Tag Archives: Sprint-T

Still nothing interesting to report

I have a visit to the Lab Rat Keeper the day before Valentine’s day for a quick blood draw and BP check, and maybe a hamberder or something at one of the several fast food joints in the area. The covfefe house is closed, but despite having a large chunk of the building burned away the Mexican chicken place almost across the street from the office is still serving food. I’ve never been there, it might be a good time to try it out.

On the Sprint-T and Mini Sprint-T front, there has been a minor change in the design around the main cage. Basically the upper frame rail is now running in segments inside the hoops instead of over the corner after I did another stress analysis that showed too much bending stress on the hoop with the rails running outside the bends and the diagonal braces welded to the top and sides through bends and/or bent gussets. What I had before was the rails welded to plate brackets on both sides of the hoops in line with the horizontal and vertical legs of the hoop. What I have now is the rail welded to the top of the hoops right at the end of the bend and also where the diagonal welds to the hoop, kinda sorta. Actually the diagonals have the same size bends as the hoops so the bends line up, and using the same bend to make the connection to the vertical leg as a gusset to carry those loads directly into both legs of the hoop without imposing bending loads that have to be carried through the bend in the hoop. That’s because the bend is the weakest part of the hoop, but by using gussets and routing loads away from the bend it can be made to not fail in a wreck.

On the down side dividing the upper rail into segments and welding it to the top hoop next to the bend slightly reduces the d4 effect for torsional rigidity, because this change moves the top rails about 9″ closer together side-to-side. Instead of being 48″ apart they will be 39″ apart across the top of the cage. That’s a 56% reduction in stiffness in that plane, but that plane only contributes 25% of the overall torsional resistance of the frame. Basically this keeps both of the side planes that resist forces up and down of the total twist vector at the same stiffness, and has no effect on the bottom plane at all so even though that one plane is reduced 56% the total stiffness is only reduced by about 13% total in torsion. Noticeable but not critical. Second downside is the reduction in the access hole in the top of the cage by 4.5″ when it was already barely wide enough to get in and out without twisting sideways. So guess what I get to do getting in and out of the car?

In things that have nothing to do with me or the Sprint-T personally, the squirrel apparently made its escape prior to the arrival of the people we paid almost $400 to remove it. Also Mrs. the Poet has fallen and bruised her backside, enough that she makes involuntary noises about every other step, sometimes very loud noises accompanied by rude words. I’m not so bothered by the rude words as I am the volume used and the frequency. If her butt hurts she needs to lie down and let me take care of her instead of running around complaining I’m not doing anything for her.

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I found something that made me rethink what I had planned for the Sprint-T

I found a You Tube video that makes me seriously reconsider using the FIRST TPI efi manifold. One of the things I had been led to believe was this manifold had an extended RPM range and kept on making power at high RPM. Well, I guess compared to the original TPI from the ’80s and ’90s that would be true, but this test showed a power peak well below the calculation crossover at 5252 RPM (HP= Torque in Ft-lbs*RPM/5252, the point where Torque = HP is 5252 RPM) and the BluePrint crate 383 makes more power and continues to breathe through a Chinese dual plane manifold and 750 CFM carburetor. Now granted the FIRST manifold made 44 more Ft-Lbs of torque, the type of head used for the FIRST manifold test was not listed, and the cam was not mentioned, but the BluePrint engine made the same torque (440 Ft-Lbs) with both a flat-tappet and a roller cam and that Chinese dual plane manifold and Holley 750 CFM carburetor. Something else not mentioned was the headers used in the FIRST test, where the BluePrint tests with 13/4” long-tube headers. Now for an engine packing 383 cubes that is a low-end torque header. Note edited 2019/26/1 to correct header tube diameter from 15/8” to the current size after getting better information.

Well, I plan on making a trip to Lowe’s today to get a couple of 5/16” bolts for the roll cage bender, a 5/8” bolt for the exhaust turnout bender, and a can of butane to make the bender work. So I should probably get to bed now.

