NineMeister

Good morning Stan, this should save you some time when you make the call...

First question is what year is the engine and hence do you know if you have the later flat top cylinders of the early spigot tops? The reason why I ask is that if you have the early type, my recommendation will be to buy a brand new set of original Mahle 100mm pistons & cylinders and fit them. If you already have the later version but the engine has done 100k miles and track days, make the same choice. I'll explain why later.

Secondly, on an engine that runs below 7000rpm you do not need to consider replacing the rod bolts or doing anything else with the bottom end, and that includes fitting ARP head studs. All the lower studs should simply be updated to the factory fully threaded 993 version and you are done. If you want to go the whole hog, fit all 24 of the 993 studs.

Then we look at the top end. If the cylinder heads have been rebuilt properly 10k miles ago you should not need valve guide work, however they will have to come apart to reface the cylinder seal surface so that would be the ideal time to consider upgrade options. Provided the ceramic exhaust port liners are in good condition you should consider a light porting job and a proper race quality three angle re-cut of the valves and seats to "RS" specification. New valve springs and retainers are a must have - many 964RS's break retainers and springs after track use, so no point in taking risks there.

Then we're looking at cams. If the existing cams are perfect you could run them again, however if not a new pair of Sport cams suitable for stock pistons would be something that you can consider - but only (and I mean only) you intend fitting a 9m Motec upgrade at the same time. Uprated cams will not work with the stock Motronic even with a MAF conversion, you would be completely wasting your time and money going down this route. So now I've mentioned the magic word Motec, there is no point in saying more than that the conversion delivers around 10% more power & torque above Motronic (with our without MAF) and will give you an amazingly strong idle with a lightweight flywheel assembly.

Which means that finally we can consider the 3.8 pistons & cylinders option. I've left it till last because putting it simply, every standard 3.6 964 engines running 9m Motec conversions have made more power than any 3.8 engine on Motronic on our dyno - and that includes factory 964 3.8RS and 993RS variants. In fact, we've recently supplied a complete 9m Motec kit to a trade customer in Germany and all his customers have driven his 964 with it on and refuse to believe that it is not a 3.8! The only advantage you have in the 3.8 (or more accurately 3.75 litre) conversion is that on Motec there is a slight increase in midrange torque and fractionally more top end power, the top end gains limited by the induction system. All said, if it were my engine I would fit a set of 3.75's if I were replacing the p&c's to gain the torque, but if you want to do the job in stages stick to building a perfect stock 3.6 as this will be happy to run Motronic until you jump for the Motec conversion.

Finally it comes down to prices:
9m top end engine build including head stud replacement & through bolt seals, fixed price labour £2500+VAT plus all parts as required.
9m Motec conversion (new M84 version, including injectors, sensors, intake, etc) £4350+VAT fitted & mapped.
9m 3.75 litre P&C set, one set only left at £2995 +VAT.

Hope that helps your Sunday decision process? Do call me in the morning though, there are many more details which I am sure you will want to ask about. I should be clear after 10:30.

evoderby

Edit : oops, typing at the same time as Colin posting

Very good idea getting on the phone with Colin! In the light of your thoughts concerning 3.8 liter P/C's being too expensive in the end (I agree) , might I suggest you have a discussion with Colin on bringing the compression ratio of your existing set up to 11.3 factory spec? That is when your set is in good servicable condition.

Many have found the compression whilst rebuilding non-RS 964 engines to be way lower than stated in the factory specs....even up to the point of 10.4 - 10.5 which costs in the region of 8 - 10 HP.

koenig_roland

Dennis D.?!

NineMeister

Might be.....

boxsey911

Stan, my car also had a top end rebuild before I bought it that wasn't worth the paper it was written on i.e. the studs and other important parts hadn't been replaced. Now that the engine has been properly rebuilt and the LWF fitted, it is night and day in how it performs. I would put your faith in Jaz's guidance on how far to go with the rebuild. It's going to be expensive enough without chasing extra power.

StanUK951

Thanks again people. Steve: I hear that, expensive enough without chasing ponies...! Especially when I'm moving house next month...

Colin: I will call after 10:30/11am. My days are usually manic so apologies in advance if it isn't around this time...! I guess the question is by cams 'not working with Motronic', is that just that they are lumpy and a bit hard to live with or undriveable? My car is a 91 with plastic manifolds so assume later type pistons? It has done 110k though but with new rings (purportedly).

If cams are a no go full stop on Motronic then I will maybe have to just do heads as cant afford Motec with the rebuild (and house!).

NineMeister

No problem Stan, call when you can, I'm planning to be in all day unless I find the time to sneak out for a well overdue haircut!

The "Cams not working with Motronic" statement refers to the fact that the only reason to change camshafts is to improve performance and you will not be able to achieve that goal with the stock Motronic system or indeed a Motronic system with MAF. Likewise for a 3.75 litre upgrade.

If budget is limited and you want more performance let's just pull the motor, do the stud repair, review all the previous work and put right anything that is out of tolerance, refit it and install Motec. The crunch is that a stock engine on Motec will make more than a modified engine without.

NineMeister

The cost and thoroughness of an engine rebuild is usually determined by the owner rather than the shop doing the job. In a perfect world you would always replace through bolt seals, head studs, pistons & cylinders, valves, valve guides, valve springs, retainers, camshafts, cam chains, chain ramps & cam followers when doing a 964 top end rebuild but since not every owner has the extra £6000 plus to throw at the job. As a result most owners make a judgement call based on the recommendation from their shop in targeting the critical parts which are neccesitating the rebuild in the first instance, hence if we had an engine apart the most usual decision path is that we allocate the customers budget to the cost of the essential repairs first then prioritise all other non-essentials on a cost/longevity/risk basis.

g-50cab

I'd also let a trained professional assess your issues - you might just be able change head studs and put it back together. You might have to pony up and get the heads resurfaced - Lots of combinations here -

Colin is definitely your guy here...

boxsey911

I would agree with most of that. Especially since I had all that done plus the bottom end bearings, little end bearings and host of ancillary items. Luckily the P+Cs were good for re-use in mine.

I think we as the customers have to form a partnership with the engine re-builder before the tear down starts. We as the customers have some input into the budget but we must also be realistic about the costs that will be involved and that those costs are going to be considerable. Skimping could have grave consequences later on and it puts the engine builder in an awkward situation of agreeing not to do X and Y just because the customer can't afford it. Yet knowing full well, that not doing those things could be bad for the future health of the engine. I'm sure no engine builder wants to utter the words "I told you so".

I guess what I'm saying is that we as the customers have very little control of the budget for the essential stuff in the rebuild but can have more say in the budget available for the upgrade items.

911Jetta

WOW! Learned a lot reading that.
Thank you for sharing that Colin.

polo classic

Colin, what parts (sensors and so on...) of the Motronic do you leave out when fitting Motec?