BoostGuy951

OK, I have been thinking about this for a while. First, for those of you who aren't familiar with this practice, Offset grinding is done by adding an amount of metal to the rod journal via welding, and then regrinding the rod journal centerline a determined amount farther away from the crankshaft centerline. The distance farther away is the amount that the stroke is increased.

Alot of guys are paying premium prices for the ever disappearing S2 cranks, you almost never see these things go for under 1000, and often closer to 1500.

I have a quote from a crankshaft reconditioning company for offset grinding. For increasing the stroke up to .25 the rate is $425. This would take us from 3.11 to 3.36. So for $425 you get 75% of the stroke increase of the s2 crank, and you get brand new machined rod journals. You also get to keep the crank that has been riding in your main bearings since zero miles. A stroke increase of over .25 requires the cheeks of the crank to be reinforced, so from this same company, a stroke increase of up to .375 is
$600, and an increase of up to .500 is $725.

Obiously there is a limit that you don't want to surpass as far a stroke goes. But, to be honest, I really don't know where that threshold is.

Purists need to stop reading here.

What is stopping someone from having the rod journal reground to the 2.1 inch journal diameter of the small block chevy, and being able to use a mass produced strong rod/piston combo, and ending the $1100 for a set of rods bull****. A rough measurement of the stock rod is 5.9 inches. (maybe someone can chime in with the actual figure) 5.7 and 6.0 inch rods are the common lengths in the SBC world. I haven't done the math as far as the distance from crankshaft centerline to the top of the deck, but its easy to get a piston pin moved when you get your pistons made to correct the difference in rod length. This opens us up to WAY cheaper prices for great quality pistons and rods.

A quick measurement of my stock pin height is roughly 1.62 inches.

I did a search and found the block deck height of the 944 to be 230mm or 9.055". The Small block chevy is 9.025". The factory SBC pin height is 1.56".

9.055 - 1.62 is 7.435. To use a common Chevy 5.7 inch rod, you would need to have the stroke machined to 3.47". (7.435 - 5.7 = 1.735) which is half of the 3.47 inch stroke. So what this means is that if you had your crankshaft machined to a 3.47 inch stroke, you could use chevy 5.7 inch rods with your factory pistons. They would just need to be machined and/or bushed to accept the porsche pin size. This would take you up to 2.77 liters with the factory bore size.

Eagle H beam 5.7 inch rods for a small block chevy run $389...... for 8 of them.

So what do you think?

johne

dang dude I like the way your thinking. That would be $600+200+extra stuff=~$1000 bucks total for an 11% increase in displacement which roughly equals an 11% performance increase so for a 350 hp motor that is an extra 35 ponies. I know I'm doing fudge math here but it seems like a decent idea.

Of course it does ignore rod/stroke ratio's and I question the strength of the new rod journals but it still may be a viable solution. Also keep in mind our rod journals tak a pounding, in a 350 4 cylinder they take the same force as the rod journals in a 700 hp V8.

Nice thoughts man,
John

BoostGuy951

Thanks John!

You are right, the rod ratio is an issue, but I dont think a prohibiting one. The chevy guys seem to do OK with these rod ratios. I am no engineer, but I have heard it said that the primary force acting on a rod is cause by piston accelleration, which is determined by the the RPM and the stroke, moreso than the actual power. Although I can't confirm this.

As far as the reliability and strength of the new journal, here is a quote from Engine Blueprinting by Rick Voegelin.

"In instances where a more radical stroke change is needed, welding is the only alternative. The welding process adds material to the outside of the rod throw... If the welding is done correctly, the additional material is virtually indistinguishable from the parent metal. Crankshaft welding is usually performed with an automatic wire welder-using a "submerged arc" process in which the electrode is covered in a constant shower of iron particles. After welding, the crankshaft journals are ground and micropolished just like an unmodified crank. Using this technique, it is possible to increase the stroke of a suitable core by as much as .625-inch."

89951DREAMER

I also like your thinking. Ironically, the SBC 350 has a stroke of 3.47". You would almost have half of a SBC 350! If you use a 6" rod, why can't you have some pistons made by JE or other so that the compression height would come out right? If the SBC can use 6" rods with a 400 crank and get a 383cid engine, we should be able to use 6" rods on a 3.47 stroke and end up with a great rod/stroke ratio. The difference of .020 between the SBC and our 944's engine's deck height is almost nothing. Excuse my ignorance, but how close is the 944's bore to a SBC? Could we use some off the shelf SBC pistons?

Ian

89951DREAMER

OK, so I did a quick search. The stock 944 bore is 100mm. The bore of a SBC is 4" or 101.6mm. This means that you would only have to bore the engine 1.6mm to fit a SBC piston, and stroke it to 3.47" or 88.1mm to use the 6" rods. Then you could order pistons for a '67-'79 350 SBC. Summit Racing lists some Ross pistons for about $550. A set of Eagle rods are about $500. That means for $1050, you could buy enough pistons and rods for 2 engines. Also, since the SBC pistons are offered in several c/r's, and the 944 block is a little taller, you are sure to find a combo that will work. All it takes is some research. This is similar to how I built a 3.1 liter Datsun 6 cylinder for my 240Z Turbo.

Ian

westcoastprshe

You can't just bore out the block and drop in some aftermarket pistons. You would have to sleeve it.

