Dougs951S

I took some measurements today, and wanted to post some commentary on the prospect of building a non interference 2.6 liter with SBC pistons.

First, some data concerning the factory motor.

The compression distance of the factory piston is ~1.599. Factory pistons sit proud of the deck by ~.003". The volume of the dish on a 951 piston is ~26cc. Additionally, the vertical clearance between the exhaust side leading edge of the intake valve and the deck surface is ~.0025" on a stock valve and seat. The factory head gasket is ~.040" when compressed, and the intake valve pocket on an NA piston is ~.115" deep. This gives an approximate piston to valve clearance of .155" at TDC. The valves in the head are at a 45 degree angle so with the stock cam lift of 12.2mm, the deepest reaching part of the intake valve extends .340" closer to the piston at full lift vs when the valve is on it's seat. So in the event the belt breaks with an intake valve wide open, the deepest tip of the valve will be .185" past the valve pocket, so of course, it bends.

Piston rock at TDC is ~.005" on a 100mm bore, so that needs to be accounted for as well.

What this means is that on a stock NA motor, the valve recesses need to be machined ~ .233" deeper to allow for .080" piston to valve clearance in a worst case belt break scenario with intake valve wide open near redline and leave no chance of valves getting bent. Essentially we need a combined clearance of ~.385" between the HG, the deck clearance, and the depth of the valve relief to have a completely non interference motor with .080" margin. I just thought I'd post this up for those considering doing this mod like I am.

odonnell

I spoke with laust about machining NA pistons to make them non interference... He told me there wasn't enough meat. I have a set I'm willing to donate to the cause though...

Dougs951S

the crown is thicker on the turbo pistons and the dish is deeper than the .115" NA valve pockets. I'm about to measure how deep the dish is, stand by.


The deepest part of the dish is .250", the part of the piston where mine has an obvious valve strike is a slightly more shallow part of the bowl, maybe .210" at a minimum. It's twice as deep as the NA recesses or nearly so. Meaning the turbo pistons only need to be machined an additional .138" (if the depth is .210" at the valve contact area) to be non interference with a .080" margin, much more doable than .233" on an NA piston.

That would mean that a SBC chevy piston with a 1.550" compression height would be non interference if ran .045" down in the hole, assuming it had a monster .300" deep valve relief.

At stock deck height, a 1.425" pin height piston would sit .170" below the deck and would need a dish only .175" deep to clear the valves. The problem with this, is that this piston will need a +6cc dome just to bring compression back to 8:1 on a 4.060" bore.

Van

How much would your compression ratio suffer?

This concept seems like the answer to a question nobody's asked... Why not just maintain your timing belt? Or, worst case scenario, keep a spare head on hand?

Dougs951S

Van, it may very well be that. I am a soon to be graduated mechanical engineer, so this is more an experiment just to see if I can come up with something that works, and also purely because crunching the numbers on this stuff is the most entertaining thing I have to do on a saturday night and I though I'd share what I measured/learned

My math shows machining factory pistons that additional ~.135" would drop the compression to 7.88 approximately. With a build using SBC, many options are out there and I am still exploring. if I find a combination that I like, I might use it in my upcoming 2.6 build.

The real point in all this madness Van, was an exercise to see if it was even remotely possible to build a motor that was completely non interference WITHOUT having to flycut the pistons at all. As it turns out, it may be doable, but the question I am still working out is will it have decent compression (to me that means no lower than 7.9:1 as an absolute minimum)? It might need a custom piston with a pin height in between the common chevy heights of 1.425" and 1.550" in order to get a good compromise between deck clearance and compression.

odonnell

I know doing the belts is easy. But going non interference is worthwhile if the pistons are out and you're running pistons that allow it. Mainly, turbo cars, because that lower CR is ok.

Dougs951S

I want to do an experiment. According to my measurments and math, the valve pockets on a stock NA piston have more than enough clearance for 49mm intake valves. This is of course true for the stock turbo piston as well. If my numbers are accurate; it seems the valve pockets are there to give margin to account mainly for rod stretch at high rpm, a shaved head which would reduce piston to valve clearance, and any carbon build up. The motor should actually run at idle with a totally flat top piston that had no valve recess at all. this would bring the clearance down to a dangerously low .040".

