Need a favor - Speed vs RPM data

[maestromaestro]

A couple of weeks ago I finished putting on new bits on my FiST - that included a full brake upgrade (calipers, rotors, pads, and in case of the rears - hubs, because I also converted to the 4x100 bolting pattern) using Wilwood parts.
However, the new setup resulted in a considerable brake drag; by my estimates, I have lost quite a bit of MPGs (from 33 to about 28 mpg on average) and about 10 mph for the same RPMs. I also see higher than expected temperatures of the rotors even after driving long distance at cruise speeds and not using brakes for stopping (at the end of the trip, I used engine braking). Granted, I went from 17" to 16" inch rims, but the overall change in tire diameter is next to nothing and I confirmed the speedo accuracy using a GPS app. The tires are also different, so there is some (potential) change in rolling resistance, but it is a second order effect, in my opinion.

So, despite the above, I still need to get the data on speed vs. rpm (at 60 and 70 mph in 6th gear) so that I can compare the "before" to what I have now. Currently, it is about 2600 rpm @ 70 mph (6th gear) and about 2100 rpm @ 60 mph (6th gear).

If you own a stock FiST, would you please post what you see on your tach at 60 and 70 mph in 6th gear?

Much appreciated!

 

[RAAMaudio]

Sounds like you might have to move the calipers out or in a bit, I had serious drag on my DIY 11.75" rotor version of the WW front BBK.

It was odd as the caliper mounts from the kit were used as templetes for the smaller rotors, just moved them in a bit to fit the smaller diameter.

I had to pull the mounts off and have .01 machined to make them clear properly and eliminate the drag.

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As for fuel mileage I get better than stock, close to 39 on the highway with a big turbo with 6/4 piston front/rear BBK's.

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10MPH for the same RPM is a big change, the only thing that could do it would be a difference in tire diameter(go by the specs not by a calculator as some vary quite a bit even though labeled the same, best is to measure from the ground up to the middle of the dust cap and double it)

Or, clutch slippage but you would have not clutch pretty quickly if slipping that much.

One more thing to consider, all tires slip a little at speed, just not often talked about.

 

[FORZDA 2]

This member has posted this question on all the FiST forums, yet still doesn't understand that the ratio of INDICATED speed vs RPM will NOT change no matter WTF you do to the brakes or tires. The ONLY way to change the INDICATED (not actual) speed is to change the speedo drive gear ratio or install a "speedo-healer" device in the speed signal line.

Now that said, the ACTUAL road speed will change with tire diameter, but you won't know it unless you observe a GPS or the local police hand you a certificate of speed...

 

[maestromaestro]

This member has posted this question on all the FiST forums, yet still doesn't understand that the ratio of INDICATED speed vs RPM will NOT change no matter WTF you do to the brakes or tires. The ONLY way to change the INDICATED (not actual) speed is to change the speedo drive gear ratio or install a "speedo-healer" device in the speed signal line.

Now that said, the ACTUAL road speed will change with tire diameter, but you won't know it unless you observe a GPS or the local police hand you a certificate of speed...

Without going into arguments, I want to point out that my question wasn't about your thoughts on my understanding of gearing (or whether your reading comprehension is where it is expected to be), but merely to share your data. You, however, chose this opportunity to pontificate.

 

[RAAMaudio]

I had to use a stopwatch and mile markers for decades before we got all this electronic stuff to play with, it was better than paying for tickets

 

[dyn085]

This member has posted this question on all the FiST forums, yet still doesn't understand that the ratio of INDICATED speed vs RPM will NOT change no matter WTF you do to the brakes or tires. The ONLY way to change the INDICATED (not actual) speed is to change the speedo drive gear ratio or install a "speedo-healer" device in the speed signal line.

Now that said, the ACTUAL road speed will change with tire diameter, but you won't know it unless you observe a GPS or the local police hand you a certificate of speed...

^ This, a thousand times. Gear ratio's do not change from brake drag. Load will go up, MPG's will go down, but gear ratio stays the same so rpm and speed won't be any different.

Either way, I can check my rpm's next time I'm at those speeds. Unless your tires are drastically different in size though, which you've indicated they aren't, then my results won't be measurably different than yours.

 

[RAAMaudio]

If you use a known value or just whatever the tire manf says the diameter is, go for a drive and see what the RPM shows versus the speed in several gears, same RMP, add in the gear ratios to the equation, calculate tire slippage at higher speeds, it can be quite a bit the faster you go.

Tire slippage is as mentioned not talked about much but it does happen.

Not likely to make a huge difference like you are seeing but a bit different actual tire diameter and more slippage can add up and help explain at least some of this.

But as it seems to be the slippage will show up in indicated road speed.

Most highways have mileage markers and your cell phone has a stop watch, pretty easy to get a very accurate speed that way.

Something about your situation just does not add up so make sure you have all the variables correctly listed.

 

[FORZDA 2]

Without going into arguments, I want to point out that my question wasn't about your thoughts on my understanding of gearing (or whether your reading comprehension is where it is expected to be), but merely to share your data. You, however, chose this opportunity to pontificate.

