Having a problem understanding CFM requirements at low rpm.

It's been a while since I've posted and touched the tuning in my truck. 3.4l engine.

It's currently running the smaller 2.8l throttle body, I believe it's about 270 cfm, I think. I am using the 4.3 injectors, and with the help you fellas, Mark especially, I've got it running pretty good.

But I have a question about using the 4.3 throttle body, which I believe is about 400 cfm.

To start, I hardly ever run this engine up to 3500 rpm. I might have hit 4k stomping the gas to pass someone, but I don't think I even hit that.

In any case, I understand that I might lose a little on the bottom end with the bigger diameter throttle bores, but since I am running a 3.73 rear, I'm not sure that would make a difference.

In any case, I am having a hard time understanding why a larger throttle body would make a difference at the rpm's I drive at.

Will I see anything?

(Please note that I am not sure I even understand what I am trying to ask here.)

i don't think you'll lose any power anywhere.... perhaps some interesting drivability at lower throttle though. too much butterfly area will cause small amounts of pedal change to translate into large amounts of airflow change.

and from the sounds of it.... you may not see a power gain either. the weakest link along the way will determine that. i imagine it will be the intake itself or the heads.

is a vacuum being generated when at WOT at any RPM? if not, the TB isn't a restriction. and even so, the vacuum could be generated by something else more restrictive.

Thanks for the reply Robert.

You mentioned vacuum being created.

I'd have to do some test runs to see. You have any suggestions at what speeds to do this at?

whatever speeds you encounter. intake restriction is more noticable at higher RPMs for obvious reasons, but i would say that it would be most effective to start checking at whatever RPM maximum torque is at, all the way up to whatever you run the engine up to.

for the purpose of testing, i would remove as many variables as possible, like a potentially restrictive air filter.

Have to make sure the throttle blades will clear throttle bores on intake first?

Easy way to see in log if there is a restriction is data log a WOT run to as high an RPM as you feel comfortable. Is data in 100 MAP column? Or 90 MAP column? 90 Map column shows some restriction somewhere...

Quote:

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Originally Posted by EagleMark

Have to make sure the throttle blades will clear throttle bores on intake first?

Easy way to see in log if there is a restriction is data log a WOT run to as high an RPM as you feel comfortable. Is data in 100 MAP column? Or 90 MAP column? 90 Map column shows some restriction somewhere...

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2.8 tbi intake throttle bores will need to be enlarged for the larger 2 11/16"(iirc)4.3/5.0/5.7 tbi. there will be noticable improvement, especially when on top of a 3.4. and yes, there will be low end improvements. the 2.8 tbi is a pretty large bottleneck, just have the intake matched whether you do it yourself or machine shop.

Will do some logging and will focus on quick WOT for it. (to see what vacuum does)

Normal driving, it seems to run pretty good, and again, HUGE thanks to Mark for helping with the bin editing.

In regards to the intake, I do have a 2.8 intake that I will have bored out later on, but in the meantime, I was thinking of using one of those Spectre "Power Plates" on top of the plate I made for the Edelbrock intake.

If you have a TBI adapter on carb intake? Then just make it fit the bigger TBI...

I could cut another plate, but I'd have to rotate the throttle body even more than where it's at now. I don't know if that may cause certain cylinders to have different mixtures or not.

I.E.: currently, the throttle shaft is not 9-3 straight across, but more like 8-2.

I don't think that'd make much of a difference would it?

:confused:

I remember your intake and TBI mount now. Doubt turning it more would hurt? Reading the plugs would tell!

UGHHHHHH.....

I forgot that to use the old style loop/clip TVS/throttle cable combo, and that I have to change out the lever on the 4.3 with one from a 2.8 throttle body, which I have.

So, using a dremel, I lightly shaved the exposed edges of the shaft that had been pressed to hold the lever. I then used a pair of pliers to pull the lever off. I did slightly bend the lever, and being cheap stamped metal, I expected as much. But, once I got the lever off, I noticed that the blades seem to stick in the bores when rotating the shaft from closed to WOT.

OOOPS.....did I bend the throttle blades, or was it perhaps the pressure of the lever being pressed on, and then being released allow the throttle stop some slack, which allowed the blades to close further than what was adjusted for.

The blades appear straight and the space visible around the blades and bore when closed looks pretty even.

I don't know, I suppose we will find out after I tap the shaft and use a bolt to hold the new lever on.

If I recall correctly, I think there is supposed to be about the thickness of 2 sheets of paper between the blades and the bores. Correct?