Building Another Stroker Engine for the Track

[inline6]

Since I thoroughly trashed my "new" engine at Road Atlanta, I now need to build another.  The "new" engine was fun while it lasted - a total of 3 and half 20 min sessions... but that limited longevity wasn't what I was after.  

 

The head appears to be ok, but will be checked out thoroughly.  The cam no longer will spin in the towers like it did - it spins by hand but keeps hanging up a bit in the same spot of each revolution.  Since there was piston contact to one exhaust valve which bent the valve and knocked a rocker off, perhaps the cam is bent.  It will be checked and replaced if necessary.

 

Some things that I have been thinking about:

 

Why did the "new" motor blow up? Basically, it looks like the #1 and #2 main cap bolts backed off while the engine was operating. This cause oil pressure loss to the bottom end.  The number #2 rod bearing experienced a metal forging process until it was many times thinner than when originally manufactured, then the #2 rod cap broke in half, the rod bolts broke, the #2 rod broke off below the piston pin... and this middle section of rod #2 then found its way into the path of rod #1, etc. etc.

 

The main bolts were torqued to 45 ft lbs.  Factory spec was looked up at the time the engine was built (I think it was 43).  My engine builder was very concerned about such a light torque spec with those bolts - he was concerned about adequate bolt stretch.  He told me so when the engine was built.  

 

To attempt to address, we will be replacing the stock bolts with ARP hardware.  

 

So, why did the main bolts back out?

I really don't know.  The rest of the main bolts were tight at the time of inspection and dissembly.  The #2 bolts were both so loose that I removed them by hand.  #1 had just enough torque that a wrench was needed to un-clock them about 10 degrees before I could remove them with my fingers.  If I am guessing, then possibly harmonics/resonance of the crankshaft came into play.  Another piece of evidence that I think points to this are the flywheel bolts:

 

 

I have no idea what caused 4 of the 6 to collect a couple of crankshaft threads at the end of the bolts... and weld all that shite together.  It made for no fun removing them.  It is interesting to me that only the last 1/8 of an inch of the bolts was affected for four of them and two were essentially ok.  

 

If it matters, there were some rather unusual driveline forces in play when, at north of 6500 rpm, most of rod #2 broke away from below the piston pin and got caught in between the big end of rod #1 and the block.  This LOCKED the engine completely at just north of 100 MPH.  Could going from high-ish RPM to NO RPM in a split second... in conjunction with a 2500 lb car on sticky race tires have yanked the flywheel so as to cause this to happen with the flywheel bolts? I am doubtful.

 

What if it is a harmonic issue?

Well, who can run their L series between 7000 and 8000 RPM then?  I've read in many posts that running in the 7300-7500 RPM range is a problem and it is to be avoided with these engines.  Even with an 8k or higher cam, depending on the track, won't there will be significant amounts of time spent per lap between 7300 and 7500?  Must we all build the 6 cyl L to rev no higher than 7200 RPM?  Or, must we all go salt flat racing where we turn 9000 RPM plus, so we never dwell in the evil zone for longer than a fraction of a tic of the second hand?

 

The current plan:

I have sourced two N42/N42 engines which the engine builder will retrieve on Monday.  The plan, for now, is to build the engine nearly the same as before with at least these changes:

• ARP main studs will be used instead of the stock bolts
• When I have the crankshaft offset ground, I may not have it lightened this time.  From what I have been able to determine, the Rebello BHJ damper I am running was tuned for an L-28 stroker crank.  More research will be done so I can understand how best to proceed.  I think it is possible to have BHJ tune a damper for an engine on a dyno.  I will look into this. 
• Dry sump oiling system - I believe it was Clarkspeed who said he was running a Nismo comp. pan and his Accusump was working in turns 6 and 7 of Road Atlanta.  I don't have an Accusump, and my Nismo comp pan was damaged quite a bit when the rods let go.  Since I don't really know the cause of my engine failure (perhaps I was just getting oil sling away from the sump causing oil starvation and main caps #1 and #2 came loose after the grenade went off inside the front of my engine), going dry sump seems wise in retrospect.
• An Electromotive Tec GT and TWM fuel injection set up will be used instead of the Mikuni 44 set up.  I couldn't get the jetting to do what I wanted with my cam and the distributor with Crane HI 6 and optical pick up was still indicating some spark timing irregularity above 7k or so rpm. Going distributorless will allow me to move the rev limit up to the full 7700 RPM spec'd for the cam.  At least at Road Atlanta, a 7700 RPM limit may have me spending less time in the evil RPM zone as the Hi 6 was holding me at a 7400 RPM limit on the back straight for a while before I would brake for turn 10a. 
• The cam I am running was developed with titanium valves. The intakes valves will be switched from stainless to titanium but I will again run the stainless exhausts for durability.

