Piston Speed - What is the limit?

[Ron Earp]

Mean piston speed, what is the limit?

My 260Z has to race against a lot of different cars in SCCA ITS. My closest analog is quite naturally the 240Z. Major differences? The 240Z gets to use good carbs that work while the 260Z has to use boat anchors. The 260Z cam has a tiny bit more exhaust lift and a tiny bit more duration than the 240Z cam. 260Z exhaust valve is 1mm larger than the 240Z valve. Not much else is different with the cars except, naturally, the engine displacement.

The 260Z has a bore and stroke of 83mm x 79mm, for 2565cc. Or 3.27” x 3.11”

The 240Z has a bore and stroke of 83mm x 73mm, for 2393cc. Or 3.27” x 2.90”

The top #1 place Z this weekend at Roebling was running a 3.90 rear end, same as me. He turned up to 7400 RPM at the end of the main straight on some 15” wheels shod with 225-50 Hoosiers.

When Jeff and I tested the car Friday we noticed that we’d easily pass 6700 RPM under the start stand and the engine wants more RPM. If you hold down the pedal you’ll hit 7200 RPM. I think if you got a good aggressive launch out of eight then you’d probably hit 7400 RPM if you held down the pedal. The question is, should we?

The mean piston speed for the 240Z at 7400 RPM is 3576.

The mean piston speed for the 260Z at 7400 RPM is 3835.

For the 260Z to turn the same piston speed as the 240Z we’d have to hold RPM to 6900 RPM.

We have some good rings in the 260Z, some Total Seal rings that are used in a lot of race engines. But still, I don’t know that it helps with the things you fight with piston speed such as ring flutter.

So what is the word out there? Maximum piston speed, 3500 ft per minute, or 4000 ft per minute?

Kevin Cameron, a technical writer for Cycle World who writes fantastic articles on engine technology, did a piece on this years back. I wish they would collect all of his writings in one place the way Peter Egan’s “Leanings” are done. Great writers, both of them.

Ron

 

[Ron Earp]

Well blow me down! They have Kevin's writings:

Amazon.com: Top Dead Center: The Best of Kevin Cameron from Cycle World Magazine: Books: Kevin Cameron,Cook Neilson

 

[Scott Calabro]

A quick search revealed this...

all_redline_rpm_vs_reliability

and this, (the section on piston speed)...

polytek


:chug:

Scott

 

[Russ Noble]

Ron,

I think the limit for piston speed occurs a split second before the hole appears in the side of the block

I don't know much about those motors but I understood the big obstacle to revving them was in fact the crankshaft due to the harmonics of a very loooong crank causing crankshaft whip. I'm not sure at what revs that comes into play. I always understood the rest of the motor was fairly bullet proof.

Ross Nicol may have some real life data on the limits.

In the meantime I'd just rev it to whatever is required to get the job done....epper: Spare Datsun motors can't be that hard to find or prepare.

Cheers,

 

[Jac Mac]

Man, if there was ever a tech thread to start a **** fight this might be it.

Without reading in depth the articles, a quick think about all the diffeent combos regularly on this site will give some indications of what works best.

The 3.25" stroker combo is a 'nicer' motor than the 3.4" stroker particularly at sustained high RPM as opposed to sprint type operation.( I am talking 7k to 8k here) In the 8.2" block rod length limitations prevent going much longer.
Now in the 9.2" or 9.5" blocks 'Nascar' stuff runs all day at around 9k with strokes in the 3.5" region.( That is a 'peak' piston speed of around 8250 fpm ). So how do they do it? Light rings- gas porting- pressure relief between ring grooves- balanced ring gaps that take heat expansion into consideration etc- long rods to reduce the rate of acceleration from TDC/BDC that can unseat the rings.

So basically your Nissan can stand a few RPM yet with its present stroke ( the valve train might not be up to it ), but it might be an opportune time with your need to do a precautionary teardown to put the old thinking cap on and take advantage of the rules and see where you can improve a few things like rod length etc in this area.

Jac Mac

 

[Ron Earp]

Jac Mac,

I tend to agree with you. While I can't do anything "non stock" with respect to bore and stroke and rod length, I could do some work on the rings. But I also fee that 7400 RPM should be in the upper end of "ok" as long as the valve train will handle it.

