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Dumb questions someone else might answer


Hot rodding is a labor of love. Not an investment. Something most online sellers don’t get apparently and wonder why their cars stay unsold for several months.


They say that to sell a house effectively, you need to depersonalize it, and make it as safe as possible to attract as many people. Same deal for cars. Which is why a white Camry will sell quickly and for a decent price despite being boring and run-of-the-mill while that 2JZ swapped S13 Will remain for months after being listed, as its target audience is much smaller. If you modify your car, you’d better be prepared to hold onto it, or have tuner friends.


Ok so I have a question regarding superchargers. When someone uses a compound setup with a turbocharger blowing into a tradition roots blower how does the blower cope? My understanding of roots blowers is that they can only move so much air regardless of engine size and rpm so what if any is the point of forcing compressed air into a blower that in theory cannot process that amount of air does it simply pass by the roots in the blower or does it act as a restriction


All compressors work on the pressure difference principal, therefore: a compressor making 2:1 in ratio will make that no matter the pressure on the intake.
High in the mountains, .75 pressure, you would get 1.5 absolute pressure. (.5 bar boost)
At sea level on a high pressure day, 1.05, you would get 2.1 absolute. (1.1 bar boost)

So, feed that compressor .75 boost (1.75 absolute) from a turbo, Tadaa!!!, 3.5 absolute. (2.5 bar boost)


I have a question about valves. Suppose you have a 4-valve DOHC system, so 2 intake valves. What would happen if you only opened one of the valves? Would that basically create the effect of halving the valve lift? Or would that just screw up the airflow?

If this works, then you could design the intake valves to have different amounts of lift, then open one or the other (or both) depending on throttle demands. In effect, three-phase VVL without any complicated or heavy mechanisms. (Four-phase if you include zero lift - cylinder deactivation.)


I don’t see any advantage that would offer. The purpose of multi-valve heads is increased airflow. There wouldn’t be any sense in restricting one valve from opening when air intake is controlled by the throttle.


For one, less valve lift means the throttle can be opened wider at low RPMs, reducing pumping losses. For another, if the valves are open too much at low RPM, the intake air velocity will be too low, causing poor fuel mixing, resulting in less torque and economy. These are the reasons why VVL systems exist. The question is whether opening only one valve can produce the same effect as VVL.


If I recall correctly one manufacturer did experiment with only opening 1 intake valve at low rpm for better low RPM performance but I can’t remember who though.

Less valve lift with more open throttle just moves the restriction… same pumping losses.


This is the point of VIR, so you needn’t worry too much about it.


Ok i understand that but… if a blower can only injest 2.5l of air per revolution it is only capable of compressing that much air regardless of any extra air forced into the blower case. Also when compounding boost its not just 1 bar + 1bar
I always thought (talking deisel here) that it was compounded at psi multiplied by psi not 0.75+0.75 but 0.75×0.75 so we are talking about a 6/71 supercharger producing 40-50psi of pressure into the intake manifold with the turbos producing only 14psi before the blower. And forget about above sea level consider this is at sea level at atmo pressure is 14.4psi. So if the turbos flow 3.5L at 14psi that compressed air is then forced into a blower that can only compress 2.5l what happens to the extra 1l of compressed air as it is forced through the rotors of the supercharger?


Honda VTEC-E.


If the air is already compressed or not, isn’t the matter. 2.5l is 2.5l. Doesn’t matter if it’s 7 psi or 70 psi.
Left side: Pressure RATIO Out / In


Thanks! So basically Honda has been doing this for like 20 years, lol.

Although, it looks like the implementation is somewhat different. Honda’s system can only deactivate one valve, so it’s only two-stage. In my proposal, both valves can be deactivated, so it’s 2x2 = 4 stages.

Certainly there are many technologies of which the point is to manage airflow to the cylinders, but I’m pretty sure that valve deactivation is not the same as variable intake runners (if that is what you are referring to).


what do you mean both valves deactivated? you mean 1 from the intake and 1 from the exhaust?

here’s a good page

the VTEC-E is the old pre 2000s economy VTEC system it’s VTEC that designed to be more efficient at the lower revs, and only for efficiency purposes. better efficiency, but same performance

but there’s also VTC, which is honda’s VVT system. but is barely even mentioned nowadays because it’s not used separately but instead it’s used alongside the VTEC-E to make the

Modern i-VTEC. which is a 3 stage vtec and vvt combination system.
this pic is a good summary of it.

so there is no more of just VTEC, or VTEC-E. it’s all only i-VTEC now, because they can use the 3 stage VTEC. to cater to all the range.

how you ask, now this part i’m not too sure of, but i remember either hearing or reading that the low RPM cam lobe that only actuates the 1st valve and the mid range RPM cam lobe which actuated the 2nd intake valve actually has different profiles, so they have different timing and lift. and then the normal high range lobe which lock them altogether is normal.

edit: i just found out, this process i described above is the normal 3-stage VTEC.
and also nowadays, it’s still only i-VTEC, but there are types of them apparently

more edit: the economy i-VTEC only have VTEC on the intake and only has 2 cam lobes. but the performance ones has VTEC on both cams (although VTC only in intake), and has 3 cam lobes


I know Mitsu had a version of the 4G63 that switched between 2 valve mode down low and all 3 up top for better power in the high end with good economy is regular usage range (yes, SOHC)


Why do car designers still use indented fog lights? I’m going to pick on the Juke here since they’re massive, but I still see tons of cars that have fog lights like this:

This has to be terrible for aerodynamics, right? Isn’t this basically the same thing as the sealed beam headlights of the 70s and 80s? Automakers fought hard to get rid of that rule, and now they’re putting them back on their cars?


This are actually day lights, fog lights are in the front lip. This was made purely design wise to be different, no real gain here.
Now fog lights are there to improve visibility in conditions where some sort of particles are in the air (fog snow heavy rain dust etc). Regular headlight can produce a fair amount of glare and due to shape of its beam cause some reflection. Now fog lights have beam have wider and thinner beam so they can pierce through and are placed closer to the road so we cannot see reflected light they produce.
Car makers nowadays are getting rid of fog lights but only if they offer some sort of intelligent lighting that means xenon or led lights on moving heads or even multi beam led headlights. This sort of light have sensors that detect increased reflection (like fog) and adapt (i.e. volvo’s active xenons become cross eyed).
As for aerodynamic: because this car is not meant to be highway cruiser but more of a city car that small aero detail is not that important especially because the biggest gains come from under and back of the car, not from detailed shape of frontal aria.


I got a stupid question, not very much related to cars but engineering:

Why some big trucks and busses still use drum brakes? Aren’t those supposed to be highly inefficient?


I believe its partly due to the fact that truck brakes are actuated by compressed air and drum brakes are the easiest way to achieve that but I’m sure there are other reasons


Not 100% on second part:
Heavy trucks and buses for braking mainly use retarders, especially from slowing down from high speeds, where discs would overheat and become useless after single descent.
So conventional brakes are used for comming to standstill and for holding veichle in place. For latter especially drum brakes are actually better because they have more friction contact patch than disc calippers.