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Which means "Evil Twin". Lets see your projects where you change boring into fun or create the fun from scratch.
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PostPosted: Wed Apr 01, 2020 10:48 am 
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And finally here's the current state of the new block:

Here's where I ended up for the weekend. Crank sensor, knock sensors and valley harness visible. These pistons have slight domes rather than the true flat tops used in '00-'05. I may be able to sell them to some 4.9 builder. This engine has 2mm smaller wrist pins than the 4.9 and older Northstars, so the builder would have to have the 4.9 rods bushed down a bit.

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PostPosted: Wed Apr 01, 2020 11:00 am 
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So... what's up next?

The new block needs to have TimeSerts installed in the head and main bolt hole locations. It needs to have the block drain bosses drilled and tapped for 1/8" pipe. Block drains are extremely handy for hot rodders, but just another leak point for OEMs. GM built the engine with block drains in '93, then stopped drilling/tapping the bosses, BUT left the bosses in the dies (don't have to pay for new dies) so the bosses remain on the block through the entire production run.

There are bay-to-bay breathing windows in the main bearing bulkheads above the crank. Since the blocks are die cast, these windows can't be cast in place. They are bored from the front of the block, and thus have sharp corners. There is actually a handful of top-end horsepower available in porting those windows by smoothing/radiusing the corners. That just takes a few long narrow strips of sandpaper and some patience.

From '93 on, the Northstar always had problems with corroding out the 11x1.5 head bolt threads at high mileage, requiring the heads to be pulled and steel inserts to be installed in the block. This is the engine's one real weakness, as once this has been done, they're bulletproof.

About '04ish GM switched to the 11x2.0 head bolt threads that LS engines had been running since '97 without problems and stopped having those problems with the Northstar.
On my prior block, I have inserts in the head and main bolt holes. The head bolts are 11x1.5 and the mains are 10x1.5. ARP has a set of studs for a VW application that happens to also fit the 11x1.5 Northstar applications, and I had a set on my prior build.

LS engines have different length head bolts, so there are only a handful of studs in an LS head stud kit that would work with the Northstar. I have not contacted ARP to find out if I can piece together a stud kit from just those long studs or not. It would also be easiest just to use stock head bolts, which should be strong enough as I'm not running crazy cylinder pressure. However, the Cometic gaskets compress differently than the stock ones, so the stock torque sequence no longer applies and I don't know where to get a torque value that does apply.

I guess that's my next piece of research.


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PostPosted: Wed Apr 01, 2020 6:17 pm 
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A LOT going on here!

And here I thought it was going to be a simple rebuild.....simple like mine I guess ;)


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PostPosted: Fri Apr 03, 2020 6:40 am 
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Nothing is ever simple. :shock:

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PostPosted: Fri Apr 03, 2020 9:00 am 
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Not in this crowd.


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PostPosted: Fri Apr 03, 2020 9:04 am 
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I kind of wish I had knock sensors like I think I'm seeing in the last pic posted.....this might be the the time for me to add them. I think last tie I couldn't convince myself that on 6 cyl bank just 1 was enough and my ECU allows only 2 so.....I wasn't SURE if Id be helping or lying to myself. but my 6 cylinders aren't really longer than a SBC so it seems ok...but BMW uses 2 on there 6cyl.....and round I go.

But I am a bit jealous seeing them on your ending.

Which tilton clutch are you planning to use? I see a lot of effort in the flywheel so I'm guessing its a big part of the plans.

I love this project. I nearly bough a fiero when they 1st came out...I graduated HS in 83 and remember reading articles about them and thinking I'd grab one when available. Then the HP was disappointing and I bought an '84 H-D RX1000 motorcyle that had nearly as much hp as the fiero I think :) Then the 6cyl cam available but I had motorcyle payments...then the GT which is what the car should have been from day one but life didn't allow. I remember seeing northstar conversion kits but the while thing was not in my budget at the time.....and then a lit car that used the fiero cabin and ne wframes front and back with a SCB and really nice suspension, again WAY out of my budget.....anyway, I love seeing the project, HATE your header that must be costing you 10+% of you HP but love the project.


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PostPosted: Fri Apr 03, 2020 10:10 am 
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Yeah, those are knock sensors in the valley. I discussed the valley harness in my first post on on tearing down the engine.
How many knock sensors do modern Ferrari V12's use?

BMWs sensors are arranged longitudinally, so you may be able to put one at the 1/4 point in the valley and the other at the 3/4 point, then apply some time-domain filtering to the signals. I'm not sure if having two knock sensors is necessary for catching detonation in the first place, for confirming that a particular cylinder is detonating or for filtering out noise to avoid registering false knock.

