Project Heavy Metal - an all steel classic, remastered

[Jonny042]

Finally got a set of plastic stacks bored, tapered, and bell-mouthed. I severely underestimated the amount of effort this was going to take but I am ecstatic to have it done!

There is a very dramatic difference between the "T" spec stacks and what I have now. I step bored them in 1mm steps from 39 to 44 over the length of the funnels, then reamed with a monster taper ream and finally machined a radius on the top.



Before and after of the top of the trumpet:




The bottom of the stacks where they meet the throttle bodies had quite a bit of core shift that would be inconsequential with the 911T setup but had to be corrected. I centered the mill on the throttle bore location to remedy this.

Side by side before/after:



The 39mm bottom bore has about 60% more flow capacity than the stock T stack. More important to my setup, the cross sectional area is 17.5% greater than the S/RS setup - which should support my horsepower goals and provide sufficient flow for the 3.2 Motor with 39mm intake ports.

[Mixed76]

Bizarre that the original casting had enough material for this big a change. What is the remaining wall thickness, and do you have any concerns about residual mechanical strength?

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[Mixed76]

Also, really nice work!

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[Jonny042]

Throttle bodies before and after:



Stock T:



41mm throttle and 39mm bottom exit. There is enough meat left over to do 42mm but it would be very, very, close. The wall thickness is already down around .110 and I'm not sure if I should risk the extra .039"....... I'm considering whittling down the throttle shafts a bit to provide more flow area instead. But then the devil on my shoulder assures me not to worry, and go ahead and do both!!

[Mixed76]

Do you mind explaining why you have the stack outlet 39mm, and the throttle body inlet 41 or 42mm? With the quality of your work so far I think you will not have that much off center.

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[Jonny042]

Originally Posted by Mixed76

Bizarre that the original casting had enough material for this big a change. What is the remaining wall thickness, and do you have any concerns about residual mechanical strength?

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The original casting (well, molding?) outer dimensions were large enough to allow for the 36mm bottom exit for the S/RS, so really it's just an extra mm off the wall thickness from that. The plastic stacks actually have enough meat to take off a bit more but the throttle bodies are a different story.

[Jonny042]

Originally Posted by Mixed76

Do you mind explaining why you have the stack outlet 39mm, and the throttle body inlet 41 or 42mm? With the quality of your work so far I think you will not have that much off center.

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The "bulge" in the intake tract around the throttle plate allows for the cross sectional area to remain consistent, given that at full throttle, the throttle shaft takes up some space. In this way the air may have to divert it's path a bit but you will not be asking it to speed up or slow down.

Probably splitting hairs and highly theoretical as most EFI throttle setups don't account for this.

[jpnovak]

You have really made a lot of progress here. Now I know where to ship my next set of MFI stacks for rework.

I didn't think you could take plastic stacks out that far due to wall thickness. I stopped at 37mm. I guess I have some room to go.

I too ran into some hole offset on the 911T throttle bodies. It worked in my favor since I am doing an offset port shape.

[Mixed76]

Makes sense, thanks. That step might give you a little anti-reversion too.

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[Jonny042]

Originally Posted by jpnovak

You have really made a lot of progress here. Now I know where to ship my next set of MFI stacks for rework.

I didn't think you could take plastic stacks out that far due to wall thickness. I stopped at 37mm. I guess I have some room to go.

I too ran into some hole offset on the 911T throttle bodies. It worked in my favor since I am doing an offset port shape.

Thanks, Jamie!

I'm looking forward to boring another set that I have as spares. Just to see if I can measure the time it takes with a watch. Instead of a calendar. Or the change of seasons, which is more often than not the pace of my progress!

Gaskets arrived from the UK so I can get back at putting the MFI pump back together and seeing if it holds fuel and oil..... gaskets came from Bosch and run a couple hundred bucks for the two sets. Worth it to have a nice fresh cover for the rear measuring port. Not sure what the graphic on the labels means. That's a damn big wrench.

[Mike Billings]

My throttles are enlarged to 40mm with 39mm ports on a 2.8 8000 rpm engine. Plastic stacks. Not too big at all.

www.Eurometrix.ws

[pdtweeks]

Not sure what the graphic on the labels means. That's a damn big wrench.

Corvette tool

[Jonny042]

Originally Posted by pdtweeks

Corvette tool

For some reason those two dudes remind me of the Village People.

[Jonny042]

Originally Posted by Mike Billings

My throttles are enlarged to 40mm with 39mm ports on a 2.8 8000 rpm engine. Plastic stacks. Not too big at all.

www.Eurometrix.ws

If you don't mind me asking, what are you using for an MFI pump?

[Jonny042]

I've been working away at stripping down my motor and figuring out what it needs. No pictures, as they would be pretty dirty pics.

But I am starting to suffer "while you're in there" syndrome. The motor was rebuilt not long ago and it's obviously an excellent, low miles example.

Even though the previous owner spent $20K on a rebuild, the cam chain sprockets didn't get replaced even though they are visibly worn. There goes another $539...

I also ordered a spiffy new sump plate:

[Jonny042]

Ooops:



I could see myself sourcing a slightly lower profile drain plug for it.

[johnsonfm3]

Now THAT is a Spiffy Sump Plate!!

[Jonny042]

Not a lot to report lately - motor build is starting. Trying to move forward on a few fronts at once.

Shiny parts!!

[911T70]

I'd been looking forward to seeing engine work. This is going to be exciting!

Still enthusiastically following your work from deep-frozen Ottawa.

[lvporschepilot]

Originally Posted by Mike Billings

My throttles are enlarged to 40mm with 39mm ports on a 2.8 8000 rpm engine. Plastic stacks. Not too big at all.

www.Eurometrix.ws

+1. I run mine with a huge cam too and it's street manners are just fine. Idle is a bit rough but I don't mind.