Several of you have indicated you'd like to learn more about EFI on the pinz. Since throttle body size came up recently, it's worth discussing.
Lot's of misunderstanding in the EFI world about throttle bodies. Which is a shame, as they are one of the best understood areas of engine dynamics. And a very simple device, not much different in operation than your water faucet.
And lot's of folks get excited about "oversized" throttle bodies, it's a very easy mod on factory vehicles where HP has been increased by changing cam, boost, etc. But that is not the case for pinz EFI for some very specific reasons:
EFI Tech talk follows..... skip if not interested!
Throttle bodies need to be sized to the airflow of the engine. Too small, and they create a restriction and rob HP. Too large and they decrease throttle control as they reach full flow at only partial throttle. (and along the way give the perception of improved acceleration)
On a your average toyota tuned for economy, then add a cam, increase boost, etc, there is indeed a benefit from going to a larger diameter throttle body. But it's because you have made changes that flow (and demand) more CFM of air. And make significantly more HP.
But if the engine HP and airflow remained largely unchanged, the only impact would be your reach WOT (wide open throttle) with less pedal input. And in doing so lose control of the throttle modulation.
WOT is pretty much defined as when the air pressure on the engine side (normally under vacuum) becomes very close to the pressure on the air filter side. Which should be close to atmospheric for non-turbo vehicles.
We measure that as MAP (Manifold Absolute Pressure), also known as vacuum. And compare it to atmospheric pressure.
Once you reach WOT, there is no advantage in HP or response in making the opening larger. Think of the engine as a pump which has a certain flow. For a box stock factory pinz, that is roughly 123 cfm or so (+-) at sea level. The engine cannot pump more air without changing the cam, valves, compression or bore.
If the throttle body bore is too small, even at WOT (minimum restriction), you cannot achieve full flow. But in an oversized TB, once the valve is open enough to reach 123 CFM, opening it further has no effect as it's no longer the restriction.
Bodies have measured loss at specific flows which translate to reduced horsepower. So you want to find the throttle body that allows full flow at slightly less than full deflection (full pedal). Any larger and you just lose throttle range. And smaller and you are losing HP.
For a factory pinz airflow/hp, that range is in the 47-55mm range of throttle bodies. Moving from 50 to 55mm, or 55mm to 60mm only result in restriction reduction that would theoretically result in a .1 HP improvement each step. But the cost is reduced throttle range.
Moving the other way, 50 down to a 45mm results in .2 HP reduction, and from 45 down to 35 a full 1 HP reduction. So you are clearly into restrictive range.
All of these numbers are relative and representative only. An individual system may see a slight difference in optimal size.
There are several factors to keep in mind:
1) CFM of a throttle body increases exponentially with bore. It's a function of area, and that is Pi * radius squared. Increase the bore, area goes up and so does CFM for a given restriction. Likewise, airflow increases exponentially as the throttle is cracked.
Another way to look at it is that the restriction and thus HP loss increases exponentially with bore reduction. So it's important to find the right size.
2) The price of an over sized TB is reduced throttle range. With a 60mm TB on my 2.5l EFI pinz I see effective WOT at 1/3 pedal travel at 3000 rpm. From 1/3 to full, the intake is close to atmospheric. No difference. So I lost quite a bit of control in fine throttle modulation. Coming off idle, the impact is much larger.
In automotive terms this is called "tip in" response. Feels like you are accelerating faster, but it's just the equivalent of pressing more pedal.
Downside: much harder to get small throttle increases, jitter at speed due to small pedal movements making big airflow changes, etc.
Because of this, we have moved to a smaller throttle body for 2.5l. For 2.7l the 60mm is less problematic, but you still see WOT at half pedal. So we've lost some control and will try smaller.
3) Engine peak CFM is largely fixed for a given cam, compression, bore, and exhaust backpressure. IE: It's a mechanical constraint, as the engine is a pump. Adding fuel or subtracting it does not change that.
4) CFM is directly related to HP for a given engine mechanical config & mixture. (really, HP is dependent on CFM, but you know what I mean) Adding more air just makes you run lean with reduced HP & higher temperatures. which leads to:
5) Most throttle bodies are rated by CFM at a given flow restriction, which is translated to a rough HP range. So size N would be good up to 100 HP, Size Q 200, etc. This allows you to apply some common sense in selecting throttle bodies. look for similar HP engines of similar displacement. Unless you are making 200 HP in your pinz, then you probably are not going to be happy with a TB sized for a 200 HP engine. You will never use it's full flow.
6) Intake dynamics in a plenum based system don't interact much with the throttle body size outside of what I described above. . The tbi size and position does have a very large impact on intake tuning in Independent throttle body systems, but that does not apply to pinz EFI offerings so far.
We really need to have a discussion of VE (Volumetric efficiency) to go further. The pinz engine is not a high VE config, for some specific design reasons. So it's maximum theoretical CFM would be 198 CFM, but a box stock factory pinz is pumps only 70% of that at peak HP, and less anywhere else. Peak VE is usually at peak torque, but the pinz torque peak is so low it still has modest CFM requirements. So all things being equal, we'd probably want a throttle body from an engine making 100'ish HP at 125-150 CFM. Turns out there are some that do exactly that!
So what does all this mean for EFI on the Pinz? It's Goldilocks syndrome. There is a
right TB size for a given engine configuration. With plenum differences you may see a few percentage variance in "optimum" size.
Too big or too small, there are huge differences. Get in the right range, and you have full throttle response with no restriction. And more importantly, very smooth control coming off idle.
This is a well understood engine dynamics, not just a personal theory. Much of it's just pure math... Ex: A 2.5l engine making 92 HP at 4k rpm at sea level at 21c degrees has a VE of .7 (70%) at that RPM.
Your average toyota has a VE in the 87%+ range. A non-turbo race car can approach 100, and some achieve 110% using tuned port dynamics, etc. But very expensive & exotic. The pinz was intentionally designed with a lower VE but a very broad torque band. Almost diesel like. It's a different beast ideally suited to it's role.
Again, don't care for the EFI background, then skip. If you want to know more, I can provide some reference links
Have fun,
Alan
