I Posted this on other forum to Greg from Specialty Z ----------------------------------- Hey Greg What is your opinion on the 3" split dumps VS 3" bellmouth dumps. wouldn't the bigger area in the bellmouth be better ? Or is the split design really better for spool? (or is it just another hype) -------------- Good question........ When the Z32 was very young, Primespeed and Random Tech both made open 3 inch downpipes for the Z. Due to the 3 inch pipe being larger than the turbo opening, these companies installed a dowel inside the pipe to be able to install one of the bolts the pipe was covering. When I installed a 300zx twin turbo engine in my 280Z back in 1994, I made 2.5" inch open downpipes for my car. I found that my 280TT made a lot more power on a dyno, with a lot wider power band then any Z32 with the same mods that had the standard 3 inch downpipes that were being sold. I tested the change from the 3 inch open downpipes and full 3 inch system, to 2.5" open down pipes and a full 2.5" system on a Z32. I found clear large gains in low end power, and found similar top end power going down in size. This led me to believe that 3 inch was not needed. In the early 2000's I was over at JWT and I noticed a turbo that had a divider built into the housing. I asked about the divider and was told that Garrett found a 15% gain in turbo efficiency by dividing the wastegate air. It was explained to me that the turbulence that is caused from the wastegate opening dramatically hurts the flow. This was the first time I had ever even heard of a divider. The light bulb in my head when on that day, I thought it would be great to try making a split downpipe with the divider on the downpipe flange. This way we could have a divider on a older turbo design. I talked to the owner of the shop that was manufacturing open downpipes I was having made and selling at that time, The owner told me I was crazy and that making a downpipe that way was too much work. He also said I was crazy because the open downpipes were selling just fine, so why would I want to make something harder that would have to cost more. I knew at that point that he was in doing the work for a different reason than I was. So.....I met with a old friend from high school, we went to our friends machine shop and set out to build the first set of split Z32 downpipes..... I tested the split design and found huge gains....... I started selling those downpipes and my friend was building them for me at his machine shop. Not too longer after I started selling split downpipes (about 2 months) the manufacture of my open down pipes started listing split downpipes for sale on TT.NET :-( Here was the guy that did not want to make the pipes and 2 months after I start selling them he has knocked off my idea. I made piece for awhile, but when I wanted to make 3 inch expansion parts, I got another ear full of how crazy i was. Sorry for the rant! At the time I made 2.5" split downpipes, I did not think going to 3 inch was needed based on open downpipe testing. As larger and larger turbos became available, I started wondering if and when the 2.5" might be a restriction. I tested a few different ways until we ended up with the original idea in my head. The testing showed crazy gains that I had a hard time believing, I tested a few different set ups including stock turbos and all tests showed the split 3 inch expansion was a huge gain over 2.5" splits or open 2.5", and we already had confirmed that both of the 2.5" pipes offer gains over the open 3 inch. The gain for 3 inch expansions over 2.5" splits was even greater than the gain from stock to 2.5" splits. So from testing I have done over the years................ The turbulence with a open 3 inch pipe and no divider causes more backpressure than a smaller 2.5"pipe, adding a divider offers less turbulence and increases flow. A 3 inch expansion divided pipe offers the greatest flow due to the least resistance, least turbulence and greatest pressure differential. .
Good read. Currently I have 2.5" split dumps into 2.5" test pipes into a 3" system (cat back). I plan on changing to a full 3" system once money permits
i wont be changing my 3inch open dumps ever, i can tell you that. I just find too many contradicting opinions about dump pipes on these cars. If i had the money i would back to back test AMS split dumps vs mine on my own car. but i dont so ill be staying with mine. If you;re really serious , external gate wins anyway
2.5 vs 3inch dumps MORE INFO Most of the gains from increasing the exhaust size, have been seen in the low end and mid range. The 2.5" downpipe has 6 PSI backpressure with just 15 PSI boost pressure. The 3" expansion downpipe has Zero to 1/2 PSI backpressure at 24 PSI boost with 700 turbos.
