Many people love the S65 in naturally aspirated form and rightfully so. This is an absolutely amazing motor with great response in NA form. For those that want the maximum possible naturally aspirated gain, the best results will come with a built motor that has increased bore and stroke. Most people are familiar with the two main options in this area which are Dinan and RDsport. Those that truly want to stay NA and desire much more power should definitely consider either of the aforementioned but also consider a custom built option.
Why a stroker?
Well, why not? The stock E92 M3 S65 has a bore and stroke of 92mm x 75.2mm. To paraphrase Steve Dinan, the motor is just begging for more stroke. The stock piston speed at 8250 rpm is 4070 feet per minute. The stroke can be raised 7-8mm relatively easily. 83mm at 8250 produces 4493 feet per minute, well within the high end of the most stressed naturally aspirated motors. Half a liter of displacement is just sitting there waiting to be tapped.
If BMW could have done it from the factory but they did not, that must mean it is a bad idea or they would have done it, right?
No, not right. BMW is very crafty, very thorough, and they definitely know what they are doing. The stroke was going to be 75.2mm no matter what as the S85 V10 in the M5 already decided it. Why did BMW give themselves this room in the S85 and S65? Well, they gave themselves a cushion if they needed it or the market demanded it. Some of you might remember early rumors of the M6 having a 5.5 liter V10 and 550 horsepower.
These rumors had a strong basis in reality as insiders knew BMW could raise the displacement of the S85 to 5.5 liters at any time they wanted to. The M5 CSL that was shown happened to be a 5.5 liter 550-580 hp S85.
BMW did not just get a new crank and new rods for one vehicle. I am speculating they already had this sitting around when they toyed with the idea of giving the M6 5.5 liters from the outset. The fact of the matter is the M5 and M6 received the same displacement to keep costs low. The same thing happened to the M3, bore and stroke was forced due to the M5/M6.
However, after the M5 CSL was announced and tested to honor the 25th anniversary of the M5, the M3 GTS was announced. What was the displacement? 4.4 liters, it used the exact same stroke as the M5 CSL test car needed to hit 5.5 liters on the V10. Coincidence? Not at all, BMW had this all planned out way in advance. Not only did they intend to carry over the stroke and the rod specs from the M5 CSL to the GTS, they gave themselves plenty of room in the S85 and S65 should they need to turn out higher performance models (CSL, GTS.) The competition never forced BMW's hand as the M3 is still the top of its class and the E60 M5 certainly was and arguably still is. Also, with the CSL and GTS, there certainly was a bit of internal pride presented through these models and BMW was showing the world they could still turn out top NA motors... but they now had to be environmentally responsible.
The S65 and S85 had the foundation to evolve for years if necessary. Unfortunately, we will never see these motors realize their full potential in official production as they have been abandoned for obvious reasons. However, this is exactly where the aftermarket comes in.
Dinan vs. RDsport
Dinan has their stroker motor listed at $24,999 as of right now. It was listed at $22,999 at one point and it was also listed above $25k at one point. The price has jumped around a bit and I am not surprised at all. Dinan is gauging how much they can get in todays market which is rough to say the least. There is absolutely no reason to pay $24,999 especially once they showed that they would take $23k. Additionally, Dinan can't make the argument that there is significant R&D to recover when he already admitted to the V10 parts carrying over. He did not need to test a new setup or use different pistons and rods. He is just applying everything from the V10 here except the crank (although the crank is for the same stroke.) Dinan also does not include install in the price or any supplemental mods which does give the user more flexibility in choosing who their supporting mods come from. You don't have to bundle the Dinan exhaust, intake, or throttle bodies. It ends up being less than the RDsport retail but more than what the RDsport motor can actually be purchased for when everything is tallied up.
RDsport has their stroker listed at $32,800. Much higher than Dinan? Yes, it certainly is, but price alone does not tell the whole story. RDsport includes exhaust and installation. Additionally, no one has paid $33k for this motor. The initial run of guys getting the stroker ended up paying $23,000, installed. If you are seriously considering the RDsport route, don't pay retail for this motor. If you do, you deserve to lose the 10k as it most certainly can be had for $23k if you put your foot down. The price between the two ends up being pretty close in reality however you do get a full exhaust with RDsport including their headers. Personally, I'd tell Federico to keep his headers.
