[Squidward]

Hi,

Did a search, couldn't find apples to apples info.

I am going to take my block down to the machine shop to get cleaned and bored. I wanted to know if there is anything substantially different about a turbo'd build that would pertain to the machine work, like looser clearances, etc.

It's a std bore '72 350, getting punched out 040, with stock N crank.

I have Probe SRS forged pistons, and 4130 Tomahawk rods. I think these will be fine for the job based on my hp goals.

My goal is for around 500 hp max on mild boost (maybe 10-12psi) from a single turbo. Nothin' crazy. I plan on building a stout 300-350hp NA engine first, and then toss on the hairdryer. I've only ever played in the world of mild street motors.

Anything weird I need to worry about, like extra cylinder wall clearances beyond what the piston manufaturer recommends?
What about ring gaps compared to normal?
Would hard-block filler be necessary?
Any special fasteners?
Additional input welcome.

I don't plan on spinning the motor over 5500. I am hoping I don't have to get too weird beyond standard performance engine building practices.

Thanks!

[Travis Q]

Here's what I would do, for whatever that's worth.

Give the block a good deburr. No need for anything terribly fancy, just clean up any flash and sharp stuff that could be a starting point for cracks, especially around the main webs.

I would give the block an extra little bit of piston to wall clearance. Call the piston mfg, and tell them what you're doing with it. A lot depends on what alloy the pistons are made of.

The piston ring end gaps need to be a little bit looser than a naturally aspirated engine. How much looser depends on your ring choice. What's the bore size? What piston ring pack is the piston set up for?

At this power level, bearing clearances should be close to whatever you would run with a NA engine.

I wouldn't worry too much about a block fill.

Good fasteners are always a good investment. I prefer head studs, regardless of power level, but if you can't swing that, at least use a good head bolt and main bolt.

500 hp should be no problem at all. At 10-12 psi, I would not be surprised at all with 600 hp or more.

[Squidward]

Thanks, Travis.

I want to use the stock windage tray. ARP makes main studs, with 200K yield strength, but no worky with windage tray. Milodon makes studs rated for 190k, but says it works with windage tray. Any big deal in the 10k difference?

I'll have to call Probe. They included specs that got looser as one stepped up from their street level pistons to their race pistons, but never had any setup differences for boost.

Concerning rings: Anything to stay away from? I've read in some threads where some people said "Stay away from moly", but I'm not sure if they were playing in a "bigger sandbox" than I will be playing in.

Thanks again!

[Squidward]

I checked with Probe's website and found some ring gap info. It had a formula for gap based on bore size. They also suggested adding .005 to those numbers for boosted apps.

Suggested piston clearances was .0025 minimum to .0065 max. They said minimum was OK for NO street motor, but go looser for boosted or race apps.

I guess I have to figure out how loose to go. .0040, maybe?

[Travis Q]

I wouldn't be at all afraid of the Milodon studs, I have not used them, but they are bound to be a great improvement over a stock bolt.

I would think .004-.005 would work just fine on piston clearance.

I have used a moly ring in some pretty high power stuff. I really wouldn't be afraid of it at all; a fair bit of the negative stuff you hear about moly comes from the early versions of the rings that had some issues. Then it gets repeated/regurgitated forever. Is it a 1/16" ring or a 5/64" ring? You definitely want to stay away from the stainless and tool steel rings, they are pretty hard on the cylinder bores.

I'm no engine builder, but hope this helps anyway.

[screamingchief]

Squid,the moly ring talk you mention is most likely in reference to the nitrous guys.

Anything over say 175 to 200 hp shot and it's not @ all uncommon for them to find it'll knock the moly coating off the face of the ring,and then you tend to lose ring seal,and oil + nitrous = very bad things.

Boost & nitrous do indeed share some traits,but from my understanding that is'nt one of them,nitrous applications often have a very high/quick initial "shock" to the engine in terms of cylinder pressure,and that can be hard on things like moly rings & headgaskets and such,where-as boost tends to be applied in a more "linear" fashion.

Though like a nitrous engine,I'd probably still shy away from low tension oil rings for an application like this.

One other thing I'd mention here is this,if you plan to use the Cometic headgaskets,be sure the machinist understands and can supply a proper RA finish to the decks & heads for those gaskets,as they have some specific recommendations for those MLS gaskets.

