Which is better, X51 engine with +25 hp or normal engine? (yes, really)

Surely the more powerful X51 engine that was standard in the 40th Anniversary Edition, optional otherwise? So I thought, but after some test drives, I'm not so sure. I've driven three X51 cars on four occasions. The first time I didn't think it was anything special and wondered if something wasn't wrong with the engine. The second and 3rd times, the car had the sport exhaust and I thought it was fine. The fourth also had a sport exhaust, but by then I owned a normal 996.2 which I knew pretty well and immediately stepped back into. Here's what I thought I noticed: First, I floored the X51 in 6th gear at 4000 rpm, and it seemed to me nothing much happened. So I tried a few more times, with the same impression. With my car, there is an instant response that reminds me I am driving a fast car. I would expect the X51 cars to be faster in this situation, but I don't feel it. Second, and probably related to the above, sound: I love the sound my engine makes (and it doesn't even have a sport exhaust). Any change in engine speed, and any increase or decrease in throttle makes a noticeable change in sound, and every sound it makes is good. It's like having 6 base players lying down behind the back seat. Whereas I felt that the X51 engine, if I exagerate greatly, is simply either on or off. And even when on, I'm not sure it sounds as good. Louder with the sport exhaust, but what's special about a flat 6 is not that it's loud ... . In short, I suspect the sound of the standard engine does much more to enrich the experience and augment the impression of speed. Could it be, though, that somehow having someone sitting beside me (an owner or sales person) is ruining the vibe, so to speak? That and maybe my test drives were too short? Although I haven't encountered a thread on this topic, I think I have read only good things about the X51 engine in other threads, including the sound. So is it just me? Very curious what everyone thinks!

The answer depends on the weight of your passenger

At a point in everything you reach the point diminishing return. Quantitive Easing, ice cream, horsepower. What is the usable performance difference between a 996 and a 991? Usable. Is it actually worth the money, or the increase in physical size of the care? That is an individual judgement but if we used the percentage of improvement and applied it to other items, would we be willing to pay the difference? Cars in competition are ranked by class...so a 996 wouldn't be in competition with a 991, it would be racing other 996s, 997s (depending on how prepared could be moved up or down. Many people equate loud with cool or performance. Same with big wheels and hard suspension settings. A car that will adapt to every section of a track will be fast and efficient, but for it it wouldn't be very fun to drive. I think a straight stock 996, when driving on public roads, is more than enough car to keep anyone (sane) entertained.

I think the power to weight ratio of my 99 is really, really nice. Okay, the fister exhaust makes it 4x better!

I agree with everything I'm able to agree with (don't know about the Fister exhaust - haven't heard it). Which means if it's between a great sound and 25 more hp, I'd go for the sound. But am I right about the sound? Based on other discussions here (where the sound is only a side topic), I wouldn't think so. And I can imagine that having never test driven an X51 car alone, for a longer period, I have missed some of its charm. (Alas, if I had the chance to find out by myself, I would already have done so.)

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Pretty funny - and right on, John Ireland! I just got my first 996 a couple months ago. It was optioned with a few goodies, including the X51 - which is what I was looking for. I figured as long as I was going to buy a 12 year old car, I might as well aim for one that had some cool options on it since they often don't appreciably affect the price at that age. My first P car is my 1988 911. Still have it, and love it. Bought it to do track days, and did quite a few of them in it before putting her out to pasture, as it were - too nice an example to risk putting into the wall. Do I notice a significant difference between these two cars in terms of power and sound? Heck yeah - 130 extra horsepower and liquid cooling will do that, without a doubt. I bring this up only to highlight what JohnI said. My 996 was the first and still only 996 I've ever driven. If there is a difference between it and a base 3.6 or 3.4 version in terms of sound, I've gotta believe it has more to do with the exhaust options (mine has the on/off noise switch - which I also really like) than any inherent characteristics of the engine and the X51 kit. But what do I know? As I said, this is the only one I've ever driven (or ridden in, I think). Sorry I can't give you anything else on your question. If you're in NorCal maybe we can swap cars some time and do our own little experiment on it. In the mean time, I'd recommend getting out there and enjoying these amazing cars.

