adrian928se

Just re-read the Tech bulletin - Nick you have casting number 1R on your rods which are the early ones, the later ones have 2R on them. As someone previously noted it says that if you unbolt the old style rods from new, then do not re-use them, but fit the later ones on all 8 cylinders.

Bit of a pain really - I need to check the casting numbers on my spare rod set now.
Anyone got a pic of the very late lighter GT rods aswell?

Adrian

heinrich

If we look at a stock GTS engine, early rods (like mine I assume) ... Assuming I replace the rods Erkka, which rods would I be able to use and slightly modify and how? And let's say I have a need for pistons for the same engine ... I have a full set of good 968 pistons but will not install them because I will not bore. So, which pistons can I use on a stock bore then?

GregBBRD

Great information Erkka. I never noticed that the 944 rods could be modified to fit a 928....that explains the same casting number. Thanks for the clarification.

Vilhuer

H, with GTS crank you need GTS pistons if you plan on using any 150mm long Porsche rods. 944 S2 and 968 pistons have slightly smaller compression height than GTS pistons IIRR. All 928, 944 etc rods are exact same lenght except possibly '89 2.7L 944 and I suspect even those are same. In order fo keeping rod angle as small as possible Porsche used same lenght rods all the way through production and adjusted total rotating assembly lenght with piston compression height when ever crank stroke was increased. IIRR using any stock rods will result 968 pistons being about 1mm below blocks top surface on TDC. This will lower CR and isn't too good idea anyway in my opinion.

From physical dimensions point of view you can use practically any 924S, 928, 944 and 968 pistons. 4 cylinder rods just need to have 2mm removed from one side on their lower end. Mixing different rods into single engine is limited by different rod types different weights. Rod bearings are always exact same part so thats not problem at all. For example friend is building low compression GT engine. He got six ok condition 928 rods from '86 ROW 32V engine. Same engine which block I'm using in stroker build. Remaining two were easily made from 944 rods at cost of few $ and all eight rods were lightened to same weight. In this case 944 rods were similar enough as they has same casting number as S3 rods.

Unfortunately any earlier than GTS/968 rod is likely going to be heavier. This means you need to add mallory to GTS crank to make them work unless they are lightened to same weight as stock GTS rods. Thus it could be better to find late GTS or 968 rods and use them. Depending on how much mallory is needed total price could end up being same even though earlier rods might be much cheaper to buy. But as late style 2R rods are hard to find as used part, using earlier parts could make sense despite the weight difference. Competent machine shop should be able to tell how much mallory and $$$ is needed to make crank work with any rod piston combination you happen to have on hand after they have checked GTS crank in balancing machine.

For this kind of mixed build any and all 5L blocks will do as long as cylinder bore and piston diameter are compatible. Lower part of the block needs to be modified a little if one wants to follow factory way of doing things. I know one case where GTS rotating assembly was installed into S4 block without any modifications. So they can work at least in some cases without any mods to block. There are few pics in earlier threads which show what kind of mod GTS block lower half had from the factory. They are easily doable to earlier block by anyone who I would trust to be able to put engine together.

Vilhuer

I'm surprised people haven't used 944 rods more. Early turbo's are probably strongest available.

heinrich

Erkka thanks A lot to digest for someone like me!

drnick

hmmm... well i might keep my GTS engine NA for a while longer. if i tear into it i would strongly think about a set of custom rods rather than re-using stock parts with the advantage of lighter weight and ajusting the compression ratio to better suit boosting. if i put together another GTS engine i would also drill the crank and strongly consider 968 intake valves along with a wilder cam - these are the things i regret not doing the most during the rebuild but i guess hindsight is 20/20!

drnick

greg, sorry but im not sure i know how to drive carefully, does this mean no more 170 mph runs - or just no more street racing at rush hour? - just kidding, before anyone takes offence - as a responsible person i would never drive my porsche in this kind of reckless manner!

i think marc makes an interesting point about 968 cars and whether the early cars have rod failure. there is a local company here in the uk who supercharge 968s and offer a warranty, i will be interested to hear there take on the early rods.

the technical bulletin is interesting but assumes the engine is out of the car in the dealership under warranty with a serious problem - i cant see any other circumstance rod bolts would be undone back in the day. and it begs the question of why they were there in the bottom of the engine and undoing rod bolts?? probably an engine rebuild as a result of excessive oil consumption...
and perhaps whatever caused the oil consumption also damaged the rods... i dont know, this is only speculation. but adrain makes a good point that there are still early GTS cars out there doing fine on early rods.

the question in my mind is whether there is any difference, from the rods point of view between boosted hosepower and naturaly aspirated hosepower.
ie if i were to put throttle bodies, bigger valves and cams vs a supercharger?

Fastest928

Erikka,
You mean "upper" block mods!! right?

Marc

Vilhuer

No, lower part mods. As nice those three air hole in GTS upper block are to have they are not needed to run GTS pistons in block. I'm sure you have seen these cuts in lower half more times than you can count. They are only changes which are directly related to rotating assembly. Even they might not be needed but for some reason factory saw need to include them.

drnick

i spoke to ninemeister here in the UK this morning, they supercharge the 968 using a rotrex supercharger. they have at least 3 kits running on the early 968 engine out of about 10 kits installed. they have not had any problems producing 400 bhp at the crank with this kit and these engines using stock internals.

adrian928se

Thats true Nick, the Ninemeister cars seem to run well, but I thought the issue was only regarding early 928GTS rods not 968 rods. The Tech bulletin only refers to 928 rods, unless there is another bulletin for 968's.
Pic attached of early GTS rods v late rods (1R v 2R), - you can see the difference at the top of the rod bolt (and for the eagle eyed, yes the bearing cap is fiited the wrong way round on the rod on the right).

(I've got a set of mixed GTS rods (1R & 2R) ready to go into a spare engine: not sure whether to give it a go or replace the 1R ones)

Adrian

Charley B

Any chance of seeing a side view of the two rods?

drnick

adrian, im just picking up on the info here on this thread regarding the early 968 rods being the same as early GTS rods. thats a tough choice youve got there about whether to install the early rods.. or plonk some major wedge and go custom/fit later ones. its a tough call as the early rods seem to do ok sometimes!

i was really interested in hearing from the ninemeister guys about there experience, i might continue with my plan to supercharge as im only looking for 500bhp at the crank. supercharging has to be a cheaper way of making this power rather than a new intake and head/cam/valve work - as long as the rods hold.

i dont think the supercharger will make my lap times that much quicker though, i need to work some more on setting up the suspension and improving my driving style. also a LSD will help more.

SwayBar

A wise man you are!

An interesting addition to this discussion:

The engine I pulled out of my GT racecar (..to install my new wet-sleeve engine) is a replacement block; the engine identification code was hand-scribed with GT specs, and the block has '92' in several locations, as well as the oil pan.

I pulled the pan yesterday, and it has the 1R rods as well as pistons without oil return holes behind the oil ring. On each side on the bottom of the piston, there are two holes straddling the wrist pin the outside edge. Does anyone have pictures of GTS pistons they can post?

The crank has 6 counter-weights so it must be a 5.0 engine for sure, and the cams are definitely GT, and are in very good shape.

The valve springs have light blue paint on them which I haven't seen before; could they possibly be a 944-variant?

Last summer I replaced the rod bearings, ran it two days without incident at Road America, and now I'm concerned because of the info Greg brought forth from the tech bulletin. According to V, since I apparently have GTS pistons, I have to run GTS-spec rods; now what should I do?