Thumper Posted December 19, 2004 Share Posted December 19, 2004 I asked a similar question before but I did not get the type of responses that I wanted so I decided too reword my question and try again. Basically I have a 3.9 rear diff at my house. My 3.545 is making all kinds of noises. So I am wondering in a drag race will it be better to install another 3.545 or 3.9 rear end. My car should be running a 12.9 @105mph. My tires are 225/50 on the stock turbo rims. Thanks for you time. EDIT: I will also be installing a na 5-spd no matter what. Ted Quote Link to comment Share on other sites More sharing options...
tyson Posted December 19, 2004 Share Posted December 19, 2004 Just a FYI, I have one in my auto(soon to be 5spd) 81 ZXT and I absolutely hate it...Mainly because of the very low topend that the differential gives me, I top out right at about 100mph (I go through the traps in the 1/4 at 97mph, and right on the edge of redline). I have heard from a few people that the 3.9 combined with the high first gear of the T5 makes first gear totally useless, the turbo does not have time to spool up before your gear runs out. I will be converting back to the 3.54 rear end when I do my 5 speed swap. Quote Link to comment Share on other sites More sharing options...
datsunlover Posted December 19, 2004 Share Posted December 19, 2004 Tyson; I would say the auto is QUITE difrent than a 5 gear is gona be.. it's only a 3 speed auto correct? If you're near redline at 100MPH with the auto, I'm sure the 5 gear will drop that a LOT and get you better top end speed.. heck, my stock 4 gear (witch sucks.. 2nd to 3rd is to big of a 'gap' and drops revs..) will get my car up to 120mph and still not quite touch redline.. Thumper; The simple answer is.. Yes. the 3.90 will make your car a bit quicker in 1/4 mile. (Compared to the 3.54, and assuming all other things being equal) Aso.. IMHO, being near or at the red line isn't really a big deal, seeing as it happens as you FINISH the drag and go through the traps. Obviously, you would not want to sustain that RPM for a long time.. IE; highway cruising/road course racing) but if you're wanting a better 1/4 time, put in that 3.90 in. Quote Link to comment Share on other sites More sharing options...
Forrest Posted December 19, 2004 Share Posted December 19, 2004 It depends on your torque curve. Use the quarter mile spreadsheet posted earlier, that will tell you exactly what it will do. If you're already trapping 105, my guess would be that it would hurt your quarter mile times. Quote Link to comment Share on other sites More sharing options...
80LS1T Posted December 19, 2004 Share Posted December 19, 2004 I am not a turbo guy but I know I have read on hear that the 3.54 is better for the turbo because it puts more of a load on the turbo so it can spool up easier? For a NA car yes the 3.9 will make it a little faster but Im not too sure about this in a turbo application? Guy Quote Link to comment Share on other sites More sharing options...
Thumper Posted December 19, 2004 Author Share Posted December 19, 2004 I forgot to mention I will be installing a na 5-spd in this car at the same time no matter what. Right now my trap rpm's should be a 5246. If I install a 3.9 with my na 5-spd then I will be at 5771. So I'm still in the meat of my power band with another 500 rpms to go. I believe that I have read that scottieGNZ is a believer of the lower gear ratio the faster you will go. Others I've talked to say higher is better. So what do you guys think? TIA Quote Link to comment Share on other sites More sharing options...
Mike C Posted December 19, 2004 Share Posted December 19, 2004 Engine load helps to build boost in a turbo car. A higher numeric number reduces the load on the engine, making it spin easier. This can reduce boost. But it also multiplies torque. Without trying itin your car, it is hard to say exactly what will happen. When Nissan went to the T5 in the turbo cars they kept the 3.54. When they went to the NA 5 speed (late) they went to the 3.90. Bottom line is overall first gear ratio is about the same. So once again, "your mileage may vary" is the rule of thumb with this mod. Quote Link to comment Share on other sites More sharing options...
Drax240z Posted December 20, 2004 Share Posted December 20, 2004 I am certain you will be slower with your list of mods. I am sure at this point traction is an issue. Getting more torque to the wheels won't really help you if traction is allready an issue, and that is all you are doing with the change to a 3.9:1. At higher speeds where you have traction I think you'll find the load from the 3.54:1 over the 3.9: gearset will have you pulling more boost, sooner. ie: you'll be faster with the 3.54. For all the trouble it is to change, I'd say give it a try and prove us right or wrong. I know someone in the past did the exact same thing and posted their results here, and they were quite a bit slower with the 3.9:1 ratio. Quote Link to comment Share on other sites More sharing options...
GrayZee Posted December 20, 2004 Share Posted December 20, 2004 The more power your engine has the lower number gear ratio you want. I'm running a 3.54 with a 83 Nissan 5spd and so far my best quarter mile et was 94mph in 3rd gear near redline. You really wanna avoid making that extra shift into 4th gear if at all possible. (5th gear in your case) I've been in Z cars with similar power but with the 3.90 and it feels like they have to shift gears way sooner and therefore waste more time shifting gears. Drax is right, a 3.90 would likely just increase traction problems. Quote Link to comment Share on other sites More sharing options...
Thumper Posted December 20, 2004 Author Share Posted December 20, 2004 Thanks for the replys I have decided to take your guys advice and run the 3.545. I will still be installing the na 5-sp because I like the gear spacing in the na 5-sd over the t5. I will also be installing my 370cc injectors and turning up the boost alittle bit more:twisted: so I'll let you know how I like the combo next time I take it to the track. Thanks for the advice. Ted Quote Link to comment Share on other sites More sharing options...