The X51 is not a special engine. Its s standard M96.03 bottom end and crankcase with the X51 power kit fitted. Some were from the factory like this (Anniversary edition) and others bought the power kits and fitted them to an existing engine.. The power kit replaces heads, cams and intake manifolds with altered items. According to when the kit was bought, it may include altered scavenge pumps as well.

not many sane people would click that option on the configurator I think it was around $13k back in 2004 and might be close to $18 now, if not higher when I was looking for a car, I wanted the x51 simply for the extra ponies really didn't mind the small premium associated with this Porsche had spent the R&D money but I don't think they sold very many so they threw into the Anniversary model (pretty good call) the low end torque might be classified as disappointing, but the engine pulls very smoothly and pretty hard from just under 3500rpm right to 7 while I plan on keeping mine for at least another 20-30k miles (just under 29k now) 991's are 400hp base but I think I'd like to be around 450hp for the next one

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Thanks Jake. I'm sure you've driven many of these cars. What about the sound?

And how about what seemed like disappointing response at 4000 rpm? I know that if a car has Variocam issues, it may be weak above ~4000, but this car was not weak. If I started at 3000 and floored it, it pulled well all the way to 7000. But if I was cruising at 4000 in 6th and floored it, it seemed nothing much happened right away. If it had lot's of power, but a slow throttle response, that would be consistent with what I noticed. (So would low torque at 4000, of course.)

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YEP!!! You just explained the phenomenon we describe as X51 syndrome! The cylinders flow a lot more than a standard head along with the intake, but with a huge compromise in port velocity and swirl. These two things lead to throttle response. A head porter always has to fight to keep these two aspects when increasing gross port flow. When we developed the R51 (which took 3 years) these two things were primary concerns. Ask Ltusler about R51 Vs X51 throttle response as he has had both on the track.

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OK, that is definitely interesting. So the X51 has slower throttle response than the regular engine around 4000 rpm, or at all rpm, or � ? All X51s, or just some of them? And is there a cure? (I mean, short of an R51. If my engine goes, I imagine I might be in the market for an R51, but I have an IMS Guardian, so that won�t happen, right?) I saw in another thread that it might be a good idea to have the lastest version of the X51 software installed. Might that do it? Or � ?

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No Porsche should be driven like this IMO. You will get more than enough top speed on public roads in 4th. You're not driving a Corvette ZR1.

Yep. An increase in port velocity, toss away that huge intake port with long runners and the high bifurcation point. The trade off for the high RPM power (which makes no sense to me because it makes all it's power right at redline, where the hydraulic lifters are losing valve control) is a loss of throttle response and low end/ midrange torque. Engines are all about reducing compromises and gaining the most efficiency where it's needed most. Lots of things going into making something perform good on paper, thats what sells things; not how well they operate. Why? Because people don't understand the equation behind HP.

Right so Porsche don't know what they are doing then.

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Nope, they knew exactly what to do to sell cars. They also knew exactly how to make a 25HP gain at peak. They knew that few people would ever have the comparisons to see what they lost to gain that.

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Is the problem 6th at 4000 on a public road? Not on a German Autobahn, as was the case. I hope the problem isn't full throttle in 6th or at 4000, because I do it all the time when I want to (re)gain a bit of speed while travelling. I'd be very surprised if this stresses the car in any way, but if it does, please let us know!

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The idea that the X51 engine is inferior to the regular engine in any way strikes me as very surprising, particularly given that so many good things have been said about X51 engines and 40th AE cars here. Very surprising, but I nonetheless suspect, based on my experience, that it's true. As mentioned, I've test driven several cars with this engine. And having heard that when test driving it's a good idea to make sure the engine pulls hard above 4000 to detect variocam issues, I've done this with at least one of the others, probably both. And I remember that with at least one of the others, I had to start at lower revs and let the power build from there to convince myself there weren't variocam issues. So that happened with 2 of the 3 cars. (And as mentioned above, I also at least remember being unimpressed with the other.)

