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Discussion Starter · #1 · (Edited)
This is ment to be an open discussion on theories behind engine internals. Please keep on topic. Any links to outside information is welcome. Lets all learn something new, lol:


1. With no other variables changed: If the rod length was shortened would this automatically decrease compression?

2. Would this automatically increase the displacemnt?

3. What other side effects would this have?

4. Would cam timing need to be changed? And could this be changed with adj cam gears and or adj cam sprockets?

5. Would the addition of a higher compression domed piston negate any loss of compression that might occur (see 1)?

6. Thoughts on "swirl augmention" to head design?

7. Thoughts on cams with variable intake vs exhaust lift & duration? Thoughts when used in conjunction with (6)?

8. Would a longer rod automatically increase compression? (see also 2,3.4 in context with 8 )

9. General effect of a light weight forged rod/piston if stock dimensions were kept?

10. Do you ever wish the tiburon was AWD, lol?
 

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Engine Builders did a test on a High Lift Shor duration Cam that made nearly the same power as a standard high lift cam but monsterous amount of torque down low instead of losing power. Just tossing that out there.
 

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ok
1.yes the piston cant travel as high in the cylinder
2. no it would decrease displacement less volume of air moving shorter stroke caused by shorter rod
3. less hp but will spool faster less weight and less distance to travel
4. unsure
5. yes
6. debatable
7.look at vtech in hondonts
8.yes opposite of 2. bringing the piston closer to the head
9.lighter less mass to turn freeing more hp and faster spool
10. no only rw
 

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Discussion Starter · #4 · (Edited)
Oh and on a note for number 7, i dont mean like a true variable timing for the cams as in vtec which is continuous correct? I just mean a static cam regrind where the two intake valves are not opening/closing at the same time (almost staggered) and same with the exhaust side.... like the tri flows from colt cams.

2. no it would decrease displacement less volume of air moving shorter stroke caused by shorter rod
You sure? I was thinking that with a shorter rod you would actually have more volume in the cylnder head when the engine is on the combustion cycle.because the piston would be lower in the cylinder, but maybe im thinking about it backwards.

I wonder what the relationship would be with the ross 11.0-1 comp pistons with a shorter rod on our 2.7s, more free reving and possibly be able to achieve higher rpms without possible damage (with oversized valves and stiffer springs)
 

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1. With no other variables changed: If the rod length was shortened would this automatically decrease compression?
2. Would this automatically increase the displacemnt?
3. What other side effects would this have?
4. Would cam timing need to be changed? And could this be changed with adj cam gears and or adj cam sprockets?
5. Would the addition of a higher compression domed piston negate any loss of compression that might occur (see 1)?
6. Thoughts on "swirl augmention" to head design?
7. Thoughts on cams with variable intake vs exhaust lift & duration? Thoughts when used in conjunction with (6)?
8. Would a longer rod automatically increase compression? (see also 2,3.4 in context with 8 )
9. General effect of a light weight forged rod/piston if stock dimensions were kept?
10. Do you ever wish the tiburon was AWD, lol?
1. When you shorten the rod, you also have to change the crankshaft. The crankshaft is carefully balanced with counterweights in order to reduce vibrations that can literally rattle the engine apart. So you would have to replace the crank with the rods. But replacing this is widely available on major platforms, and it's called a stroker kit. Yes, this will decrease compression.

2. Yes, this increases displacement. The Tiburon already has a bored-out version of a 2.5L V6, so the cylinder walls are already relatively thin. To increase displacement by boring them out further can be risky. But the motor definitely has room to be stroked.

3. The advantage to stroking a motor is increased torque. The connecting rod is shorter, thus giving the piston more leverage to turn the crank. This gives you pretty sizable gains. The disadvantage is that you're increasing the displacement relatively drastically. Since you need to fill up a larger cylinder size with air, it gets less efficient in the higher RPM's since the intake tract (intake, manifolds, heads) were designed with a certain velocity of airflow. Now that it needs to supply more, you really need to change this to take full advantage of the added displacement. This includes the cam profile.

4. Cam timing doesn't NEED to be changed, but it would be highly advantageous if you did. Yes, this can be adjusted with cam gears, but for the full advantage, you'll want to have camshafts ground to new specifications to take advantage of your new displacement.