Good luck with this!

Bret P.

[BruceWilkie]

I wouldnt run the factory windage tray... they are prone to cracking. Under 6000 rpm I dont think I'd bother... but a higher volume pan would take priority... Canton has some nice pans and sandwich mount windage trays...

Careful on your cam pick... you dont need much. It's not uncommon for tq to peak @ 500 rpm sooner and hp to extend @ 800-1000 rpm later over NA. Not optimum but an 068 works ok on a 400 with a turbo but an 067 might be better on a smaller motor

A custom grind would be far better. Travis can set you right with that better than I.

[Squidward]

Originally Posted by Travis Q

I wouldn't be at all afraid of the Milodon studs, I have not used them, but they are bound to be a great improvement over a stock bolt.

I would think .004-.005 would work just fine on piston clearance.

I have used a moly ring in some pretty high power stuff. I really wouldn't be afraid of it at all; a fair bit of the negative stuff you hear about moly comes from the early versions of the rings that had some issues. Then it gets repeated/regurgitated forever. Is it a 1/16" ring or a 5/64" ring? You definitely want to stay away from the stainless and tool steel rings, they are pretty hard on the cylinder bores.

I'm no engine builder, but hope this helps anyway.

I measured the ring lands, and came up with something like 1/16, 1/16, and 3/16.

I talked with the shop, and they were leaning toward .005-.006, but I told them that I didn't want to go straight for the upper end of the recommended clearance. I wanted .004-.005.

Does anyone have a brand and product line they would recommend?

And Bret - I haven't yet thought about head gaskets. I know that 350 options are limited, so I guess I have to look into that. I appreciate the heads up since I would hate to haul an engine back to the shop for putting the right finish on the deck.

[screamingchief]

As of now,the only decent/functional 350 headgasket choices are the Cometics,and the Innovative Seal (formerly ROL IIRC).

There have been some discussion about that in the street forum not too long ago,a search should bring up those threads.

All the other headgaskets are made for the larger bore engines,and thus have too much "dead space" around the top of the bores.

One of the main reasons some of us are looking at the 350 for a boost appl. is because the smaller bores should allow for slightly more headgasket material between the bores which hopefully may add some % of strength to a known weak spot in the larger bore applications,this would be especially important with regards to the center cylinders with the siamesed exhaust ports (3-5 & 4-6) as that's where pontiac headgaskets are most likely to fail.

HTH

Bret P.

[Squidward]

OK, thanks. I looked at the Cometic. Just over a bill - definitely not your run of the mill "Mr. Gasket" stuff.

I've searched for Total Seal rings, with 3.915 bore and 1/16-1/16-3/16 rings. Found a couple of places. Is it better to get something a tiny bit bigger and file back to the proper gap, or does ordering the exact size usually get ya in the ballpark with just a little bit of tweaking needed?

[screamingchief]

Originally Posted by Squidward

OK, thanks. I looked at the Cometic. Just over a bill - definitely not your run of the mill "Mr. Gasket" stuff.

I've searched for Total Seal rings, with 3.915 bore and 1/16-1/16-3/16 rings. Found a couple of places. Is it better to get something a tiny bit bigger and file back to the proper gap, or does ordering the exact size usually get ya in the ballpark with just a little bit of tweaking needed?

Sorry about the late reply...

Yeah,cometics aint cheap,but they are very good gaskets,you pay for what you get & vice versa.

Anyhow,on to the main question.

I dunno if I'd say it's "better" per-se,but yeah some ring sets are sold .005"-.010" over size to allow for "file fitting" to the given bore size allowing more precise final gap spec.

IIRC these end up being the same size rings that a 283/307 SBC uses,so they come in a good variety of oversizes and such.

HTH

Bret P.

[79bluebird]

I plan to also build a turbo 350 Pontiac after I find a good core block. clean it, block fill it, stud it. I would like to get between 750 to 850 hp from it.

[Squidward]

SC, I looked into the sbc stuff. Most of the 283/307 stuff has 5/64 compression rings. But total seal makes 3.915 std and file fit versions with the 1/16, 1/16, and 3/16 setup. My machine shop says he found them locally (one of the few benefits of the big city). Gonna go with TS rings and cometics.