In the end, I would never think about doing any of this because engine internals are one thing that I would not want to cut corners on. Would I rather spend $1k for an S2 crank, or $500 for an offset ground one? I'll pay the extra $500 any day. Increasing displacement is an expensive way to get more power, any way you look at it, and I would not skimp on parts for it. That's not being a "purist" I just wouldn't trust my engine running an offset crank and chevy parts, especially if they were "engineered" by people like you and me

BoostGuy951

Dreamer, you've illustrated my point perfectly. With this crankshaft mod, All the block machining you need is a .020 deck surfacing, and a .060 overbore and you have got yourself 1/2 of a small block chevy. You can find a buddy to do the same, and you can share a badass V8 rod/piston combo.

BoostGuy951

Not so. You can also Nikasil treat the alusil bore to run a standard aluminum piston.

WestCoast, I respect your opinion, but I don't understand how this is cutting corners. Offset grinding has been a recognized practice in domestic drag racing for generations. If Rick Voegelin is of the opinion that its a viable option for stroke increase, then so am I. IIRC, the Stroke on the s2 crank is 3.48. If someone like you didn't like the crank idea, you can still apply this reasoning to an actual s2 crank. Just cut the Deck surfacing down from .020 down to .015. You can still use an SBC Piston/Rod Combo. All kinds of domestic drag racers are running 8k, 9k rpm on drag motors with some hella piston accelleration due to the 3.47 stroke. If its good enough to power them into the 8's or 9's in the 1/4 mile, why is it cutting corners in a street driven 951 that isnt going to see much north of 7k rpm?

Having said that, I do understand that it is a radical approach, and its not for everyone. Different STROKES for different folks, as they say.

westcoastprshe

How many miles does your average "domestic drag racing" car see out of one of these engines? I doubt they expect to get 100k miles.

When it comes to offset grinding, I also doubt any engine maker will tell you that an offset ground crank is better than one than a stock one of the same size.

I think you should ask someone who's familiar with the 944's block about this, since neither one of us know what we're talking about. Just because the numbers are the same... doesn't mean it will work. I think one reason that people use pauter and other expensive rods is because they're really good, and if you want the engine to last, and you're going to spend the huge ammount of money it takes to tear it out of the car, and apart, it's worth it.

BoostGuy951

And I suppose its the quality of the pistons and rods that cause this? There just a few more factors involved in the lack of longevity.


I won't argue that. But how much better? If the offset one isn't going to grenade, which it wont, then its good enough.

Speak for yourself.

Ummmmmm..... Yes it does.


They are really good. But I think the fact that they have a monopoly is the bigger issue.


So far it has cost me $0.00 to remove the engine from my turbo and my NA parts car and completely break them down. I'm 19. I don't have 1000 to spend on a crank and 1100 to spend on rods. I have some money, and I want to make it go as far a possible.

westcoastprshe

"And I suppose its the quality of the pistons and rods that cause this? There just a few more factors involved in the lack of longevity."

No, but if you start saying "it's good enough for them" you have to take into consideration how long their engines last. Maybe it's not good enough for a 944, because you don't know what kind of longevity you'll get out of it.

"Speak for yourself."

I didn't mean to offend, but you said yourself... "I am no engineer." Looking at numbers and throwing in a bunch of parts because they fit is not a way to design an engine, that was my point. More displacement at the expense of (possibly) heavier parts and suspect reliability is not the right way to do it, unless you want to do it again in 6 months. If you want to tinker with it and rebuild your engine twice a year, go for it! this is perfect for that, but if you want to build an engine that will last another 10 years, I would think hard and consult with an expert and do it right the first time.

BoostGuy951

We are talking about two items that are shared with the dragsters I referenced. Pistons, and rods. I cannot understand how you think that the quality of either item can in any way be inversely related to the longevity of an engine. There are hundreds of other factors that contribute to the accelerated wear on these engines. The big ones that come to mind are High RPMS, High dynamic compression, High BMEP, very large tolerances that allow a lower coefficient of friction at the cost of bearing wear. Also, large piston to wall clearances that promote piston slap which wears the cylinder walls. Burning NITROMETHANE, or Methanol, or spraying copious amounts of N20. Its doesn't have a thing to do with the quality of the metal in the pistons and con rods.

You'll save over 300 grams per rod by going from the stock rod to a good aftermarket H beam. I think the piston weight savings is similar, but I can't say for sure, they are in the parts washer and havent been weighed yet.

The damn thing is a block of aluminum with a bunch holes in it. Its not magical. It has lengths and dimensions. If you get the dimensions correct, its happy. It doesn't care who made the parts that slide or spin around in its holes. With careful measuring and calculating, and accurate machining, it can be made even more reliable, and last longer than it came from the factory. There is no voodoo, or sacrificing animals, or chanting over burning magnesium Cup Car wheel shavings.

dmoffitt

it's one thing to have an engine rev to 9k 5 times on it's way down the strip. it's another to have it hold 5-7k for 90 min. in an enduro...

i like your innovative thought, and i think you should try it, but in the mean time i'll keep piercing a bill gates doll, biting the heads of roosters and dancing naked in circles around my kinesis k57s -- as i think the way to really get our engines to shine is bore & sleeve, not stroke (ask rage2 or tonyg) - i stroked mine, it's great, but i don't think it's the 'idea' way by any means...

Koda

I like your idea...I think you should try it...if it works start selling kits.

BoostGuy951

Thank you for the encouragement guys. I am not trying to redesign the 951 engine. The goal of this is to find a cheaper way of reinforcing the bottom end. I am planning on taking the crank out to within .010 inch shorter than the s2 crank which alot of guys are using, and then taking maybe .010 to 0.020 off the deck. We are talking about the difference of a few hundredths of an inch. Its soooo miniscule of a change. I know the offset grinding sounds far out there, but its been done in race engines for years. With a quality weld, it can actually be stronger than the metal around it.