DSS’s New Stroker Pistons – Extreme or X-Stream?

DSS’s dedicated approach to stroker piston design: With new patented features they are worth way more than just casual consideration!

You may well ask if a whole feature just on a new and possible somewhat innovative piston is really called for. Tell you what – I’ll let you decide. But why a special stroker orientated piston? What so different about a stroker piston other than it’s for a stroker motor? The answer here is more than you might think so let’s start at square one.

First Off – Who is DSS?

The chances are you have heard of KB, Ross, Wisco, Sealed Power, Mahle, JE and the like. They are all well known and respected names in the piston industry. But who are DSS? If you are a small block Ford guy you almost certainly will know of DSS. This company, based in St. Charles, Illinois, produces a lot of small block Ford (push rod and modular) hard core parts. Over a period in excess of 27 years and even through economically lean years, they have more than just survived. Indeed though most of their years in operation they have expanded progressively which suggests they are doing something right.

A number of years ago DSS started to make big investments in CNC equipment and, looking to the future, they decided to buy more equipment than they had current use for. This was to cover the intent to push out into other product markets. With sales of their short block assemblies blossoming they were buying a lot of pistons. The numbers were such that it made sense to produce their own – and that is how come they got into the piston manufacturing business.

For DSS the piston demand was almost all in terms of pistons for stroker Fords of one kind or another. Pistons for a stock motor almost did not figure into the picture here. The demand they saw was pistons for high output motors usually of maxed out, or nearly so, in terms of displacement. What this meant was that, for the most part, they were not in the business of producing pistons for near stock engines that had an easy time of things. Only pistons that were likely to get a heck of a beating and had to survive it not just once, but time after time!

Since DSS would have to eat comebacks should they occur the prime requisite here was a piston that would survive. After that power was the next item on the list of things to take care of. Well time shows they managed to do that but at the end of the day we can achieve anything including putting a man on the moon – given enough cash. This was the crux of much of DSS’s move into the piston manufacturing business. What ever they did had to be affordable as well as functional. The Ford guys (and that includes me) will tell you for the most part they have achieved reliability, performance and cost economy – but now they are moving into the Chevrolet market, and for one good reason. They believe they have something that once the Chevy guys know about it they may well want!

The Age of Stroker Motors

Forget the Chinese calendar; as far as the US based hot rodder is concerned we are living, if not in the year, but in the age of the ‘stroker motor’. With the proliferation of low cost stroker cranks it just does not make sense to build (or re-build) a performance motor (assuming it is not constrained by race rules to a given displacement) with a stock crank. Even the lowest cost aftermarket cranks offer advantages in terms of additional strength and wear resistance as well as more cubic inches. Unlike a re-ground stock crank the amount of their fatigue life that is used up is zero so, all in all, installing a stroker crank from a reputable supplier is a good move toward increased reliability as well as performance. But making a piston work in a stroker environment is a little more involved than just moving the pin nearer the piston crown. Everything about a stroker motor is more demanding on the piston. And that is why it is important to address those issues more pointedly than for a stock re-placement piston. Let’s look and see just what needs to be addresses in a more purposeful manner for a stroker piston.

Design Demands

The whole point of a stroker motor is to have an engine that produces more torque and hp everywhere in the rpm range. This means the intent of the engine build, for a performance enthusiast who is going to use that extra output, is to go fast. No-one builds a stroker motor for outright mileage. This means that the piston has to be of the ‘heavy duty’ variety – especially if forced induction or nitrous injection is involved. But, as important as it may be there is much more to it than just being tough. The whole point of a stroker motor is more power and if that is not realized then the point of the exercise is lost. Let’s look at a list of things that are important for a stock stroke motor but become even more critical for a stoker motor.

#1 Increased stroke length increases potential for increased power loss due to piston assembly to cylinder wall friction.

#2 Increased rod angularity caused by longer stroke increases piston side thrust forces into the cylinder wall thus further increasing power lost to friction.

#3 A shorter piston skirt brought about by the increased travel up and down the bore and the need for the skirt to clear the crank counter weights means the potential exists for accelerated skirt wear.

#4 Stroker pistons require deeper valve pockets for the higher valve lifts called for when a stroker crank is used. These deeper valve pockets mean lowering the top ring groove to avoid the valve pocket and groove intersecting. Lower ring positions can be bad for output so minimizing it is essential.

