Yorcci

I was somehow convinced that I should get a noticeable change with my 383 cid engine with Stealth Ram and a custom cam, when changing from stock 48 mm TB into 52 mm TB. But no. No change at all. No change in MAP readings. Well, actually I mean the drop in MAP readings (in addition, the hp and tq numbers didn't change either).

First: Bare stock TB (without air filter or air bridge) in engine dyno



100.0 kPa @ 3 000 rpm, 97.4 kPa @ 5 950 rpm 2.6 kPa drop



Second: Stock TB with air filter and air bridge in dynomite dyno (which is bolted straight to rear wheels)



98.5 kPa @ 2 300 rpm, 96.0 kPa @ 5 950 rpm 2.5 kPa drop


Third: 52 mm TB with air filter and air bridge in the same dynomite dyno a few months later, several runs
1) 100.0 kPa @ 2 175 rpm, 97.1 kPa @ 6 000 rpm 2.9 kPa drop
2) 100.4 kPa @ 2 125 rpm, 97.8 kPa @ 6 075 rpm 2.6 kPa drop
3) 100.8 kPa @ 1 950 rpm, 98.2 kPa @ 6 075 rpm 2.6 kPa drop

No change in corrected hp or tq numbers with the two latter dyno sessions:
first run 558.2 Nm @ 4 600 rpm, 405.0 hp @ 5 600 rpm (48 mm TB)
second run 558.0 Nm @ 4 500 rpm, 404.8 hp @ 5 600 rpm (52 mm TB)

I've heard of gains with LT/Miniram intakes, which could be due to the significantly lower plenum volume compared with Stealth Ram.

QCVette

Good test. I had heard mixed reviews on the bigger throttle body, but most thought very little change. Your testing proves it.

It also make me wonder about the C5 and later. There are people going from a 78mm to a 90 mm or even a 102mm. (yes, I know it is one instead of two on theirs, but the area of the 78 is still larger than two of the 52's) Many are making less power than you. Makes me wonder how much is hype.

vader86

Doesn't surprise me.

pologreen1

I think the info has been around decades that the 48 will feed 400+ hp. A wild 383+ you may have seen restriction IMO

blackozvet

the stock 48 mm throttle body flows 783.0 cfm, and that is probably plenty for most 383 and smaller engines,
here is an interesting article where they went from 390 cfm to 850 cfm carb on a 383 engine,
http://www.superchevy.com/how-to/148...ey-carbs-test/

Yorcci

My cam is probably on the mild side...
228/234 @ 0.050"
0.580 / 0.592 gross lift
114 LSA

BTW, the first time the engine was started after rebuild
In the engine dyno http://penthouse.homelinux.net/fs14/yorcci_dyno.mp4

One dyno pull from the last session filmed from behind with my cell phone http://penthouse.homelinux.net/fs15/...tus_210515.mp4

Yorcci

All this data was just a byproduct of the quest for optimal performance (AFR) of the newly built motor.

I was just assuming that the MAP readings should not drop, if the throttle body doesn't restrict the air flow. Because I saw the 2.x kPa drop in the MAP, I thought that the bigger TB should/would/could cure the assumed problem.

In addition, my TB had bad (loose) bearings on the axle (shaft?) of the blades, so I had to have it fixed anyway and I had those 52 mm blades just lying around.

Supposedly the drop in the MAP readings is due to increased velocity of the air flow?

Christi@n

My OEM tb 1995, is not so good conditions, anyone know if is it still available on sale as a brand new? I've seen many used on ebay, otherwise should i purchase a new 52mm from BBK or edelbrok?

blackozvet

the 52mm TB may be capable of moving more air,
you first dynoed with open 48mm TB, and subsequent dyno's with 52mm TB were wheel driven with air hose, bridge and filter attached ?
the air bridge over the radiator usually is the bottle neck in the inlet system and probably wont flow as much as the 52mm TB. The early air bridges were rated t 499 cfm.

GREGGPENN

Except I'm not sure we've seen a post where the TPI Vette air box was flowed. This link shows the 499cfm quoted above but that's for Camaro/Firebird. I saw similar posting at 3rd Gen. I'm pretty sure those cars use a (somewhat) different-looking air box. That said, the bellows (hose) and 90-deg bend shows flow lower than the stock 48mm TB. In short, I'd agree the air box (bridge?) MAY need to be modified to let your 52mm supply more air.

If you look hard enough, there are a couple of threads on this.

Note, there's also a good thread about MAF issues you might have to deal with. (I'm pretty sure I reposted a link to that thread when it was created. Search for threads created with my userid. You should find it. I'm thinking TequilaBoy was the main contributor.)

blackozvet

Because yorcci has MAP (speed density) he could get a 4" length of tube and run it from the TB over the radiator and I would hazard a guess that it would make more power on the dyno with the 52mm TB ?

anyway 405 hp at the rear wheels is a pretty stout engine !

1963SS

I always try to remember that the GMPP crate engine Ramjet 502 uses a 48mm throttle body to make about 550 tq and over 500 HP. I've always thought bigger was better. I put 2 Holley 4bbl carbs on a stock 1967 289 Fairlane 500. Stomp the gas and the car would almost stop. Then I put a 1050 CFM Holley 3 bbl on my new, stock 1969 Charger R/T. I figured 440 cubic inches could handle. it. Stomp the gas and it fell on it's face. I learned early that bigger ain't always better but I do have a 58 on my LT4.

BOOT77

I was trying to find something awhile back and one post said something like 500, but it was a hear say number and not something they flowed themselves.


I'd still be interesting in an actual test if anyone knows.

Yorcci

No, the 52 mm TB was only on with the last session.

There was no difference in the drop of MAP readings whether the air bridge and air filter were on or not, so I presume "wouldn't"

Unfortunately, those are only flywheel numbers

blackozvet

"anyway 405 hp at the crank is a pretty stout engine !"
I will just talk it up for you then

looks like 2 x 48 mm is enough to feed a 400 hp engine ?

ghoastrider1

Laughing at this post. Reason is it sounds like something I would do/ did. Took a carb off brothers in laws 396 55 chevy and put it on my 6 cyl (240) ford pick. Had camed and headers,ect, the 6 cyl. It ran and drove, but what a dog.Yep. floor it and talk about hesitation..whew.

Yorcci

Thanks!

...and beyond, depends on the dyno and the numbers it gives.

Yorcci

I forgot to mention in the first post, that I had the 'legendary' air foil attached to the TB on the first and third dyno sessions, but not on the second session..., no change whatsoever. But to my defence, it was already on the TB when I bought the car.