Thom the only thing with the flow is that you say its 10% bigger but you have maybe 50% more power so it might not be optimum. My calcs are based on the size you need to flow enough air to make 400hp without restriction. The 1/3 rule is universal, of course a thinner core will not restrict flow as much as a thicker one but the rule applies, especially as you now have it in front of the radiator .....ORIGINAL: TTM Thanks for that Nick, I'm replying now as I saw yours posts on RL in the 968 supercharger thread. I checked the SFR IC cross section before ordering, it has about 10% more than the standard IC so this should be good. With regards to spool I have disabled the boost controller for now, meaning the turbo is working only against the wastegate spring at the moment, as I have found pointless to put unnecessary boost load on the engine at low revs when it's not in the rev range where it naturally makes most of its torque (which seems to be a bit around 4000rpm, and the turbo seems to blow most of it lungs out at about 4200rpm). I was advised that building boost "too soon" would greatly increase intake temp, not good, but I will have lots of extra spool left in the boost solenoid if I need to when I install the SFR IC. Interesting comment on the IC only needing 1/3 of its frontal area for its air intake duct. I guess that's why the original IC works so well, and I was sort of concerned how to bring fresh air to the whole frontal area of the SFR IC which will be "hidden" behind the front bumper. Now If I only need to duct 1/3 of its frontal area somewhere through the bumper then it should perform well, if I understand you correctly? Is the 1/3 rule universal? I have the feeling it might vary with the thickenss of the core? If so, and as the SFR core is less thick than the original unit, perhaps I will need more ducting than 1/3 of the frontal area? Did 600 miles last week with the car, the engine is feeling freeer the more I drive it. It's getting worrying at 120+mph though as the rear end squats so much when I floor the throttle that my new set of rear S02 feel on the limit of traction. One great thing though about the 3.0 is that it's sooooooooooo much more flexible that the 2.5. Even when boosting it at only 10psi it takes off so effortlessly on a subtle but vigorous wave of torque that I hardly ever feel the need to floor it at those speeds. I LOVE it so hard it's almost embarassing.[] Sly has done 500miles since the rebuild, engine feels more responsive at low revs and less exhilirating than before, certainly caused by having ditched the airbox - less restriction, more linear torque delivery.
Engine build progress
- Thread starter TTM
- Start date
[/quote] Interesting point about ducting Nick, what is your opinion on adding an additional duct in the badge panel for the standard intercooler? [/quote] If its to help exit air thats a good thing, just look at the size of the exit air intercooler scoop on the bonnet in a 968 turbo RS. It has to be in the right place though or it wont be as effective.
In theory if your air temp readings are right it is not possible for an air to air cooler to cool the air lower than the ambient temp.....? It might flow enough for 400hp but could still be causing a restriction....A big front mount IC that is visible prob doesnt have the right ducting...lots of these around on Jap cars! The bar and plate IC's from the factory are very good with the standard ducting but bigger front mount tube and fin IC's will flow more but need to be close to the rad to work well.ORIGINAL: DivineE I think Scott you'll be pleased to know that all of those things are being done at the moment - so we'll see. The LR intercooler uses the same core but different end tanks to flow better and even out the flow of air through the cores (or so they say
TTM
Well-known member