#5 The shorter piston skirt along with the added piston to wall clearance for high performance use means that the piston can rock in the bore to a greater extent. This brings about it’s own set of oil control problems.

#6 The longer stroke and usually the desire to turn the engine to higher rpm means piston weight is more critical. Piston weight and piston strength diametrically oppose each other so to make a successful stroker piston it is necessary to pay closer attention to design details and make the right overall compromises between strength and lightness.

#7 So that the piston has wide appeal anything out of the ordinary that is ‘built in’ has to be done in a cost effective manner.

#8 The hot rodder is a far more demanding and critical customer than the regular re-build customer. This means the product they buy has to meet far higher standards.

Now we know what we are up against let’s tackle these items, one at a time, and see how DSS, through their considerable experience in stroker piston design, has re-acted to each of these issues.

Design Refinements.

This DSS piston goes under the name of GSX where the X signifies ‘X-Stream’. At first sight it looks like an attempt at a catchy marketing title but as far as catchy phrase titles are concerned both Extreme and X-Stream (Trade Mark name registered and Patent applied for) have some significance here – especially the second term. Take a look at the piston shot below and you will see an X groove in the skirt. This is where the X-Stream moniker comes from. Read on to see how this simple skirt modification might just be one of the most significant piston innovations to come down the pike in a long while.

Having now made a start on our dissection of what DSS is offering here to specifically target stroker motors let’s go through our previously numbered list starting with friction and what ever function that so obvious X groove may play in the grand scheme of things.

#1 Increased stroke length increases potential for increased power loss due to piston assembly to cylinder wall friction.

#2 Increased rod angularity caused by longer stroke increases piston side thrust forces into the cylinder wall thus further increasing power lost to friction.

#3 A shorter piston skirt brought about by the increased travel up and down the bore and the need for the skirt to clear the crank counter weights means the potential exists for accelerated skirt wear.

All three of the forgoing points relate to the primary mechanical problem the piston designer has to face – friction – and if a short skirt is involved then accelerated wear can also be a problem. This is a major stumbling block and the high performance engine builders number one enemy. My experience here is that starting with a stout and acceptably good stock piston assembly (piston, pin and rings) and swapping out to a really good piston assembly in a nominally 500 horse stroker motor is worth in the order of 15 to as much as 30 hp. An illustration here might just make the point.

About 15 years ago I had a 350 small block Chevy to rebuild as a 383 stroker. When I called a certain piston company about pistons the chief engineer there told me they were finalizing a piston for the job with a revised skirt profile and ring design along with a material specification change. He told me that they were very subtle changes so don’t expect to photograph them. He was right – I got the pistons and eyeballing them saw no differences what so ever from this companies stock replacement style piston. Still I had been warned not to expect to see any visual differences so went ahead and built the motor. I had expected to see about 505 hp with this build. I have been doing this long enough that I can usually second guess an engines output (especially a small block Chevy’s) to usually within 5 hp. Instead of the 505 hp expected the engine topped out at 493 hp and I was far from pleased about those new pistons!

While I am brooding over the missing HP I get a call from my chief engineer friend telling me the shipping department sent me the wrong pistons and another party has what I should have run. That was some consolation but I can’t say I was overwhelmed with the shipping department’s efficiency. So I pulled the motor off the dyno stripped it and made a few passes with the hone through the bores. When the correct pistons arrived, I found they did actually look sufficiently different to be noticeable. Anyway to make a long story short with the new pistons installed the motor made 12 lbs-ft more torque and with power topping out at 511 hp so output was up a solid 18 horses. So, all in all, that was a good piston – it just was not cheap – but regardless of cost it does demonstrate just how much difference a piston can make to the total output. Since blow-by (as measured on our Dwyer blow-by meter) was virtually the same in both instances I have to say that the difference in output was probably due almost entirely to reduced piston assembly Includes rings) to cylinder wall friction.

So how does the subject of friction and efforts to reduce it relate to the DSS X-Stream stroker piston design?