Displacement: 4.608 cc
Compression Ratio: 12.0:1
Horsepower: 502 hp without supporting mods, 527 hp with supporting mods (exhaust, throttle bodies, under drive pulley.)
Piston Speed: 4493 feet per minute
Displacement: 4.619 cc
Compression Ratio: 12.0:1
Piston Speed: 4601 feet per minute
So how much power do these strokers really put down? That is really what it comes down to. Here, we have a set of graphs from BimmerBoost member GT3 who was one of the first guys to get an RDsport stroker:
This car is a DCT cab with 3.62 gears. The highest number here was achieved with race gas mixed in. GT3 also opted for a different header design than the other strokers. The highest number here was achieved with a 4-1 header design 2mm larger than the other strokers and with further tuning. I have not see a higher dyno from any other stroker motor. Using the accepted 15% driveline loss on the M3, this stroker achieves 519.8 or ~520 horsepower on race/pump gas mix. Well within what RDsport quotes for it however on solely pump gas it will be a bit shy of the number, as 417 whp is on 91 pump gas.
So how does this compare to Dinan? Well, we don't know. No Dinan stroker has undergone an independent dyno test. Why not? Well, Dinan has everything to lose and nothing to gain from independent dynos. Dinan has a long paper here explaining their thoughts: DYNAMOMETER TESTING AND THE MODERN BMW ENGINE: http://www.dinancars.com/bmw/technia...ern-bmw-engine
, which really amounts to a list of excuses he can point to for why his parts don't put down similar numbers to the competition. I'm not trying to bash Dinan here as they have wonderful products that are overpriced. You are paying for the Dinan name and the extremely misleading and hyped up aspect of Dinan parts maintaining the warranty which in reality just amounts to Dinan providing their own warranty. The BMW warranty is still voided with Dinan parts.
So Dinan states a list of variables that can affect dyno output and that is fine. Dynos are susceptible to a degree to various variables and conditions but even the worst case scenario should show a significant change through the curve on average from the baseline stock vs. the stroker. A large reference of dynojet runs exists for M3 owners to evaluate and compare runs. Even if one is opposed to dynos and is going to argue that there is not sufficient airflow, that the hood being open or closed skews results, and that engine management needs to adapt, we still have real world metrics such as 60-130 times and trap speeds that can compliment the dyno. Dinan does not provide either of these as well.
So how does the stroker compare to the stock car?
Well, here is a dynograph of a stock car on 93 octane:
You may notice that the power difference is rather larger, as is the torque difference. You may also notice the RDsport stroker graph has a torque dipfor 2.5-4k RPM which was attributed to the collector on the headers and is supposedly resolved now. So, on paper, the stroker has more power through the entire curve. A ~67-91 whp difference due to the increase in torque.
Not bad you say? Well, what if we compare the stroker to a mildly modified M3 such as this one with a pulley, ecu, and secondary cat bypass:
The stock motor can be modified into the upper 380 whp range pretty easily. This is also on pump gas which is important to note. The difference in peak power is not quite so extreme now, amounting to 35 whp on pump gas. The stock motor would also see gains on race gas and with the cats completely eliminated. Does not seem like quite a large difference now, does it?
So what is great about the stroker motor? Well, it is the way BMW would do it if they had to. As the M3 GTS shows, BMW does not have an issue with raising the stroke to increase torque through the curve. There is also no weight added to the front. The DCT can handle stroker motors easily as the torque is well within the limits. This goes for other parts in the driveline as well.
The throttle response is also superior to other options namely supercharging. As an owner of a supercharged M3, I can accurately state that the response down low is not the same as the car has in NA form. In the lower rev ranges (2k-3k) it just does not respond as quickly and it feels as if it does not have as much torque in this range. The stroker clearly has an advantage namely over the supercharged and the stock cars in this area. Increasing the stroke adds torque in the very low rev ranges and really adds to the overall driving pleasure of the car. One will find themselves downshifting less often and noticing that nice push when putting the foot down while in 6th gear on the highway. It is most noticeable in those situations where the car is not being wound out to 8k.
That brings me to this important point which is when going for max performance the car will be wound out to redline. The stroker does not show much of an advantage here. On paper it clearly has higher peak figures but the powerband also has been shifted a bit to the left. The stroker makes peak power earlier.