Russ, The engines are easily obtained, but preparing them to ITS specs is time consuming. For instance, what if I told you that the top ITS racers got 175-180 rwhp, or about 220 flywheel hp from their "stock" 240s? Stock cams, stock carbs - no webers, but two oil filled stock Datsun SU clones, stock valve sizes - the only major work being header, exhaust, and attention to detail? Can't even port the head beyond gasket matching.

The Nissan enthusiasts say "no way is that possible", but in fact it is possible and happens all around with good IT motors. IT builds have a lot of attention to detail and it takes time. The "stock" motors won't cut it in the racing so they do take a while to prepare. Mine has some flat top carbs that are extremely trick but legal, excellent valve job, springs, rings, line bore with torque plate, very good fitting pieces, and a extremely good balance job balanced to the lightest parts picked from a pool. The way this engine revs compared to a "stock" engine, which I had to run a couple months back when my race engine was dead, is like night and day.

So, I do want to keep this one alive as it is quite a lot of work to build another one like it. The 3.90 gear is ideal for CMP, Rockingham, and Roebling if I can go to 7200 or so. It appears this should be surviable. It won't be that great at VIR, but maybe doable.

I could go to a 3.7 gear and that would make it perfect for VIR and probably Road Atlanta.

If I went to a 3.54 gear it'd be awesome on the ovals like Lowe's Motorspeedway and Daytona.

The 240zs definitely seem fine at 7400 RPM and I don't think I should be a whole lot different. With the 260z being the "red headed step child" of racing Zs I have little info to pull from. There are lots of 240s, a few 280s, and I know of three 260s - one of which holed a piston about two months ago the same weekend I had issues. Damn carbs......

I need some dyno plots before I can pick the optimal ratios.

I think I'm going to get another diff and equip it up for racing. The expensive part is the LSD since I have a Quaife in the 3.90 rear end. I'd like to have another with a Quaife.

So Jac Mac, you say rev it?

 

[Jac Mac]

Just been re-reading an article by some guys called Tilton /Knepp who had a SCCA C production Z -Car out to 2850cc. 300 HP at flywheel. They ran a 7500 limit due to the torsional vibration factor ( conservative in their view ) not so much due to the engine but the damage it could cause further down the driveline- clutch trans etc. Stock bottom end-Rods Crank, but with Venolia pop up pistons. Could scan the article if your interested- that will be a learning curve for me!

Jac Mac

 

[Ron Earp]

I'd like to see it if scannable. Production (which isn't "production" at all) they can do a WHOLE lot more modifications than we can in Improved Touring. Sky is the limit there, tube frames, etc. They can do a lot of head work, cams to certain lift, etc. But I wouldn't mind checking it out.

R

 

[Ross Nicol]

Ron
The biggest difference between a 240z and a 260z is weight.The 240z was made with very thin metal and usually if you hold a fluro light underneath while you look at the inside floor you will see pinholes. So it's not just the better SU carbies that make them go.Word is it took a turbo in the 300zx to match the 240z for acceleration, so all post 240z cars were slower due to increased weight.The extra displacement of the 260z was supposed to make up for it and emmissions stuff, but didn't.
If your allowed to put the car on a diet.Drill holes in everything, not the important stuff though.I have a fibreglass dash if your interested.Check gearboxes for the best ratios.I think 260z is best but look for a Nissan option 1 box, 5th gear 1:1. You have to change diffs for different circuits but that's the price.
Ross

 

[Russ Noble]

Ron,

I don't think you'll hurt the actual motor if you rev it. From what I have heard, the problem is that the flywheel keeps falling off......

Have fun.

 

[StanJ]

Russ, The engines are easily obtained, but preparing them to ITS specs is time consuming. For instance, what if I told you that the top ITS racers got 175-180 rwhp, or about 220 flywheel hp from their "stock" 240s? Stock cams, stock carbs - no webers, but two oil filled stock Datsun SU clones, stock valve sizes - the only major work being header, exhaust, and attention to detail? Can't even port the head beyond gasket matching.

Ron, how sharp are the tech inspectors? From a practical standpoint, how much time do they have to spend on each competitors engine? For example:

When checking the cylinder head, does the inspection process go beyond measuring port volume, checking for visible grinding/polishing, and maybe cross-sectional dimensions that can be reached with a pair of dividers and/or dial calipers?

What about the carbs? Are the critical venturi and bore dimensions checked with anything more than i.d. mics and "no-go" gauges?

Are the camshaft lobe profiles plotted per degree of crankshaft rotation, or are only the opening/closing points and peak valve lift verified?

