Dyno #'s This Weekend For 308 Intake/TB Mod

That was our expectation going in, but that's not what the dyno showed. What I'm wondering is whether adjusting the mixture to richen it with the bored TB on might have overcome the extra air draw.

We definitely want to try it again in a few weeks with Tommy's car since he has the bored TB and Plenum combination, and after dyno'ing his car, we'll retrofit my stock TB and see what happens.

 

Tommy, the rated HP was between your memory of 240 and mine of 230, being 235 in the owners manual. And either 4th or 5th gear would be equally near to 1:1 though of course 5th gear won't hit redline. Actually, not having that information, we actually ran all the tests in 3rd gear.

 

On the old carb cars, one thing that you'd do would be to add a spacer between the carb and the plenum, raising the carb up an inch or so.

That would often get a better fuel/air combination/mixture, but it would also slow down the fuel/air velocity.

Conversely, the tighter that you squeeze down the nozzle of a fire hose, the faster the water will spray out.

Well, a larger TB is like opening up the nozzle of a fire hose in that the *velocity* of the air is going to decrease.

So if you already have a well-matched TB, then a larger one will cost you horsepower because your air intake velocity is reduced by the larger TB bore.

If you have a TB that is too small, then a larger TB is a good idea, of course, but not every TB is too small from the factory. Some factories get it right on occasion.

On my old 5.7 liter Corvette, I went from the OEM 48mm TB to a 52mm TB and saw a pleasant HP increase. Then I went to a 58mm TB and saw a decrease not only in HP but also in throttle control/drive-ability, etc. So it was back to the 52mm TB for me back then.

I'd guess that this effect would be even more pronounced if the TB in question wasn't the main restriction in the air intake on any particular car.

 

3rd gear - 1: 1.693
4th gear - 1: 1.244
5th gear - 1: 0.919

I have always pulled in 4th gear. I'm sure someone out there can do the math to extrapolate you 3rd gear pull to a 1:1 based pull. It's too late in the evening for me to do it. That might explain some of the questions as to hp readings.

 

On my dyno pulls we used 4th gear as the 1:1 and start a pull from 2000 - 7500. When I dynoed my carb boxer we also used 4th gear for the 1:1 ratio.

 

What's with the huge lip around the butterfly valve-flap inside the TB?? That's like putting an airbrake on your intake.

 

The person who runs the dyno didn't tell you to run in the closest gear to 1:1?

 

Sorry, I meant on the brass-looking valve-flap itself, not on the intake plenum that you had bored to match (which Mike doesn't have).

That valve-flap looks as though it has a spoiler lip on it! Also, the two screws aren't counter-sunk that hold it onto the TB-bar in the center of the picture.

That's a lot of unnecessary air resistance for a performance TB. The internals should be smoother via better screws and a perfectly flat valve-flap.
Image Unavailable, Please Login

 

I don't think so... wouldn't pulling in 3rd gear make it look *higher* than pulling in 4th gear, making my real numbers even lower?

 

It does if your mind doesn't properly interpret what you're seeing. It kind of does look like that at first glance. What you're seeing as a "lip" if that was the far side is really just the flap thickness on the NEAR side. See below.
Image Unavailable, Please Login

 

That's a good analogy and good observation. I guess that's the real question we're trying to answer... will the added flow of a bored TB -- at least on a 308QV -- result in more power here or does the loss of velocity become the bigger factor and cause a loss of efficiency? Only the dyno knows for sure!

 

Theoretically it shouldn't make a difference, in practice the difference is huge.
Depending on the gear used my car puts out 280kw (3rd gear) -350kw(5th gear) at the rear wheels on the same dyno.

 

Yes, my math was off, sorry

Am I to assume these are not brake dynomometers, that hold you back at a steady speed? I dont understand how what gear your in would have any bearing unless its calulating torque based on acceleration. If thats what its doing, parts like lighter flywheels would make it look like you had more power when in fact you wouldnt have any more at all.

Also, I can see now why you would lose power with that TB, I can almost see the air tumbling over that ridge from here. Good test though, when you going back????

 

Excellent information. Thanks guys for taking the time to post this. I would also guess that the disruption of linear laminar flow at the tb/plenum opening and possible 'bouncing waves' played a part. What we can say for sure based on your data is that solely increasing tb size without anything else is definitely bad, as well as the importance of mixture.
Again, many thanks!

Just as an obtw, I had always heard to do and always did my pulls in 4th.

 

i am in Lost Wages. And yes i am reading this, and no, i never sent in my plenum/TB. Ok, guess i need to wait until i have $$$$ as i REALLY would like to dump the distributors and also go with a blower

If you see a homeless guy living out of a red Italian car on the side of the road with a sign saying "Will Work For Custom Ferrari Blower," please donate generously

 

Thanks for the explanation and photo illustration...My mind was seeing the top valve-flap *away* from me instead of towards me.

 

Exactly what I was going to say. If you have a 2 ft section of 2" pipe and you cut out a 6" piece in the middle and replace it with a 2.5" pipe you will disrupt the flow. Not only going from small to big, but then from big to small again. Fluids don't like this kind of stuff. That's why exhaust have mandrel bends so that the turns don't "neck" down.

At least that's what I think.

I'll be interested in seeing what numbers come up with Tommy car.

 

That's basically how they operate, calculating the speed to accelerate a known mass.