ptuomov

Those pistons have a different dish design than the 968 pistons. Any insight into why go with the circular squish band instead of the 968 squish zone matched more to the intake and exhaust squish pads in the heads? I think the spherical dish (or at least circular) and circular squish band are more common in modern engines. I'm kind of torn which way I'd go myself if I ever have to make that decision.

Relatedly, are you guys computing maximum squish velocities for the chamber/piston combos and if you are, what kind of MSV are you targeting?

Rob Edwards

HI Tuomo-

I'm going to have to go learn what MSV is, and I'll get back to you. Any suggestions on where to look for good info on the subject? Always happy to learn!

ptuomov

I've been trying to learn about that as well. It's slow going. MSV is the maximum squish velocity. It's an estimate of how fast the gas has to flow out of (into) the squish area when the piston and head approach the TDC. It usually peaks at about 10 degrees BTDC towards the center and at about 10 degrees ATDC aways from the center.

When I was a kid I probably knew more about this than I know now, since I was tuning small two-stroke motors. I didn't know what I was doing then, but I know even less now... it was just so easy and cheap to experiment with a single 70cc cylinder. And those motors run very differently based on what you did to the head and the piston.

In any case, I've been told that at very high boost and pump gas large squish pads and tight squish clearances can be a negative, because the burn is already very fast because of the high temperature and pressure in the combustion chamber. Squish increases that burn speed, and if it's too fast already then you don't want squish. Now, why can't you just retard the ignition somewhat to offset that is a mystery to me, I think it has something to do with the temperature of the end gas that can detonate. But as I said, mysteries are numerous. However, there are 30 psi boosted Skylines and Supras that die if you leave the squish pads in and live if you grind them out, even if you keep the compression ratio the same. This is a consensus view among experienced builders, so I think it's not just some old wives tale.

There are a couple of threads on speedtalk.com in which I expose and repeatedly demonstrate my ignorance on squish, in case you're interested.

Tom in Austin

Holy cow Rob, those pics are amazing!

BC

I was on yellowbullet. Great place if you can stand... some personalities - you would probably do great.

Anyway - they were talking about this exact thing - and the analogy/explanation was that the bigger the squish, the smaller the tuning window. And vice versa, (to a point).

I will try and find the thread. They guys are all about finding power in 2V pushrod small block and big blocks. Great info.

the flyin' scotsman

we used to play quite a bit with squish bands with roadracing 2 strokes.

The heads, pistons and barrels were essentially the same starting with single cylinder 125cc, 2 cylinder 250cc and 4 cylinder 500cc.

4 strokes are a totally different ball game; leave that to the masters.

GregBBRD

When these cranks got delivered, I called Rob and told him that although his new crank had arrived, he couldn't have it, because I wanted to build a set of end tables for my living room.....

There are no engines running with this version crank...end of this month stuff.

There's a couple of these style cranks, in the longer stroke version, running in street cars.

mark kibort

so, is this crank going into Rob's new version of the Zombie? (shorter stroke version GTS?) i might have missed that point.

Rob Edwards

Yes. Track engine for the Zombie. Shorter stroke, less chance of wheelspin while I'm passing you.

mark kibort

hahaha..... Your going to need all the wheel spin possible if you are going to pass me!

On a serious note, it will be HUGELY interesting to see the stroke effects on the HP, since most of the changes are only the crank,right? same cam, or did you go with something even hotter than what MAderson had to take advantage of yet higher revs? as i look at his HP curve with the longer stroke, he got good power through 7krpm..... so, again... im still interested in the goals here. this isnt to dumb down the torque down low, because that was a street driving goal, right?
Or, was it to lower forces on the drivetrain? but still keep peak HP near the same, while also lowering bearing speeds because of the smaller journals? i would think even that would be offset by the higher RPM operating range. I mean, how much lower is the bearing surface speeds by the size reduction of the rod jounrals?

Rob Edwards

Lighter crank, smaller rod journals, better rod bearings, lighter pistons, different rod ratio, lightweight DLC'ed lifters, a little more cam lift and better springs, bigger intake valves, Greg's revised intake, revised combustion chambers, new dry sump pan, windage control, crankcase ventilation, more revs, and better control over fueling. So many variables besides the crank, it'll be impossible to tell what was responsible for me passing you.

ptuomov

I think they are pulling your leg a little bit, Mark.

For a race or a hot-rodded street car, you almost always want the maximum displacement that you can reliably run within the rules. With the intake, exhaust, and cams, you can shift the torque curve left or right pretty freely, so "too much torque" is not a good reason to lower the displacement. If you have too much low-rpm torque, just make the IVC later and pick up your higher torque at high rpms.

In my experience, the 928 S4 without any electronics has no problem with productively putting down 500 rwhp with a slight controlled wheel spin. Furthermore nobody's changing the crankshaft just to reduce wheelspin.

It has to be something else. For example, to test the GTS crankshaft to verify that it's reliable and working before its sold to people who want stock stroke for the GTS and are only replacing the crank, rods, and bearings. Or some racing class that requires stock stroke. They may tell you what the real reason is, or may not.

Strosek Ultra

Quote:
I think it has to be for something else, but if you look at Gregs comment below, he talks about the 400rwt stokers being a problem on the street, so it got him "thinking".

Can´t you allow Mr. Brown to brag a little in order to enhance the sale!
Åke

ptuomov

I think everyone thinks these crankshafts are really nice and consistent with modern best practices.

RFJ

Is it not possible that with that crank and light weight parts you will be spinning even more??? I do think it is.HAHAHA