That's where I am for now.  I'm sure things will change a bit as we go... It's too bad I can only devote part of my time and efforts to my hobby.  That makes the learning come slower and at a much higher cost.

Edited September 1, 2015 by inline6

[Bob_H]

I will be watching this one to see what you eventually find out on the new one as well as the analysis of the failure of the old build.

[Xnke]

If the cam, with its reduced valve spring pressure, was designed for TI valves and retainers, you must either run TI parts or run lower RPM's.

 

If the valve spring is spec'ed for TI, then it's TI. Either you'll need to run them or you'll have to run a different spring or rev to a lower RPM. The valve spring required to keep the rocker on the lobe is determined by RPM and controlled weight-meaning the combined weight of the rocker, retainer, valve locks, the valve, and some people include 1/2 the weight of the valve spring itself. Changing the controlled weight will mean changing the spring.

 

I would be sending the torque wrench in for a calibration, too, or at least getting it checked out. Tripley so if it's a clicker type wrench.

 

For harmonic problems, you can do your best to reduce torsional stresses by running a good damper (you're doing this already with the BHJ) and getting the flywheel weight down and the flywheel diameter down. Reducing the diameter helps more, as you're running the smaller diameter clutch.

 

An engine with a smaller diameter, but equal weight, flywheel will have a lower torsional stress than the larger diameter wheel. The smaller diameter exerts a lesser torquing force than the larger diameter flywheel does on the back end of the crank.

 

This reads like crap but it's 2:15 AM and I've been using power tools. Hopefully it makes sense in the morning.

[roger280zx]

It would be my guess that the bolts backed out when everything was coming apart.  Turns 6 and 7 are known engine killers of all makes, models, and performance levels.  If you don't have a way to keep the oil fed (accusump, drysump, flop tube, etc.) it is not a good idea to go tearing around back there.   Lighter pistons make rods stronger, so less mass is required of the rods, lighter rods and pistons make cranks stronger, and mass can be taken off the crank.

[inline6]

If the cam, with its reduced valve spring pressure, was designed for TI valves and retainers, you must either run TI parts or run lower RPM's.

 

If the valve spring is spec'ed for TI, then it's TI. Either you'll need to run them or you'll have to run a different spring or rev to a lower RPM. The valve spring required to keep the rocker on the lobe is determined by RPM and controlled weight-meaning the combined weight of the rocker, retainer, valve locks, the valve, and some people include 1/2 the weight of the valve spring itself. Changing the controlled weight will mean changing the spring.

 

Thanks for the info - I appreciate it.  I will get weights of the valves and relevant hardware.  Logic tells me that the smaller exhaust valves will weigh less than the intakes.  On that basis, we'll see how much the new exhaust valves weigh in stainless vs. the intakes in titanium.  Sourcing titanium valves isn't exactly easy, I guess, as I just spent a few minutes looking on the web...  

 

It would be my guess that the bolts backed out when everything was coming apart.  Turns 6 and 7 are known engine killers of all makes, models, and performance levels.  If you don't have a way to keep the oil fed (accusump, drysump, flop tube, etc.) it is not a good idea to go tearing around back there.   Lighter pistons make rods stronger, so less mass is required of the rods, lighter rods and pistons make cranks stronger, and mass can be taken off the crank.

 

If only I could know... I have a lot of anxiety over building essentially the same engine again as the harmonics, if the problem, may wreak havoc to it again.  If only I could get it tested on a dyno after it is built and tune the damper, or change whatever else as necessary (smaller flywheel) to know that I will not have a harmonics issue in an important rpm range... (again?).

Edited September 2, 2015 by inline6

[clarkspeed]

Balance the lower assembly, get the bearing clearances dead on, ARP rod bolts and main studs, accusump, then go race. Stuff happens sometimes.

[Xnke]

Call Del West or Ferrea for you Ti valves-most places don't list them as they're a custom part.

[260DET]

Ti valves are a potentially problematical item by the sound of it, what actual difference are they going to make? I bet that there are a few conventionally valved engines running around that make more power than yours and don't blow up.