More on the new seat for the Sprint-T

I have been sitting in the new seat to further evaluate it for further adjustments. Right now the biggest concern is the rib support impinging on the latissimus dorsi when I try to turn the steering wheel. The upper corner of the rib support is all I need to adjust, because everything else is Big, Too Big. The seat is 14.5″ wide at the back of the seat pan and I can get a hand between the seat and my hips, thickness, not width. And I can get a finger between the rib support and myself about ½ up the support, but right at the top corner it like negative clearance when I simulate turning a steering wheel, like right under my armpit.

Secondary concern is the fact the seat is aluminum, and like all aluminum structures is subject to fractures from work-hardening. What that boils down to is “adjusting” the seat could cause a catastrophic structural failure in a wreck that could cause jagged pieces of aluminum to be exposed right under my heart. This can be prevented by annealing the metal. This requires heating the seat to a point well below melting but still pretty warm, the standard test being to take a wooden matchstick and rub it across the surface and see if it leaves a streak of carbon on the surface which Google says is 800 – 1000° F and can be done with a propane or butane torch but only just barely because of the heat required to raise aluminum to that temperature. Aluminum moves heat like mad away from the heated area, called high thermal conductivity, and also I think it has a high specific heat the amount of energy required to raise the temperature of the material. But anyway, if I am careful I can soften the aluminum enough to make the adjustments without damaging the seat.

And I have to get up early for an RPG session that starts at noon and requires catching a bus at 1030 to catch the other bus that leaves the station at 1100 and gets to the stop closest to the game about 1135 or so, and then it is a 0.65 mile (1 km) walk from the bus stop to the house where we play, which gives me about maybe 3 hours of sleep if I quit now and play solitaire until my brain shuts down from lack of use.

More stuff arrived, part MMXXIX

Well, something for the 1:1 scale Sprint-T arrived, inspired by free shipping and Mrs. the Poet giving me the engine hoist for Christmas, and it’s something essential for driving the car second only to the steering wheel. Let me show you what it is.
The cover was in a different box inside the delivery box and I was in a hurry to check the fit in the car and on me

As you can see, without the mounting bracket it doesn’t set straight inside the body. The black stripe is where my personal centerline is supposed to be inside the car, and the seat is a bit off to the side and a few degrees off plumb. Also I was in a hurry to check the fit both inside the car and on my backside, so I didn’t take the seatcover out of the separate box inside the delivery box. That delivery box was almost as big as I am. This was also before I tweaked the rib supports to not bite me in the latissimus dorsi quite so much. The seat was actually a bit wide through the butt, but painfully tight in the chest area the first time I tried to sit in it even without the cover. The only way I could breathe was to keep my arms up and my lats stretched out. Getting back out to adjust the rib supports was really tricky and brought much amusement for Mrs. the Poet.

A better view of how much room it really has in the car.

Lots of room when the proper seat mount is bolted to it to raise the seating surface 3 inches above the bottom of the bucket

These pictures were taken prior to “adjusting” the left side rib support which you can see in the top picture is a bit closer than the right. I might need to “adjust” both sides a bit more before I start building the seat mount, because I literally can’t put my arms down from the “9 and 3 o’clock on the steering wheel” pose even without the cover. By using my body weight and holding the seat down with one foot I was able to use the other to move the left side rib support out until it was even left to right with the right side. Now I need to figure out how to move them both out by about an inch so I can sit in the car without holding on to the steering wheel, which comes out by the way. If Mrs. the Poet didn’t recycle the old phone books I think I might be able to do it without scratching the shiny aluminum seat.

And it’s kinda late at night/early in the morning (0500) so this seems like a good spot to wrap this up and go to bed.

It might happen

I have been watching Facebook and catching really cheap SBC engines and corresponding transmissions. Cheap as in $350 for a running engine and $500 for a rebuilt transmission. Or a mostly-built complete powertrain for <$1000. I might even be able to compete equally at Goodguys with it against the pro builds, if I get the suspension sorted properly. Even with an iron block and iron heads, and what they call an “RV” cam which is what most rebuilds get because they’re cheaper than a stock replacement cam when the cam is worn out, the power to weight ratio will still be sufficient to fry the tires if I put too much pressure on the right pedal. At this point I know I will need to dial the suspension a bit on the “tight” side (understeering), because I can get sideways (“loose”) with pressure on the “loud” pedal. This is called “power oversteer”.

