Piston Decisions - Tuners please step inside

[ElementR]

I'm building a new engine for the S4. Current mods in sig. Car runs extremely well, but clutch recently gave up and have some oil seal leaks on the engine so time to pull and upgrade. My intention is refresh the engine with upgraded components but not make any more power. Reason is that I like the way the car is tuned and do not want to mess that up.... down the road, I foresee E85, but that's after fueling upgrades. Onto my dilemma...

I picked up a used shortblock and heads at the start of the year with a build in mind down the road. So here I am. My intent was to use the block, crank and pistons and refresh all bearings, seals, studs/bolts and rings along with a set of H beam rods. Unfortunately, it turns out that the cylinders need enough honing to require 81.5mm pistons. I put some feelers out for another block in better shape but so far no luck. So, if I go with new pistons, they all have good and bad. I want an OEM like piston so I'm leaning towards Mahle Powerpack http://www.ctsturbo.com/cart/product...-3370-516.html

As with any piston I've found, they generally come in 8:1 or 9:1. I'd prefer 9.3:1 like the OEM Mahle pistons, but that's not an "off the shelf" offering. My engine builder has a good relationship with Diamond and they are right around the corner here in Detroit. Since they don't have a suitable forging available, they will CNC a set from billet to my specs... with thermal coating, pins and rings that'll set me back about $1500 (my builder's cost). That's several hundred more than the Mahles for 3 tenths. Question is will the tune care about 3 tenths or am I worried about nothing?

Perhaps there are better options, but I'm not aware of them. I know 034 offers JE pistons with 9.5:1. Technically, overboring 0.5mm will drop compression nearly a tenth so the 9.5:1 option seems like a close match. However, I do not like JE's choice of alloy which requires larger tolerances.... not to start another debate. Point again is I want an OEM like piston which Mahle comes closest to. Diamond is still an option, but $1500 vs $900 for 3 tenths. Is it worth it and again will the tune care? Aside from the upgraded shortblock, I will be upgrading the heads with the 034 valvetrain (springs, retainers, valves) and doing the usual timing belt kit, oil pump, gaskets, seals and hardware. I'll be re-using the Tial 605's, exhaust, fuel system, induction, tune, etc....

[aysix]

If you're taking everything apart anyway just get another shortblock with good bores for a couple hundred bucks, reuse those pistons. Done! Suck up the loss from the old block.

[ElementR]

I hear you as that was my first thought so I put a wanted thread up and made some phone calls but so far no luck scoring one.

[slowSfaux]

Technically, overboring 0.5mm will drop compression nearly a tenth

Are you sure about this? You're not changing the volume of the combustion chamber of the head, but you are increasing the volume of the cylinder. Logic dictates your increase in bore will raise the compression ratio, not lower it. Be sure to talk with your piston maker to be sure you are getting your target compression ratio.

[landfill]

I have a spare APB short block if you want it.
$250 + freight shipping.

[ElementR]

You're right. I even checked myself on that, but looks like I mixed up my results. You can easily play with the numbers here...

https://www.rbracing-rsr.com/compstaticcalc.html

I wonder if the piston geometry is changed to compensate for larger bore size in order to maintain the same compression ratio. If not, then a 83mm piston would be closer to 9.3:1. than the same piston at 81mm diameter with 9:1. Yes, I certainly will confirm before I buy any pistons.

[ElementR]

pm sent landfill

[MacDaddy]

Well your probably decking the block and machining heads which will raise compression, so 9:1 Pistons will probably have you close to 9.3:1 anyway.

[ElementR]

Yeah, question for my builder to answer. I still wonder how much 2-3 tenths matters to the tune. Anyone know?

[B501S4]

http://www.importperformanceparts.ne...wpak-audi.html

You can snag these up for the tighter wall clearance that you are looking for. Better price than CTS or others. Believe I snagged them for about $640 plus shipping.

[ElementR]

Awesome. Thanks for the link. Leaning towards using these if in fact compression is bumped a bit with the overbore.

[Schtug]

The problem you're going to run into with the Mahle's is that they are on backorder from Mahle until the end of January. I searched high and low for a set because I'm doing a build right now as well and I wanted to use them. The only readily available shelf option I could find was JE. I had a long talk with one of their reps about my setup, power goals 500-600whp, ect and he said I could run them pretty tight, like 3.5 thou piston to wall. I ended up ordering an 83mm set from them with their tuff skirt coating. Total cost was just south of $1000. Oh and if you are looking around online you'll find some places offering an upgraded tool steel wrist pin for an additional cost. It's probably not going to be necessary for most of the builds that ever happen with these cars. JE rep said the standard mild steel wrist pin that comes with the shelf sets can handle 175hp per cylinder, so just north of 1000 crank horsepower on one of these. Also, the .3 of a compression point isn't going to make much of difference in the grand scheme of things. The shelf JE's that I bought were 9.0:1.

[ElementR]

Thanks for the feedback. Seeing how the car won't be coming out until Spring it's not the end of the world if the pistons aren't read until end of January. I can wait.

[julex]

Yeah, question for my builder to answer. I still wonder how much 2-3 tenths matters to the tune. Anyone know?

Not in the slightest. Or rather very little on timing front but not on anything else. This assumes you get a good tuner doing the tune, he will probably adjust timing anyways since every car is different anyway.

