Carburetor sizing for a blow through turbo

[Houseoffubar]

Hey all, there is a ton of information on carb sizing, based on your engine size, rpm and HP expectations, but I am wondering if those with experience have advice for carburetor sizing when blowing through a turbo?
Does sizing it for drivability work fine, because the turbo is forcing air through, or do you need to size it for your expected horsepower, or somewhere in between?
Thanks for any advice you can offer!

 

[rumblefish360]

Does sizing it for drivability work fine,

Yes, to error on the small side will have a higher velocity through the Venturi. This is good before boost kicks in. While under boost, the smaller carb can be power limiting.

because the turbo is forcing air through, or do you need to size it for your expected horsepower,

Yes, striking the balance is about as easy as a NA engine.

or somewhere in between?

Yes, the balance is where your comfortable at in having a machine drive, react while driving and it’s driving behavior at any rpm.

Details of the build would be super helpful as well as what the vehicle is intended for. How it is to be used.

 

[Houseoffubar]

Yes, to error on the small side will have a higher velocity through the Venturi. This is good before boost kicks in. While under boost, the smaller carb can be power limiting.

Yes, striking the balance is about as easy as a NA engine.

Yes, the balance is where your comfortable at in having a machine drive, react while driving and it’s driving behavior at any rpm.

Details of the build would be super helpful as well as what the vehicle is intended for. How it is to be used.

Thanks! Interesting, the Holley online calculator is very conservative. I was worried that making nearly 600hp with a turbo and N/A at 300 they wanted pretty much the same carb (600cfm)
I have a 780, looks like I will be trading it for a 570-650cfm soon?!

 

[rumblefish360]

Details of the build would be super helpful as well as what the vehicle is intended for. How it is to be used.

Please read the quote!!!!!!

 

[Houseoffubar]

Please read the quote!!!!!!

Sorry, yes , of course! I was asking in general, however, I have two vehicles I am inquiring about. My D-150 Truck Which I just want to make modest power 400-450 with a 318, and use as a daily.
My other is a 65 Datsun 320 with a turbo Small block 302. It made over 500hp as a street car, now going into the Datsun to be a “pro-Street” type deal.
Turbo, about 550-600hp and 1,800lbs.
Calculators say about 8.4 and 167 in the quarter. I will be happy with a time slip in the 9’s anywhere.
This will be driven to shows, and played with on weekends.
The big Dodge will tow it on occasion, but mostly haul motorcycles and lumber.

 

[dusted]

650DP is the easiest to tune or buy a super sniper from holley. Unless you are good/great at carb tuning. Use the hanger 18 mod to convert carb to blowthru. Get a wideband! Can’t stress that enough!! I’ve been there on the Turbo 318 D150. Loads of fun and no one expects it. Late model hemi now.

 

[Sprayin73]

Don’t forget you need a high flow / high pressure capable electric pump, return line setup, boost reference fuel pressure regulator, so fuel pressure rises with boost or you loose fuel flow around 8 psi. Also want that boost timing master 6AL from msd or something similar to pull timing out when coming up on boost. Carb size? As the other member said it’s hard to say without knowing your whole build. You want to stay double pumper as vacuum secondary doesn't work too good on a blow through setup.Like Dusted said, wide band 02 regardless.

Got to watch them calculators though, I’ve always found them a bit small on the fuel side, and a bit of fantasy when it comes to their ET predictions. Too many variables to calculate a real close ET, track surface temp, prep, ambient temp, How much wind and direction, actual car weight at race, how much slip, Etc.