Also the extra weight will help with the ratio between the spring/unsprung weight I brought up in an earlier post talking about the weight of the front axle assembly compared to the total weight of the mid-engine version of the car. Basically I need to get the weight distribution as close as possible to 50/50 because of how heavy both axles and associated elements are. There is only so much I can do for the rear axle, basically go as light as I can with brakes and brackets, but there are limits because I need to get a rear axle from a full-size car or pickup truck, and the parts have to fit and work with that. Even a relatively puny V8 build will destroy all the lightweight rear axles I can get in local junkyards, just look at what happens to Gen3 Camaros with the factory 7″ ring gear axle that started its production life under the 2.3L 4 cylinder Vega from the ’70s. Even with the OE 305 engine good grip and a heavy foot would cause broken differentials, ring gears, and pinions, what have you it probably broke on the V8 Gen3 Camaro. And for those thinking that won’t be a problem with the Sprint-T as light as it is, the load is independent of the weight of the car to a large extent, being mostly determined by power applied and grip. And grip is more dependent on tire choice than applied weight, and is highly dependent on where on the grip curve the particular tire is for the applied load. Which brings us back to tires being the most important choice in the design process…

So, there might be a budget to build the Sprint-T with an SBC and automatic transmission after all.

Nothing to write about still

Nothing of interest has been going on the last few days except paying the phone bill and buying a lottery ticket. I have decided to start getting the materials for the frame because except for a few brackets the design doesn’t change with any of the engines I’m looking at. The reason I’m doing this is Mrs. the Poet has declared my Christma$ present this year is an engine hoist which won’t be much use aside from building a hot rod or frying a turkey. And I’m not kidding about deep frying a turkey . Well, at least I’ll have something I can use it for after I build the Sprint-T.

I have more data

I finally found out the weight of a couple of different Subaru transmissions for comparisons with other engines and their intended transmissions. Especially for the Subie, using a transmission designed to bolt to the engine saves a ton of work.

Anywho, what I found out was the 2002 5 speed weighed in about 135-140 pounds, the later model 6 speed was 155-160, and the automatics were about 200-210. Also removing the front drive differential and drive shaft and welding the center diff solid took about 35 pounds off when converting AWD to RWD. So for the normally aspirated 2 liter engine bolted to the 5 speed with the front diff removed we are looking at an all-up weight of about 300 pounds and about 200 HP, to 460 for the 2.5l turbo bolted to the late automatic with the front diff propelled by 300 HP. And those power numbers are factory ratings, tuning for higher octane can raise those numbers considerably especially for the turbocharged engines that can refuel with E85 and really crank the boost.

And thinking about this kept me awake most of the night and caused me to wake up way early, after only 4 hours of sleep. I need to go back to bed now. Except I need to clear my inbox before I sleep.

Flogging of the deceased equine continues

I’m running out of topics, but not running out of words. I was lying awake after going to bed last night and of course I’m thinking about the Sprint-T, as that seems to be the topic I can’t not think about. What I was thinking about was various engine and transmission combinations and relative performance advantages and disadvantages. Specifically, I was comparing the Pentastar V6 and the Subie turbo engines and their available transmissions.

Since I was trying to go to sleep I left my various calculation and information tools beyond arm’s reach so I wouldn’t spend the night punching buttons or tapping screens instead of shutting down and sleeping. That… didn’t work so well. Since I didn’t have a calculator handy I did everything in my head, which is the opposite of what I needed to do to get to sleep. When I’m trying to stay awake I do math in my head, a trick I learned from making cross-country trips as a solo driver back when I owned a business in Nashville but lived in Dallas. Anywho I didn’t reach any new conclusions, or even any conclusions at all. What I did reach was I need more information to reach a definitive conclusion. The question is will the reduced weight and higher potential peak power of the turbo Subie result in better autocross performance than the fatter low-end torque and perfect gearing of the Pentastar and ZF 8 speed transmission? Between the variable valve timing and increased displacement of the 3.6 liter Pentastar there is about twice as much torque on tap below 3000 RPM compared to the Subie 2 liter engine, but the Subie and transmission weighs about half the Pentastar and ZF combination, so it comes close to a wash and really depends on the course, with longer straights and more sweeping turns favoring the higher peak power and light weight of the Subie where lots of single pylon turns favor the Pentastar and its higher low-end torque and better transmission matching.