5. I don't think this is very popular. You now would have to redesign the heads, pistons, and valves to work together with this new design. You're already making more power from the increased displacement - who cares about compression?

6. Swirl promotes better air/fuel mixture, which is key to efficient combustion. Poor mixing results in increased emissions (particularly NOx and CO) and incomplete combustion, which gives you less power. Nissan is currently spending a lot of money on advanced direct injection nozzle design to promote immediate vaporization of the fuel. This is definitely a good thing to incorporate into new engine design, but retrofitting is very difficult.

7. Varying valve timing helps emissions, fuel economy, and power. This was a pretty popular research topic of the last 20 years, but the automotive engineers have taken it as far as it can reasonably go. Again, good design, but very difficult for an aftermarket company to do something like this.

8. If you fit a current engine with a longer rod, yes, it would increase compression. But you're losing displacement, so you'll lose power. Also, many new engines are interference motors, which means that the piston (at the top of its stroke) would contact the valve (at the height of its camshaft position). Adding to the piston height would almost certainly shear off the valves. In general, longer rods are not recommended. When designing an engine, the measurement of the bore against the stroke is a very fundamental design criteria. If the bore matches the stroke, the engine is considered square (bore/stroke=1). If the bore is larger than the stroke, the engine is considered over-square (bore/stroke>1). Most new engines are over-square and are around 1.2. Finally, if the bore is smaller than the stroke, the engine is considered under-square (bore/stroke<1). Under-square engines are all but phased out these days. I think the old Miata engines were some of the latest ones to have them. They were reliable, so long as you did not over-rev them. Taking these engines to redline for extended periods of time were much more damaging to these types than over-square engines.

9. If everything is kept the same, including weight, it wouldn't matter that the internals were forged. It helps under forced inducation. When an engine is under a lot of stress from boost, the weakest link is the first failure. Usually, it's the connecting rod that experiences a classic 45 degree shear. Making the rods out of a forged metal will make it fundamentally stronger. If it's designed exactly the same, it won't change power. It'll just make your engine safer.

10. It would be an all-around better platform to modify if it were AWD. But that doesn't change the fact that the engine is still underpowered, inefficient, weak, and harder to modify than competing platforms.
 

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Discussion Starter · #6 · (Edited)
thanks for the info bosco. Couple more questions if the engine is stroked you gain displacement but would lose some compression ratio. Is there any way to gain that compression back easily. Because in theory wouldnt you want the highest displacement with the highest compression (that pump fuel allows) or can you forego the higher compression and gain back the lost power by adjusting the timing (without knock on pump fuel) And is the reason we do not have stroker kits for our 2.7 because no aftermarket has tackled the crank? does it have to be a new crank or a ballanced stock one?
 

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If you stroked the engine, who cares about compression. Any ill-effect from lost compression is more than made up from a larger overall engine displacement. That is the most important factor when assessing engine power output.

Nobody has produced a stroker kit for our 2.7 because there isn't a large enough market for it. It's a one-off product that requires a relatively high initial investment. A company would have to reverse-engineer the crank, size out new rods (after a lengthy R&D process), and then create a design for the updated rod length. And the cost of each unit is high - usually in the thousands range, plus installation. And since we own Hyundai Tiburons, we are usually young and we can't afford expensive modifications. Therefore, the market is small. However, there are lots of kits available for muscle cars, both new and old.
 

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1. With no other variables changed: If the rod length was shortened would this automatically decrease compression? Yes

2. Would this automatically increase the displacemnt? no

3. What other side effects would this have? If the only change is a shorter rod you lose compression, create more cylinder side-load, unbalance the engine, and chance the piston hitting the crankshaft counterbalance weights

4. Would cam timing need to be changed? And could this be changed with adj cam gears and or adj cam sprockets? does not matter. shorter rods by itself is a disaster

5. Would the addition of a higher compression domed piston negate any loss of compression that might occur (see 1)? yes if calculated correctly

6. Thoughts on "swirl augmention" to head design?swirl = good

7. Thoughts on cams with variable intake vs exhaust lift & duration? gimmickThoughts when used in conjunction with (6)? none

