Hi,

I know that the question about 52mm vs 58mm throttlebody has been asked over and over again and I hate to bring it up again; however, I was just offered an interesting proposition. I currently have a 58mm BBK throttlebody and port matched manifold in my 93 6 speed car with a modified airbox and muffler eliminators. I am however just starting to build the motor and plan to go with a Lloyd Elliot stage 2 heads and cam kit with some long tube headers with the stock 350 bottomend. A friend of mine is building a 396 LT4 and has a 52mm throttlebody (also BBK) and wants to trade with me saying we will both see benefits from the trade. I have seen countless articles stating both sides of the arguement but I am wondering if at 0 cost to me, would this trade be worth doing in the long run concidering my heads, cam, and headers plans for the future.

Thanks for your input
-Tommy

 

I would suggest a 396 needs a 58mm throttlebody more than a 350 does ?
do the swap and tell your friend that he is indebted to you for the rest of his life !

 

Chevrolet sells a 502 cubic inch engine complete with fuel injection. It makes 502 horse power and comes with a 48 mm throttle body. I'd make the swap.

 

Do not make the swap; you will gain nothing from going to a smaller throttle body. I can tell you from experience that if a car is properly tuned, a larger throttle body will never cost you any power and will usually gain you something on top end power. Throttle bodies only flow air not fuel, the velocity of air in the runners will not drop (key point) with a larger throttle body. To the contrary it will increase or at least stay the same if the engine cannot use additional air potential of the larger throttle body. The drop in velocity of air and fuel in the carb (and subsequent loss of fuel in the air) is what causes the loss of power on a carbed car when you overcarb, this problem does not happen with a throttle body only flowing air.

A side note on a stock LS1 the 75mm throttle body is greater than twin 52mm throttle bodies, on latter stock LS2 and LS3 with 90mm throttle bodies the throttle body is greater than a monoblade or nearly twice the size of twin 48mm throttle bodies. GM did not do this to kill torque and power.

 

I'd pass. No real gain for you and I would reset the idle so it would be work. Why? You gain nothing. Now if he is letting you have something good for it, sure.

 

58 is good for about 850cfm
52 about 750cfm maybe a little less

If it helps a friend, and it does not hurt you. I'd do it.

TJM

 

Thats what im leaning towards cause he is also giving me a deal on a set of Lloyd Elliot stage 2 heads. So far I have not heard anyone say that it will hurt me at all. I'm not worried about the effort it takes to dial the idle back in cause my TPS sensor has already been drilled out to be adjustable.

 

Ah well. That's different. BTW you dial the idle back easier with IAC counts if you want it easier.

 

The science of air flow tells us somethings contrary to what seems right...

In many situations the smaller TB will do better BECAUSE having TOO much available air causes a drop in Vacuum, which translates to a REDUCTION is air flow velocity....

The velocity is your best friend in a naturally aspirated engine. It causes the air to speed up (accelerate)and slam into the cylinder as the intake valve opens, where the reduced vac from a big TB will slow it down and end up with less air in the cylinder...

I proved this to my own satisfaction by experimenting with a BIG induction system that was at least 1" larger duct, big TB and big MAF. The engine got sluggish and would not perform until it got to the upper rpm range. Low end was worse than poor. Embarrassing would be more like it. After reinstalling the stock duct work and stock size MAF, all was well again. The lesson? the air intake velocity was the ONLY thing that changed and that killed the performance down low, where we actually WANT some guts in a street car.

Those folks with a MAF engine are always going to be restricted by what will flow thru the MAF, NOT the TB....true story. Think it thru...

Big TB's are most beneficial IMHO to engines that are going to see extended high RPM where the air flow is consistent and not subject to varying conditions. For a street engine, I'd never go more than 52...and there are plenty of guys here with 383s and larger that run the stock TB. The biggest advantage to ANY aftermarket TB is the throat shape and the nice smooth ports that allow great air acceleration minus the turbulence that comes from the stock TB and its many edges in the throat the air must go around to find its path.

There is only "so much" air that a naturally aspirated engine can possibly suck in...a hole larger than that is absolutely pointless...you simply cannot put 1 gallon in a one pint can. Your 1 pt can can only hold/suck 1 pint...

And yes, you CAN compress AIR, BUT, you need a "compressor"....a super charger. Then with positive pressure you still don't need the 58 since the air gets pushed in regardless of TB size...

 

[QUOTE=leesvet;1584796674]The science of air flow tells us somethings contrary to what seems right...

In many situations the smaller TB will do better BECAUSE having TOO much available air causes a drop in Vacuum, which translates to a REDUCTION is air flow velocity....

The velocity is your best friend in a naturally aspirated engine. It causes the air to speed up (accelerate)and slam into the cylinder as the intake valve opens, where the reduced vac from a big TB will slow it down and end up with less air in the cylinder...

QUOTE]

I am not trying to start an argument but I do not agree with this post. Any vacuum in the intake plenum will slow down the velocity of the air in the intake runners not speed it up. The runner volume on an engine does not change so the highest air speed or velocity through that runner will be at the lowest plenum vacuum or in other words the highest plenum pressure (closest to atmospheric pressure). As I stated before a properly tuned engine will not loss any power or torque from a larger throttle body.

 

Take this FWIW. My friend with a stock Vette made a few passes with stock and with the 58mm TB. Went slower consistently with the larger one. Perhaps it just isn't right for a stock system.

 

Tuning or different atmospheric conditions from the 48mm test.

 

Was the stock intake manifold opened up to match the 58mm throttle body bores?

 

Nope. 2 runs with, 2 without, 2 with, 2 without. Consistently slower with. No other changes.

 

Nope. Just bolted on. Of course it felt faster with but somehow the timeslips didn't agree.

 

[QUOTE=bjankuski;1584798676]

Several of the claims don't make sense to me from a pressure drop/restriction standpoint.

 

If the opening were not opened up to match the throttle body that could have been the problem. You could have been creating turbulance at the throttle opening causing a reduction in flow.

 

Not sure if serious. If anyone else wants to trade their 58 I have a 52 I will gladly trade.