[inline6]

The titanium valves are really a separate issue.  Through my engine builder, I've been in contact with the guy that developed the Sunbelt cam I am running, and it wasn't known by me until now that the cam and valve spring set up I am using was designed to utilize titanium valves.  So, just going to do that.  The blown engine was not related to running the stainless valves.

Edited September 11, 2015 by inline6

[inline6]

Been searching, reading, etc.  Might go a different direction.  I am thinking of building a 3.3 L stroker instead of what I had.  I am still investigating, but it looks promising.  We'll see.

[ryant67]

3.3L?  RB crank, or sleeved LD28 block, or?  

Ambitious, but could be pretty special if it all came together properly.  Any particular reason for wanting more than a standard 3.1 (or even 3.2) stroker?

[30 ounce]

Your engine was very similar to mine. My machine shop and engine builder insisted on balancing the entire rotating assembly together; harmonic balancer, crank, rods, pistons, flywheel (which they lightened) and clutch even though it was purchased from Rebello Racing. Everything was marked so we could assemble it correctly. We used ARP bolts and studs everywhere. I dyno'd it before taking it to the track. My cam was probably a lot milder than yours as my peak power was at 6500 rpm. I set my rev limiter to 7500 rpm (Rebello says 8k was ok) but I don't see any reason to rev that far past peak hp. That being said I find that on the track I've been on the rev limiter a few times unexpectedly. No harm though. I've got probably 25 track days and 5k miles of runs through the mountains. One thing I've always worried about was oil starvation through those long sweeping corners but so far I've been lucky. Might still add an Accusump just to sleep better.

 

Sorry to hear about your engine. Don't cut any corners on the 3.3 and it should be badass! Can't wait to see this come together.

[inline6]

3.3L?  RB crank, or sleeved LD28 block, or?  

 

Ambitious, but could be pretty special if it all came together properly.  Any particular reason for wanting more than a standard 3.1 (or even 3.2) stroker?

 

RB crank - 89 mm stroke...

 

 

Your engine was very similar to mine. My machine shop and engine builder insisted on balancing the entire rotating assembly together; harmonic balancer, crank, rods, pistons, flywheel (which they lightened) and clutch even though it was purchased from Rebello Racing. Everything was marked so we could assemble it correctly. We used ARP bolts and studs everywhere. I dyno'd it before taking it to the track. My cam was probably a lot milder than yours as my peak power was at 6500 rpm. I set my rev limiter to 7500 rpm (Rebello says 8k was ok) but I don't see any reason to rev that far past peak hp. That being said I find that on the track I've been on the rev limiter a few times unexpectedly. No harm though. I've got probably 25 track days and 5k miles of runs through the mountains. One thing I've always worried about was oil starvation through those long sweeping corners but so far I've been lucky. Might still add an Accusump just to sleep better.

 

Sorry to hear about your engine. Don't cut any corners on the 3.3 and it should be badass! Can't wait to see this come together.

 

I liked my engine a lot. Though I only got to run it in its element for a little over an hour, it was sweet.  So much more power, and a sweet engine sound at the upper limits of rpm.  Power was plenty.  

 

I wish I knew what broke it.  If I knew for sure it was turns 6 and 7 causing oil starvation, I might proceed differently.  But I am freaked out about the torsional vibration issue.  It's no fun to spend... let's be conservative and say $8k plus over 100 hours of time building/tuning an engine to have it come apart in a big way during the first track session.  

 

The first temptation was to repair the crank if possible, buy two replacement rods, another set of pistons, straighten the cam, another set of titanium retainers (one got dinged badly), and replace the exhaust valves (one was bent, and they easily have over 100k miles on them).  And... you have to add to that all the work to the new block: boring cylinders, honing, align boring the mains, assembly, etc. 

 

However, the mods done to my crank hurt it from a standpoint of torsional vibration: smaller rod journals...  crank lightening...  I have no idea how much worse I may have made the problem. 

Edited September 14, 2015 by inline6

[inline6]

I've been continuing to investigate the 3.3 L option.  Right now, it's just ideas, but thinking of going with this:

• N42 block, sonic tested to verify 89 mm bore is safe
• Billet crankshaft with 89 mm stroke
• 137 mm rods
• Forged pistons - flat top with valve reliefs
• P90 cylinder head with chambers @ final size of 49 cc 
• Kinetic Sunbelt cam and matching springs with titanium retainers, 45 mm titanium intake valves, 36 mm stainless exhaust valves
• TWM throttle body fuel injection
• Electromotive Tec GT fuel injection/spark management
• Dry sump oiling system

With the extra stroke, keeping the compression ratio down to 11:1 is a challenge.  A welded N42 is not a good option.