8. Would a longer rod automatically increase compression? (see also 2,3.4 in context with 8 ) yes.... until the piston hits the head

9. General effect of a light weight forged rod/piston if stock dimensions were kept? stronger, more abusable, typically heavier

10. Do you ever wish the tiburon was AWD, lol?no
 

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ok i have 1 ? the displacement is measued buy the amount of air the engine can move right so if shorter rod piston will not reach as high in the cylinder and will still stop at the same bottom for theory perpose. so tdc air space minus bdc air spcae would be less than stock. less displacement not the other way around. stroker motors larger crank more travel more displacement not less!!!
 

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ok i have 1 ? the displacement is measued buy the amount of air the engine can move right so if shorter rod piston will not reach as high in the cylinder and will still stop at the same bottom for theory perpose. so tdc air space minus bdc air spcae would be less than stock. less displacement not the other way around. stroker motors larger crank more travel more displacement not less!!!
There's the rub.

Displacement is volume of each cylinder X number of cylinders. Volume of cylinder is bore or area of piston (Pi x radius squared) times the distance the piston travels from top to bottom. The distance traveled by the piston is entirely a function of the stroke (or arm length x 2) of the crankshaft.

So the piston diameter effects displacement and the crank stroke effects displacement. Nothing else matters for displacement. Not piston height. Not rod length. Not pin diameter or cam specs or valve size or volumetric efficiency..............
 

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thanks for the info bosco. Couple more questions if the engine is stroked you gain displacement but would lose some compression ratio. Is there any way to gain that compression back easily. Because in theory wouldnt you want the highest displacement with the highest compression (that pump fuel allows) or can you forego the higher compression and gain back the lost power by adjusting the timing (without knock on pump fuel) And is the reason we do not have stroker kits for our 2.7 because no aftermarket has tackled the crank? does it have to be a new crank or a ballanced stock one?
Forced Induction
 

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Uhh, what? Stroker kits DEFINITELY increase displacement. The flaw in your logic is assuming TDC is at the top of the cylinder head. When you stroke the motor, TDC is at a lower point, leaving all of that room for air to fill up. Draw the sketch yourself and then draw a box around the working boundary. The stroked area is larger.
 

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Uhh, what? Stroker kits DEFINITELY increase displacement. The flaw in your logic is assuming TDC is at the top of the cylinder head. When you stroke the motor, TDC is at a lower point, leaving all of that room for air to fill up. Draw the sketch yourself and then draw a box around the working boundary. The stroked area is larger.
His question was not about strokers. I think you may have misunderstood. All he asked is what the effect of a shorter rod is.

A stroker is a shorter rod combined with a longer stroke (or longer arm x 2) crankshaft.

re-read my post. bore x stroke = displacement. stroke changes are related to the crankshaft, not the rod length.
 

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Discussion Starter · #14 ·
If a new crank and rods were used to raise the stroke to would cam timing or ignition timnig need to be altered? Obviously a/f ratio would because it would still be putting in fuel like only 2.7L of air were there as aposed to like 3.0L.

Do any mitsu motors share the same block and or crank specs as the delta 2.5 or 2.7?

Also do we know for sure that the 2.7 can NOT be bored out anymore? Has anyone taken a broke block and just kept on boring till they reached the coolant lines and measured the bore? I only ask this because the Sigma 2.5 was first a 82.5mm bore then 3.0 was 91.1mm, then again 3.5 was 93mm.

Thanks, keep any interesting thoughts coming.
 

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Yes they can be bored, but I dont know how much more it can be bored. I'll tell you later ;)
 

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3. The advantage to stroking a motor is increased torque. The connecting rod is shorter, thus giving the piston more leverage to turn the crank. This gives you pretty sizable gains. The disadvantage is that you're increasing the displacement relatively drastically. Since you need to fill up a larger cylinder size with air, it gets less efficient in the higher RPM's since the intake tract (intake, manifolds, heads) were designed with a certain velocity of airflow. Now that it needs to supply more, you really need to change this to take full advantage of the added displacement. This includes the cam profile.
Ok, I guess I have a few "what if" questions for you