Does anyone know why stock motors have so much PtV clearance?

I find that interesting and am at this point just rattling off comments concerning the mechanicals of the motor for future posterity.

Additionally, all 2v motors run a really tight quench of ~.038". This may be part of the reason for their good fuel economy despite the limitations of the head design and motronic fuel injection.

V2Rocket

It's all due to cam timing...

Remember that the intake valve hasn't even cracked open at TDC but ramps up fast just after TDC so there needs to be a lot of clearance there.

And the piston is chasing the exhaust valve closing on the up-swing.

Dougs951S

Yeah I realize TDC is not the point of minimum clearance, but I still think our motors have a lot of clearance there? The problem of working out the absolute minimum clearance just using math is a lot more complicated than figuring clearance out for TDC so I started there. I've been ruminating on a good way to work it out without claying the valves. Again, this is partly academic masturbation of sorts. I'm taking a stab here, but I believe the ramp rates are similar on the intake and exhaust lobes. With larger than stock valves, the intakes will hit first but if I'm not mistaken, on a stock head the clearance is actually smaller for the exhaust valves? I have yet to confirm this, just a hunch.

After a lot of deliberation concerning running pistons down in the hole and attempting the mental exercise of building a non interference motor free of piston machine work; and after doing much research on the effects of quench and proper valve timing on performance and taking the factory cylinder head design into account; I intend to try to stay close to factory design in terms of valve and deck clearance.

I am surprised by how close the factory runs the pistons up against the head; the ~.038" clearance is damn near what a steel rod can stretch at high rpm. I'll be shooting to have ~.005"-.010" deck clearance. I'll probably end up having the pistons fly cut, and I've selected 2 pistons that will give me either 8.5:1 or 9.5:1. I have not decided which I'd prefer but I am leaning toward the higher compression currently.

Van

What size is the SBC wrist pin? Do you need custom connecting rods?

I understand you're doing this more "theoretically", but you could always order JE pistons specifically to your specs - maybe with just the valve reliefs, and no dish, with the pin location like stock, you could make this work.

If you're going with a custom turbo build, I'd shoot for a 9:1 compression ratio...

V2Rocket

SBC pin is 0.927", 944 pin is 0.945"...
you could just have a custom small-end bushing made for the 944 rod to fit the SBC pin.

Van

No way would I trust a bushing with only a 0.010" wall thickness to handle those forces. The only (sane) option is to bore the pistons out for the larger pin - that's if the pin location is correct for the connecting rods.

V2Rocket

Van
Remember that the 944 rod has a bushing of its own already...so you would actually be making the bushing thicker

Van

Ahh, you meant a new bushing altogether - I thought you meant a secondary sleeve between the pin and the existing bushing.

Dougs951S

I appreciate all the discussion guys!

The "Theoretical" portion of all of this was "building a non interference motor without needing to flycut the pistons". For posterity, it is indeed possible.

In reality I'd like to stick close to some of the design points the factory engineers saw fit to go with, and as Van said to me, it's worth doing this right if I'm going through all the trouble. All that being said, cost is a huge factor for me, I just got done redoing an NA engine and funds are low.

A sleeved short block with factory pistons and rings will run me about 650 dollars.

Adding in the cost of buying total seal rings @ 360/set brings the cost nearer to the realm of simply having the block bored and honed and buying oversize wossners, which is about 1200 dollars.

A sleeved short block with SBC pistons gives me an extra ~160cc of displacement and lets me bump the compression to either 8.5:1 or 9.5:1 (it will be an E85 motor limited to around 21 psi at most so I'm leaning toward 9.5:1) but will also cost in the neighborhood of 1100-1200 dollars.

Obviously, using the factory pistons is half the cost so I'd love to do that, but I'm still researching using factory rings on a steel bore. I'm trying to make a decision and I'm asking for advice. I need to make a choice before I can move forward with making any of this happen. I'm not looking for perfection, just a running motor that doesn't leak oil and has good compression. To that end, option #1 is looking mighty good if I can make factory rings work (since I already have a set).

It's also worth noting that of my four spare blocks: One has nice bores but has a crack on the deck surface near the water neck that will need to be welded, two will need a .040" overbore to clean up (so if boring, I'd have to go to 101mm) and the last one needs at least a .020" overbore to clean up.