Thanks Nick, good food for thought again. To be honest, considering how the car performs and feels now I don't think 400bhp will ever be within reach at a reasonable boost level, I mean if running at less than 18psi, and with super unleaded. To improve the power signficantly I would need to replace the restrictive original exhaust with some 3" Fabspeed unit or equivalent and perhaps also fit a more agressive camshaft like a 944 NA or a Milledge, but budget doesn't allow it at the moment and the car doesn't really need to be any faster as I have a very nice and exploitable torque curve thanks to the excellent turbo. If I get to make proper ducting through the bumper the SFR IC will probably and hopefully perform just as well as the original unit but at least it shouldn't be affected as much by heat soak, so should bring further reliability to the current set up which feels already quite solid. The 968 turbo S/RS used the 951 intercooler with a max boost of 15psi (1bar). For what it's worth Tom M'Guinn in the US who has 3.0 engine similar to ours checked intake temps and noticed the original unit performed very well at 15psi, over which intake temps sharply increased. The intercooler still reduces much the temp out of the turbo but the intake temp is more than proportionnaly much higher than at 15psi. http://reutterwerk.com/forums/showthread.php?t=10574&highlight=temps
Thom, the reason the temps start to go up very quickly after a certain psi is because once the intercooler is pushed past the point at which it is at its optimum flow the turbo has to work harder and harder to get every extra psi in the intake manifold. So if the IC is at optimum flow at 15psi it might have to work to turbo twice as hard to get from 15psi to 18psi as it did to get from 12psi to 15psi if that makes sense. That is why temps rise so fast after a certain point. Therefor if you have a really fast spooling turbo you could prob give up some response to go to a larger more laggy intercooler setup to keep temps lower and you prob wouldnt notice much increase in lag. That is the reasoning to look at the power you want to make before sizing your intercooler, if the size of the cross section is restrictive you will always pushing your turbo harder than you need to. The biggest restrictions on your car are the exhaust + downpipe, followed by the head/ cam/ throttle body. But like you say your car is already fast enough, its all about making it more reliable. On that basis the exhaust and downpipe should also be considered to reduce backpressure.
TTM
Well-known member
Ok I see. Let's consider things the other way around now - considering its dimensions (24" x 8"x 3.5"), how much bhp do you think the SFR IC would allow at its peak efficiency, regardless of boost pressure? Can you calculate this? Yes ideally I would upgrade the exhaust but to be honest if (when?) I decide to go that way I would be very much tempted to go for a full Garrett turbo. One of the reasons I initially turned to SPS for a hybrid turbo was that I wanted to reuse the original exhaust, otherwise I'd have certainly installed the GT3037S the previous owner had purchased for what was to become my 3.0.
I took the most conservative calcuation at 300 hp and it tells me your core should be 9.5" high at 300hp, 11" high at 350hp. However this is based on your core also being the optimum length at 12 inches, but yours is 24. This means your experiencing more pressure drop than required from the added length. If you factor in the length it will be more like 15" high at 300hp and 17.6" high at 350hp. I also have an IC that is longer than its optimum length as a compromise to having the IC in front of the rad allowing all the air that flows through to hit the rad with no end tanks in the way. I also only have a 2.5 inch thick core so as not to slow down the air too much on its way to the rad. As with all these things there is always a compromise to be made and these calcs are only out of a text book and real world situations are of course much more varied and unpredictable. Having said that I built my IC based on these calcs and it works very well according to the Motec Data logs and live readings. However I have yet to test it in anger on the track as my car is once more work in progress as I fine tune the install before the next mapping session.
Someone once told me that it was a mistake to use an NA cam on the turbo as there would be too much overlap which is not good for the turbo engine. Not sure how true this is but maybe someone else can confirm? If funds allow its always better to get a purpose built cam in my opinion.
DivineE
New member
The original turbo cam is the n/a cam. Supposedly better suited to the turbo than it was the lux (hence why they revised it for the lux and used the original cam for the turbo, or so I read). The cam in Wuf has increased duration as well as lift, so assuming it still has a similar shape dyno curve when I re-dyno it at the end of the summer (i.e. producing most power at peak revs) that would surely suggest overlap is not causing a significant problem?