[ElementR]

The reason why I asked is that I don't want to change the tune. I have a VAST tune that is extremely solid and given the state of VAST and the need to go find another tuner and open up a can of worms, I'd rather not make changes that will impact the tune right now. I'm happy with the power level. This is just a refresh/upgrade of the internals to withstand the power level (H-rods, new bearings, rings, bolts/studs, valves/springs/retainers). Other than these items and the required gaskets, seals, timing belt stuff, the engine will not be modified. I'll reinstall the same Tial 605's, FMIC, exhaust, fueling, induction, etc.

At a later date, I'll explore E85 and a new tune. The work I'm doing now will give me the foundation to do that.

[julex]

The reason why I asked is that I don't want to change the tune. I have a VAST tune that is extremely solid and given the state of VAST and the need to go find another tuner and open up a can of worms, I'd rather not make changes that will impact the tune right now. I'm happy with the power level. This is just a refresh/upgrade of the internals to withstand the power level (H-rods, new bearings, rings, bolts/studs, valves/springs/retainers). Other than these items and the required gaskets, seals, timing belt stuff, the engine will not be modified. I'll reinstall the same Tial 605's, FMIC, exhaust, fueling, induction, etc.

At a later date, I'll explore E85 and a new tune. The work I'm doing now will give me the foundation to do that.

It should work but don't be surprised if it doesn't. Even a minute change can render tune useless if said engine starts exceeding boost level due to increased efficiency and such things.

[ElementR]

That's the sort of concern I have. Someone on the forums offered up a shortblock. Need to get details about condition and shipping cost, but if all checks out it seems like a good option. Other leading option is the Mahle Powerpack pistons.

[ElementR]

Info for anyone else that reads through this... Wossner will make a set of pistons to my specs for $200/piston including pins, rings and clips. $20/piston extra for skirt thermal coating. They use 4032 low expansion aluminum like Mahle. Diamond will CNC a set from billet (not sure what alloy) for $200/piston but does not include rings or thermal coating. Similar price on thermal coating as Wossner, but the rings are another $250 or so. So, if you are in the market for custom pistons, check out Wossner.

My builder already has a bit of time invested in the block so it makes financial sense for me to continue using it. He's convinced the 2 to 3 tenths of compression will not matter so I'm going to go with the Mahle Powerpack pistons. Custom pistons are around 2 months lead time so not much difference if Mahle is truly quoting end of January for ETA on their pistons.

[ElementR]

FYI - Just got this from IPP (Import Performance Parts)... As of October 27th, 2015, I am retiring Flatlander Racing and IPP (Chevy Cavalier World will remain open for business), and we will no longer be taking orders other than for our Overstock Sales Inventory.

[slow ride]

Splitting hairs with that small of a SCR change. I might have a source for the Mahle stuff as long as domestic dealers can get the import stuff. That sucks about flatlander as they had good pricing.

[ElementR]

Second strike on the Mahle pistons. APM engine parts says they are not available and not sure Mahle will ever run more.

Slow ride - let me know what you can do. Fall back plan is Wossner custom at this point.

[slow ride]

I'll check, but I need to know if they are in production anymore also. I know autotech does some domestic Pistons in 4032 on a custom level. You can also build them as you like with a thicker top ringland etc.

[Schtug]

I wish I remembered wossner existed. That's not a bad price for a custom set with pins, rings, locks, and in 4032 alloy to boot. Multiple places I called on the mahle's said end of january and I can't imagine mahle would be lying to their vendors. If you end up paying for custom you might as well turn this into a 3L build

[ElementR]

Yeah, 3L is not gonna happen. I am sticking to my conservative plans. Been down that road before...lol

I love this car for the fact that I can get in it, do some spirited driving and still be able to take the kids to the park or head to dinner with the entire family without wondering if something is going to break.... if I want to push something, that's what my mustang is for. :)

As for pistons, I'm going to call CTS tomorrow and check one other place that supposedly offers them. If neither can get them, then it's probably safe they don't exist and Wossner is my next best bet. $200 a piston including rings is not bad at all for all custom. Even at my builders cost, Diamond wants a bit more than that (granted they have to CNC from billet). I mean, JE is in the same ballpark as Wossner pricewise and it's essentially right off the shelf. Yes, here I am justifying paying more for pistons. All in all, I'd be fine with the Mahle's if I could get 'em. We'll see...

[Schtug]

Haha yea you already know you'll be better off. I nearly went down the 3L road, but it's such a terrible idea for all practical purposes that I talked myself out of it and committed to 2.8L. I'd like to spend more time driving the car than working on it, crazy idea, I know. I hate to admit it, but I was deep into 1990's honda products in my teenage years and they spoiled me. Those cars have so much aftermarket support it's insane. I could probably go on ebay right now and find 20 different sets of forged pistons for a b series block that are all in stock and ready to ship. Here we are struggling to find a non-custom option that isn't made by JE. Well, you're struggling to, I've already made my deal with the devil lol. I'm sure the wossners will do really well for you, if I could send the JE's back and pay an extra $250 or $300 to get them I would, just for the low expansion alloy.

[Avantugofast]

Wiseco pistons? AMD has a custom Wiseco set up that seems to work well.

[ElementR]

OK, good news... Butler-macmaster will get the Mahle's for about $700. They spoke with Mahle and were told about 2 weeks to make them.... with the holidays, likely early January before I see them. I ordered a set.

[ElementR]

FYI - http://www.butler-macmaster.com/aud209209i07

[ElementR]

Shockingly, the pistons already arrived yesterday. They drop shipped from Mahle in TN. Got them exactly 1 week after ordering. Just FYI for others who may be after a set.

[B501S4]

Great find. Certainly harder to come by and you still made out with them at a solid price. You should be very happy with the functionality of these.