 

I am still trying to figure out the final package.  Will be getting some pricing info soon, and then it will be time to make some decisions.  

Edited September 20, 2015 by inline6

[clarkspeed]

I guess I just saw your earlier post, what RPM were you shifting at?  It's not a trival task to have a 8k RPM engine with the big bore and stroke.  Better to shoot for something that will deliver power < 7200RPM and use that big flat torque curve to your advantage coming out of the corners with a crappy ratio Nissan transmission.  I'm sure it can be done if you pay attention to the details, and Rebello may report a high limit, but in the last couple of years I have seen 3 oversize engines wipe bearings when rev'd often over 7k.  And one was mine. I know there are a lot of theories on this with vibration and all, but the simple solution is keep the rev's down. 

[inline6]

I guess I just saw your earlier post, what RPM were you shifting at?  It's not a trival task to have a 8k RPM engine with the big bore and stroke.  Better to shoot for something that will deliver power < 7200RPM and use that big flat torque curve to your advantage coming out of the corners with a crappy ratio Nissan transmission.  I'm sure it can be done if you pay attention to the details, and Rebello may report a high limit, but in the last couple of years I have seen 3 oversize engines wipe bearings when rev'd often over 7k.  And one was mine. I know there are a lot of theories on this with vibration and all, but the simple solution is keep the rev's down. 

 

Oops... I just realized you had asked a question - I read this post a while back.  I was shifting where I had the rev limiter set on the hi 6 ignition - 7400 rpm, or just below.  Yes... keep the rpms down below 7300 - it is what I should do.  However, I am so addicted to the sound at the higher rpms!  

 

The late 280zx box is really not that bad in my opinion with the ratio's... I am running a 4.11 rear and 245/45-16 tires. It was a pretty good combo for Road Atlanta.  That said, the new engine (if I go the route above) is going to be too much torque/hp for a stock 280zx box.  I've started looking at newer options like the 350z 6 speed.

[260DET]

If money really is no object http://lescollinsracing.com/engine/projects-engine/3435cc-z-racer

[johnc]

The flywheel bolts are interesting, especially the damage to the first coupe threads.  Did they bottom out in the crank?

[inline6]

Same bolts and flywheel were undamaged upon removal after the first 1000 or so miles of operation on this engine.  I had some cam break in issues and ended up pulling the engine out, pulling the head and sending the short block off for examination and a re-hone while I had things apart.  Plus, 2 (opposing) of the six are ok.  So... this damage to the threads was definitely not from bottoming out.  Flywheel is Fidanza aluminum.  Pressure plate and disc are Clutchmasters - I'm trying to remember, but I think the FX100 in the 240 mm size.

 

I have a guess as to what happened.  Looking at the flywheel...

 

 

The flywheel bolt holes have thread marks only on one side of each hole as you can see in the pic.  I mentioned before that the middle section of rod number two got wedged between the big end of rod number 1 and the bottom of bore 1... and locked up the engine at something like 6500 RPM.  I think when the engine stopped rotation that suddenly, the mass of the flywheel, pressure plate plus the fact that the car was in fourth gear all combined to jerk on the flywheel hard enough to allow it to shift under the heads of the flywheel bolts - causing the threads of the bolts to imprint the sides of the holes.  This created a sideways pre-load on the bolts, and even though I "cracked" them loose... diagonally opposite and rotating around, before backing any of them out very far, the flywheel was still tweaked and the holes were not centered on the bolts as I continued to remove them...  While I was completely oblivious to this, I kept trying to back them out and met very heavy resistance.

 

I remember being totally confused as to why the bolts weren't coming out as they always had, but I couldn't figure out what was wrong.  I got rather frustrated after I knew the threads were "f"ed, and had to power them out with a breaker bar.  

 

This seems much more likely than friction welding from torsional crankshaft vibrations now but I still don't know much about those (torsional vibrations) or how the damage from them manifests itself.

Edited October 10, 2015 by inline6

[inline6]

If money really is no object http://lescollinsracing.com/engine/projects-engine/3435cc-z-racer

 

Yes, I have been emailing Les quite a bit as of late.  He shared that video with me and I've watched all of the others that his CA customer has put up thus far.  Pretty awesome.  

 

I've been considering building the 3.3L with Les' kit, but I'd like to use my Sunbelt cam which revs to 7700 and that really isn't what the 3.3L is about.