The last of the engine hardware to appear before Christmas turned up yesterday. Not a huge amount but quite useful nonetheless.
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I've always used Arias pistons since the early days. These are a spare set with slightly lower compression (6.8:1) than those currently in the motor. They're teflon coated to give some extra durability. As far as pistons weights go, these are on the heavy side. The bore size is slightly larger than a big show Top Fuel engine (4.250" vs 4.187") but we use a shorter stroke so they're a bit more revvy.
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Ferrea produce some very exotic valves for very exotic cars (Ferrari, Lamborghini, Bugatti etc). In terms of the Corvette though, these are custom made, 2.00" exhaust valves. Very nice pieces and should put up with a lot of abuse. When time allows I'll get an intake and exhaust valve side by side for a comparison.
In theory the body will be back on the chassis in a few days, so I might have some photos of what looks like a car...quite soon.
To pick up where I left off regarding the pistons being slightly different to a regular one, the photo below (from many moons ago) shows the physical attribute differences between a blown nitro piston & rod assembly, and a regular steel rod & piston assembly (from a traditional naturally aspirated big block Chevy) - note the rings are missing from the fuel piston, and the buttons which slot in when the wrist pin's installed.
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This is the job card for the Vette motor pistons, numbers, lots of numbers:
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Although I'm using early BAE Fuel heads rather than the current AJPE billet heads, due to the chamber CC volume being nearly identical, it still works nicely. Later heads used in the big show cars have much bigger chambers, which flow much more air, but it comes back the old thing of needing the fuel output to match. In this combination with a small pump, everything gets downsized from big show Top Fuel.
Regarding the compression available (6.85:1 vs 7.2:1), the idea would be that due to weather conditions (or perhaps track conditions), a full rack of rods and pistons with the lower compression would be installed if the weather was better (cooler conditions, lower corrected altitude etc). The high compression rack would be used during the hotter months and better track conditions.
Whilst I'm going through paperwork... one of the more secret aspects on an engine build such as this, is the camshaft. When the guys in the US are racing competitively and every little counts, a camshaft spec is a closely guarded secret.
It's common to see a cylinder firing order swap (No. 4 & No. 7 typically). This serves two purposes; it helps reduce crank deflection under high load (the crank will physically twist from prolonged high energy events) and it allows the motor to reach peak RPM sooner, and run smoother at said high RPM.
Since we're combining the old stuff and the new stuff in terms of technology, a little bit of that crosses over into the TFX motor in the Corvette. The basics of the current camshaft (to be swapped out late next year once some testing's been done - hence this isn't really top secret data as this is a 'just get from A to B' setup:
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While the big show cars will have cams with a duration of over 300 degrees, we use something a little more tame. The operating window in terms of RPM can be a moot point as the goal is to get the engine up to 8k RPM as quickly as possibly to ensure maximum fuel pump volume is available. There'll be an RPM dip when 2nd gear is selected, and this can potentially cause damage as the motor will temporarily go into a lean condition from the drop in RPM and hence a reduction in fuel throughput.
Piston(s) would get nipped, which is called 'backsiding' - where the ring land area / skirt gets overheated from the temporary starvation of fuel, causing a lean condition (remembering that the fuel has a role in cooling the motor as well as fueling it). It's common for it to happen on the burnout and any subsequent damage on the run which manifests, can be quite substantial.
Back to cams and stuff...valve spring technology has improved over the years, allowing greater total lift for both intake and exhaust. In this application, .714" lift is fairly large. Anything over .700" in a nitro engine is on the aggressive side. Spring pressure recommendations are a bit of an odd one here, I've always used Manley Nextec springs which had an open pressure North of 1000lbs, so backing it down to just over 600lbs is a change from the norm.
In recent years, the evolution on valve-train elements has centered around push rods and in particular, rocker arms. I'm using standard (albeit racing spec) ratio rockers. More extreme applications will use a higher multiplier on their ratio to achieve greater valve lift i.e. 1.65 vs my 1.57 for example. In some applications you might even see individual rockers with differing ratios i.e. your 8 intake rockers might have differing ratios for specific cylinders, and similar for the exhaust rockers.
This is something that only comes from a lot of R&D and access to blower dynos to assess intake charge in real time. At the nostalgia level, it's not really a thing - the work involved in ensuring a cylinder's combustion is as efficient as possible (all things considered) is approached from the opposite end by using a different lengths connecting rods to provide differing compression throughout the motor (certain cylinders being inherently stronger or weaker by design or as a byproduct of the firing order).