Friction Sources

Piston friction with the bore is dependant on two aspects – the pistons itself and the ring pack. We will deal here with the piston forging itself. The first move and one that every piston manufacture has to deal with is the contact pattern and contact area the shirt has with the bore when up to working temperature. These days so much data exists on that as to be of little concern to us as end users. But for a stroker piston the fit within the bore and the thickness/volume of oil on the cylinder wall can be an issue. A crowded ring pack and a short skirt can make for a less stable piston in use and that in turn can lead to reduced oil control capability. The fix is an oil ring with a higher radial compression (usually wrongly but commonly referred to as ring tension when actually it is ring compression). Unfortunately extra radial compression leads to a measurable increase in parasitic frictional drag – and that simply eats into the power we are seeking to gain in the first place. So to better control both oil control and the amount of oil the skirt is riding on DSS have introduced something totally new – the X-Stream grooves.

X-Stream Function

Function of the X-Stream grooves appears somewhat subtle so here is my best explanation of how they are supposed to work. Let’s start with placement of the oil return holes as illustrated below.

 

The function of the X-Stream groove is basically tied in with how the oil groove oil return holes (yellow arrows) operate. Most pistons have the complete diameter of the oil return hole (or width if it is a slot style return hole) in the back of the groove. This provides a good pan return route for the oil scraped off the bore by the top ring of the pair that make up the oil ring assembly. However the oil scrapped off by the more important bottom ring of the pair, which gets to the oil first when the piston is on the way down the bore, and is otherwise responsible for the better part of oil control, has a far more tortuous dispersal route. The consequence is that it may need more outward radial force to take the oil off the bores and return it to the pan. By drilling the oil return hole lower so that they intersect with the oil ring groove we now have route for the oil scrapped from the bore by the lower ring to more easily get back to the pan. Although there is a chamfer that creates a small reservoir volume just below the oil ring groove it is of far less volume than the upper ring of the assemble has available to dump the oil for pan return. When the piston comes down the bore the hydraulic action of scrapping the oil from the bore tends to push the oil down the X-Stream grooves. As the piston accelerates up the bore the acceleration tends to push the oil down toward the bottom of the piston. Either way oil motion is more positively directed back to the pan. With an easier return route the oil ring has less work to do and a less aggressive ring pack will now get the job done.

With the potential for better oil control there is a need to consider piston skirt to cylinder wall lubrication. Personally I have seen a short rod/stroke ratio engine have sufficient piston side loading on the major thrust side to displace oil between the surfaces to the extent that is was inadequate for the job this resulted in rapid bore and piston wear. For the sort of applications we are dealing with here I don’t believe that inadequate lube will be a problem but if the X-stream grooves do bring about much improved oil control then who knows? Making sure that there is enough oil at the piston skirt/cylinder wall interface might just be something that has to be considered. If this is the case the presence of the X-Stream groove actually deals with a second facet of oil control. If there is the potential for too much oil to get ‘trapped’ between the skirt/cylinder wall interface the X-Stream groove channels it away. If there is insufficient the X-Stream groove meters oil out onto the surface as the piston moves up or down the bore. Bottom line here is that the secondary oil control function is to make sure there is just enough oil between the piston and cylinder wall to get the job of minimizing friction done while also minimizing oil drag.

Three and Three Quarter Plus Strokers.

Any time the stroke length of a 6 inch rod length stroker combo goes to the commonly used 3.75 inches or more the wrist pin will intrude into the oil control ring groove. As with most stroker pistons the DSS piston deals with this by utilizing a steel rail that snaps in place in the ring groove after the piston is assembled onto the rod. Bear in mind here the purpose of the rail is to bridge the gap left by the pin bore intersecting with the oil ring groove. Where the DSS piston differs to others in this area is the way in which the rail support ring is located so that it cannot rotate and leave the rails gap over the gap in the ring grooves lower face. The illustration below shows how this is done.

Forging Spec.

So we have a novel piston design here but let’s ask that all important question – just how tough is this deal from DSS? Let’s start with the forging blank.

The alloy that DSS have chosen to use is the tried and proven 4032 spec. This alloy is about the most popular choice by piston manufactures world wide for high and very high performance street applications. It is not quite as tough as say the RR 58 alloy which was a really popular choice for Formula 1 engines a while back but it has far better wear properties, a lower expansion rate (good for quieter running pistons as less initial clearance is required.) and is nearly as tough.

