how efficient the chamber is which will have a direct correlation on best power ignition timing, (swirl, plug type like projected tip vs non, etc). An engine that can make best torque at a lower ignition timing indicates the chamber is more efficient at getting the job done of combustion converted to push on top of the piston. Higher compression ratios do help as well. The GM small block V8's are a good example. The traditional SBC with typical pre-Vortec heads prefer 34-36 degrees ignition advance. The LT1/4 and Vortec, 32-34, the LSx, mid to upper 20 degree range. The chambers and how the ports allow the air/fuel in and out has been evolved into more efficient designs requiring less ignition timing to get the job done. With that said, having a low ignition timing numbers due to knock preignition/detonation is not an indication of an efficient chamber, just a band aid for an inefficient chamber that isn't able to make use of the fuel being burned in it.

[z-ya]

 

Pete,

I too like hard numbers.

 

Those are two different engines with two different tunes, (AFR's reflect such). Unless the entire exhaust systems were the same from the header to the tail pipe, and the inductions is the same, from air filter to manifold, injectors/carbs, cams the same, head work, if any, the same, etc, those two dyno sheets are apple and oranges. Even if those were identical twins in every respect, it would show a trend, but again, apples and oranges.

 

 

Yes, but that much difference? Your saying a 3% increase per point, so the total if everything is the same would be 4.5%? Where did the other 15% come from, tuning? Yes I agree, the stock CR motor had and AFR of 12:1 at peak power where the higher CR motor had a AFR of 14:1 at peak power. You might be right, but I don't think that increasing the AFR by 2 points is going to gain 15% on that motor. Again, I have never seen a stock motor make over 140WHP. That stock 10:1 CR L28 will make 160WHP all day.

 

How about this. We know that compression ratio increase is typically worth 3% power gain per full point of increase, all else being equal. So next time you guys have a dyno day/session, using a known stock compression Z car with quality pump gas in the tank, after the base line run has been established, retard the ignition timing 5 degrees, run it again, then retard the timing another 5 degrees, run it again. Note how much power was lost for 5 and 10 degrees of ignition retard.

 

I think the better test would be to use a flat top 10:1 CR motor and do the same test you describe above. Because the increase in CR in the 10:1 motor will yield less HP lost per degree of timing pulled back.

 

The timing on the stock CR motor was 34 degrees, and the high CR motor 28. Both had stock ignition systems (280Z). Premium unleaded (93) was used in both engines.

Edited November 11, 2010 by z-ya

[z-ya]

The one in my 76 Fairlady Z 2+2, bone stock EFI and 225,000 miles on it. Taken from a 1980 280ZX 2+2 that was purchased at police auction for $100 (abandoned and towed at the port of L.A.)

 

Spins the Dynojet to 147rwhp.

This was very close to what I got with the G-Tech.

The car with my 255# frame in it scaled at 2695# at SanAntonio Dragway, and turned 26 1/4 mile passes that day in a range from 15.50 to 15.60 at around 89mph.

 

I thought the scale said 2965 as time passed, but the horsepower to run that time, at that weight, and that trap speed is no where near possible...

 

I have replicated these numbers in several other vehicles. This is the car that had triple Webers, a header, and Crush-Bent 2.5" exhaust on it when I got it (and dynoed at 88 RWHP). Pulled off all that stuff, put a 1976 CA Emissions Spec ECU, with a Federal non-EGR EFI Intake and stock cast iron (unported) exhaust manifold on and went to the dyno...then to the track.

 

It was so consistent at the San Antonio Convention (with 186,500 miles on the clock then...) the guy with a nice shiny silver 280Z having a big throat and headers actually SWORE at me claiming it had a cam, it HAD to have a cam, since he ran consistently 16.47... a full second slower than me, no matter what he tried!

 

So much for the 2+2 hater myths of being slow!

 

He actually started SWEARING at me! That still amazes me to this day.

 

I stand corrected! Cool, never seen one make that much. I'm not a fan of using time slips to determine power. To many variables. Your 2+2 is quick (not too pretty, but fast). I remember you doing a 100 or so runs at the drag strip in San Antonio. You were getting the most out of it for sure.

 

Gave Art Singer from Nissan Sport a ride on the track in Nashville this year:

 

This is what he said about my 180WHP track car:

 

 

 

It always feels great when a Corvette has to point you by. Took him a couple laps to figure out that there was no way he was going to loose me in the turns.

[Tony D]

Not 100, only 26, and they all were within 0.1 second of each other. For the longest time I knew where those time slips were...

They were good for a reference because a lot of guys put stock in the 1/4 mile calculators and that shows (from my recollection) 202HP...which I think is total BS, but they all come out around the same. Guess that's 'flywheel' so my drivetrain is a 55HP loss...what's that "30%" I'm sure this will find it's way into someones inflated claims now when their RWHP numbers show up shy... "Tony D had a 30% driveline loss"!

 

That car was where I confirmed "Butt Dyno Correction Factor" the Webers and Header FELT fantastic, but only spun the wheels out back to 88HP!!!

 

Did a tune up involving plug wires, putting on an EFI harness and ECU with 146,000 miles on it, etc and eventually got it to 147HP. My best time ever was a 15.30 on a 40 degree day---San Antonio was like 95-110F!!! I'm not sure my technique is right, I just hold it at the line with the e-brake, rev the car to at least 3000 and dump the clutch feathering the throttle to keep it from spinning. It usually hooks more or less and I shift at 6300 or 6500 out of 1st and 2nd, and cross the traps at 89-90mph just before I'd shift into fourth.

 

I've gotten similar performance out of several other vehicles in that range, lighter cars in the high 14's. It just takes attention to details and generally I don't spend a lot of money on 'performance parts' until I get a good consistent baseline figure of power near that point.

 

If more people did that, I think the performance market would transform...or go broke! Once you got it dialed in, you can really see what changes when you do a modification. I took the CAI kit off because I thought it was too noisy...put the stock filter back on with a K&N Replacement Filter. Didn't see any appreciable change in anything! So now, I just get the filter and leave the stock box---haven't cut the box top yet, but might in deference to the old "SharkZX Air Cleaner Mod" from the old days at ZC.C! Never know what will happen. But it's nice to have something you daily drive and KNOW so when you make a change...it shows Good or Bad right away!

[z-ya]

Not 100, only 26, and they all were within 0.1 second of each other. For the longest time I knew where those time slips were...

They were good for a reference because a lot of guys put stock in the 1/4 mile calculators and that shows (from my recollection) 202HP...which I think is total BS, but they all come out around the same. Guess that's 'flywheel' so my drivetrain is a 55HP loss...what's that "30%" I'm sure this will find it's way into someones inflated claims now when their RWHP numbers show up shy... "Tony D had a 30% driveline loss"!

 

 

30% loss? I think you need an oil cooler!! But seriously, a good example of how 1/4 mile calculators can be way wrong.

Edited November 11, 2010 by z-ya