slate blue

Hi to all, I don't mind if anybody puts there 0.02cents in but it is a fairly technical question and please excuse me for thinking out loud too. As the majority are aware I have had to put my big engine project on hold as there was a disaster with the block. I was at the time just going to go for a 5.8 liter engine. With a rod to stroke ratio of 1.8

I was thinking if I revamped the project if some things were to fall into place I would prefer to use the standard stroke length of the stroker crank i.e 3.75" or 95.25 mm. I wouldn't do this with the 4 valvers as I would like to rev it real high but since my 2 valver would be limited to 7000 to 7500 the rod to stroke ratio becomes less critical.

The options are sell the pistons and rods as listed below, or reuse the existing pistons with a slight dishing to cope with the comp ratio. The new pistons could have a rod to stroke ratio of 1.653 whereas the existing ones would have a ratio of 1.633. It seem only to be small difference but will it count? That's the big question. As to comp ratio we now have commonly available 100 octane petrol, so I think you could go as high as 13 to 1 if you wanted to but I would keep it at 12 to 1.

Most strokers have a rod to stroke ratio of 1.56 BTW but I think their skirts are longer. This would help their durability.

The reason I'm showing these photos is that there is less skirt material on these newer style pistons while that is great for high rod to stroke engines it may not be so good for short rod to stroke motors, anybody know what the total length of the skirt is on a 968 piston?


Cheers Greg

Vilhuer

Don't know but can check it if no one has number on hand.

What rpm you think is safe with 95.25mm crank and 968 pistons?

atb

Greg, I'll try to take some measurements for you. The compression height (crown to pin) is definitely taller on the S4, I don't know that there is much difference from below the pin to the bottom of the skirt.

I like your Mahle slippers, I'm hoping to be running the same in my stroker. I'm hoping that the lighter weight of the piston will help offset some of the potential stability issues of a heavier long skirt piston.

I don't think the rod ratio by itself is going to make a noticeable difference on the dyno. It's only a .02 difference. A stock SBC rod is 5.7" with a 3.48" stroke. 1.64 ratio. The common long rod replacement was a 6" rod, which bumped the ratio up .08 to 1.72. That produced a worthwhile difference in power output according to Smokey Yunick, but what you are proposing is only a 25% gain in rod ratio. I don't know that that would show up on a dyno, let alone be noticeable on the street.

Durability wise you are going to be better off with the stock pistons. Piston stability is going to be better because you can run the longer skirted OEM piston because of the tight tolerances. I didn't think you can run the aftermarket Mahles at the same tolerance as the stock units can you? I've heard that they run looser than the stockers.

I think my motivation would be whatever parts I could get more money out of if they don't go in the motor, rather then which may provide a little more power if they go in the motor. In the end, I think either set up is going to produce the some results if they are installed.

Just my opinionated opinion.

atb

I haven't heard of any stroker failures from being revved to redline, so I think you're going to be okay there. With ITB induction system like Louie's, and the aftermarket cams focused toward top end HP, the stroker motors can get pretty high revving. If there are going to be any stroker failures due to high revs, we'll probably start hearing about them now that there are products that will allow a stroker to easily rev beyond redline.

I think you're limited by the stock induction system on this one. The strokers really seem to drop off in HP at around 5500 rpm. You can take them higher, but why? Just grab the next gear and land fat in the torque band.

1.56 rod ratio + high revs = we're all living on borrowed time

slate blue

It is the bolts that will break, but I will use my multiphase bolts which are almost unbreakable, certainly not an in issue at 7500 rpm these bolts are rated at 285,000 psi versus the Arp 2000 bolts at 220,000 psi.


The measurement I need is of the skirt only, don't mearsure the ring land area. The ring land or comp height area doesn't matter as this doesn't add to stability.

Well this will be answered by the relative skirt lengths as the Mahles I have are made from 4032 which is the low expansion tight clearance material. I think Mahle used a material called 124S in the stock pistons but even Mahle themselves admitted to me the difference is miniscule.

