mark kibort

these are pretty small factors. one test you can run , really easily, to show that there is not "massive "differences is to run the car up to 5000rpm, lift and kill the ignition. You can do this on the dyno, (and by the way, this is hardly calculated. its measured on the dyno. It's rate of rpm (KE) change = hp)
I've done this before. decel, with ignition turned off and then moving the throttle plate to open and closed. Decel rate doesn't feel like it changes much. First of all, I think much of the decel forces on the engine is friction anyway. next , the compression stage, obviously at high RPM and the throttle plate closed, you wont have the density, and then pulling vacuum against the the throttle plate is a big force, probably the most weighted of all of them, and thats the reason that a smalll change in the throttle plate position , doesnt change much, and wide open throttle actually lowers the engine braking. as long as the throttle plate is reasonably closed, you will get a tremendous amount of engine braking due to this vacuum caused by the throttle plate being a huge air restrictor. for all practical purposes, it seals off then engine. any small idle leaks dont do much to change that. just look at the measured vacuum. under decel.

mark kibort

the LHjet systems do full gas cut off until about 2500rpm. you cut that wire, and there is no cut off from 2500 to idle, so the RPM falls much more slowly, no hunting as the RPM gets to 1000rpm.
if your GTS vs GT are actiing different, something is wrong because the GTS should have more engine braking. any fuel in the system, off throttle will change this dramatically.

RKD in OKC

Most likely has a loose nut.

dr bob

[ ]

littleball_s4

Sorry if I am part of the tribe. I try to be precise and correct, but sometimes I'm not.

mark kibort

pretty pompous Bob. its a discussion. i have information. some empirical data, and actual measured data, yet you are the know it all and cant seem to communicate your point with out being offensive and class less. pretty sad as i think if we were in a room talking about it, it would be a better exchange of info.

proof in your last sentence of your comment. ("little changes in throttle position can MASSIVELY change engine braking") I would be willing to bet, NEVER in your life have you tested or measured engine braking. true???? be honest. Here is the hard cold truth... I have . Yet , i am the "caveman". ? again, pretty rude and arrogant, considering the company you are in here.

Speedtoys

You've been ignoring brake advice actively for 4 months here Mark.

How are those pop rivets holding on those 2pc rotors?

mark kibort

you knuckelhead!! the rivets were a joke, anderson and i thought that would be funny to post. in thinking about it though, all those little bolts and that little flat washers only job to is keep the rotor position on the had. since there are no real shear forces laterally, I dont see a problem..... the bobbins take all of the shear forces radially. so, maybe ill just use the steel rivets and ignore the extra weight .

dr bob

[ ]

mark kibort

dup

mark kibort

Bob, we are all friends here and i dont appreciate the condescending tone. really, clean it up or dont post. You think im some guy on a dirt track with a few teeth missing? My experience is pretty wide. ive drive and instructed many years now. coached new racers to help them figure things out faster than i did going through hard knocks. Ive helped folks even here, not as much as you and others have helped me, but still. remember the intermediate plate discussion? even the experts had NO clue of what i figured out just by being logical and analytical. and yet, some how, i log more races than anyone here with a car that has never blown up even running faster times than the benchmarks of the past. (with a hell of a lot less equipment) you might want to see what im doing and wonder, what the things i do know, rather than focus on those you think i dont.

I listen carefully and respectfully to all that post a contrary view.

you make such statements as above... (if i had proper bias i would end up backwards... or workaround to some special style.. or 200hp braking you dont want.) all you have is ancedotal evidence at best that this is not true or correct. so, please elaborate if you want to make a statement like that. i would do so for you or anyone.
you say "fact" by those with experience??? non drivers that have worked with 1000s of DE guys, or guys that raced 20 years ago? com'mon..... racing is an art and a science. most of the grass roots guys dont know why things work, they just cut checks for the things others say work and off they go. I tend to try to do that, but then validate and calculate. its served me well on most all my endeavors.

every racer uses their engine braking whether they like it or not. its there and there is no way to get around it. the "GOOD" drivers know this and use it to their advantage. ive proved with some data, that the compression braking is likely to be near max in a threshold situation.
OTHERs have tried other things, but look at Dennis for example. he is a great driver (good instints and hand control) but he was led down a path of lock up rear diiffs, and other set up ideas that made his car , almost undriveable based on "suggestions and advice of the others with experience". did you see his video. 1:46..... in an S4, by a good driver, yet thats 10 seconds off my time. why is that?...... because his car is a little heavier??? Bob, ive been racing without missing a season for over 15 years now. I have more or as much wheel to wheel experience than anyone on this list. the fact that ive done that in a car that has issues, is a good thing, not a bad thing. When i get in a good car, like a 458 or a AM vantage , 911 turbo, M3 racer, or others ive driven, its so easy. its like everything is automatic. its one of the reasons that anderson was so good when he got the cup car. he had been driving garbage for so long, that when he got in the cup car, it was a piece of cake. however, his banging gear technique broke some stuff, but he won a lot of races.

so, all i ask is to be respectful and illl do the same.

By the way, i dont trash the advice. i investigate all of it and take it into consideration. If i see something i dont understand, i ask.... but i always talk, not insult to make my point, and i try to be as factual as possible. For example.... you say, no one wants or needs engine braking.... yet all good racers, and you can see this on ANY video you watch , pro or good club racer, uses. it. its there... so, its frustrating to try and talk to you when you make such glaring generalizations. based on what????
you're pretty emphatic about the compression braking based on the aux air meter or throttle position, yet, i can prove that its not "massively" different as long as there is no fuel when off-throttle, for different throttle positions. can you prove it, other than just talk about it being massively, or maybe back it up with some solid reasoning?? its always easier to put someone down than explain yourself for proving you are right. thats a systemic problem here on the list. someone disagrees and instantly they are an idiot and their data is fake.

