LS1 TB and Bigger?!

[alldaysteve]

I have a N* TB now but I am going to port my blower to match a LS1 TB that is 80mm in diameter from my 75mm N*.
My question is can we upgrade to the "PORTED" 85mm tb over the standard LS1 for more air flow?

[1bad6]

Yes, but not sure if you need that big for power! Transitional porting to accommodate the much larger throttlebody will have to happen, to smooth up the tract. Installing the larger TB, will slow velocity. Not sure if the equivance of 5 mm will be gainful for you, suffice it to say, the diameter before TB remains the same. Inspite induction, installing the 85mm TB is useless, if the rest of the induction tract rests on a 3" ducting to create more useable airflow to power. Inducting port velocity requires thoughful construction to create proper engine air needs vs air speed. Large diameter ducts (4") opens breathability for horsepower for example, but slows intake charge, which in effect, reduces port velocity, yet supports upper horsepower. Inherently, smaller duct work (3"), incorporates faster velocities, which support lower horsepower and torque. It all depends on the style, diameter, length, material and any frictional bends used in creating your intake tract, up which includes performance mods. done to your car. Large camshaft? Larger ducting(4"). Stock cam? Stock opening! Proponents to support more air, to support more power, will rest solely on design. Basically, squeezing in the much larger 85 mm TB between unmated and smaller diameter ducts might drop your horsepower levels, because of diameter indifferences.

[alldaysteve]

I have a 4in intake reduced down to 3 to fit the Nstar. I hear alot about Velocity blah blah blah, but If everything is ported to match from the blower I don't understand why I would make less power. There was a significant gain in power with the N* from stock once I calibrated the maf so why wouldn't it be beneficial?

[1bad6]

You'll be able to see different power levels if you were to re-install the 3" duct, all the way across the board, instead of the 4". Whether or not you see a gain, is really based off your mods. Mostly stock, power will rise; using a cam, heads etc,.... you will drop!

[1bad6]

I noticed a small improvement of power going from stock to a 3" duct. However, when I installed my ported LQ4 MAF, I went with a 4" size duct with a K&N cone filter. You can feel and hear the difference with this set-up, especially mid/high speed. World of a difference with ported heads, bigger cam, headers etc. If I had to resort to 3" ducting, I'll promise you a significant drop in power. Strange how airflow can make or break.

[alldaysteve]

Yeah, it is kinda funny how air velocity works. I have stock heads and cams so I should be ok to upgrade to the LS1 and gain as long as everything matches. It's kinda like an engine is only as strong as its weakest part just like you can only flow as much as your smallest diameter. Too big with smaller pipe will create restriction thus the loss in power essentially is what you are saying and you would just be better off with the smaller that would match... right?

[Miko K]

Just a reminder, look at the area of the TBs, not just as diameter. 75mm TB has an area of 4415mm2, an 85mm TB has an area of 5671mm2, in other words the 75mm TB has an area that is 77.85% of the 85mm. Back in the day I switched to an LT1 TB which was 2x48mm, not knowing that it had an area of only 3617mm2, which is equal to single a TB of 67.88mm2, not even taking into consideration the restriction that the design had.

[12.99someday98]

Ok... First off these aren't indy cars, or nascars, looking to get .01hp in our applications is pointless. That said, ON A SUPERCHARGED car that SUCKS the air in the bigger the opening the better, there is not velocity b.s. i can put 500 different intake setups onto a flow bench and see all kinda of different velocities, then stick them on the car, the one that allows the most air in, wins. period. There aren't any issues on a FI car with velocity unless you're over the point where you need to make certain angles just right or the air doesn't flow correctly. Do you HONESTLY think we have an issue like this on a 3800? C'mon, give me a break, this is ridiculous, bigger t/b with matched s/c opening=more air (if needed). period. Worry about velocity when you're trying to make a 350cu motor put out 700hp N/A.

[Miko K]

Well, we are just talking about making the right decisions and not spending a fortune to come up with a decent setup. I have had a good luck with a 75mm TB:
GM High Flow Throttle Body VORTEC Carburetor w/ cruise : eBay Motors (item 170444750603 end time Mar-12-10 16:27:29 PST)
which is dirt cheap and works very well, I did have to scale the TPS with a variable resistor, but I made it perfect. I also slotted the holes of the TPS to make it read 0% when closed. All you guys with big TBs- scan your TPS and make sure that they reach 100% correctly and also 0% is really 0%.
I have also had a good luck with a stock descreened delphi LS1 MAF, loaded with a map for a descreened LS1 MAF, scaled to fit the smaller number of cells on the V6. I think I paid $40 for the MAF shipped from car-part.com. My LTFT's were a little off initially, but AFR was correct even with the 60lb injectors, appropriately scaled to 1.66