As far as capability is concerned DSS is in a prime position to ascertain just what their pistons will deal with. Remember that hardly any of the engines that leave their shop are stock or even that near stock. On the other hand the number that end up at the drag strip is a major proportion. I asked DSS what they thought the power limit on this piston was and was told that the Ford versions were holding up reliably on the drag scene at the 1,500 hp mark. I have already used small block Ford DSS pistons in high output nitrous applications were I have made a whole bunch of pulls at the 800 -850 hp mark with nitrous and the pistons looked totally unscathed at the end of the tests. So just from my own experience here we can assume that the DSS piston is a tough player.

Pin Design and Valve Pocket Geometry

We are now on to the next point which was:-

#4 Stroker pistons require deeper valve pockets for the higher valve lifts called for when a stroker crank is used. These deeper cutouts mean lowering the top ring groove to avoid the cutout and groove intersecting. Lower ring positions can be bad for output.

The need for high valve lift is greater with a stroker motor than for a stock displacement unit. Not only that but also the lobe centerline angle required for a stroker motor is tighter than for a stock stroke unit. What this means is that the valve cutouts in the piston crown need to be deeper to accommodate valves that are substantially further open in that TDC and up to about 20 degrees before and after TDC period where piston the valve clearance is most likely to become non-existent. This being the case DSS elected to put in a valve pocket that would accommodate all but possibly the most monstrous of cams. Also the valve cutouts are symmetrical. This in itself actually has a minor down side because the exhaust valves don’t need such a large cutout as the intake. However this proves only a minor issue because getting compression with a stroker motor is easier than with a stock displacement motor. The reason DSS has elected to do this is that it allows any piston to fit in any bore of a small block Chevy thus avoiding having 4 left hand and 4 right hand pistons. This cuts cost.

Back to the valve cutouts. Under normal conditions there are limits as to how deep a valve cutout can be made that is imposed by the position of the top ring groove. If the cutout is too deep it intersects with the bottom of the top ring groove. Right at that point the limiting valve pocket depth has been reached. To allow for even more valve cutout depth and to accrue the advantages of a superior ring package DSS opted for a top ring that was not only narrower than typically used in lower cost pistons but also had less radial depth. A regular 1/16 or 5/64th thick compression ring is 0.185 wide where as the one utilized by the DSS X-Stream piston is 0.170 wide. This allows for a significantly deeper cutout some of which however is traded off for a good cause. To get a reasonable cutout depth with a wider radial width ring the ring almost inevitably has to be set lower down from the piston crown. The ring land volume is a power robbing crevice volume (read the upcoming Motortec article on crevice volumes. The smaller the crevice volume is the less the power loss it incurs.

The wrist pin used for this piston is also a little lighter than what would normally be expected for use in inexpensive pistons. By using a thicker wall section in the middle of the pin where it is most needed the wall section of the part of the pin that is in the piston boss could be reduced. The result is a pin that is just as strong but is typically 12 – 15 grams lighter than normally seen.

Pin lube is by forced oil via a drilling from the oil ring groove. A good oil film here is of paramount importance for any type of high output supercharged or nitrous application otherwise pin bore scoring will, for certain, happen. Pin retention is by means of double True-Arc Locs (Circlips if you live in England) or Spirolocs. Now it has to be said that many pro engine builders are leery of double-locs because they have, in the past, had reputation for coming out. The bottom line here is that they never come out if the clips and grooves are dimensionally correct and they are installed properly. That means checking, with an eye glass, that each and every clip is seated home in the bottom of it’s groove! That said if you want to use Spiralocs DSS can oblige.

Up Market Rings.

We now start to move into the realm of ring design and how it applies to compression, oil control and minimizing friction as summed up by point #5 below.

#5 The shorter piston skirt along with the added piston to wall clearance for high performance use means that the piston will rock in the bore to a greater extent. This brings about it’s own set of oil control problems.

Let’s deal with oil control first as this can be an issue with big stroke increases. What is used for rings here can be ultra critical. More outward radial preload to aid oil control costs power – and as any experienced cup car engine builder will tell you – it inevitably costs much more than you might think! However with the X-Stream grooves helping oil control the DSS piston gets the job done with an oil control ring set having a narrower radial depth.

