What Horsepower does the Stock TB support?

I did some searches and didn't find anything.

thanks!

 

The TB doesnt support HP . It just allows air into the intake, so the bigger the TB (52mm,58mm) the more air is allowed in.

 

Well at some horsepower rating, your car will require more air then the TB is able to supply.

what is that number.

smartass. :P

 

I think I have read somewhere here that the stocker starts choking off @ around 400HP. I could be pulling that out of my butt though

 

Nah, I think you're right. I was told about the same thing.

 

The stock 48mm TB should be able to work fine up to whatever HP you can get out of the engine, until you have the intake ported. As far as a number I would say probably fine up to 450hp.
PS. I was just answering your initial Q. Im not being a smart ***.

 

The engine tries to pull in a certain volume of air.... its defined by the displacement and the RPM. While the volume is relatively fixed, the MASS (~weight) of air in the cylinder is determined by its temperature and pressure. Let's ignore temperature and focus on pressure.

When the piston drops, there is only one thing that can "push" the air into the cylinder, and that is atmospheric pressure. (yes, I'm ignoring wave tuning, but let's keep it simple). You have 14.7psi(absolute) of pressure pushing the air into the cylinder. But, the air loses its pressure as it passes through each component of the intake system - filter, duct, MAF, TB, etc. So, the object is to minimize the pressure loss in each component, without making it so large that is it ridiculous, or could in some way alter the dynamics of the flow into the cylinder.

A stock LT1 needs a max air flow of about 470-500cfm. At that flow, the air is moving through the stock throttle body at about 200 ft/sec. There is a pressure drop associated with that. Make the TB smaller, the pressure LOSS goes up, cylinder gets less air mass. Make the TB larger, the pressure LOSS goes down, the cylinder gets more air mass. But if the pressure loss through the stock TB is already very small, making it larger will not make a "significant" increase in air mass flow.

If you look at this thread on Advanced Tech....

http://web.camaross.com/forums/showt...hreadid=228052

...you will see that at 28"H2O pressure loss, the stock 48mm TB will flow almost 800cfm.... well in excess of what the stock LT1 needs. At 20.4"H2O pressure loss, it still exceeds the air flow required by the LT1.

I did some extensive flow measurement and tuning on an engine dyno with my 500HP 381ci stroker. In an engine that can run at over 7,000rpm, with a 58mm TB, the air flow to the engine peaked at about 77% throttle opening. Opening the TB further, did not increase air flow to the engine. If you work out the area, the actual flow area being used was about equal to the area of a 52mm TB. That tells me that a 52mm TB should support 500HP comfortably. If you prorate by area, that puts you at 400HP for the 48mm TB. That would cause you to question the validity of the method though, when you consider that GM put the same 48mm TB on their 502ci/502HP Ram-Jet crate motor.

 

heh I know. I worded the question incorrectly.

 

Thanks, scary thing is that made sense to me..

I'm scared, hold me? :P

thanks!!!

 

Well damn. Thats better than my responses.

 

I love it! Great information.

Thank you Injuneer.

 

Inguneer,

Great response. I love it when responses are based on data.

Does that same thing hold true for the exhaust backpressure?

You need your exhaust system to flow at least 470-500 cfm not to see HP losses due to the exhaust?

I'm asking this because I recently switched to a single CAT when I went with the MAC headers instead of the dual CAT stock setup I had with manifolds. This has been bothering me, knowing that my high flow CAT doesn't flow as good as the two factory CAT's did together.

Looking at flow numbers that I've seen, I know that my single high flow cat still doesn't allow as much flow as the two factory CAT's. Am I loosing much HP from the one Carsound CAT, which flows about 437 cfm @28" H2O, versus the two stock cats that flowed about 665 cfm @28" H2O?

Engine is stock otherwise, except for CAI. Exhaust is now '94-'95 MAC headers and Y-pipe, single 3" Carsound CAT, Hooker Catback.

Thanks,
Dan

 

Injuneer you da man

finally some research based, hard numbers on the eternal "tb size" question.

 

I always thought it was pretty funny when I see people with one of their first mods being a 58mm TB on an internally stock engine. Didn't the guys on HP TV do a before and after dyno with a stock and aftermarket TB on their "Project Nightmare" 4.6 stand and see zero gains? That was hilarious.

 

Obviously, a system that flow less at the same pressure drop will have increased pressure drop (= backpressure) at the same flow. So you are restricting the exhaust. Whether it is a significant restriction remains to be seen.

There is no direct relationship between inlet air flow volume and exhaust flow volume. You have all the nitrogen (80% of inflow mass) that just passes through the engine, but is heated up to over 1,000degF (volume increases by a factor of 3X), and all the combustion products, which includes the fuel that flowed in as a liquid, and comes out with the oxygen as water vapor and carbon dioxide. The volume is way larger than the air inlet volume. I guess you could work it out on a molar basis, assuming stoichiometry plus the excess fuel at peak power A/F ratio.

Then consider that the exhaust volume is dropping due to cooling as it flows through the system, and it becomes a more complex issue. And since the exhuast is pushed out of the cylinder... with the piston forcing it out... the magnitude of the pressure drop can be greater. Not that you want any pressure in the exhaust... but the impact of the pressure on the volumetric efficiency is going to be less for 1"H2O pressure loss in the exhaust, compared to 1" H2O loss in the intake. And exhaust flow benefits from the "inertia" effects, or scavenging, so its a complex problem.