My motivation was more durability given the short skirt, the differences do appear small and I read an article last night in hotrod magazine about an engine, the 400 chevy with a special deck height, identical to ours and they were useing the same length rods as I would use, i.e. the 6.125" and almost indentical comp height on the piston and didn't even mention the crappy rod to stroke ratio. That engine made big power and its heads flow slightly less than mine. Here's the article Quest for Torque

My engine would only be 6.4 liters versus its 7.4 liters but I have a few slight advantages over it, like the headflow and probably the cam too, not too sure on that one as 0.50" figures don't tell the whole story but mine are bigger at that figure. I will also run 12 to 1 comp and a dry sump and we know that our engines are lower friction also. I think I will try to use Honda journals because if you look at it the difference is only 1.3 mm in overlap versus the standard Chevy 400. If you look at this engine that is featured it has a lot less overlap than that. As it is 4.125" in stroke versus the standard 3.75" stroke of the 400. Remembering too that these stroker crank are made from some mighty fine materials.

The other upsides are less weight in the crank, both in the form of heavy metal and crankshaft journal. The piston and rod weigh 1.1 kilos in my case. so less heavy metal, two, the crank journal creates less friction, about 7 hp less.

All good stuff.

Cheers Greg

atb

Wow, a 1.48 ratio. Maybe we stroker builders don't have it so bad after all. Conventional wisdom is 1.6 minimum right? Then again, conventional wisdom also said that stock 32v 928 motor couldn't take boost. Guess we've come along way.

6.125 is mondo rod, could make some serious torque with that.

What kind piston to cylinder clearance are you going to run with the aftermarket mahles? With the stockers you can go to 8 ten thousandths. I heard with the Mahle Motorsport you would still be at 1 to 1.5 thousandths. I'm having my pistons made to match the bore (long story) so I guess we'll see where Mahle puts them.

slate blue

Adam I have taken the liberty to repost one of your photos, as to the skirt length atleast in the photo it doesn't appear to be that much difference, it would be great to get the actual length though. Have a look at the pin and it reference to the skirt and then have a look at the bottom of the pin and the distance to the bottom of the piston.







They don't to me look that different.

slate blue

I just measured the skirt on my pistons and it is 30 mm, so it would be really interesting to find out what is the skirt length of the 968 piston. The area measured was only the area that contacts the wall of the bore, so effectively very close to the ring land of the oil ring down to the bottom of the skirt.

Cheers Greg

Tony

http://www.stahlheaders.com/Lit_Rod%20Length.htm

JET951

off topic a little but 13:1 compression ratio will make it a difficult to start. it will give your starter motor a workout thats for sure

slate blue

You can if need be buy those lightweight Hi Torque starters for the 928 on Ebay, the're quite cheap and save a few pounds too.

Cheers Greg

atb

Okay Greg, had a chance to snap some pics today. The pic you posted above shows two 968 pistons. The one one the left was modified to clear the throws on the stroker crank. Sorry for the confusion. To further confuse things, I was way off on the skirt dimensions. In my mind thought the S4 skirts were the same as the 968, with just an extra "tab" on it, but they are much more substantial than that.

Here are the pistons hanging on a common wrist pin. The S4 has much more skirt.




Similar shot, only this time instead of the wrist pin hole lining up, the oil control ring lands are aligned.



Sorry for the blurred pics coming up, but the digital camera I was using didn't lend itself to one handed use by a lefty.








Hope the pics help.

slate blue

Hi Adam that is exactly the way I wanted you to measure it except I can't read the scale on the caliper as it is too dark. Can you remember how many mm the skirt is on the 968 piston?

Thanks again Greg

atb

968 is 36.68mm from oil control ring land to bottom of skirt. S4 is 58.37mm for comparison.

I've got to be honest with you Greg, I was in a rush and holding the caliper while working the camera may not have yielded an absolutey precise measurement, but I think it gives you a pretty good idea and is really close. If you like, when I have a litte more time I'll try again and double check that I am 6-axis square. I'm probably +/- .02mm in the photos.

slate blue

Thanks Adam, if you put the caliper into the bottom of the ring land, the difference is 4 mm. The reason I said 30 mm was that I measured from where the machining on the skirt started. When I measured from the bottom of the ring land it was 32.5 mm. So there is a difference but a small one, that I'm sure I can live with.

Thanks Greg