In this segment of Ask The Experts, we dig into ten piston-related questions sent in by EngineLabs readers. Graciously, David Fussner, Research & Development Manager at JE Pistons, has offered his time and expertise to us. This ongoing segment on EngineLabs has already seen input from Nolan Jamora of Isky Cams, and we have upcoming segments planned with many of the performance industry’s smartest folks. Without further ado, here are The Answers.
1) Where should gas ports be located around the piston or more importantly,where should a gas port NOT be located to not impact the pistons top ring land integrity or the pistons stability in the bore?
David Fussner: Drag racers tend to prefer top gas ports, while lateral gas ports are preferred on engines that experience extended run times that could clog top gas ports. Top gas ports and lateral gas ports are sometimes used together. As far as strength, I would not suggest placing a top gas port on a piston except through the top land, and not into a valve pocket that extends to the edge of the piston dome. The gas port, whether located in the top or laterally from the side, should not intersect the root of the ring groove, and the hole should have .005-inch minimum clearance from the root of the ring groove in either case.
In the quest for low blowby, one important point that is sometimes not considered is the significant role the lower side surface of the ring and the lower flank of the ring groove play in sealing. This interface can be greatly improved by lapping the side faces of the top ring. Our tests have repeatedly shown blow-by reductions of 35-percent or more. JE can provide this lapping procedure in house on our equipment.
2) Does the future for higher performance piston material(s) lie in steel capped, forged, hypereutectic, or exotic (titanium)? Please comment from your experience.
David Fussner: It depends on the specific high performance application. I think forged aluminum (2618 alloy, in particular) will be around for many years to come, due to its toughness and superior resistance to mechanical damage from detonation. Today’s forced induction engines with power adders rely predominately on massive cylinder pressure rather than high RPM like a naturally aspirated engines. So these boosted engines require a very strong piston structure, but in most cases can work with reasonable durability without a steel dome.
3) I have an ’06 Harley V-Rod fitted with a Garrett T25 turbo. Have ported heads, Jones turbo cam, beehive springs, 1mm oversize valves ( intake & exhaust), Carrillo rods, case brace with ARP studs. Total bike weight wet is 585 pounds. Mainly street cruising. What CR, coatings, head gaskets & possible dome configuration would you suggest, and why? Boost is currently between 8 to 12 pounds with stock compression.
David Fussner: The standard compression ratio of the V-Rod is listed by Harley-Davidson as 11.5:1. This is high for a turbo application. I would like to see 10.0:1 or less. This will likely require a reverse dish dome configuration. Cometic makes excellent MLS head gaskets for the V-Rod.
A very good feature that the V-Rod is equipped with is Ion Sensing Knock Control. This feature instantaneously pulls back the ignition timing if it detects knock, which would be a great engine safety feature with a turbo.
4) My question about pistons is, would it be better to have the lower edge of the piston skirt thrust side left sharp to help scrape oil from the cylinder wall or smoothed to allow oil to help reduce friction between the piston and wall?
David Fussner: It would be best to be smoothed – for two reasons.
First, to allow entry of oil to lubricate the piston and cylinder and secondly, sharp engine parts are more prone to crack due to fatigue.
5) We are building more high end street engines under the “Pro Touring” and “Pro Street” categories. Being a “street” engine that is tracked occasionally, I like to use the 4032 material, even in a custom application, for its quiet operation. With the horsepower levels, larger strokes, and power adders in some of these engines I am concerned with the strength of the material. Is there a horsepower level/piston speed area I should consider a threshold not to pass with the 4032 material?
David Fussner: It sounds like you have had good success in the past with 4032. It is a very good material, with good expansion control and wear resistance. However, when you move into the use of power adders, the chance of getting into detonation becomes greater. That is the time to move to 2618 alloy, it is really tough and withstands detonation better than 4032. Always best to err on the side of caution in that situation.
The good news is that JE can apply its Perfect Skirt coating to 2618 alloy pistons. It is a specially formulated patented coating designed to reduce the clearance and allow the piston to run quietly, so you could have the best of both worlds.
6) A smaller pin height leaves room for a longer rod but reduces room for rings and puts the rod side thrust off the skirt and into the ring land area. Can you discuss these issues and recommend a minimum pin height for various engine uses, such as atmo, low boost (10 psi) street and high boost (20+ psi) race?
David Fussner: The compression height (pin height) is the main player in the determination of piston mass. So, a short compression height piston can yield a lighter piston. This is important in a high-rpm naturally-aspirated application. Extremely short compression heights leave less room for the rings and can cause the dome to crack along the wrist pin axis.