What if used injunction with a turbocharged set up? Would that help with the extra air requirements?
Also, if nitrous oxide was ever introduced into the equation, would raising the boost even higher aid the additional air requirements on top of that, would it eventually balance out?

this thread has sparked my curiosity:3_worship
 

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Discussion Starter · #17 · (Edited)
this thread has sparked my curiosity
excelent =)

To answere your question yes, any type of boost would utilize a stroker kit very effectively. With a n/a car though the heads would have to be reworked pretty extensively with some oversized valves. Intake and upper/lower manifolds would also have to be able to supply enough air (basically zero vacuum at WOT, anything less and there would be a bottleneck). Since your question is about boost zero vacuum has already been achieved and surpassed. Nitrous would work too, as soon as you hit zero vac you have achieved everything you need. It would just be harder to hit that n/a if you have a stroker kit.

On a side note the new genesis RMR car is stroked =) and just broke the pikes peak rwd record.

Yes they can be bored, but I dont know how much more it can be bored. I'll tell you later
Tell me now! j/j It sounds to me as if you have something up your sleeves. Keep us all updated on any good info you find.
 

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We'll info on the block it will be awhile,but something will come sooner or later.
 

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Discussion Starter · #19 ·
What about offset grinding of the stock crank?

By offset grinding the rod journal you move the centerline of the rod journal away from or toward the centerline of the main journal. This will result in increased or decreased stroke. Figure 3 above illustrates the case we are interested in, the rod journal is ground in a manner to increase stroke. Keep in mind that when the rod journal is offset ground it now has a smaller diameter. The motor will require special connecting rods with correctly sized bearing bores


A high rod angle or low Rod Ratio creates a greater potential for accelerated wear to cylinder walls, pistons, and piston rings.

By lengthening the rod, as stroke is increased, we can offset the increased rod angle. However, this requires further shortening of the piston. The further the piston is shortened the more likely the piston pin will intersect the oil ring groove, creating a potential for increased oil consumption.



Either way, there comes a point when you cannot shorten the piston any further before dependabilty is compromised. As in the discussion about offset grinding, we have reached a limit to how far you can stroke a motor before some component or function is sacrificed.

The consensus amongst engine manufacturers is that a ratio of 1.50" is the lowest acceptable rod ratio for a street motor. Realistically, rod ratios between 1.65" - 1.80" are ideal.

An often overlooked factor that contributes to the advantage of a stroker motor has to do with piston dwell time, the amount of time the piston remains at the top and bottom of the stroke. The increased stroke and rod length of a stroker motor yields a longer piston dwell time. Longer dwell time allows for better flow of combustion and exhaust gases since the piston accelerates slower in the transition between "up" and "down" strokes. Intake gases have a longer time to enter the cylinder while exhaust gases are given more time to escape. This translates into more natural torque over a longer range of rpm. Power and torque can also be enhanced with valve event timing and cam profile.

Even though the piston accelerates slower in transition, the piston ultimately reaches higher speeds to cover the additional stroke. This increase in piston speed means greater component strain. Another factor to consider before simply going with the kit or components that give you largest stroke increase.

As you may have guessed, there are certain issues which must be addressed when actually assemblying any stroker engine. First and foremost is the issue of clearances. Due to the increased stroke and rod length changes, it is common for the rod and crank to interfere with cylinder bore end, pan rails, piston skirts, windage trays and other areas inside the block. Therefore it is mandatory that you preassemble the engine components, mark the areas needing grinding for clearance, dissasemble and make the neccesary clearances, and then reassemble and check again. As a rule of thumb you should have at least 0.030" clearance between any interfering points. Another set of considerations unique to stroker engines is rotating assembly balancing. Whether the stroker kit is custom made, or off-the-shelf, the use of new or offset ground cranks, longer rods, and stroker specific pistons ensures that the assembly is not going to spin evenly. Any stroker kit, even off-the-shelf ones, must be balanced by a competent machine shop. Not doing so is a recipe for failure. Always perform the balancing with the harmonic balancer and flywheel you intend to use.
Info gathered from: http://www.fordmuscle.com/archives/2003/09/stroker/index2.php

So any thoughts regarding an offset grind on our stock crankshaft?
 
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