Smaller TB....more power?

96z · 07-01-2002, 01:29 AM

This is the same old question that has sparked debates but I want someone to explain to me how more air as in a 58mm TB can produce less HP than a 48 mm TB?
The reason Im asking is Craig at GTP told me this is the reason for my lower than average dyno numbers and that the 58mm TB is too big for an A4.
What do you guys think about this? Better and bigger flow numbers in heads work good, why not in throttle bodies? Does it slow port velocity?

Oh and I searched for this discussion in LT1 tech and couldnt find it.

Get as technical and theoretical as you want....just dont break out the quantum physics on me.

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[This message has been edited by 96z (edited July 01, 2002).]

Injuneer · 07-01-2002, 06:55 AM

I can tell you why an oversize TB won't help. Just because a TB can flow more air at a given pressure drop doesn't mean it will cause more air to flow into your specific engine. Air flow is determined largely by the heads and cam. The TB is simply there to act as a "valve".... allowing the engine to run at less than 100%.

Yes, if you had a severely undersized TB to start with, you would choke off the amount of air that can flow into the engine, but put a grossly oversize TB in place of a correctly sized TB, and it isn't going to magically make the engine demand/flow more air.

A measurement of flow means nothing if it isn't accompanied by a pressure drop. Pressure drop is roughly proportional to the diameter raised to the 2.5 power. Once you reach a certain point, the reduction in pressure drop is negligible. And the only thing that is going to change the amount of air filling the pistons is the pressure drop in the air inlet track. A fraction of an inch of water isn't going to alter volumetric efficiency any appreciable amount.

As a point of reference, when we first set up my engine, we used N-alpha programming, where fuel is essentially determined by throttle position and rpm. At 77% open, the 58mm TB was flowing about as much air as the engine needed. In effect, the engine could have made the same amount of power on a 52mm TB. Going larger did not cause the engine to flow more air, or make more power.

I can't tell you why Craig is saying the 58mm is costing you power because you have an A4. I doubt that the velocity of the air through the TB bores is of major significance, since the volume of the plenum relative to the volume of the runners should effectively decouple the TB from the runners. I "think" that's the case... can't prove it for the specifics of the LT1 plenum.

But the idea that putting a larger TB or a larger MAF sensor or a larger CAI magically makes your engine flow significantly more air is a popular misconception. The increase in flow is proportional to the reduction in pressure loss, relative to full atmospheric pressure.... a few 1/10th's of an inch H2O from an oversize component means virtually nothing.

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Fred
94 Formula A3: 381/TH400/N2O

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kmook · 07-01-2002, 07:42 AM

Not saying that this is your reason too, but Cody (97 RedSS) just had his GTP stage 2 heads flowed because he was only making about 370rwhp and found that the heads flowed much less than Craig said they did...

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[This message has been edited by kmook (edited July 01, 2002).]

NOMAD · 07-01-2002, 11:43 AM

The throttle body will only flow the volume the cylinders flow. I dont' really see how it can hurt except for the shift problems with a 58mm on a auto car without programming. The ECM gets confused due to the larger amount of air flow at a different % of TPS reading.

I usually judge by HP numbers. The stocker is good for some nice HP, even a 52 is good up until about 500 or so.
The only time I suggest a 58 is in a very high HP application or forced induction application.

-Shannon

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Eric Bryant · 07-01-2002, 11:50 AM

Quote:

Originally posted by Injuneer 94FormM6:
I doubt that the velocity of the air through the TB bores is of major significance, since the volume of the plenum relative to the volume of the runners should effectively decouple the TB from the runners. I "think" that's the case... can't prove it for the specifics of the LT1 plenum.

One of the Impala SS folks, when working with Joe Prince on some heads for his car, found that the throttle body does indeed have an effect on intake port velocity (after spending a lot of time on a flowbench).

When running a lower-stall (~2500 RPM) converter, the 48 mm TB seemed to work best, as confirmed by testing at the strip (FWIW, he was running 12.6 with a stock 350 bottom end and a full-weight B-body). When a change was made to a higher-stall (3500 RPM) converter, a 58 mm seemed to work best. This would seem to indicate that the increased velocity helps increase power on the lower-end, which may or may not be important depending on the TC, gearing, etc.