Boosted engines, on the other hand, rely more on higher cylinder pressure for power and require a heavier dome structure to withstand the additional cylinder pressure. Light weight and short compression height are not as important as high strength in these applications.
So, the minimum pin height would be less for a naturally aspirated high-rpm engine and more for the boosted engine. Also, aside from the rod length influence on the compression height, the reduction of room for the ring package can be a friction advantage if the second ring can be eliminated in N/A applications. But this would be much less of a consideration in a boosted engine where more advantage can easily be gained by simply turning up the boost.
7) For a street car: How do you determine whether you should use a cast or forged piston? Why do engine builders bore an engine that doesn’t need to be bored, for example .030-inch over, when the block is fine? Do you recommend any coating on the pistons? If so, which one and why?
David Fussner: It’s important to understand the primary differences between forgings and castings. Castings have no organized grain structure in the material and are therefore brittle. A forged piston, on the other hand, has “Feature Aligned Grain Flow” that is established in the material as it is being formed. This is the premier attribute of a forged piston that separates it from a cast piston. Feature aligned grain flow is the number one reason that forged pistons have such toughness and fatigue strength, and just as importantly, a property known as ductility. Ductility allows the piston to tolerate distortion without fracturing.
Ductility could be considered the opposite of brittleness.
If you were to step up to forged pistons, you would have the confidence in knowing that, properly installed, they would be able to handle nearly anything a street car would ever be capable of.
The engine builder might want to bore and finish the block to his own tolerances for roundness, taper and honing finish.
As far as coatings, JE can apply its Perfect Skirt coating to 2618 alloy pistons. It is a specially formulated patented low friction coating designed to reduce the clearance to nearly that of a cast piston, and allow the pistons to run as quietly as a cast piston.
8) I own a 1916 American LaFrance Pumper with an 850 ci (5.50-inch bore x 6.0-inch stroke) inline “T” head six-cylinder engine. The cast iron pistons weigh over 8 pounds each. I would like to use lighter aluminum pistons with a modern ring package. Can I install the much lighter pistons with no effect on the engine balance? The crank has no counterweights and the redline on this motor is 1,300 rpm.
David Fussner: The reciprocating motion of the piston is the origin of engine vibration. Reducing the weight of the piston by changing to aluminum will reduce the inertia force of the reciprocating piston by 60-percent. I would assume that this six-cylinder engine has the crankpins staggered 120 degrees from one another. If this is so, it is one of the smoothest running crankshaft balance arrangements known, even today. Since you mention that this crankshaft has no counterweights, I can only think that this engine would be happier with aluminum pistons.
9) Application is a blown big block on alcohol, max is 30psi boost, max rpm is 8,500, aluminum rods, Bruno drive, 3-speed Lenco, 4:11 gearing, 2200-pound Altered.
What should the piston dome thickness be to live under boost conditions? How far down should the first piston ring be from the deck of the piston? Recommend piston-to-bore clearance for above application.
David Fussner: These are guidelines, of course, but based on your description –
- The dome thickness should be .300-inch minimum.
- The top land should be .400-inch minimum, and as thick as you can make the second land (+.200-inch), depending on how much room there is to accommodate these land widths.
- To be on the safe side, .008-inch clearance would be a good place to start.
10) Can you make custom pistons for maximum efficiency with 108-octane fuel?
Background info and facts:
We’re worried that we’ll interfere with fuel efficiency if we do. In the past, combustion chambers weren’t sophisticated. Now they’re very sophisticated. Maybe we’ll interfere with the combustion process if we squeeze the charge into a much smaller space. We are aware that we should have a custom grind on the cams, then harden it, and that we may have to take responsibility for ignition timing work, as well.
The fuel: E100, 100-percent ethanol, not this E85 junk.
You may wonder why we don’t read up on compression-raising with piston design, for alcohol fuel. Well, we can’t. All the alcohol engine work we do in this country stinks because it’s not done for uncontaminated alcohol fuel, but so engines can also run on much lower-octane gasoline.
David Fussner: You are correct about the sophistication of today’s combustion chambers. The project you are considering would require extensive development testing. Perhaps you could have some pistons made with several dome geometry iterations of your choice, and test them to see if you gain the efficiency you are striving for.
We here at EngineLabs wish to thank David for his efforts to answer each of these questions, and hope you have enjoyed reading his responses—and perhaps learned a thing or three in the process!
Thanks for following along; we have another one of these sessions scheduled to hit shortly with Keith Jones of Total Seal Piston Rings, and more to follow in the future with other experts including Ben Strader of EFI University. You can check out a past article with Nolan Jamora of Isky Racing Cams. Stay tuned to EngineLabs for more!