TitoPR221

I read in the dtcc.cz28.com website that the flow through a 48mm TB at 28"water was measured at 783 cfm. My 383 at 6800 rpm would theoretically aspire 753 cfm (at 100% volumetric efficiency). This would indicate that the 48mm TB has enough capacity for the 383.

For a normally aspirated engine, 753 cfm would result in approx. 501 hp.

So why is it so common to see 52mm and 58mm TB's installed?

96supersport

One thing to think of is how fast can you cram that much air threw the 48mm holes?
Logically with a 52mm or a 58mm, you could get more air in faster. In result better throttle response and with tuning maybe more power.

Im not an enginner, but i would think that having the throttle body that close to its max cfm could cause a loss of power.

Injuneer

Because at 753 CFM (and assuming the measurement of 783 CFM @ 28" H2O is valid) the 48mm TB will be causing a pressure loss of (approx) 25.9"H2O. Pressure lost in the intake track reduces the density of air in the cylinders, reducing HP proportionally.

Assuming simplified calculations (and basing it on the quoted numbers for the 48mm TB), for 753 CFM, pressure loss would be:

48mm = 25.9"H2O
52mm = 22.1"H2O
58mm = 17.7"H2O

The simplified basis for this calculation is that pressure loss is roughly proportional to the square of the veloicty.

If you consider the fact that standard atmospheric pressure is about 407"H2O, you can see that the pressure loss in each TB size is reducing your HP by:

48mm = 6.4%
52mm = 5.4%
58mm = 4.4%

TitoPR221

The density change is proportional to the absolute pressure, not the pressure drop. So comparing intake plenum absolute pressures, between 48mm and 52mm for example, the difference would be 3.8"wc and the air density would drop about 1%, which would be difficult to notice when taking into account other factors such as temperature, which could have a larger effect.

96capricemgr

Does not sound to me like you want the answer instead you want to argue that you discovered something and the rest of us are doing something wrong.

Let me throw this out there though, how about instead of just considering the pressure drop Fred details also consider that a reciprocating engine does NOT have a steady demand but rather spikes of demand and the LT1 intake's plenum volume is relatively small.

Also if a stroker is only making 501hp flywheel it was done poorly or at best a very mild setup.

Sometimes measurements mean a whole lot more than theory, my motor sees over 2" by 6300 rpm and the most restrictive part of the intake is a descreened f-body MAF.

mdacton

Go buy a new TB....dyno it with the stock one then swap the bigger one on. See what happens

I would like to see

dangalla

i still have my stock one, when i do get my car on the dyno maybe in a month or so i would be glad to bring it along and try it if no one else has by then.

i think if nothing else throttle response would be improved and i cant imagine running that close to maximum efficiency can be good

AdioSS

That last statement made me laugh.

Personally, I'd prefer to run at an even higher efficiency if I could.

Injuneer

Not sure what your point is. I told you the pressure drop on a 48mm TB is costing you 6.4% density loss = HP loss:

25.9 / 407 = 0.0636 = 6.4% loss

Then I told you the 52mm TB would cost you a 5.4% density loss = HP loss:

22.1 / 407 = 0.543 = 5.4% loss

6.4% - 5.4% = "about 1%"

Which is what you just restated.

What is your point?

All this ASSumes that other variables are held constant. Raising the issue that the temperature might change and affect density it irrelevant and an obfuscation. Why state the obvious? That's like saying you shouldn't upgrade your cam because any gains may be offset by a slippery track......

At 500 flwyheel HP, you could pick up roughly 2%, or about 10HP by running a 58mm TB instead of a 48mm TB. You have to decide based on your required "rate of return" whether that 10HP is worth investing $350 for a TB and a bit extra for porting the intake to 58mm.

You asked a question, and I gave you a technically correct and accurate response. What problem do you have with that response?

mdacton

Its over my head, please add pictures

Injuneer

Sometimes I wonder why I waste my time

I could have spent that 2 minutes polishing my car.

dangalla

ohhh pretty

thanks fred

onebadponcho

Well, let's see here... A stock throttle body with an airfoil will flow over 600cfm (from what I've read). This is good enough to feed a 350cid engine up to 6000rpm at 100% VE (volumetric efficiency). Plus, the ZZ502 (fuel injected version) makes over 500hp with a stock L98 throttle body.
With that being said, if you increase your displacement, rpms, or VE (heads, cam, exhaust, etc.), the bigger TB will benefit you. I have a book called "Small Block Chevy Performance" by Dave Emanuel that has a series of dyno tests on a Camaro LT1. They dynoed the engine with ported heads by CNC Cylinder Heads, CAI, and a stock 48mm TB. Everything else on the engine was stock. They bolted on a 52mm TB from TPIS; and gained 12hp and 6ft-lbs of torque. That's probably why you see so many people with bigger throttle bodies.

Injuneer

I guess it always comes down to your personal point of view. We regularly see posts here from someone who wants to put a 58mm TB on a stock engine (with the correct intake manifold bores), and people generally tell them they don't need it and to save their money. The response is "the gain may be small, but every little bit helps". Then you show someone how they will probably pick up 10HP on a high RPM/high HP stroker, by using something larger than the stock TB, and they write it off as "which would be difficult to notice".

You can't keep everyone happy.

TitoPR221

Sorry if I offended anyone, not my intention. Does anyone have flow numbers for 48mm, 52mm and 58mm TB's? or better yet dyno runs with different TB's for the same engine?

It would be wasteful to spend $350 to gain 5 HP.....