The M96 3.4 makes what, a whopping 160-170 HP @ 4K? WOT @ 4K RPM is definitely not the most fun way to turn gasoline into speed. The X51 concept has always kind of irritated me...it feels like paying a tax (or being extorted) to get a car that hasn't been detuned to make less power. "Oh, you'd like a car that goes as fast as it should? That's going to cost extra." I miss the days of manufacturers deliberately under stating HP numbers to sneak around emissions requirements. "Horsepower? Um, let's go with...250. Sure, that sounds low enough."

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Is there less low/midrange torque? Some dyno results have been posted in another thread showing more torque at every rpm shown. And while one started a bit above 4000 as I recall, the other started much lower. Also, is the lack of throttle response just around 4000, or is it everywhere, or ... ?

But is it really all so bad? X51 fans, Anniversary Edition owners, tell us it's not so! Please tell us you only get slow throttle response and a disappointing soundtrack if the oil wasn't changed recently, or that reflashing the computer with the latest X51 software solves everything , or ...

Bigger is not better so many times... This is especially true with street driven performance engines. What I just said will be surprising to anyone who hasn't driven the same car with different engines multiple times. In the Aircooled days I had one vehicle that had 80 different engine combinations applied and I drove them all. The best performers were the medium displacement engines with good port velocity and chamber filling. The high revvers made more power on the dyno but that trait didn't follow them to the street. To first understand the differences you must have a huge amount of experience driving different cars with different engines. I tested 105 cars last year and I'll sometimes test 3-4 cars a day on the same 3 mountain roads. These are not short trips around the block. What you learn from those experiences and data is amazing. The X51 is a potent arrangement for a factory option and it does what it was developed to do very well. I don't appreciate what that goal was nearly as much as the next guy. Sorry :-)

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Yes, it's you. I'm going to have to chime in here. I have driven the normal vs. X51 on the track. At 3750ish+ rpms, I can't even say there is a comparison here as the X51 "at 25 hp" was deemed under-rated AND it felt very much so since it ws so much faster in my testing. I've driven the normal 996 at Watkins Glen and my X51 C4S back to back to back and there is no way that they are even close. Add to the fact that my C4S was heavier than the RWD 996 is an additional testament. This is very much evident when I was going uphill at the "Esses" at Watkins Glen and I'm pulling through the revs all the way to the back straight (assuming you're familiar with the track). The X51 pulls much harder. Caveat was that this was at the track. But hey, if you couldn't feel the difference, then save your money and get the regular one. But beware of people telling you that one is faster than the other. Decide for yourself if it's worth it. I never trust anyone who is trying to sell me something; directly or indirectly.

Yeah, X51 engines suck. I think I'll put that someplace everyone can read it. Maybe, on my rear bumper?

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I believe that - it's consistent with my experience. I have no doubt the X51 is more powerful and delivers more straight line speed. Though because of the sound and because some of the extra push comes at higher revs, I'd guess this is a car that is faster than it feels - at least compared to the regular 996. What concerned me was a feeling of slow throttle response in certain situations. Situations that don't matter on track, but that can matter on the road. Also, on the road there is something to be said for a car that feels faster than it is, rather than the other way around. But now that more X51 owners have joined the conversation, what about the other part of my question: Am I the only one who (based on incomplete experiences) prefers the sound of the regular engine?

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Straight line, corner, any time my right foot wants to it IS FASTER go drive one and then a regular 996 and difference is irrefutable unless it's one dog of an X51 engine. Slow throttle? The X51 package specifically reprograms the throttle to be similar to those aftermarket boxes. ALL n/a 996 sound like crap stock send mufflers to FisterD for mod and problem solved under $500. Then let me know which sounds better when the varicams kick in.