Your compression is... 10.5? I would say keep 93 in it and don't worry about it. My prior Northstar ran fine on 93 with 11.5 and a bunch of oil in the chambers from bad bore finish. It would pull cleanly and accelerate uphill from 1000 RPM in 5th gear.
BMWs V12 for the McLaren F1 runs 11:1 with no knock sensors at all.

Classic Mark Logic (CML) wants the BEST knock sensor arrangement, which may be four sensors... but even two sensors would be better than zero sensors.

One of the things I haven't understood about knock sensing is that detonation happens with the piston way up in the bore, so the shock/noise has to go through the bore liner, through the water jacket and through the outer wall of the block before it hits most knock sensors. And the head gasket interface can't be very good for transmitting shock, especially with composition gaskets... maybe MLS gaskets are better but still have several interfaces to travel through.
I would *think* that you'd want knock sensors on the head because that's the most direct path for the shock/noise to travel from the chamber to the sensor... but not only does NOBODY do that, EVERYBODY (that I've seen) puts them on the block.

And AAAAAAAACKSHULLY... there is a way to get TWELVE sensors ;) Spark plug gap impedance sensing/plasma ion sensing... whatever you want to call it. I've seen aftermarket modules advertised that do that. Bugatti does it on their W16 because apparently 45 degree firing intervals are not far enough apart to distinguish with conventional knock detection methods.


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PostPosted: Fri Apr 03, 2020 10:34 am 
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Tilton clutch: I want to run a 7.25" organic dual disk. It just BARELY fits in the original Fiero V6 five speed transmission that I'm running. The car will get a 6 speed eventually, but that adds 50# to the vehicle, so I'm going to hold off for a bit.

Tilton doesn't like selling dual organic disk sets, as their disk supplier tends to come in thick and then the dual organic setups have trouble disengaging. I'm certainly not above shimming my pressure plate a smidge, then running those disks until they don't work, then pulling the shims to get a little more service life out of the unit.

However, Powertrain Technologies will sell me a dual organic clutch. Their stuff is more or less interchangeable with Tilton's, as all the race clutch MFGs want to be able to pull customers from the others. Quartermaster will also sell dual organic sets, but they don't like to give me the engineering data I need to design something new around their clutches.

I'm looking at building a single-stroke pressure intensifier to adapt the stock Fiero pedal throw to the Tilton throw out distance. If I don't do that, I'll end up with something like 70# of pedal load. Bleh. That can also bend Fiero clutch pedals.


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PostPosted: Fri Apr 03, 2020 11:19 am 
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The tilton I have is a 7.25 triple carbon-carbon and it came with 2 shim things....a thick and a thin to extend service life

Way back when I changed the linkage at the trans to get the 308 pedal travel to match the new clutch travel....stock it would have been well under 1" pedal motion and high force. After the new linkage it was freakishly easy to operate...honda civic force easy. The V12 meant making everything shorter so I switched to a short hydraulic throw out bearing (the old setup I had to add an adapter on the throw-out so the bearing would engage the fave of the tilton correctly) then I think its a 5/8" master and I have about the same result as the mechanical setup but short enough to fit the smaller space.


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PostPosted: Fri Apr 03, 2020 2:15 pm 
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mk e wrote:
The tilton I have is a 7.25 triple carbon-carbon and it came with 2 shim things....a thick and a thin to extend service life

Way back when I changed the linkage at the trans to get the 308 pedal travel to match the new clutch travel....stock it would have been well under 1" pedal motion and high force. After the new linkage it was freakishly easy to operate...honda civic force easy. The V12 meant making everything shorter so I switched to a short hydraulic throw out bearing (the old setup I had to add an adapter on the throw-out so the bearing would engage the fave of the tilton correctly) then I think its a 5/8" master and I have about the same result as the mechanical setup but short enough to fit the smaller space.


The Fiero V6s used a Getrag 5 speed that's decently strong for how it was originally rated. It holds up as long as you don't drive it like you're 17. The transmission I have now has the 3.61 final drive, with 3.50 first and 2.19 second. It also has the external release arm throw out linkage that Fieros had. The slave cylinder pushes on a lever outside the bellhousing, which rotates a shaft that has the throw out fingers inside the bellhousing on it.

I am working on a transmission with the 3.94 final drive used with the Quad 4 engines, and a custom 2.50/1.85 first/second set. That will use the hydraulic throw out bearing & compatible bellhousing from a later transmission.

The beauty of the pressure intensifier setup is that it *SHOULD* work with both.

If I multiply out the Tilton throw out load times throw out distance, then divide by Fiero pedal travel, I get something like an 18# pedal load... which is ridiculously light, as you said. But then again, isn't clutching a huge amount of power with a light pedal a hallmark of good engineering?

Also, your carbon clutch gets the shims because the disks are the most expensive part of the clutch, so they want to be able to use them up completely.


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