| Well, after MANY hours/days of research, I have just about summed up the IC piping needs of our Z32s (on paper, of course)….thought I would share with the community. Disclaimer1: I am not an engineer and do not claim to be, just a guy who has done some research and choosing to share what I have learned…so draw your own conclusions. Disclaimer2: Again, this is on paper…who knows what dyno results would yield? I have discussed IC piping with a few experts on the phone, read many, many websites on the subject, and done some math based on my research and the specs of our cars. The bottom line is: Our stock IC piping is VERY much sufficient to support *most* of the higher horsepower Z’s in our community. If your Z is on top of the bell curve in horsepower (beyond 550 rwhp?), your cold-side IC piping and your stock throttle bodies may become a limiting factor. Keeping airspeed velocity in the IC pipes at an ‘acceptable’ high rate is the one of the keys to limiting pressure loss and lag. If the IC pipes are too small, the airspeed velocity will be too high. Additionally, the pipe’s ability to flow air will become a limiting factor. If the IC pipes are too big, airspeed velocity goes down, and lag increases due to the fact there is more space to fill from the turbo to TB. Bell Intercoolers told me they suggest keeping airspeed velocity in the IC pipes at a max of 350 fps. Bell Intercoolers also suggests keeping the airspeed velocity at the TB at a max of 300 fps. On to our stock Z piping – Stock piping I.D. is 1.85”. This is the case for 4 of the 5 IC pipes from the turbo to TB. If using a hard pipe kit in place of stock boost hoses, the final pipe prior to the TB is roughly 2.15” I.D (2.25” O.D.)…depends on your kit. Using my Z’s engine setup as a base (HKS2530s @ 24psi with stock IC piping), I used the following website to calculate my airspeed velocity in the stock IC pipes: [ http://www.toymods.org.au/throttle_body.html ] . This website coincides with the information I got from Bell Intercoolers, as well as the information found on this engineering website: [ http://www.engineeringtoolbox.com/air-duct-calculator-37_813qframed.html. ] This latter site was used as a ‘checks and balances’ to my calculations. Using the first website above, I calculated my “Boost Airflow Rate” for my application, which at 183 C.I.D., 7000 RPM, and 100% VE is 973 CFM. I divide the 973 in half to account for our divided IC piping setup…which is 486.5 CFM per side. Side note – I used 100% VE, as I have read in more than one place that turbo-charged cars are able to go above 100% VE at full boost. For now, I am assuming 100% VE. This nets the following airspeed velocities, assuming the following pipe sizes: 1.9” I.D. (2.0”) piping: 349 fps
2.15” I.D. (2.25”) piping: 321 fps
2.35” I.D. (2.5”) piping: 269 fps Stock pipes not looking so bad, eh? Seems to me that the stock 2.0” inch pipes are doing well at 24 psi…adhering to the suggested max of 350 fps. Keep in mind this is at 7000 RPM. I don’t think there are many Z’s out there running more than 7000 RPM and 24 psi of boost. If you drive a monster Z, it *may* benefit you to run 2.25” piping from the turbo to IC, and 2.5” from the IC to a 60mm TB. For all other applications, our stock 2.0” IC piping should be sufficient. On to the throttle bodies – Let’s entertain Bell’s suggestion of a max of 300 fps airspeed velocity at the TB. .Our stock 50mm TBs have an I.D. of 2.0”. Running 24 psi at 486.5 CFM, 183 C.I.D. and 100% VE will see airspeed of 371 fps! Since I did the 60mm TB conversion, I ran the numbers for this TB, which ended up being 237 fps. Big difference from stock! When considering the short length of the TB, it may not account for much of a change in performance, but worthy of bringing up. In summary – If you run “Boosted CFM” beyond 1000 CFM, it may warrant turbo piping of 2.25”, since this level of performance is pushing the 350 fps limit of the 2.0” piping. If there are Zs out there warranting 2.5” piping from turbo to IC, they are few in number. If you question your piping needs, run the numbers on the websites I listed. From IC to TB, the goal is to keep airspeed at a max of 300 fps. The stock TB can contribute to higher airspeed due to its smaller I.D., but this is only in higher horsepower Zs. For my situation, I am glad to be at 60mm on the TB. I may choose to run 2.25” piping from IC to TB, which will bring my airspeed down a bit…closer to the 300 fps mark. You have to consider that the larger IC to TB pipe is 2.0”…and only a short section is 2.25”. I’m guessing that airspeed in this pipe in this section of pipe is between 300-350 fps. My intent was to fab up a full 2.5” IC piping kit, but given the above information, I don’t think it will be a good mod for most of us. Based on my research, I don't think the stock piping is enough of a restriction to warrant this size of piping. Further, I am thinking the 2.5" piping will negatively affect the power curve due to the large space to fill...who knows how bad (lag). I’m pretty much sold on a 550 + rwhp Z going with a 2.25” IC to 60mm TB setup….but just my opinion. To me, this would be a nice dyno comparison from the stock setup. Of course, any dyno comparison would be nice, just to prove these points. James
Feedback from forum gurus:
1. I assumed 100% VE. Any thoughts?
2. I did not account for denser air on the cold side of the IC. Thoughts on the impact to airspeed velocity from IC to TB given the colder air charge?
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18:42:26 06/23/05
