Benefits Of Tb's

[benlilly 0]

I have been reading up on plenum manifolds as I want an improved version to use on my 1.9 8v. This was started by reading the 205 8V head article on Guy Croft's website. He found a flow loss when the manifold was fixed to the head. Both flowed well on their own.

 

From the info I have found on the net and from a couple Vizzard books, 50% - 60% of the engine capacity seems to be the norm for the plenum volume. This works out more than the std GTi manifold which I calculated to be 900 cubic cm, approximately (never quite got round to blocking the runnners and filling the plenum with rice!).

 

Most of the literature I found said that none of it was an exact science so my final design has the same length runners and the same volume as the std manifold but has nice straight tubular runners that mate to the head in line with the ports. It also has trumpets inside the airbox. Bogg Bros. are starting it in the next few weeks and I'll get photos on here as soon as I can along with flow measurements, on its own, and on the cylinder head.

[Sandy 189]

The spindles, if properly profiled, make no perceptible difference usually. Dyno tests back to back with barrel bodies show little or no measurable difference. For the multple tracts into a plenum to breathe as independently as TB's into open air, the plenum would have to be enormous, far too big for most engine bays. The other point on this is that at part throttle when there's low pressure in the plenum, the cross breathing effect is exaggerated, where as TBs will still be independant. If you're running a fairly serious duration cam in a road car, this makes all the difference in terms of smoothness, idle, economy, throttle response and throttle transition.

 

The complications and expense of fabricating plenum inlet manifolds as being discussed above will often wind up being more than TB's, this is absolute madness (except for boost, where it's essential). You have to know that many TB conversions are poorly executed in terms of tract shape/length and air feed, often giving less than perfect results, because as I said above, they are not as well developed as the plenum inlet, or simply pull hot air off the back of the rad (step forward most TB'd 205s!)

[benlilly 0]

The complications and expense of fabricating plenum inlet manifolds as being discussed above will often wind up being more than TB's

 

Hi,

Sorry this is kind of off topic, but from my sketches, Bogg Bros. have estimated £250 to make the plenum manifold! I was really surprised. As you have said, I was expecting it to be too much money to make it viable. Wether or not this will be the final cost, I'll have to see but they have done a couple before, so they are not doing the work totally blind.

Ben

[Doof 11]

Ok so it appears charge robbing is the biggest factor for individual throttle bodies producing power. But why do TB's with an airbox where this no longer applies make more power?

[Sandy 189]

Tuned length mostly, TBs and air box still has the benefits I described at part throttle mostly and the entry aperture area is much greater than a plenum typically.

Do you think the majority of competition engines and some production cars like the BMW M range use TBs for the hell of it? The only good reason to use a plenum over TBs is where cost, space and cold air supply is extremely limited or most often were the regulations dictate it. The recent touring cars and Super 1600 Rally cars etc have been forced to adopt single throttles in the interest of levelling power and costs, which seems to have led to a generation of observers who think they were chosen for power. I've even heard of tuners promoting plenum single inlets as superior to TB's, mostly on 106's; they clearly haven't tried a decent TB set up on the same engine I reckon. I've mapped and driven back to back and the difference in peak power can seem trivial, but power everywhere else and the driveability can be chalk and cheese.

[Doof 11]

Don't get me wrong, i know TB's are much better...i want them! But i'm just interested in the theory as to why. This thread has been great so cheers for your input.

 

Can anyone recommend a good book on induction systems? I'm reading A Graham Bell's book "forced induction tuning" which is good but i'd like something on this sort of topic too.

[benlilly 0]

"Carburetors and Intake Manifolds" by David Vizzard is quite good. Perhaps weighted a bit too much towards American cars.

Does cover intake tuning pretty well, including plenums.

Ben

[Sandy 189]

One thing I forgot to add, with Noise regs getting tighter at tracks, TB's can be a problem on drive by tests.

[niklas 1]

Sandy309: I don't doubt it's much harder to design a plenum intake with the same performance as ITB's. And I also agree it's mainly an option when regulations require it!

ITB's are easy and as you say easier to tune. It's not like you change the length on the intake runners on a plenum intake for the fun of it

[Sandy 189]

I didn't mean it to sound accusive or lairy btw, it's been a long few days with short nights, so I might not seem as diplomatic as I intend to be! I'm building a 1.8 16v for my 205 soon and keep thinking a single inlet (ie GTi6) would be cheap and effective, but I just know i'll wish i'd TB'd it!

[niklas 1]

but I just know i'll wish i'd TB'd it!

 

Of course! The MI I'm building has TB's as well

My point was not to defend plenum intakes as the ultimate, just to give some thoughts about them. Since noone else was defending them I thought the discussion was getting a bit too single-sided..