Overlap is not supposed to be good for turbo engines, but this is where my theory gets a bit sketchy as cam profiling is not my strong point. What I do know is good na cams normally run more overlap than turbo cams. The best person to explain this theory is Barry from Hartech as I cannot claim to know anything other than second hand knowledge on this one.....
sawood12
New member
Funnily enough I had a conversation about overlap today with one of our engineers. When he talks most of what he says goes way over my head but the general gist of what he said was, since overlap is a method of trying to get more charge gas into the cylinder and evacuating more exhaust gas you need more overlap for a 8v engine as there is less area of the open valve. In a 16v engine you need less overlap because with the two valve you have more area therefore more flow of the valve. The downside of overlap is that you lose some of the charge gas unburnt straight out of the exhaust port which is not good for fuel consumption, and in a turbocharged car it increases turbolag as you lose charge pressure as soon as the inlet valve opens as the charge gas is simply squirted out of the exhaust port and the turbo wont have enough capacity to maintain boost pressure. Therefore based on this my (potentially flawed) logic tells me that a turbocharged car would need less overlap than an N/A car as the charge gas is coming in under pressure with a much higher flow-rate - but if you have a higher capacity turbo with higher boost pressure then you have higher flowrate of charge-gas so can get away with the larger overlap of an N/A cam to get better exhaust gas expulsion at the expense of fuel economy with little effect to turbolag as the larger capacity turbo can maintain boost pressure?? The big upside is the less exhaust gas you have in your charge gas the more fuel/air mix you have and the better and more powerful the combustion process - more bang per power stroke.
George Elliott
New member
Very interesting thread for me, especially the recent pressure / temperature comments Nick outlined above. My own view is that beyond 1.2Bar, you really need to know what your doing or the pistons melt very quickly. I know very little about the subject of cams on any engine, and I hope we get some views. I have always been curious about how the cam profiles on a 3.6turbo of the 1993 era, or a 993 turbo compare to a 951. My view is Porsche used the cam profile - like the Snorkel, IC end tanks, Down-pipe and exhaust etc to curtail the 951 performance. If we want to see their true engineering capabilities, we need to compare a similar generation of engine where they were more inclined to do their best work. A 3.6T has 600cc cylinders, 2 valves per Cylinder, similar bore:stroke ratio, Motronic ECU. It does not have the benefit of water-cooling so the engine should if anything be more conservatively spec'd. I would love to know the lift, duration and timing of Inlet & exhaust valves. My daily driver 944 turbo X50 (SPS), has just been fitted with a free-flow IC (Lindsay Stage 1) and 3" Downpipe (Stainless TIG fabrication to OE mounting positions) and it really complements the upgraded turbo on this remarkable example of an engine. The exhaust is OE from the D/p back (although the wastegate entry has been flowed as it was really badly made - another example of Porsche managing the 951 performance into a mid-200bhp region as the 911 could only manage 300. Simon was consulted through-out as I did not want to spoil the engine. George 944T 964
TTM
Well-known member
Thanks for your input George. So you are saying that fitting a bigger downpipe while retaining the double-walled original exhaust line behind still has improved turbo performance? I always thought that upgrading both was required to see a significant difference... Would you mind sharing some more details about the downpipe? Most I see from aftermarket suppliers make without the mounting point to the block, and having one that still has the mounting point would be nice.
George Elliott
New member
Thom, am i saying there is a difference? Yes there is a difference for sure, and for me it is worthwhile, but I'm not saying it adds 100hp. It is a subjective difference - if you could find a rolling road that was accepted as accurate, and you ran a car which measured 300lb/ft and 300hp, with a standard D/P, and then fitted a 3" D/P to the same car, I would not be surprised if it read 330lbft and 320hp. For me, thats around 7% and it feels nicer too. It is part of the package which makes a 951 a nice car. Sharing? - I learned a lot from the forum over the years, so I am happy to share my hobby with like minded enthusiast but pls dont think I am saying my solution is brilliant or I have any special expertise. I will attempt to upload some images of the jig I made to produce the 3"D/P when the kids come back in. I don't have an image of the D/P as the digi camera which I used in the garage has quit. I plan to make a 3" Exhaust as phase two. George 944T 964
George Elliott
New member
Thom, file too large - sorry cant attach this today. George
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