ptuomov

This argument doesn't hold water. The variable cost of precision powder forged rods is very low. Given the GT volumes, tooling for a separate rod type almost certainly cost more than cranking out some additional s4 PPF rods. (I recall from a presentation from the mid nineties that the variable cost of PPF rod is less than ten bucks at OEM production volumes, don't quote me on that it's from my memory which is like Swiss cheese.)

A more likely explanation is the following. This is just a theory, don't shoot me. The GT has a higher rev limit, right? The factory may have judged that the higher rpms and top-heavy S4 rods would make the rod bearings even more vulnerable. Therefore, the factory desgined a different rod for GT. My theory is that the factory designed the GT rod to have a lighter small end to keep the reciprocating mass and therefore the inertial rod bearing loads about the same as in S4. The GT rod may be weaker overall but still plenty strong for the stock GT, and may lead to much more reliable rod bearings at high rpm than the S4. If this theory is true, the GT rods are preferred for high-rpm NA engines and S4 rods for lower rpm FI engines.

One way to test this theory is to take an S4 rod and GT rod and weigh them. If the GT rod is lighter and in particular if the small end of the GT rod is lighter than that of the S4, the the theory may be true. If the GT rod's small end is as heavy or heavier than that of the S4 rod, then my theory is rejected.

Can anybody weigh them?

blown 87

If I can get to the shop tomorrow I can get within a gram on them.

I will have to rig up some kind of knife edge balance to weigh each end.

Mike B

Those appear to be rods similar to those from an early 951 which are sintered forged..My '86 951 had the same rods....here is a dated thread from the 944 turbo forum for caparison. See post 12.

https://rennlist.com/forums/944-turb...ting-rods.html

ptuomov

Here's my highly imperfect method. Who said books are useless?! ;-)

"Australian" aftermarket rods:
Big end measured 410g
Small end measured 220g
Sum 630g
Big end percentage 65.08%
Small end percentage 34.92%
Rod weight measured 634g
Big end weight 413g
Small end weight 221g

blown 87

I tried to weight them with my bullet scale, which is a old triple beam scale and it would not go high enough.

I will take my digital kitchen scale to do it, it goes up to 5000 grams.

slate blue

Blown the rods you have pictured, the 944 stamped ones are the forged type that I sold to the other rennlister who also got the set of GT rods too. It is our belief that the early 944 rods are the ones to use. He has put the early ones in his engine and seen over 7,000 rpm.

ptuomov, those are indeed nice rods you have, I like them, if I had USD I would probably but them also, the lighter weight of the rods IMHO will not help the rod bearings live longer at all. This problem has been I believe correctly diagnosed By GregB. It is a detonation problem, you wont have a problem if you don't get detonation either, as you are probably aware detonation is a violent rise in pressure in the cylinder, this sharp rise pounds the bearings, when they are soft like the 928, they are quickly damaged.

Something some of you may be interested to find out is the direct petrol injected engines are much harder on their bottom ends, they have a more violent combustion and in race engine have destroyed their bottom ends due to this factor, so much so that some manufacturers will not being returning to direct injection, at least in the near future.

Greg

namasgt

Hi Greg

exactly which one was blown the GT ones pictured or the 944 S4 ones ?

the "early 944 rods" are they the early 87 928 rods or your actually talking about a 944 car.

sorry Ive read theis thread several times and Im still confused.
is there a picture of GTS early and later rods to be compared with the GT ones ?

slate blue

The rods that are on this page are the 944 stamped ones that are also fitted to the 928, it just depended on what thickness the big end was machined to.

Greg

blown 87

Detonation will just beat the chit out of every thing, it is a engine killer.
The old way of tuning fuel motors was in part, by looking at the mains and rod bearings.

Back to the rods, there is no visible parting line on the rods that is usually found on forged rods on the 944 ones.

I wish that Porsche had some info on what is what and some testing info that we could get.

I am getting down to a time factor on making a choice on which rods to use, which ones will be best for a base for a blower motor.

It would save me a bit on money to use the rods that came with my motor, just because I can get by with out balancing the motor, (I am not sure what it is going to cost) I do think I need to have it done regardless of which one I use though.

I just want to use the best parts for the application.

blown 87

I think I am going to start signing my posts as "The other Greg"

The two pictures I took of the rods came out of my early S4 squirter block.

Many of the parts on a 928 start with a 944 part number, and as a general rule, when calling for prices on parts, the ones that start with a 928 part number is usually followed with the words "you got to be ****ing kidding me, right?"

The other Greg

blown 87

Just so there is no confusion here, Greg Brown does not want to cash my check for these rods until we get to the bottom of this.

He does not know me from Adam and sent them before he got payment.

He is a stand up guy and wants to know the answer to these questions more than any of us, he builds more 928 motors than most of us see.

the other Greg

fst951

Interesting discussion.

The import rods have come a long ways. I have a set of rods for a VW that I will put up against a carrillo any day for over all power capacity versus weight. My buddy has them made in China. Pretty badass stuff. I think that you guys might be surprised by what is coming out of there for the price. You have to deal with a good supplier and do your testing. He has sold thousands of sets and never had a failure....800hp vw 4 cylinders! They survive nicely!!!

It is amazing. They rods cost about $150 per set of four shipped!

I didn't believe it either.

blown 87

Exactly how are we going to "Do our testing"?
I am not trying to be harsh here, rather wanting to know how we can test them.

GregBBRD

This stuff is hard enough for me to do, without throwing in the variable of Chinese steel. I'll have to pass.

blown 87

Thanks for the link Mike.