Depending on the answer to the above questions, you could be leaving on the order of 30-35 horsepower on the table, despite the time and attention to detail that you're already putting into the engine. Let me know if you'd like to discuss feasibility.

It would be a shame to work for months forging, purifying, folding, and tempering the finest possible steel for your katana, spend even more months forming the perfect blade from it, and even more time honing your skills with the finished sword...only to have some ogre show up with a crude, brazed together scatter tube filled with black powder and rocks...and be able to blow you to pieces with it for no other reason than the fact that he considered the possibility.

 

[Jim Cowden]

Ron

You rev it to hard I recon the crank will flex and it will run a bearing usually one of the rear ones.
It will more likely happen on back off as the throttles closed and the cylinders are not filling ,free spinning.

I did this 3 times on a Nissan eng I raced (not an L series)it was an fj20.
Nitriding the crank did allow me to go 500rpm above the factory redline.
I found a formula (Graham bell performance tuning I Think).
When I worked out the rod length crank throw bla bla bla funny thing it worked out exactly to the factory redline.

Obviously this is my 2c worth as I know there are knowledgeable engine boys on this site.
It was a first had experiance.

Be careful.

Jim

 

[Jac Mac]

Jim,
When you run bearings due to a closed throttle deceleration it is caused by deformation of the rod big end,usually across the parting line between cap /rod and the bearing insert has its 'crush' factor reduced/removed, starts to move in the big end and then a combination of -heat-lost brg clearance across the part line-and drag as the oil film is wiped away by the reduced clearance at that point results in the bearing spinning in the rod and what remains of the insert trying to weld itself to the rod journal. The fact that it happened on the rear journals in your case is more likely due to the point the oil is fed into the main gallery. Standard oiling system SBC usually fail the rod brgs either side of #4 main , SBF seem to fail around the front of the motor.

Jac Mac

 

[Jim Cowden]

Jac mac

As you have explained it that is pretty much much way it happened.
From memory (10 years ago) oil pump fed from front to rear.
Always ran # 3 big end.
4 was perfect so I put it down to crank whip.
Rods are reasonably strong in that motor so after nitriding crank the issue went away at the time I put it down to that.

A bigger budget would have fixed it, story of my life.
Maybe Ron will need to consider if his rods are up to the task.

Jim

 

[Ron Earp]

I don't know what the 260Z did overseas, but in the US it was a one year only car, 1974. The car shell is identical to 240Z shells, same build, weight, etc. The car had a slightly better cam than the 240Z, but the carbs were really useless as delivered from the factory and the compression was a bit lower.

In our racing series the 240Z weighs in, with driver, at a spec weight of 2430 lbs, the 260Z is speced at 2480 lbs. Both cars have no trouble meeting those weights with a driver in the range of 180-200 lbs. Mine is about 20 lbs over, with me in the car suited all up and very little fuel, so I'm okay with that.

As Stan says there is power left on the table for sure. It isn't ever likely that this car will be torn down in a tech inspection. There is certainly more I can do to make power, but a lot of it will be in the gray area of the rules. SCCA IT racing is self-policed to an extent. Other competitors would have to file a protest to sort of a really technical issue but for the most part the insides of engines are not seen.

We all know each other and for the most part people are legal. When someone rips off a lap time that is just unheard of a protest typically follows. Teardowns do happen at the championship race for the top three or five, but I'm not in danger of that! I want the car to be as legal as possible and follow the intent of the rules. Of course, we'll take advantage of them.....

There is much that needs doing to this motor. Header development would help as well as exhaust tuning. More A/F tweaking, cam geometry, etc. All parts of development.

Right now I need a reliable car that runs decently so I get more seat time. After about a year or more with poor reliability and so forth I'm looking at finishing some races!

 

[Ross Nicol]

Well you've picked the right car for reliability Ron, I thrashed the heck out of mine and couln't break it. Keep an eye on the harmonic balancer bolt if you over rev. A white paint line across it is a good Idea. A reverse lockout is a good Idea too.

Ross

 

[Ron Earp]

I did like you fellows suggested and put a paint mark on the new balancer. Except for my silly overheating, which is my fault, the car has done okay. I just put a new head gasket on today with some help from Jeff and should have it buttoned back up tomorrow night. Nice and easy to work on, that is for sure, and EXTREMELY cheap to get parts for. Don't know many cars that you can buy complete new calipers, pistons, seals, and all, for $49 each. Other parts are similarly priced although not always worth it - you have to be choosy.

R