Mark

Jim Devine

Easy way to demonstrate importance of balance-
get on a bicycle at a reasonable speed & brake hard using a LOT more front brake than rear. It will pitch forward & unload the rear & become unstable. Repeat using more rear brake- stops better & more controllable. Same thing is going on in a car.
When you put on bigger rotors you increased the front bias. Now you have to shift more bias to the rear. Do a quality 4 wheel brake job with the proper pads & bed them in EXACTLY as instructed by the pad maker/ supplier.
Two tires (fronts) can only supply so much braking, you need to get all four to work at their best.
Continue to use the temp paint & record the results.
In the long run your costs will go down as your brakes will last longer & work better.

mark kibort

jim you will always use the brakes in front at their max. there is only so much you can expect out of the rears, and thats my major point. the engine of my car has almost the calculated max braking at the g loading on decel. even optimizing, might only be fractionally different, because what do you expect to gain. you are right , the brakes are needed because you cant lift the rear wheels under the best case braking senaero . but you get it wrong, and you get some instability.
for DE guys that are not going really 100%, this is wise advice. good braking, using a lot more of the rears, but the slow down times and distances are quite abit longer. some coaches even advise , never threshold braking... again, much easier on fronts adn more dominant on the rears. 911s, need 2x the rear brake bias than a 928, and 20% front braking force. huge differnce.

brake analogy... if you can get a bike to slow so fast as to pitch up, the trick is to keep the rear from lifting . at that point, wiht almost no load on the front... you are at max decel. the rears cannot help. however, if you want comfort or trail braking, then, more rear bias is required. fortunately, our cars cant stand on their noses, if they did any rear bias would cause lock up, which wouldnt be a big deal when the tire was up in the air, but when it came down to turn, that could be an issue.

again, you have to think about this. weight transfer is a function of 4 things. weight, length, CG height, and g loading. you get the car to 1.5 decel g's and there is only a few hundred lbs on each rear tires. engine braking in a straight line, is pretty much near max here. not expect too much more to be gained to help the fronts. and if by chance you dont have max rear bias for threshold braking, adding some will increase G loading, and lower weight on the rear even further in an ironic twist. but becuase there is so little force slowing the car from the rear, this is usually not an issue. folks that over bias the rear, just have rear lock up, or the ABS starts to engage in the rear, and saves them OR the drive removes overall braking pressure. thinks he is at the edge, but actually is modulating due to rear wheel instability caused by too much rear bias.

to your point of adding larger rotors to the front, and having to increase bias to the rear is corrrect. (if no proportioning valve) but, that falls apart when you were getting into fade and with the porportioning valve) in fade, more pressure is required to get the fronts to get near lock... this adds to the rear brakes , slighly due to the proportioning valve, but generally, not much changes based on the graphs we saw. so , if old rotors would go to the limit of the front tires before, and you go larger rotors, then yes, you just lost the effectiveness of the rear tires and stop times would be longer. But, if you had fade before, and you now are getting near the same pedal pressures as before, the bias willl be the same and stopping times would be shorter, because the greater leverage is slowing the car better during fade.

in other words, a little less brake force pressure on the pedal, doesnt lose much rear bias brake force. and again, we are only talking about 10% more leverage in most cases. so by looking at that graph if you were near threashold and have pretty high brake system pressure, backing off a little is not going to do much to your brake bias force in the rear..... see graph.

Speedtoys

"jim you will always use the brakes in front at their max. there is only so much you can expect out of the rears, and thats my major point."
---
And that's what youve been missing since only upgrading HALF of your brakes to higher mu material.

You must MAINTAIN bias. When YOU change it via a longer brake lever, or a better front pad, you have to reacquire that original bias.

I think you should do what Jim say, even if it will take you 5yrs of your race schedule to get enough data to start making decisions from.


Id love to see you attempt to setup a spec limited class car with half of what you think you know.


This is why I begged Petty Jr to start with spec class cars and fundamentals, instead of building too much confidence off of second-hand theory and..as I told him he would...ball up his 928 by misapplying it too quickly.

mark kibort

your absolutely right about littl petty. He balled it up by not spending the one day he needed with me. that was just a car control thing. its somehthing i get into before they get to that point. (hopefully)

I dont agree with you jeff. Did you think about how the bias works? look at that graph. it seems to be in direct conflict with your statement.
The small amount of leveage ive gained, doesnt change the rear bias, unless you DONT have a proportioning valve, and then it doesnt matter, unless you start with real tiny rear brakes.
Also, did you miss the point about the rear engine braking forces. at a pretty high confidence level, i calculate the rear braking forces of the engine at pretty close to limits in the area in question. how can you dispute this? 1.5 g decel leaves only about 300lbs on each tire. decel forces can be much more than this on each tire, just by the engine forces. and, i know im using a some rear brakes too, because i can feel that on trail brake. PLUS, i gave you the number of engine braking forces. 200ftlbs !!!!! multiply that by the gear ratio and you get whats on the rear tire. even if its 50 to 150ftlbs, multiplied by 5 or 7:1, gets you well over the weight on the tire. at 300lbs vs the 300lbs sitting on teh tire, thats near 1g.... it can handle more than that if it has a coefficient of friction of .5 or something..... still that engine force is getting close to maxing out the rear tire grip and slip %.

Ill run a spec mustang soon.. funy, i drive against the best of those cars right now, and they dont have any on my braking, yet on newer better tires and huge, well designed brakes. my only issue, is fade in that last 1second. . don't think ill spend the money in the miata, but those cars dont even need brakes really. Ive driven one.

Let me take you for a ride sometime on my next instructor day. I'll show you a few things . and you can ask me to do anything you think might help.