GrayZee Posted December 20, 2004 Share Posted December 20, 2004 What kind of downpipe are you running? With the Nissan tranny you may be able to upgrade that now too. You can SQUEEZE in a 3 incher! Quote Link to comment Share on other sites More sharing options...
Drax240z Posted December 20, 2004 Share Posted December 20, 2004 Honestly I can't see a lot of gains from a 3" downpipe over a 2.5" downpipe with the stock turbo. The outlet of the stock turbo is ~2.125" from what I recall. Ideally you'd leave the turbo outlet at 2.125", flare at less than a 10 degree angle up to 3" or so, and then run 3" all the way back. Quote Link to comment Share on other sites More sharing options...
Thumper Posted December 20, 2004 Author Share Posted December 20, 2004 I am not ready to do a downpipe just yet. My plans are still to transplant this engine into an earlier z. Although it prob won't be with the 240z I have now. So when I get it into the new z I will defiantely do it. But I agree with drax it doesn't help as much as it would if you had a t3/t4 with a 3" outlet. Although two 3" 60 degree bends are much better than two 2.25"-2.5" 90 degree bends. Quote Link to comment Share on other sites More sharing options...
Scottie-GNZ Posted December 21, 2004 Share Posted December 21, 2004 Honestly I can't see a lot of gains from a 3" downpipe over a 2.5" downpipe with the stock turbo. The outlet of the stock turbo is ~2.125" from what I recall. I am with you on this. Never really understaood the obsession with wanting a 3" DP on a stock turbo or even a t3/t04 with the stock wastegate. There is only so much HP you can make with that turbo. I am making 520 AT THE WHEELS and doing just fine with a 3" DP on a "P" trim exhaust wheel so why is 2.5" with 2 45* bends not enough for a 275-325hp L28ET with a 2.125" wastegate? Quote Link to comment Share on other sites More sharing options...
fl327 Posted December 21, 2004 Share Posted December 21, 2004 Overkill and Obsession are the reasons some of us Build these cars. Quote Link to comment Share on other sites More sharing options...
GrayZee Posted December 21, 2004 Share Posted December 21, 2004 Actually I think you guys (Drax and Scottie) are right about not getting any higher peak HP from the 3" DP but don't you think it would help the turbo spool up a tiny bit faster? I mean look at the improvement you get from going from the stock DP to the 2.5 dual 45degree bends, wouldn't a 3" be even better? Once the turbo is spooled up, as you say.. it will only flow so much anyway. I can definatly see it cutting back on the lag factor though. Also I forgot that Thumper is running a ZX so a DP with 45 degree bends wouldn't be possible. However if you are forced to use 90 degree bends then isn't that all the more reason to go 3 inch? Quote Link to comment Share on other sites More sharing options...
Drax240z Posted December 22, 2004 Share Posted December 22, 2004 I bet a properly designed diffuser from 2.125" up to 3" will make more power than just a straight 3" system. How's that for an answer? The 3" system vs.vs the 2.5" system... well I'd say the 3" will make SLIGHTLY more power, but packaging will be an issue. I was able to do 2 45's with my 2.5 system, no way I could have done that with a 3". Quote Link to comment Share on other sites More sharing options...
GrayZee Posted December 22, 2004 Share Posted December 22, 2004 Did you not see the "diffuser" in the pic of my DP I posted a while back? Mine is 2.5 to 3" and it also uses two 45 degree bends like yours. Although I will admit that it is VERY tight! http://members.shaw.ca/icecube/DPclose.jpg I'm sure you are aware of the huge difference of the surface area of a 2.5" circle VS a 3" circle. All other things being equal (degree of the pipe bends, ect.) There would be alot less back pressure and turbulence on the 3" system. Otherwise why is it that people use a 2.5" DP and then flare out to a 3" exhaust system if there were no gains to the larger pipe? Quote Link to comment Share on other sites More sharing options...
Drax240z Posted December 22, 2004 Share Posted December 22, 2004 Ah, you see I like that downpipe a LOT. (not sure why the flange is made of 2 - 1/8" thick pieces and welded together though?) It's not just about volume of the pipe though. Any kind of step from the 2.125" to a 2.5" or 3" section will cause turbulance when the flow seperates. (ie: if there is an abrupt change) This could concievably cause slowing of the turbine wheel depending on where this step took place. At a minimum it will reduce flow, and the bigger the step, the more flow will be reduced. This is what I am not entirely convinced a 3" will be worthwhile over a 2.5" system if there is no diffuser design incorporated. Quote Link to comment Share on other sites More sharing options...
heavy85 Posted December 22, 2004 Share Posted December 22, 2004 I think this has been hi-jacked by 2.5" vs 3" debate so to continue that .... let me start by saying I have no first hand experience with turbo Z's but from what I've seen from CFD (computer analysis that models fluid flow) of air systems in general, the MAJORITY of pressure drop occurs at the bends so the larger the bend raduis and lower bend angle the better. From a whole system perspective little of the pressure drop occurs in the straight sections unless it's virtually all straight or the straight is relatively very long. Angle was mentioned above but not bend radius which might even be more important than the actual angle. I'll guess that a swooping 2.5" section would be better than a tight 3" section but you would probably still want the 3" from then on because of the relatively long straight section to the back. Main point I guess is to consider bend radius not just angle and minimize the bends rather than maximize size. Again this is all theoretical so take it for what it's worth as it applies to 1/4 mile times. Cameron Quote Link to comment Share on other sites More sharing options...
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