You can take it however makes you feel better. Your question is irrelevant to your perceived problem. Several have explained why your brakes may be dragging, so maybe you should go and check it out. The most accurate method to measure your speed is via hand-held GPS.

 

[maestromaestro]

See above. Speed checked with GPS.

 

[maestromaestro]

If you use a known value or just whatever the tire manf says the diameter is, go for a drive and see what the RPM shows versus the speed in several gears, same RMP, add in the gear ratios to the equation, calculate tire slippage at higher speeds, it can be quite a bit the faster you go.

Tire slippage is as mentioned not talked about much but it does happen.

Not likely to make a huge difference like you are seeing but a bit different actual tire diameter and more slippage can add up and help explain at least some of this.

But as it seems to be the slippage will show up in indicated road speed.

Most highways have mileage markers and your cell phone has a stop watch, pretty easy to get a very accurate speed that way.

Something about your situation just does not add up so make sure you have all the variables correctly listed.

Rick, noted. I did check my speed with a high frequency GPS and the speedo is dead on balls accurate. The gear ratios argument is certainly correct, for some reason I must have had it in my head that the RPMs were lower. But, the lower MPGs are very real (and so are the elevated temperatures), so there's added resistance in the drive train.

 

[Waterfan]

For the data sample...

60 mph, 2400 rpm, 6th, 19k miles on original stock tires, 42psi front

 

[dyn085]

Rick, noted. I did check my speed with a high frequency GPS and the speedo is dead on balls accurate. The gear ratios argument is certainly correct, for some reason I must have had it in my head that the RPMs were lower. But, the lower MPGs are very real (and so are the elevated temperatures), so there's added resistance in the drive train.

It's entirely possibly to have the exact same rpm and gear with wildly different fuel mileage, and that's due to load. If you were on the prefect decline to maintain 70 mph while in gear with DFSO engaged you could literally coast forever. On the flip-side, that exact same speed being maintained at 2 load while climbing a hill would bet extremely crappy mileage all the way until your car overheated (or worse). Both scenarios involve the same gearing and speed, but have a drastically different amount of work being done by the engine.

If you pedal a bicycle at 100 rpm in a certain gear it will be the exact same speed on a flat road as climbing a hill, but at the end of the hill you'll be much more tired and sweaty than the same distance covered on the flat road.

 

[RAAMaudio]

Put the car on jack stands, out of gear, turn each front wheel to see if you can hear or feel any drag, listen to see if from the brakes.

If not able to tell pull the wheels and do it again but first take a good look at the pads, if one side of the caliper pad is touching the rotor and the other is not then you need to space the caliper properly, why the kits come with shims.

You want the calipers to be a centered to the rotor as possible and lined up top to bottom as well.

 

[maestromaestro]

Rick, all good advice and all was done. The caliper is centered with shims. The rears only move about 30 degrees when spun. I can hear and feel the drag on all wheels.

 

[RAAMaudio]

I reread all the posts and it made me think back a bit and I did have a bit of brake drag when I first setup my DIY BB's but I have had that on other BBK's so just drove it a bit and it went away as things settled in. That was after the initial tweak to the front caliper mounts.

Is the higher rotor heat on both ends of the car or just the rears? Perhaps the parking brake is not releasing far enough.

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Wheel bearings properly installed and torqued? Tight wheel bearings will generate more heat and of course accelerated wear, the heat will transfer into the rotors though less with aluminum hats.

I would pull the pads and see if there is still drag.

 

[Hijinx]

Brakes throwing off speed at rpm? Dafuq?

 

[maestromaestro]

Brakes throwing off speed at rpm? Dafuq?

Asked and answered.

 

[maestromaestro]

I reread all the posts and it made me think back a bit and I did have a bit of brake drag when I first setup my DIY BB's but I have had that on other BBK's so just drove it a bit and it went away as things settled in. That was after the initial tweak to the front caliper mounts.

Is the higher rotor heat on both ends of the car or just the rears? Perhaps the parking brake is not releasing far enough.

------------

Wheel bearings properly installed and torqued? Tight wheel bearings will generate more heat and of course accelerated wear, the heat will transfer into the rotors though less with aluminum hats.

I would pull the pads and see if there is still drag.

The pads are too tight. The bearings are fine, the hub spins freely. Drove 300 miles and I don't see any less drag so far. The fronts seem to be the primary issue, and the driver side is marginally worse than the passenger - based on the temperature readings. The braking is straight though, doesn't pull.

 

[RAAMaudio]

Possibly the front pads are a bit of a tight fit in the calipers and not moving back enough.

If they feel tight it could be the pins going through a misaligned hole or the ends need deburred or slightly trimmed.

 

[maestromaestro]

Possibly the front pads are a bit of a tight fit in the calipers and not moving back enough.

If they feel tight it could be the pins going through a misaligned hole or the ends need deburred or slightly trimmed.

It is relative. The rears only spin 30 degrees before drag stops them.