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As it relates to engine sound... I have an X51 with cat back muffler delete pipes installed... at the last 5 DE's I've attended, without exception, I was approached by 1 to 3 people who went out of their way to find me in the paddock and compliment me on the engine/exhaust note and to find out what I had... most of these were Air Cooled guys to boot! This independently reaffirmed how I already felt about the motor and exhaust sound... man I love it! Something about the intake resonance and power pulses out of the exhaust and the dual tone morphing nature as it climbs all the way to redline... its simply one of my favorite features of my car! I will say that the throttle response and Torque low in the rev range is somewhat anemic, but I have no regular M96 experience to compare it to... no doubt eliminating the mufflers and related back pressure likely hurts these attributes, but the howl to redline and related deceleration into turns is addictive! I try not to spend much time below 4k anyway so no big concern of mine. my $.02!

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This thread is way way dead, but I am reviving it. I not only have a factory X51 car, but one that is not a 40thAE. I have a narrow body 2003 C2, with X51 from the factory...but also PSE from the factory. Having owned now 7 Porsches, I can say that this is absolutely the best sounding one I have ever had. Many people comment on the sound of the car with the PSE open...and the sound from 4K-up when on the throttle is just super. I can also say, having owned regular 996 and also a 987.1 Boxster S, the difference between an X51 car and non-X51 is really surprising. In any gear and at any rpm (yes, even low rpm) the car pulls much harder than a standard 996. Above 4500rpm the difference is huge, it really is much quicker. Remember, this is not just a tune, this package included the following parts in the engine which were replaced, exchanged or modified to achieve 345hp:- new sand cast aluminum intake manifold with modified cross-section - new intake pipe supports adapted to the form of the intake manifold; - new exhaust manifolds with larger cross-section and optimized flow behavior; - new cylinder heads with optimized, CNC-milled inlet ducts; - new camshafts with larger valve stroke on the inlet side and modified inlet and outlet timing; - inlet valve springs adapted to the enlarged valve stroke; - modified partition box in the oil pan; - modified maps for the DME control module. The following steps are also taken on the vehicle: - new underside panel in the transmission area; - additional radiator in connection with modified front spoiler So just adding an exhaust, a crappy filter and a tune will not get you this effect. This was an integrated package, optimized for low and high rpm performance, utilizing both hardware and software to get the best out of everything. My car, a one of one, stickered for $96K in 2003. Sport seats, PSE, 030 chassis and other options make it so. It is the best driving car I have owned, and a lot of it is the engine. So, yes, the sound is great, but more than that, the whole package is amazing. With the 40AE appreciating rapidly, people are starting to see that a huge part of the appeal of that car it is the fact that it came standard with the X51 and 030. Short of a GT3, it was the car to have for NA performance. I think the cars with the X51 as a factory option (and with the third radiator) will be recognized for this also. Cheers.

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I understand that you appreciate the X51 package, but it's not based on reality. 25 hp increase on a 300+ engine isn't going to be felt as much of an increase. "Above 4500rpm the difference is huge, it really is much quicker.". No, it's not either huge or much quicker. Also, the AE40 is not appreciating because of the engine. Limited edition cars will usually appreciate in the long run compared to the regular lineup. Car collectors love rare cars, no matter what the specs are. Porsche did what they are in business to do. Sell cars and make money. The X51 option was part of that business model.

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Wasn't X51 built in parallel with the gt3 engine for possible race cars in case something went sideways with gt3 production? Or something like that? Regardless, love sound (PSE) and extra kick above 5k.😎

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X51 with an additional 25 hp? I'll stick with my "stock appearing" F6I which has much more than that and is useable over a much wider range. However, to each his/her own... Stay safe.

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2/21 can't come soon enough for me!