MotorTec tests here indicate that this oil ring pack has a little less radial pre-load than a regular depth oil ring. I would like to quote the precise numbers involved but without some dedicated equipment it’s a hard figure to measure accurately. However that said our less than perfect measurement of the sample ring sets we had from DSS indicated that the assembly (rails and expander) that comes with these pistons has about 10% less radial expansion force than a regular ‘tension’ oil ring set. If we consider just the oil control rings themselves (excluding the expander) then very substantial difference is seen in radial expansion force. The numbers look like this – 255 grams for the regular ring and 115 for the DSS ring!

Compared to some of my Cup Car engine building acquaintances my ring testing is somewhat limited. But what I can say from a couple of dozen or so tests in this area is that the reduce drag of the oil rings alone is going to be worth a solid 3 lbs-ft in a long stroke small block V8. This equates to 3-1/2 hp at 6000 rpm and 4 hp at 7000.

So much for the oil control rings – now let’s look at the compression rings. Like the oil rings the compression rings on the DSS piston are of a narrower radial width than usual. This, with a 0.0588 (1.5 mm) thickness, serves to significantly cut the mass involved. For the top ring the figures are – for a 30 over ring set – 19.7 grams for a 5/64 ring, 15.9 grams for a 1/16 ring and 12.7 grams for the DSS top ring. This all stacks up and by the time we take into account the entire ring pack for one piston we see some very favorable final totals. For a 5/64 -5/64 – 3/16 set the total is 56.8 grams – for a 1/16 -1/16 – 3/16 set this tallies out at 49.2 grams and for the DSS set the figure is 38.3 grams! But ring weight is not the only issue here. With less radial width the ring conforms to and seals up better than it’s wider equivalent.

Overall Weight

To refresh your memory here point #6 was:

#6 The longer stroke and usually the desire to turn the engine to higher rpm means piston weight is more critical. Piston weight and piston strength diametrically oppose to each other so, to make a successful stroker piston, it is necessary to pay closer attention to design details and make the right overall compromises between strength and lightness.

Make no bones about it, piston weight is always an issue. Every time the piston gets lighter the amount of counter balance mass required becomes less. This means less windage losses from the whirling crank. But super light weight pistons are not without their problems. Past a certain point lowering the finish machined piston forging mass comes with an upward spiraling cost increase. Sure you can buy a piston that weighs in some 10% lighter but you could be paying 50% more for such a set. If searching out cost effective power is the goal then it is almost certain that the money possibly spent on super light (instead of just light) pistons could have more effectively been spent elsewhere. To see where the DSS piston falls in terms of other similarly priced pistons I weighed three different brands that did the same job. They ranged, with pins, locs and rings, from 578 to 594. The DSS piston came in at a commendable 554.

Last Points

#7 So that the piston has wide appeal anything out of the ordinary that is ‘built in’ has to be done in a cost effective manner.

#8 The hot rodder is a far more demanding and critical customer than the regular re-build customer. This means the product they buy has to meet far higher standards.

Above are the last two points that are relevant to the typical performance buff. Let’s take cost. These days you can buy almost anything you can imagine as far as piston design goes and some you probably cannot. For instance I was working on ringless zero expansion F1 pistons that were for the most part grown a molecule at a time (took six months to grow) about 11 years back. A Chevy sized piston came in at about 200 grams and would have cost about $45,000 – apiece! We, as regular hot rodders, are way over at the opposite ends of the scale. Value for money rules the day here. At a street price of about $699 a set, including the high tech rings, the DSS pistons look like a pretty good deal so they certainly have that aspect well covered.

As for quality the sample pistons that have been sent to us at MotorTec look good and certainly above average for the cost. I certainly was happy with my test set and I have a reputation for being somewhat picky at times (who would have thought eh!).

OK here we are at the end of the feature and it looks like only one question remains – how well do they work? My now unfortunately Ex partner in crime (he decided a real job would earn more money!!!) and did 383 build for an auto industry client using the DSS X-Stream pistons. I am pleased to say they more than lived up to our expectations. When this engine was torn down for internal inspection a well respected Indy engine and Cup Car engine builder friend of mine walked into the shop. He spotted the pistons (which showed near zero wear) on the bench right away – picked one up – looked at the X-Stream grooves and said ‘this looks a great idea – where can I get some of these? Answer – only at DSS!

David Vizard.

 

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