Sorry for the lack of specifics, but I don't have any more data in front of me. Someone with a flowbench and some parts could dig into this further, I imagine.

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96z · 07-01-2002, 06:19 PM

Quote:

Originally posted by Eric Bryant:
One of the Impala SS folks, when working with Joe Prince on some heads for his car, found that the throttle body does indeed have an effect on intake port velocity (after spending a lot of time on a flowbench).

When running a lower-stall (~2500 RPM) converter, the 48 mm TB seemed to work best, as confirmed by testing at the strip (FWIW, he was running 12.6 with a stock 350 bottom end and a full-weight B-body). When a change was made to a higher-stall (3500 RPM) converter, a 58 mm seemed to work best. This would seem to indicate that the increased velocity helps increase power on the lower-end, which may or may not be important depending on the TC, gearing, etc.

Sorry for the lack of specifics, but I don't have any more data in front of me. Someone with a flowbench and some parts could dig into this further, I imagine.

How much power did he pick up when he did this?

96z · 07-01-2002, 06:21 PM

I understand the pressure idea Fred. I couldnt get the A4 idea either???

96z · 07-01-2002, 06:24 PM

Quote:

Originally posted by kmook:
Cody not saying that this is your reason too, but Cody (97 RedSS) just had his GTP stage 2 heads flowed because he was only making about 370rwhp and found that the heads flowed much less than Craig said they did...

Interesting.....but dont different flow benches flow different numbers?? Ill guess I never know until the motor gets torn down in a couple years.

kmook · 07-01-2002, 07:44 PM

Oh when I said less i didnt mean what could expect to see between difrent benches a few cfm, They flowed substantially less.

96z · 07-01-2002, 08:07 PM

Quote:

Originally posted by kmook:
Oh when I said less i didnt mean what could expect to see between difrent benches a few cfm, They flowed substantially less.

Really? That doesnt make me too happy...

96speed · 07-01-2002, 10:09 PM

Quote:

Originally posted by kmook:
Oh when I said less i didnt mean what could expect to see between difrent benches a few cfm, They flowed substantially less.

Ken: How about encouraging him to post over here?

Where/who flowed them recently. How about some actual numbers across the board? I also, thought his combo seemed a little "low" on peak rwhp.

Ryan

kmook · 07-01-2002, 10:11 PM

I will email him and see if he'll make a post.

I recall that they flowed like 26x intake and 18x exhaust @600.

97 RedSS · 07-02-2002, 01:13 AM

Yes I had them flowed over at ProPerformance over in Shrevport,LA..They did flow a lot less than I thought...I was expecting 269/209 but only got 260/180 at .550"..The exhaust was killin me escpecially with my big cam..with a smaller cam I woulda made better power seeing where my heads flowed decently down low which my cam was out of that rpm range..I had brought a set of bare LT1's to his place that he was gonna port and had my GTP's in the car and he wanted to see what they would flow..This was all after they were sold

Cody

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96speed · 07-02-2002, 04:29 PM

Quote:

Originally posted by 97 RedSS:
Yes I had them flowed over at ProPerformance over in Shrevport,LA..They did flow a lot less than I thought...I was expecting 269/209 but only got 260/180 at .550"..The exhaust was killin me escpecially with my big cam..with a smaller cam I woulda made better power seeing where my heads flowed decently down low which my cam was out of that rpm range..I had brought a set of bare LT1's to his place that he was gonna port and had my GTP's in the car and he wanted to see what they would flow..This was all after they were sold

Cody

Thank you for the info, Cody. Did you speak with Craig @ GTP about this?

Ryan

97 RedSS · 07-02-2002, 07:03 PM

No I hadnt..I was even at GTP 1 mnth before to get him to change my valve springs and put new ones on it..I asked him if he could flow them but we didnt have time..He was a cool guy though. I just got a good deal on another set of heads so I sold mine thinkin they flowed what Craig told me..

Cody