htis guy knows his **** ( and he should, he works for Garret)


http://www.hotrod.com/editorial/arti...=text&id=42798

 

This is why I run a big exhaust housing on my TO4 (1.32 a/r) I may even step up to a 1.5 ?
Also why my maximum exhaust ports work well ....

Nice article, good work !

quote "Here’s a little background to appreciate the exchange of bottom end for top end: If you run little nozzles the boost response is early and you’ll make more power at 2000 rpm than you would have with a bigger nozzle because of the increased boost. However, the downside is that your back pressure will be higher with the small nozzles on the top end because the wastegates are open and less and less of your exhaust has to do "all the compressor work." What do I mean? If memory serves me correctly, it takes around 60 to 70 compressor horsepower to supply my engine with 11 psi compressed air at 6000 rpm from the turbo’s compressors. Where does this energy come from? Exhaust heat (which is "free") and exhaust back pressure (not free). With really big turbine nozzles, (no wastegates), 100 percent of my exhaust generates this work and back pressures the motor minimally, less than 11 psi, in fact. With really small turbine nozzles, the majority of exhaust is going through the wastegate (not through the turbine) to keep the compressor from overspeeding. Thus, if you will, "40 percent" of my exhaust has to do all the compressor work. If there’s less flow through the turbine but you need the same power, it must be accompanied by a higher pressure—you guessed it—back pressure."

 

Verrrry interesting!

 

on that note Rice, i am switching to a 1.3 housing tomorrow but i have a concern...launching, how are you able to launch your car? two step,antilag? i have a auto with a 6400 stall and i dont know if i will be able to get it to spool fast enough for a pro light. i have NOS but i did not want to spray a 100 shot 'on tha brake'. i know that once it is up it will stay up but there is not much time on a pro light.


MWW

 

i like this part, it explains why LFW are not the best thing for turbos



in a turbo car, boost vs. rpm is a transient event, so with a 5-speed, you really have five different torque curves to design for. By gearing the car steeper (4.11, 4.56, etc), engine rpm grows faster than boost can keep up if you will, so in a rpm-sense, this is slowing down boost response. Add on top of this the fact that the effective mass of the rotating group of the engine goes up with gear ratio squared. So there are two downsides to more gear, while the obvious up-side is the increase in torque multiplication (which is why people do it in the first place). This is definitely a juggling act, and turbo cars for this reason tend to do better than expected with slightly taller gears because they have two downsides fighting the up-side, not just one vs. one when you’re normally aspirated.

 

hmmm, i've always wondered the same thing about wide tires. surface area between the two interfacing surfaces isn't part of the equation when calculating friction, however, all the big power cars, like top fuel and funny car run super wide slicks. i'm sure w/ the millions of dollars in r&d that nhra teams put in, there is a reason for the huge slicks those guys run.

 

fdracer, something to consider is that we're dealing with traction, not friction. also notice that the "super wide" slicks are not super wide when rolling, they are super tall yet narrow (lots of stretch) and in fact at they are so flexible that they are actually "square" at some points! the only time they are super wide is when they aren't rotating.

 

ahha, we just went over this last part in physics class The frictional laws (coeff. friction * mass) only work with 'non-sticky' substances. The tires on drag cars (or anything else, for that matter), are stick to some extent, especially when they get warmed up. Hence, you want a big-*** tire back there to put down all that power

 

It is circuit so it is a bit soft under 4k, I get within 95% of full boost by 5500rpm, it shows 20psi on the gauge from 5k to 8.5k but on the data logger it shows the "real" MAP pressure, it is on full noise by 5.5k.

I don't do much launching

 

I'd like to add that when rolling (after the wrinkle effect) the tire actually goes beyond "square", and the contact surface will stretch into a convex shape minimizing the contact patch for less rolling resistance. You'll notice this when they are smoking the tires. The top will look like a bubble tire, and it's designed to do that...

 

true they do get very tall at the lauch and settle back down about half track. let's say for arguments sake that they put skinnies out back, would they in fact have the same amount of traction, which is simply just friction.

 

ok FD racer I AM NOT FLAMING YOU, JUST CORRECTING YOU. on funny car and top fuel cars the tires grow as the cars go down the track, my car does the same thing. they are in top gear when in the burnout and the tires are spinning 200mph + which makes the tires grow in height but shrink in width. as the car goes down the track and the chassis settles down the tire deform drastically and they are not round anymore , the backside of the tire is actual perpendicular to the ground and the bottom of the tire goes parallel!! this makes the contact patch multiply and thus enabling 5000+ h.p. to connect to the ground.


MWW

 

huh, what do you mean they grow? i thought they were taller at launch and then get shorter and wider as they make it down the track?

 

sorry man, that is false!! even in my car my tires grow 1.25" @ 150 mph , this is data from the tire mfg themselves. i actually have to calculate this into my gearing choices. you know i really do not want to argue with you since you think i am a "dick" so i will just wait until someone else can verify this before i continue. centrifugal force is spinning outwards how can the tire get smaller?

MWW

 

Tires do expand at high speeds... the tires the 'Fuelers use expand a LOT. But even passenger car tires will spin fast... I remember one time (KIDS DO NOT TRY THIS AT HOME) they were allegedly trying to find a vibration/noise in a car, think it was a 3rd gen Eclipse when they just came out... anyway the car;s 4 feet off the ground, one guy is holding the driver's side front wheel, and they're listening to the pass side wheel. Then the guy in the car runs it up till the speedo says 70mph, meaning the RF tire was spinning at 140mph. Tire was much narrower, tread was no longer flat but was a constant curve from one bead to the other. Very scary! I was expecting the tire to come apart, or the differential to sieze and do serious damage to the poor sap holding the other wheel!

Ok enough storytime... That article has some good info. Some other good info that he only hinted at is that for pneumatic tires, no matter what size (within reason), the contact patch will be the SAME SIZE for a given air pressure and vertical load. If you put larger tires on (width OR diameter) and run the same air pressure that you did previously, you will have the same contact patch area. You are also overinflating the larger tires, because you don't need that much air to support the vehicle properly anymore. Flip side - put smaller tires on. If you still run the same air pressure, the weight of the vehicle will sag down the new undersized tires and the contact patch will be the same. Just imagine running 32psi in a space saver spare - the tire will have to spread out so much for the contact patch to be the same size as a standard tire that the rim will hit the ground first. This is the "within reason" part!

Note that this does not apply to cornering - cornering loads are a special case because they are applying lateral load to the tire, which can cause all sorts of problems if the tire can flop around and run on the sidewall instead of the tread.

Also note that a stiff, non-pliant sidewall will hamper the ability of the tire to flex enough to get its full contact patch down. Wonder why Z-rated and/or low profle tires generally suck at the dragstrip?