Thread: For Don Shilady

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That's great that "the BOSS" can enter/exit easily and you can share the experience together. My wife went for a short cruise last week, we went for a short ride and then stopped for ice cream. Later went over to some friends and she said that she did her annual ride and that's enough 'til next year..

I don't really mind tho.. as she talks the whole time she's in there! takes all the fun out of the cruise for me!! So I'm good 'til next summer!

Well 34-40, since my last post "we" had a break down coming home from a local cruise-in but the wife still likes the car because she rode home with me from another local drive-in two weeks later. Due to the tight placement of my electrical coil under the simulated tank eyebrow on the fire wall, the top of the coil broke with bouncy vibration and the problem was mysterious for a while until a crack in the neck of the coil was noted. Replacing the coil fixed the problem but I added a foam rubber "nest" between the new coil and the fire wall to soak up future bouncing vibration. The coil spring rear suspension gives mostly a good ride but the front end is subject to bumps and on the way to the cruise-in we crossed a long one of those new slab-type bridges with a bump for every boundary edge. That is probably what bounced the coil up and down? Now for a promised report on the mpg. The results are disappointing since I had hoped to get to 20 mpg. The stock specs for a 1977 Z28 (3828 pound car) were 14-17 mpg and my mild 350 is basically a 1976 350 built to 1977 Z28 specs with slightly more compression. Sooo, I took a short 63 mile jaunt from home near Glen Allen to Bowling Green Va and back again and the AV8 used 4.201 gallons of 89 octane (10% ethanol). Thus (63/4.210)=14.9964 rounded to 15 mpg. I checked this with my 1998 Sunfire and got 32.1782 or almost 32.2 mpg over the identical route. Parts of the course were traveled at 65 mph but most was at about 55 mph. Noting the tach showed most of the time it was between 2000-2100 I was probably traveling in the 0.7 4th gear of the 700R4 trans.

700R4 0.70 trans 4th gear
3.55 8" rear gear
P235/75 R15 rear tires (fairly wide and large)
Tach Reading over most of trip: 2000-2100 rpm
15 mpg over mostly a rural black top road (Route 2 north, Route 1 south)

I guess when you consider a V8 engine as two 4 cyl engines the near 2:1 gas consumption makes sense but I thought I was deliberately building a "conservative hot rod" with a high ratio rear, OD and big rear tires for a mild V8 in a light car. The acceleration performance of the car is fun but I had hoped for better mpg with the 700R4. In any event I put the side curtains on the car for the winter yesterday and will have time over the winter to think over the range of driving to local meets next summer.

Best Wishes,
Don Shillady
Retired Scientist/teen rodder

Postscrip: I went out to Sonic for a snack and noted one rear tire looked a bit low. Checking showed both rear tires were at about 10 psi! maybe that hurt the mpg a bit but I doubt that it would bring the mpg up over 17 mpg so I will just tell myself its 16 mpg until I can run it around the course again.

Don Shillady
Retired Scientist/teen rodder

Low tire pressure adds a lot drag and could effect the milage drastically.

and that thing has a terrible drag co effient----------------

Hi Don! Thanks for the update(s).. 15 for a pretty conservative build is poor mpg. But having the tires at 10 or less surely dragged it down, but by how much? I'll be curious as to the next measurement results.

And breaking the coil tower is curious. I don't think the material between the coil and firewall is helping, I'm thinking it may allow movement but I'm not holding it in my hand(s) so it's only my imagination of what it looks like (Hint / Hint /Hint re; pics)....

One of my last trips out my car just died! I wasn't far from my shop and my uncle came out and flat towed me home. I later found I had run out of gas! The gauge says I have a 1/2 tank, but when I dipped the tank with a stick.. It was dry! Lousy TPI Gauges! I've had enough of them.. I just ordered a new set of Classic and wil replace them over the winter.

Jerry is correct again. A "flying brick with a 2 inch chop" is still a "flying brick" aerodynamically! For reasons I don't remember I had a 3/16" spacer between the coil and the firewall allowing the mount to flex and pushing the top of the coil toward the lip of the firewall eyebrow. Removing the spacer and adding a foam rubber pad (pipe insulation) reduces the mount flex. I will rerun the test as soon as we get some clear weather here and I can pump up the tires to the 35 psi maximum. I think the 1st-to-2nd shift is too sharp with almost always a tire chirp which may have loosened the seal of the tubeless tires to the rims. Hopefully using the 35 psi pressure will seal again or maybe I will have to remove the tires and apply some sort of sealer to the rims or use tubes as a last resort. I'll post new results when I can run the course again with better tire pressure. The interesting thing is that the car is light enough so that even at 10 psi the tires were barely noticeable as underinflated. Another chore I need to do is to weigh the car but I expect it to be in the 2600 pound range. This kind of open-ended discussion with experts is what makes this Forum valuable!

Don Shillady
Retired Scientist/teen rodder

Originally Posted by Don Shillady

Jerry is correct again. A "flying brick with a 2 inch chop" is still a "flying brick" aerodynamically! I will rerun the test as soon as we get some clear weather here and I can pump up the tires to the 35 psi maximum. I think the 1st-to-2nd shift is too sharp with almost always a tire chirp which may have loosened the seal of the tubeless tires to the rims. Hopefully using the 35 psi pressure will seal again or maybe I will have to remove the tires and apply some sort of sealer to the rims. I'll post new results when I can run the course again with better tire pressure. The interesting thing is that the car is light enough so that even at 10 psi the tires were barely noticeable as underinflated. That is another chore I need to do is to weigh the car but I expect it to be in the 2600 pound range.