Good news... crank was in great shape. He said it was easily at standard, and should be good with a micro polishing. Not bad for spending a month or more submerged in rainwater! Prev owner let it sit with valley cover off out in his yard, and AZ DOES get some rain.

[Squidward]

Originally Posted by 79bluebird

I plan to also build a turbo 350 Pontiac after I find a good core block. clean it, block fill it, stud it. I would like to get between 750 to 850 hp from it.

I wish you the best. It was a little challenging finding forged pistons. I think I got lucky. Head gaskets are also a bit dicey. It seems like the aftermarket parts people pulled the plug on 350P's shortly after all the hobbyists pulled the plug on wanting to build them. Your goals are more aggressive than mine. I am shooting for a smooth streetable 500 or so with my foot in it.

I want to start conservatively on the boost and power levels. I am sure I will find a weak link farther down the road, either with tranny (TH350) or rear end (8.2). But I want to make sure that I build a strong enough motor to keep me content as I step it up. I currently have a 72 block, 40 over, forged pistons, forged rods, stock N crank. I will be using ARP main studs. I want to do a little bit of "weird factor", especially if I can turn in some good timeslips at a Pontiac Heaven event with my redheaded stepchild motor.

[marks73ta]

Squid. I'm in Glendale. Who did you find to do your engine work? l had my engine built by Paul down in Tucson but I would like someone closer to take the little stuff to.

Mark L

[Squidward]

Baileys in mesa. Bob's a good guy, been doing it forever. He's never done me wrong, but I've never done a build before at this level. We'll see how their work holds together!

[Squidward]

Actually, I just got back from there. They were checking the balance with my probe pistons, tomahawk rods, and my stock 72 350 crank. I guess it was way out by somewhere around 200 to 300 grams? Anyways, he had to make some calls. We were in the classic discussion of what exactly is internal, external, and neutral balanced on a Pontiac assembly. So... balancer is neutral, flywheel is not? The flywheel has extra holes on one side, so I would assume that the flywheel has an external balance?

Sorry. Some of this stuff may be basic to some of the key players in this forum, but I am in newer territory here.

[marks73ta]

Yes Pontiac used a externally balanced rotating assembly. They put some into the flex plate/ flywheel. Whenever I built one of my Pontiacs, I would internally balance it. Then I used zero balance flywheels or flex plates. Not a big thing but it's easier to balance the motor and change parts later.

Mark

[charlie66]

Originally Posted by Squidward

Hi,

Did a search, couldn't find apples to apples info.

I am going to take my block down to the machine shop to get cleaned and bored. I wanted to know if there is anything substantially different about a turbo'd build that would pertain to the machine work, like looser clearances, etc.

It's a std bore '72 350, getting punched out 040, with stock N crank.

I have Probe SRS forged pistons, and 4130 Tomahawk rods. I think these will be fine for the job based on my hp goals.

My goal is for around 500 hp max on mild boost (maybe 10-12psi) from a single turbo. Nothin' crazy. I plan on building a stout 300-350hp NA engine first, and then toss on the hairdryer. I've only ever played in the world of mild street motors.

Anything weird I need to worry about, like extra cylinder wall clearances beyond what the piston manufaturer recommends?
What about ring gaps compared to normal?
Would hard-block filler be necessary?
Any special fasteners?
Additional input welcome.

I don't plan on spinning the motor over 5500. I am hoping I don't have to get too weird beyond standard performance engine building practices.

Thanks!

Sound like you're going to have some fun!

Was wondering what size turbo you're going to use, mm/AR ?

Are you going blow through or injection.

I bet 500 hp for you will be under 10 psi.

My 4 cyl made over 500hp @ 15 psi.

Good luck and I hope you keep us posted on you progress....

[Squidward]

Originally Posted by marks73ta

Yes Pontiac used a externally balanced rotating assembly. They put some into the flex plate/ flywheel. Whenever I built one of my Pontiacs, I would internally balance it. Then I used zero balance flywheels or flex plates. Not a big thing but it's easier to balance the motor and change parts later.

Mark

I got the call today everything is done. Rotating assembly balanced nicely. It turns out that there was a balancing equipment setup error that was discovered when they went back and rechecked again why it was so far off. After that they said it balanced pretty easy. Check twice, cut once...

I had them balance it with the factory flexplate, so there is going to be some external balance involved, I guess.