Is this bad? Well, the S65 is designed to make peak power toward redline as the torque curve is flat which is what allows the rev multiplication to have the impact on power that it does. By shifting the power band to the left the torque curve falls off faster not allowing the stroker to show full potential with the stock cam profile that was not designed for it. Neither Dinan or RDsport address the cams.
Additionally, the stroker motor has shown a best 60-130 time of 11.05. We have seen bolt on DCT cars go 10.71. Now, not everyone buying a stroker necessarily wants to be the fastest but I think one would at least expect to be ahead of stock motor cars decisively for over 25,000 dollars. The stroker has a much fatter torque curve but in straight line acceleration it is uncomfortably close to stock motor cars, especially for the money.
The amount of money RDsport and Dinan charge is probably the largest downside of all. Both of these companies have V10 stroker motors. Well, the majority of parts have already been sourced in that case. One is paying for name brands here and I will commend Dinan for dropping their prices rather significantly as of late. If one buys a stroker motor, it will be a pain to sell later as it can not just be bolted off like a supercharger. It is a permanent modification and one that will not carry over into resale.
Hey, I have a great idea, what if I supercharged a Dinan or RDsport stroker and have the best of both worlds?
Nope, you won't have the best of either world and it is a terrible idea. Why? For one, what holds back the stock motor from big power with boost is the same thing that will hold back the stroker motors and that is the compression ratio. The stroker motor will not have a large advantage over a stock motor anywhere except down low as the amount of boost will really be the deciding factor. A stroker motor on 6 psi vs. a stock motor on 7 psi will result in basically a wash if not a slight edge to the stock motor. That can be achieved without even having to go into the motor. We have seen a boosted stroker lag behind stock motors thus far. For the amount spent, there is no justification in performance as the compression ratio is the limiting factor. The stroker can only run so much boost just like the stock motor before detonating on 12.0:1.
If one simply has to supercharge a stroker I recommend doing it with the RDsport stroker. Why? Thicker cylinder walls. Dinan bores their motor to 94mm which takes the cylinder walls to a dangerously thin level of approximately 3.25mm. The maximum acceptable range before failures in motors is 3.15mm. Dinan really is pushing it to the max and not staying conservative. They also are not giving themselves any room for the future.
RDsport by contast uses a 93mm bore and the cylinder wall thickness is about a mm more. It is safer to supercharge the RDsport stroker vs. the Dinan. Here is what the block looks like at 93mm bore:
The stock motor actually would have the most material vs. the strokers and therefore the strongest block. If supercharging, it is pointless to build the motor and not drop the compression, just a waste of money.
What about doing a custom stroker?
This is a great idea. Not to mention, you can do more for less money than the Dinan or RDsport strokers. There honestly is nothing magical about Dinan or RDsport. They source parts from Mahle and other big names just like anyone else can. They are just names. Their products are great, but you are overpaying to a degree.
There are many good shops that can do this work for you if you desire. The majority of the cost will come from the labor and tuning keep in mind, not the hardware itself. If going the custom route one could even do things that Dinan and RDsport do not offer such as sleeving the block and combining the Dinan Bore with the RDsport stroke for a 4.7 liter motor. Maybe even push the stroke a bit further and do something like a 4.8 liter that no one else would have. There are all kinds of options available when not limiting oneself to a setup that is set with a certain bore and stroke on the stock block.
Just talking to local companies the Dinan and RDsport prices can be beat. If one is truly interested in a stroker ask some of the top tier BMW shops. Do not just immediately feel you have to be locked in with these two options (which are solid.) I am extremely surprised no one has tried a set of aggressive Schrick S65 cams (such as the 292 degree cams available) with the strokers to shift the powerband to the right. The torque down low will not be missed and the gains in the upper rev range should move peak power to the right. With a custom setup, there are many more possibilities for the same if not less amount of money. The issue is making sure one is dealing with a capable/reputable shop.
The Dinan and RDsport stroker motors are fantastic options available to us. The BMW aftermarket is a better place thanks to them. It is great to have so many options available to us and for enthusiasts to be able to build their dream M3 in any fashion they desire. The quality of parts used by both are top notch and sourced from the best manufacturers available. Personal preference and cost really should be the deciding factors. I have followed RDsport builds closely and the hardware is nothing but the best. Dinan's extra mm of bore probably gives a slight edge up top as increasing bore usually does vs. stroke which usually gives torque gains down low. However, without dynos or real world results that is just speculation.
Which would I choose? My choice is the blue line here overlaid on the stroker: