Engine Design
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Not all 4.0L Engines are the Same

Not all 4.0L Porsche GT3 Engines are the Same


This paper is written after receiving many phone calls asking, “What should I do?” It may read a little whiny and it’s intended to be critical of how many of these engines are sold.


You have decided you want a 4.0L GT3 engine. As the title suggest, a 4.0L engine can be built many different ways. Bore, stroke or bore and stroke. All have their advantages and disadvantages which need to be known in order to decide which way to go. Typically customers just want a 4.0L engine and have no clue about how it can be configured and what the end results will be. As a customer you only know you want a 4.0L but you really don’t know what you are buying. The really sad part is many of the “shops” offering these “hot rodded” engines have no clue what they’re selling!!


I wrote another paper about this subject but that’s a little more technical. This is more about informing the customer of some of the questions to ask and have answered. My issue is not how it’s done but the fact customers are never informed of the options and that the bore only upgrade is done by the shop because it is easy and the parts are purchased “off the shelf”. Particularly, the piston as its one size or configuration fits all. I’ll point out the disadvantages of this piston later because the disadvantages need to be understood. You’re spending a lot of money and should be told the pros and cons of this upgrade.


Many 4.0L upgraded GT3 engines are built with a bore increased to 105.40mm from 100.00mm or 102.70mm depending if the base engine was a 3.6L or 3.8L. This is a fairly simple displacement upgrade, the only “technical” part is the machining of the engine case and liner housings to fit the larger OD cylinder liners. I say fairly technical but most shops send this work out to be done so their technical input is pretty much “who” to send it to. The bore increase uses the stock stroke and often the stock crankshaft. Now here is the first of many issues I have with some of many of these upgrades done this way.


I have seen in the promotion of such engine upgrades by many of the well-known shops, knife edged crankshafts. They look pretty, real racy and are often shown to show off the engineering level of these shops. This is a completely bogus upgrade and one that should never be done. Here are the reasons.


  1. We have learned from computational analysis since the mid 90’s and maybe earlier that knife edged crankshafts do absolutely nothing to help lower wind age friction inside the crankcase or block. There is no oil around the crankshaft as it’s all stacked up against the wall of the case or block not in the path of the counterweights.
  2. There are 12 counterweights on a GT3 crankshaft, two per rod journal and they are approximately 0.250” wide opposite the rod journal. The rod is approximately 0.875” wide. So if there is a reason to help the narrow counterweight cut through the thick oil film inside the case or block, why are the rod beam widths not considered?
  3. The counterweights are there for exactly what their name suggests. They are there to counter the weight of the piston and the portion of the rod that is reciprocating in its motion. You have now removed weight from the counter weight and in the case of many of these big bore 4.0L engines added additional piston weight and a steel rod. The factory GT3 engines come with titanium rods. After the counterweights are knife edged and material removed, the crankshaft can still be balanced. These flat 6 engines can have their crankshafts balanced without the need to add weights to the rods journals or what is called “bob” weights. As the throws on the crank are even numbered and essentially opposite one another the cranks can be balanced up to around 750 RPM without “bob” weights and typically balanced end for end. If the crankshaft is balanced at 750 RPM it will not be any different at 8000 RPM. Typically the mass weight of counterweights is a calculated value based upon 50% of the piston and upper rod weight along with big end weight with bearings fitted. Even oil that is between the crankshaft journal and the rod bearing is factored in. Sometimes we over balance the crankshaft particularly in race engines. The GT3 engine should be one engine that is over balanced, yet the crankshaft has been knife edged and had weight removed? Backward for use. They look pretty on the bench. If only they looked as pretty spinning around at 8000 RPM. This weight is there to counter the inertia of the piston and small end of the connecting rod rotating at a larger radius from crankshaft centerline. Counterweights are there for a serious reasons so don’t remove any weight or touch them. We have all done the practice of removing weight and knife edging the counterweights in the past, but some of us have kept current on modern engine building practices where others have not. It’s your engine so be sensible about what is done to it. The issue of harmonics is another factor but I won’t go into that here.