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I don't argue on the internet because that is for losers. However, I'm going to assume you haven't driven an X51 car. Unless you have driven a 40thAE the odds you have driven one are incredibly slim. Dyno plots show a +25hp increase, but also +23lb/ft of torque, which yes, you do feel. The torque increase is constant from 4500K on. It's a much stouter engine. It pulls way harder. Other verified dyno plots have the engine at 317whp, which means much more than 345hp at the crank. "Limited edition cars will usually appreciate in the long run compared to the regular lineup." Like the Millenium Edition? Or basically any Corvette "limited edition? Ha. True limited editions do not have 1,963 copies like the 40TH. The rise in price is because it came with the 030 and X51 standard, two options which make the car a better performer in all ways--better handling and faster. Any 911 with the powerkit commands a premium because there is no simple way to add 25hp to this engine, unless you call TPC. If the IPD Plenum actually works, it most likely adds 7hp, and people line up to pay for that and brag about it. So why the hate for the X51? I find it amusing.

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Yes this is why I would not pay any extra for the option, few Porsche's have adequate low rpm torque.

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wonder if the pandemic is going to change that?

To get the power out of an X51, you have to be willing to drive the car with a heavy foot. I love this about the engine. I've owned both a 996.2 regular engine and X51 Anniversary. It isn't even close how dynamically different these two driving experiences are. The X51 is the only reason I got back into a 996 and I'm so thankful that I gave it a chance. It's truly a great engine and the extra HP is VERY noticeable IF you drive the car with a heavy foot up to the top of the rev range. Which isn't a problem at all for me. As far as the exhaust sound... all stock 996's (non sport exhaust), have a really wimpy exhaust tone. But the larger X51 headers in combination with the throttle body/plenum really shine when you add a smooth intake elbow, x-pipe and sport mufflers. I've had the same experience as others who have had air-cooled guys track me down to say that my car sounds as close to an air-cooled as they've heard out of a water engine. In short, after being a bit disappointed in my first 996, I had all but given up on the generation until I gave the X51 a shot. I'm so glad I did because it delivered everything I was hoping for in the 911 experience. It's a little bit more of a raw experience, you have to be willing to push the car a little harder to get the extra power out of it but it's definitely there.

Just picked up a 40AE and I can attest to what's being reported by most owners or folks who've driven these back to back - the X51 motor pulls harder than my other 996 and it's very noticeable, but the personality of the two motors are different... the X51 likes to breathe and it surges in ways that the non-X51 car just cannot match... not even close. There's a reason most folks believe it's more than 25hp... it sure feels like more

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Not on my end fortunately and FSI is on tract.

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thought I read that^

"the X51 likes to breathe and it surges in ways that the non-X51 car just cannot match... not even close. There's a reason most folks believe it's more than 25hp... it sure feels like more" I love these fact based declarations...

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I fixed my quote for you, so yes, fact and experience based 'declarations'. What was your impression driving them back to back? If you're looking for dyno plots those exist already on this forum and bear out what I and others have actual first-hand driving experience of. If facts are your friend, so is search.

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Having owned a 2004 996 C4S X51 and access to a regular 2002 C4S, I can attest that my car pulled and sounded THAT MUCH better than the stock one at intown and highway speeds. The owner of the regular one and I drove them back to back to back to back. There was no contest. We ran them side by side running gears in 2nd, 3rd and 4th. Again, no contest. I wish we took videos but we didn't have any of those "fancy cellphone holders" during that time and we both were driving manuals. Because of this knowledge and that I am actually back in the market for another 911, I'm searching for a GTS this time. Unless I can find a Carrera S with the X51 and other options that I want. But no way I'm getting a "regular" 911 anymore.

Curious what number people are willing to pay for the extra 25? Is there a big premium on BaT?

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I doubt it. People with money can easily jump up to a turbo for not much more.

What would be the price difference if buying a used 996 3.6 engine or a used 996 3.6 X51 engine? If both had around the same mileage and were in similar shape? Thanks