Tuning engine volumetric efficiency theory

[gliding_serpent]

Not sure if this is the right place to post this, but here I go anyway. I am trying to learn the theory of engine tuning through design. My current focus is revisiting what I knew of intake and exhaust theory to tune your powerband (peak Volumetric efficiency, which usually correlates with peak torque). Would appreciate input on the theory...

Without getting into the really fancy internal engine stuff, or helmholtz resonance... larger diameter intake manifolds/header primaries push your peak volumetric efficiency/VE (Torque) to higher RPM's, and smaller diameter does the opposite. Longer tubes push peak VE down the rpm band, and short tubes push it up. Internally, an engine is what it is, and intake and exhaust mods help increase (or decrease) efficiency of air flowing into it (VE).

Opening things up before the intake manifold is free power because you are just reducing restrictions. Same for removing the cat and muffler on the exhaust side (although the muffler loss can hurt midrange if it has an x-pipe function that is now lost, like in the OEM and dansk models).

The intake runners and header lengths and diameters are more key in "tuning" peak volumetric efficiency. In theory, calculations can be made for ideal lengths and diameters that can synch your volumetric efficiency at a set narrow rpm band... giving max power in a defined region... great for racing and short gears, and bragging with dyno numbers, but the motor will be "peaky" as VE will be reduced in other areas of the power band. You could also choose to flatten your torque curve out, either by choosing larger diameter, but long tubes (or narrow short tubes). Or by paring an intake that favors high rpm performance, and an exhaust that favors low RPM, or vice versa. Thus you lose out on really high peak engine torque, but spread the VE/torque over a larger area, increasing "drivability."

Also important to note that internal engine mods aside, and after removing overt restrictions (muffler, cat, air filter, AFM, larger TB), you are just playing with shifting, concentrating, or spreading out the volumetric efficiency. You increase high rpm VE at a cost to low rpm VE. A wider band of VE will hurt peak VE. You can't have your cake and eat it too. In other words, you have 6 baskets, and 12 apples, and you can choose to spread them evenly in all the baskets, or put most of them in one or two baskets... but you still only have 12 apples.

So, in the case of my car.. if I have a well flowing intake, but a comparatively undersized headers (SSI's), than that will hurt my peak VE/torque/hp, but it will also give me a flatter torque curve. Throw on some larger B&B headers, and I have just helped to synch the VE at higher rpm levels... increasing peak VE at high rpm, but over a more narrow range, and with a cost to lower rpm performance.

Thoughts?

[boosted79]

How did you come to the conclusion that your headers are undersized?

[jluetjen]

I shared some thoughts on this subject in the past. Warning, you can definitely start to go nuts with some of this stuff, especially when you keep in mind that the tuning of exhausts and intakes are related.

Enjoy the holidays!

[gliding_serpent]

That seems to be the dogma from dyno data.. I have a 3.4 with 964 cams.


But i ran my engine numbers (stroke, displacement, exhaust valve size, and a lot of exhaust valve timing specs, combined with the targeted rpm for peak torque) and it said i needed about 31 inch primaries with 1.5id... Which ironically is the length and diameter suggested by many for our 3.2 engines. As porsche knows a thing or two, it kind of tells me that these calculations might be decient. Porsche motorsport, bursch, b+b, ssi long tube style headers all have about 31 inch primaries. That being said, they are just calculations and all calculations make assumptions. I suspect these calculations under estimate the truly ideal ID of the primaries... Presumably because they do not incorperate bends into the formula. Thus, the predicted rpms for max VE are probably being over estimated by 300-400rpm for a given primary... Or the sizes are too small by a small factor.

If i plug in ssi numbers in and solve for the rpm of max VE, it is somewhere around 4000rpm if memory serves (don't quote me). So i call that undersized as it generates a point of max VE that is lower than oem (about 5400rpm). But, combined with my liberally flowing intake, this should give me a flat torque curve, and very good drivability, at the cost to max hp.

However, my calculations suggest that a b&b header setup with 1.5id 31 inches primaries would be ideal for a peak VE at 6200 rpm. But, correct that a bit for bends in the primaries, and the peak VE is just at the 5.5-6k rpm range... Which is pretty close to stock.

[gliding_serpent]

Originally Posted by jluetjen

I shared some thoughts on this subject in the past. Warning, you can definitely start to go nuts with some of this stuff, especially when you keep in mind that the tuning of exhausts and intakes are related.

Enjoy the holidays!

I will read your posts. I agree, it is a bit of an exercise in intellectual masturbation. In truith i have chosen my components, i just wanted to see if i could generate numbers that support what we se as working in the real world.

And for sure on intake effecting exhaust. More air in results in more air out, so it is all give and take. Most calculations do not consider the entire system, or their unique characteristics (bends in tubes). I guess there is complex pro modeling software out there, but it costs around 60,000$

P.s. Backpressure is in my mind complex. I think it is a poorly used generalization for a number of air flow dynamics happening in an exhaust. On one end, it incorperates helmholtz resonance, and in another way it is based on optimal flow rates that allow max rate of exhaust gas expulsion (bigger is not always better). This is dependant on things like rpm, exhaust valve timing, primary length, primary diameter, crossovers. I have yet to fully understand all of these elements.

Your second thread is excellent. I skimmed it, and will digest tonight.

[gliding_serpent]

I need to order that p smith book!!!

[Costa P]

Originally Posted by gliding_serpent

I guess there is complex pro modeling software out there, but it costs around 60,000$.

Good news...not all pro software costs an arm and a leg!

Google EngMod4T....very affordable and most importantly very potent. If you take enough care to model your engine accurately you will be rewarded with very good correlation to your dyno measurements.