What configuration will make the most torque?  Both will make around the same if all other factors are done correctly. The larger bore has more surface area for the compressed expanding gases to push against, but a shorter piston travel to work against time wise. The long stroke smaller bore has the opposite. There are many other factors to be considered. The long stroke engine will shift the peak torque value lower in the RPM range which, for a street engine, should make for a better driving car.


Add to the longer stroke engine a better designed piston for this application (long rod) and you have changed the time motion of the piston as it goes through TDC. The long rod will slow down the piston over TDC and help the gas exchange become more efficient. It helps lower the demands upon the intake system, gives the fuel more time to atomize and helps the stock ignition handle the work it has to do. Cam timing and ignition are somewhat fixed based upon the crankshaft rotation and speed. What I am talking about is the time the piston dwells at TDC. Think about the opening and closing of the valves during the overlap period. If the piston is travelling through TDC at higher rate of speed and opening and closing times of the valves is set by the Crankshaft speed the air exchange is quite different. As the engine speed increases this time diminishes which is one of the reasons why a long stroke engine makes its peak torque lower in the RPM range. Another factor to be considered is the flame travel time. This is controlled somewhat by the engine compression and piston design and also by the energy level produced by the ignition system. Slow it all down by slowing down the piston travel over TDC and all areas become better.


A shorter stroke will typically have a longer rod so many of these advantages are had with the bigger bore. A longer stroke engine with an equally long or longer rod is even better. These are options that are available but only if the shop doing the work is prepared to do the work and not just buy “off the shelf” parts.


I see no fault in building this configuration just the fact that the customer is generally never told the differences and the shop decides what the customer gets because it’s easy for them. I could bring up many other contributing factors that are present in either configuration but that’s not the point of this paper.


Another very important piece of knowledge the customer needs to beware of is the machining process required when rebuilding one of these engines. Shops that do it in house do so for two main reasons. They control the quality and the time. For example, these engines come with very soft valve guides. I have seen them worn out within 5000 miles. Replacing them requires care and finesse. The valve seats may need re-facing regardless, but any time you replace guides you have to go back in and touch the seats again to get the seat concentric to the new guide location. These heads do not like the seat being messed with. Each time the seats are touched they get lower in the head. This affects the flow. Experience has shown us that as soon as you lower the seats they require different shape and widths than what came stock to obtain the same stock flow rates. It’s no coincidence that our flow bench is arm lengths away from the machine that cuts the seats. We know what seat angles are required at any seat height to gain back flow. Regardless of this knowledge, every time we recut seats, the first intake seat is measured on the flow bench to confirm we are correct. The same goes for the exhaust seats. Then all of the others are done the same. Do you think a production machine shop is going to go to this trouble? Absolutely not. They want the job done as quickly as possible and paid for. The real killer is the shop they are doing the work for doesn’t know the difference. They get your heads back all assembled, surfaced and looking pretty just like your knife edged Crankshaft, but hiding a lot of ills. You get the engine back with substandard head work but feel the extra power of the bigger displacement so who cares. You should because you can be down a lot of HP that was left in the seat cutting machine.


Shops that do their own machine work should be sought out over places that “shop it” out. Why? Because they have decided to invest huge amounts of money in machinery for the benefit of their customers that use them. They have decided that quality is important and they want full control of the repairs and assembly of your engine. These shops will have all of the required measuring tools, understand and know about low tolerance work, “clearances and fits” that are so important to control in these engines.


Something I have heard many times before is, “We have never had a problem before.” Maybe so, but wrong doesn’t always manifest into broken parts. It can be loss of performance and isn’t that the reason you bought this car in the first place?


Get the idea of this paper? You have choices and should be told what they are. Unfortunately you are never told because the shop doing the work has no clue either. I may have ruffled some feathers by writing this but I don’t care if the level of work offered out there is raised to a level befitting these cars. Not all shops are like this, it’s your car, your engine and your money so seek out those that do it right, ask questions and demand quality.