ive been toying with split dumps and bellmouths for a while but this is my theory only... the problem with split dumps is not the design. its the actual divider i.e the small block.... the design is the best for flow if fitted the way its intended to be. its purpose is to seperate the turbulance between the chambers and by the time rejoined the gasses by theory are to flow into and mix as one with the least turbulance acheivable.... usually what happens is when split dumps are fitted with the divider, they are too small or do not completely 100% seal the wastegate area from the exhaust section... these small gaps etc and not only disturbing the flow but also making the turbulance go crazy... you are still getting a mixture of exhaust gasses and wastegate gasses mixing which defeits the purpose of what a split dump is... so in saying all that if you can completly 100% seperate the wategate area from the exhaust side with absolutli no mixture before the pipes are remerged then you have succesfully used a split dump to its full purpose. this will give you the best flow and performance etc.... this is why turbos that have a externally seperate wastegate are the best setups.... in saying this though bellmouths have been proven to show great results especially with rob260, uas etc.. bet you any money any of you guys out there that have split dumps will most likely have some leakage between the chambers which defeits the purpose of what a split dump is made for... thus i have heard kingyz made a custom divider which was bigger and said it sealed alot better on his dumps, but still does leak minimally.... also i think mrzedx did something like this too but i dont think he has tested his out yet... way to test it out? cut the wastegate pipe. if you hear exhaust gasses leaking out that pipe at idle etc then its not 100% completly sealed... because of the disturbance of the gasses with these little gaps etc it doesnt flow like their intended for so this can have an effect with pushing bigger power i think as seen in kabir's zed... so in some setups trying to push bigger power bellmouths have been proven to give better results, obviously because the flow is majorly disturbed with these shitty dividers in split dumps.... sizes of pipes will depend on power etc intended to push, bigger isnt always better depending on the setup you are going for and power intended to pushh...
Well I have 1 3/4" split down pipes running in to a 2.5" twin system and I make peak torque and power at 5250 RPM I know I can make more power by opening up the pipes etc but I like the way it drives, bigger pipes will effect eng torque prior to boost & may cause spool that is so rapid the car will be too hard to drive :br::br: 350Kw is enough for a road car anyway
Top thread Rob... I was going to start a thread about this on Tuesday and then though on the way home this arvo... better jump on and start a thread. ...already done
IMHO, i think that possibly if you tried a decent set of 3" expansion splits like the AMS ones i'm now running you may find less lag and full boost at slightly lower revs Niall. (and yes it's always easier said than done, as you mentioned ) As you know i used to have the NPower (tuneagent clones) on mine while still hitting 430hp, and my back to back "seat of the pants" comparison is probably similar top end power feel - but noticably less lag! ...might be another thing to consider as an aiding advantage of the larger 3" split design as stated in the OP...
Now there are two things debated here ! (without people realising) 1/making more top end power 2/spooling quicker due to less turbulance Now there isnt a huge difference in power between bell and split. Now it was tested on GTR skyline that it made 10rwkw more with bells in comparison to splits at the 450rwkw level. but nothing noted on spool differences...(want to see results on a vg30dett before I find it relevant) If the the splits help spool the turbos sooner due to less turbulance well that means torque/power will come on sooner and give you a better powerband . I would rather this Remember THE BOYS COMPARE PEAK HORSEPOWER THE MEN COMPARE POWERCURVES
this is important too. My last engine had CES dumps which I checked and actually had to file the divider down as it was a tight fit touching turbine housing ..so sealed pretty good. Also To all the AMS split dump guys who have installed with GT28RS without checking the TURBINE opening on their dumps.... The opening is only 46mm ..the turbine housing is 48mm (to cater for 47mm wheel).. This should be match ported and divider seal checked before installing .
*Happily waves his new time sheet in the air* I can compare now too! I've decided to go with the bell mouth design come the big rebuild in a few months time. I'll be shooting for a minimum of 350RWKW and I think most of you know by now I'm pretty anal about noting down every single occurrence along the way. Would be nice if someone doing a similar build but going the split pipe design would record as meticulously as I am going to. Clearly that should help stop this guessing game. MoulaZX MoulaZX
theres still too much variation. There are 4 people i know of in QLD with the same turbos as me. over 100hp difference in power levels on boost levels that arent that different. All motors are different and dynos are very different. If we could get min and Benny c's on the same dyno at the same boost level it would be a good indication , but my motos is running different compression and a bit of port work. Still, might be a good comparison for split vs bellmouth dumps
this is where all the bullshit starts ! 2 different cars/engines comparing a split and a bellmouth ?... back to back testing on same car
If SZ are so confident about their split dumps over my bellmouth's, they can send a set over and ill fit them, test back to back no worries