So I guess the answer is the best is whatever I get first. 🤑 So whatever is available when I have the money, that will be the best choice. And the post title is “beating a dead horse” with more expensive words.

I found another problem

I was doing a little SWAG on the Sprint-T and tires and I came up against another problem. I can only hook up so much HP no matter how big I get the tires, because of weight. Traction is a function of weight and tires, and there is a hard limit to traction with a car as light as the Sprint-T trying to use UTQG 200 treadwear tires. I can get a little more power to the ground with sticky race rubber, but depending on the weight of the car I’m looking at 300 to 500 HP without boiling the tread off the tire before getting the pedal to the metal. More than that and I’m never getting my foot to the floor.

What I’m looking at is with the Subie engine and the T5 transmission I can stop cranking up the boost around 300 WHP, with the Pentastar V6 and the 8 speed transmission I can stop trying for more than 375-400 WHP because that transmission just weighs that much, the LS3 and the 4l70 I can pretty much leave in stock tune, and the LS7 I will need to put a block of wood under the gas pedal to keep rear tires on the car unless I’m running the softer compound of the race slicks I have available. Street tires will require really feathering the gas to maintain traction with just the stock 505 HP tune.

Now these are just what they used to call “back of the envelope” calculations but they are the best I can do with the data I can get, and the low precision of the data. Seriously I’m looking at ±0.1 G precision on a 0.8G predicted grip for the 200 wear tires, or 0.7 to 0.9G and a huge difference in performance between those numbers, so lets say we are talking about the LS7/3 we are looking at maybe 400-430 WHP on the low end, just shy of 500 at the high end. And as far off as I was on the unsprung weight at the front end I’m not putting much faith on these numbers either. But if the data is correct and I used to correct formulas, If I’m running an LS7 then I don’t need to use the dyno for anything more than fine tuning for gas mileage and verifying the stock tune is working correctly. Pretty much the same for the LS3 except I have more room on the other side of the equation, I could get away with maybe the 480 HP tune, but not the 525 HP tune.

And I didn’t get much sleep last night, and I’m starting to get a headache, so bedtime.

Not making much progress

I have been thinking and researching and thinking some more, but I still don’t have much in the way of progress to report. Also the injured finger has been breaking open and bleeding all over things, which can get a little messy as well as not safe because of spreading a growth medium for germs, so I took a walk to the CVS and bought a fresh box of bandages using the last bit of money left in the gift card this one gig I work uses to pay me. But seriously, I really had to buy a box of bandaids yesterday. When I take the old bandaid off the clean the finger I’m constantly snagging the flap of skin still attached to the finger and pulling it up and causing the injury to bleed like mad, but the flap is still alive so I need to keep it attached to the finger so it will heal. The situation is unless I can keep that flap alive I will have yet another nasty scar on my finger…

The issues with the finger also affect the Mini Sprint-T. My dexterity is reduced and my grip is hindered and that means making the kind of precision cuts needed to build the frame is a no-go until the finger is healed enough to work without the bandage. A slip of 0.01″ translates to a part that is a scale 0.25″ too big or too small, and wasted raw stock if I make it too small. So I have to wait until I can handle my tools safely before I can get to work on the Mini Sprint-T. If there is one thing I have learned in my 60 years, it’s patience. I will have to wait.

Another thing I have had to worry about is the weight of the front axle assembly with all the bits and pieces attached compared to the sprung weight of that end. There is nothing I can do about the excess weight except turn the heavy bits into “drillium”, also known as “swiss cheesing” the parts. The process involves a drill press, a lot of time, and usually several worn-out drill bits as holes are drilled in areas that don’t get a lot of stress in use and shouldn’t suffer fatigue failure. This only goes so far, and the parts that have the highest weight can’t be drilled out, like the spindles and wheels and especially the tires. Drilled tires would be useless. 🙂

And my finger is starting to hurt from typing, so this is pretty much it for the day.