When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
i think those tests were about 20-21 psi, and those numbers are @ wheels.
To make 800 rwhp with a 928 engine that is normally aspirated and that runs equally restrictive throttle body as Shawn's 65mm throttle body, one would need a 136mm throttle body. To make 600 rwhp, one would need 117mm throttle body.
To make 800 rwhp @ 15psi boost with a 928 engine and that runs equally restrictive throttle body as Shawn's 65mm throttle body, one would need a 96mm throttle body. To make 600 rwhp, one would need 83mm throttle body.
To make 800 rwhp @ 25psi boost with a 928 engine and that runs equally restrictive throttle body as Shawn's 65mm throttle body, one would need a 83mm throttle body. To make 600 rwhp, one would need 72mm throttle body.
The same air mass can go thru a smaller hole if it's compressed more.
What the practical take-home message from this is that unless one is running a very high boost, low compression, and high octane fuel, making huge power numbers thru the stock S4 75mm throttle body is inefficient and there are probably some gains available from opening up the throttle bore.
howabout knocking off the S4 intake side plates and making new TB-mount-plates that bolt on there, instead of having a single center/bottom feed?
2x 65mm is nearly the same total area as 1x 96mm
2x 70mm would be almost as much as 1x 100mm.
you can get 65 or 70mm TB dirt cheap off of Modular Fords.
Possible, but will it be worth it? Here’s some computations for a perspective:
At 6300 rpm and 90-95% volumetric efficiency, the 5.0 liter engine consumes about 500 CFM of air that needs to be pushed thru the throttle body. Mine consumes probably a little more, but I picked the number 500 CFM because it’s probably pretty close to the stock S4 air consumption.
If the stock throttle body flows 896 CFM at 28 inches of water or 1 psi, then the pressure drop for a normally aspirated engine thru the throttle body is 0.32 psi. If we get the throttle body to flow say 1060 CFM, then the pressure drop goes to 0.23. Assuming about 14 psi in the manifold after filters etc., it’s about 0.6% impact on power or about 2 hp on a stock. That’s not much, which also explains why Porsche picked a 75 mm throttle plate. An infinitely large throttle body would give about 7 hp gain in a stock S4.
Now, suppose we boost the air box with 2 bar and make about 1000 hp. Now, we’re looking at 0.95 psi pressure drop over the stock throttle body. Going to a ported throttle body that flows 1060 CFM at 1 psi. Now we’re looking at only 0.67 psi pressure drop. So here we’re looking at a direct gain of 6 hp.
There’s an indirect effect of increasing the manifold pressure in that the overlap will work better, so that’s a little extra.
Finally, the impact of ported throttle body is larger in percentage terms if either the VE of the base engine, rpm, or displacement is increased. So for high rpm strokers with bigger cams, the throttle body modification is going to yield more gains.
The largest possible throttle valve that goes into the stock housing is 80mm. The stock 928 throttle valve is 75mm. It can be compared to a single 55mm throttle valve at each cylinder for a good stroker ITB setup. I have just taken a BMW Alpina 4.6L V8 engine apart, it has an 80mm throttle valve and a plastic intake manifold (picture) of similar design as the one Mr. Brown is making.
Åke
The largest possible throttle valve that goes into the stock housing is 80mm. The stock 928 throttle valve is 75mm. It can be compared to a single 55mm throttle valve at each cylinder for a good stroker ITB setup. I have just taken a BMW Alpina 4.6L V8 engine apart, it has an 80mm throttle valve and a plastic intake manifold (picture) of similar design as the one Mr. Brown is making.Åke
The Alpina engine needed to produce 340hp from 4.6L displacement, so the engineering team there needed to pick up a couple hp here and there I'm guessing. Thus 80mm throttle body was justified.
I'm estimating the potential gain from larger throttle bodies in normally aspirated engines using the following formula.
((((HP*1.6)/TBCFM)^2)/(14.7-0.7))*HP
HP is the power that the engine makes. (14.7-0.7) = 14 is a guess of the pressure in the plenum downstream of the throttle body. TBCFM is the is the CFM of the throttle body at 1 psi pressure which is very close to 28" of water. 1.6 is an empirical constant of how many CFM the engine consumes per hp.
Plugging in 896 CFM for the stock S4 throttle body and using the stock power of 316 hp, we get a number that is about 7 hp. That's the potential gain from completely eliminating the throttle body restriction. Porting the throttle body to flow 1060 CFM will drop that to 5 hp, which then means a 2 hp increase.
Contrast that with a hot-rodded 928 engine that makes 500hp but needs to pass a visual inspection in some dystopian totalitarian society. There, the 75mm stock throttle body causes a restriction that is ((((500*1.6)/896)^2)/14)*500 = 28 hp. Now, by making the throttle body element flow say 1060 CFM reduces the loss to 20 hp and picks up 8 hp. 1300 CFM throttle body would reduce the restriction to 13 hp and pick up 15 hp over the stock throttle body.
In turbo car that is run with pump gas, there are additional benefits from getting the intake port pressure up during the overlap. That's why John has been going thru all the trouble of redesigning the intercoolers and using the largest boost pipes that fit. We're trying to pick up every penny, nickel, and dime in there to reduce unnecessary pressure drop while still maintaining the nice high low-rpm torque of the stock intake manifold that help before the turbos spool.
You're building an exhaust capable of 1500hp. What are you guys expecting to make?
I don't see the pipe sizes as being discretely related to hp capability. It's just either more or less efficient system, depending on the pressure loss that the system has at given gas velocities.
At 500-600 rwhp or so, the true dual 3" system was already reading about 1.5 psig at the turbine downpipes. The true dual 3" exhaust was perfectly fine on the 700 rwhp iteration, but going to say 1000 rwhp (hypothetically) would have caused too much downpipe pressure with these pipes. If we'd see, say, 5 psi at the downpipe and run an absolute pressure ratio of 3 at the turbine, that's like 15 psi more at the exhaust manifold (not quite, but humor me). We can't allow that because the whole blue engine design with these exhaust manifolds relies on boost being close to the exhaust back pressure. The cross-plane V8 is a very different animal to turbocharge compared to, say, a boxer six because the cross-plane V8 has the 90-degree exhaust blowdown interference issue at high rpms (and 180-degree exhaust blowdown interference at low rpms). Because of the exhaust interference, we really need to keep the cycle average exhaust manifold pressure down and make our power in the 3500-6000 rpm range. That's why we stepped up to dual 3.5" system with a cross-over. In the end, it'll make whatever it makes (the compressors will not go to 1500 hp so that's not even theoretically possible), but hopefully pretty efficiently.
Exhaust piping size is directly related to HP capability. For all the reasons you listed
I didn't say your car will make 1500hp with that exhaust but you won't be limited like your current configuration. Based off of all the reasons you stated.
10-13-2017, 04:03 PM