In about a week or so, the body will be on the chassis and it'll be full steam ahead to get the last of the tin-work, chassis and body mounting completed. In the meantime, I'll be starting the bottom end check and re-assembly of the motor next weekend. Piccies and drivel shall follow soon after... data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7
A picture says a thousand words...or something like that. The body needs a few bits tidying but is nearly ready to be draped on top of the chassis. From here on in, it's full steam ahead hopefully. I might be able to update things before Christmas, we shall see...
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The rear wing, as mentioned before needed raising on the left hand side and un-twisting. Some clever cuts and new glass grafted in sorted that.
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The nose required some work; this was more an aesthetic than corrective - to get the body at the right height it needed a little bit of glass grafted in to just get the ride height sensible in the finished state.
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The briefest of updates before the Festive break...
The new rear end gears have arrived in the UK. They wont be collected until early January, but that's the last of proper hardware to sort. I think I touched on it before, but during the last event the car raced at in 2015, it hurt the ring & pinion gear (4.11:1 ratio 9.5" gear set).
The only spare available was a 3.92:1, that isn't ideal for this application. That gear set had been installed and run, but I wanted to revert back to the correct one, so that's the plan in the coming months (they can be a bit of a bugger to set up sometimes). I'll sort some snapshots out when time allows.
All that's left to say, is a very Merry Christmas and a Happy New Year to you pleasant chaps following the thread. 112 days until Easter...data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7
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Happy New Year folks data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7
Today the body went back onto the chassis. This was the first time the two had been combined since the body first arrived in the UK. Given all the theoretical dimensions we were working to, and how wonky the body was prior to it being cut, sectioned, trimmed, cut & sectioned some more - it wasn't necessarily a given that the body would sit right, nor the roll cage be at a sensible height.
As it turns out, the maths and artistic eye that led the process, meant it went very well. Before we get to that though, an explanation of the last bit of body glass & filler work that was undertaken.
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The front end was a straightforward filler and sand job, so not much to explain here...
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The sectional cut beneath the A pillar is a similar story, it just needed some filler and sanding to achieve a nice line prior to handover to the painter (that wont be for a while though).
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This is the start of the escape hatch in the roof. Whilst the driver would enter the car with the body propped up, he or she would generally get out of the car from the top. The latches and brackets come from McKinney in the US; one of the biggest chassis builders in the sport who provide kits for this kind of stuff.
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The rear of the body is where all the issues were; I've mentioned the twist in the wing and the height being 2" from left to right. What didn't help was the fact that the light cluster section (across the full width of the body) was also low on one side.
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This was a bit of a pain and had to be corrected. The finished article provides a good basis for paint - overall, given what we started with, it's a miracle that we've managed to turn it around.
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With all that done, it was the moment of truth - draping the body over the chassis with the revised roll cage in place. Job's a good'un - when the car's on the floor with wheels and tyres in place, it ought to look damn near perfect in terms of stance.
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The regular fabrication can now commence and it should start to come together quickly. I'll have another update in a few days data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7
"This was a bit of a pain and had to be corrected."
You could say it was a pain in the rear end.....
I'll get my coat data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7
A brief one to keep things ticking over...
Yesterday I popped to Silverstone to pick up the new ring & pinion gears. The story behind this goes back to 2015 and the National Finals, when in Chaos guise, the car did some damage to the diff.
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The spare gear set that went back into the car for it to be able to complete the weekend, wasn't quite right for that application, nor this one - so the new gears are merely a corrective action to get the diff back to the right numbers. In the future, they might change again if performance requirements dictate it, but the change would have a knock-on monetary effect i.e. the engine's likely to see higher RPM and therefore will shorten the service life of valvetrain components and connecting rods. Time will tell on that one.
More updates towards the end of the month data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7
Update time...
Last week's storm was a bit of a bugger. It managed to rip the roof clean off part of the workshop. Not so good. Saturday was a damage limitation exercise of moving as much kit as possible into the remaining workshop space whilst a replacement roof is sourced.
In other news, the rear tree of the chassis is almost complete. I mentioned previously that it's a bit of an engineering task itself; there's a fair bit of kit to mount into it (fire bottles, Co2 bottle, fresh air bottle and oil catch can). The result of some clever design though is a simple, lightweight framework that allows everything to nestle neatly within it whilst being easily serviceable. Once the tree is complete, the car will be off the jig and the front end can be tackled; it'll look like a car once again.
The top end of the motor (intake manifold, blower, injector & restraints) is back with me for final assembly. I have some changes to make to tidy up a few pieces, but I'm hoping to have it finished during the next week. I'll try to post a couple of updates as it goes data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7.
Have you kept a log of the man-hours burned so far?