Don Shillady
Retired Scientist/teen rodder

Don,
Running 35psig may give you better mileage, but it will also prematurely wear out your tires due to them running "crowned" with only a portion of the tread pattern contacting the ground fully. To get the best mileage plus the best wear you need to chalk the tires, drive a short distance (100' is enough) and look at the chalk line. Raise or lower pressure until you see the whole line disappear at the same point, showing you that the whole tire is in contact with the road. You'll then get the best of both performance and mileage from those tires. I'd venture that about 20 to 24 psig will be the right numbers for the back.

Don S., I'll agree with what RSpears said. 35 PSI is going to be to much. The car will ride harsh also. I started at 35 and did the chalk thing. I think I'm at 25 / 26 now.

But, I'd be curious as to what the MPG totals at that pressure!

34-40 and others, Thanks for the suggestions. I did the chalk trick but only at 30 psi. It may be slightly on the high side but I want to favor mileage. Anyway you can see from the chalk pattern that the whole width of the tread is present. I also saw a suggestion on a Corvette web site to just use 5 psi less than the max rating. I also saw other suggestions ranging from 22 to 33 psi. Anyway the pattern looked good to me so I used 32 psi in the front tires and 30 psi in the rear tires and took the identical 63 mile trip as before including a stop at the same spot in a McDonald's parking lot and a quick lunch. This time I used only 3.889 gallons of fuel (WaWa 89 octane) so this run gave (63 miles/3.889 gallons) = 16.19954 mpg or about 16.2 mpg for an improvement of about 8.02%. Thus I will have to keep checking the tire pressure and 32psi front with 30 psi rear looks like about all I can do to optimize the mpg. Of course this little experiment was fun because I got to drive on rural roads but I did keep the speed roughly between 57-62 mph since I saw some traffic pull over tickets along the way. Again the Tach stayed under 2100 rpm the whole trip. On the previous run I hit 70 mph a few times but that is about the top speed with the top up due to vigorous flapping of the top at speed. I also recall seeing a wind-force curve versus speed somewhere and it is pretty steep after about 35 mph. When the Model A body was new most highways had a 35 mph limit so the flying brick aerodynamics was no problem then but when you get up to 60 mph the drag coefficient becomes important as Jerry noted. As far as the ride goes the choice is between "harsh" and "harsher" anyway although most of the trip was on smooth roads. However there is one pothole in Route 1 just north of Ashland that I forgot about and when I hit it again this time it almost took out some of my eye teeth! Well I did not even get to 17 mpg and 20 mpg seems impossible even with my 0.7 4th gear. I do note that the 1977 Z28 had a 3.42 rear gear compared to my 3.55 rear ratio. Thanks to several of you for suggestions as well as some unknown folks on Corvette chat rooms. I look at the Corvette comments because technically my engine is from a 1976 Corvette. The first picture is for 34-40 as to the new padded mount for the coil. I am not happy about all the wires which resulted when relays were added for my dual fan setup and the plug wires could be hidden better but you can see the edges of the black foam around the back of the coil and the mount is now flush with the firewall. The second picture shows the chalk pattern for 30 psi in the rear tires as Roger suggested. One good result is that I was toying with the idea of changing to smaller rear tires for better acceleration but obviously this experiment shows that I better stay with the 235/75r15 tires!

Don Shillady
Retired Scientist/teen rodder

Tire pressure might help a bit Don, but you've got to remember that the car is about as aerodynamic as a barn door!!!!! Unless it's going through the air smoothly, which a 20's and 30's car (with a few exceptions) were designed as a low speed rig, and aero was never a concern......

Don,
Just to be sure that you understand, the chalk process is not to see the pattern on the floor, but to see what's left on the tire after driving a short distance. Your floor pattern really doesn't tell you anything. For the process, a large parking lot on a Sunday when the store is closed, or a big church parking lot during the week work well, and you'll likely need an air "bubble" tank to add air during the process. With the tires warmed up, check your pressure with an accurate gauge for a reference point - I only use a digital gauge that has 0.5 psig resolution. The old mechanical gauges are useless for this - not repeatable, and not accurate enough. Draw a chalk line across the tread, not necessary to "paint" the tire, a 1/2" wide line is good, side to side across the tire. Drive about 100' or so and stop, check the line and see what part is scrubbed clean. If the center is gone but the edges remain you're over inflated, and would let out 1/2 pound of air and try again. Repeat the process until the nominal 100' drive scrubs the chalk clean, or leaves an even pattern across the tire. If you're lucky you'll hit it without going too low, where the outer edges are cleaner but the center has more chalk remaining. I'd bet a cookie that your 30psig is still at least five pounds too high.

let it down 5 lbs at a time til too much then reverse and add a set amount ( 2 or 3 lbs) til too fat that way and then reverse with smaller increment---should be able to do it in 4 trys-----called bracketing in artillery

I used the 'chalk trick' to get the optimum tyre pressures for the Cobra.
Finished up at 22 psi for the front, and 18 psi for the rear.
With the pressures set by this method handling improved markedly.

Thanks for the pics Don! Best part of your write up... you were having fun!

Keep playing with the tire pressures to find the best balance between ride / economy / handling.

But most important, keep having fun!