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ITT: we talk about dual VVT cam tuning
This has been consuming my thoughts and ponderings lately.
Let's start with the basics of post below: (this is the most cohesively written example, by far not the best, but makes sense to me, feel free to supplement/critique) On four stroke engines, it is important to realize that the cam rotates once for every two rotations of the crankshaft. Volumetric efficiency is based on cylinder fill. If a 2.0L engine is filled with 2.0L of an air/fuel mixture, we say its volumetric efficiency is 100%. If a 2.0L engine fills with 3.0L of an air/fuel mixture, we say its volumetric efficiency is 150%. A forced induction engine will have a larger than 100% volumetric efficiency since the intake charge and combustion chamber are being pressurized. A naturally aspirated engine can also have a slightly larger than 100% volumetric efficiency, but it will only happen for a short duration, and is usually only in the peak of the powerband. The art of designing camshaft profiles is meant to increase the volumetric efficiency in the RPM range that the powerband requires. Camshafts don't make magical horsepower from nowhere, they simply move the powerband around by changing the volumetric efficiency to attain the desired results. The four strokes of the engine are: Exhaust Intake Compression Combustion Looking at a camshaft, the sequence would be as follows: The exhaust lobe pushes open the exhaust valve and the piston comes up to push the exhaust out, then starts to close. The intake starts to open, just as the exhaust is closing, piston goes down, and the intake valve closes. Then both valves stay closed for the compression and combustion strokes. This means that the first lobe to come through the rotation will be the exhaust lobe, immediately followed by the intake lobe. Overlap is the point where the exhaust valve is closing, and the intake valve is just opening. To increase overlap, you have to RETARD the EXHAUST, and/or ADVANCE the INTAKE. To reduce overlap, you have to ADVANCE the EXHAUST, and/or RETARD the INTAKE. Simple cam tuning rules for NATURALLY ASPIRATED engines: Advancing both cams => more low-RPM power, less high-RPM power Retarding both cams => more high-RPM power, less low-RPM power Less overlap => more low-RPM power, less high-RPM power More overlap => more high-RPM power, less low-RPM power In a naturally aspirated engine, the extra overlap is called "scavenging". Scavenging is using the out-flowing exhaust to help draw in the next intake charge (partially causing lumpy idle). Simple cam tuning rules for BOOSTED engines: Advance intake and exhaust => more low-RPM power, less high-RPM power Retard intake and exhaust => more high-RPM power, less low-RPM power Less overlap => lower EGTs, faster turbo spool, less fuel More overlap => higher EGTs, slower turbo spool, more fuel Boosted engines don't like overlap. The incoming cold air and fuel cools down the outgoing exhaust charge, condensing the exhaust gasses. This is VERY counter-productive in a turbo application since the engine needs no help from scavenging to fill the cylinder. I've heard this being called "turbo chill". Cool, condensed gasses in the same space push less hard on the turbo, causing lag. HOT gasses are better at spooling the turbo, thus the advanced exhaust timing to open the valve sooner in the power stroke. This steals some of those hot, expanding exhaust gasses to help spin the turbo a little faster. When the piston is near the bottom of the bore, hardly any energy is going into rotating the crank anyway, so stealing expanding gasses won't hurt anything. The retarded intake just helps cut down the overlap further. Retarding overall cam timing: Retarding overall cam timing is better for high-RPM power. This is because the valves are closing later. The intake valve is closing AFTER the piston has started to travel back up the bore for the start of compression stroke. This is terrible at low RPM because the intake air velocity is low, and air that was once in the cylinder is now being pushed back into the intake manifold and causing turbulence. At high-RPM, the rules change. Air has weight, and thanks to Sir Issac Newton, we know that once it is moving, it doesn't want to stop moving. This means that the air can continue to flow into and fill the cylinder, EVEN AFTER the piston has begun to travel UP the cylinder bore. This can allow an engine to exceed 100% volumetric efficiency, if even by a small amount. Advancing overall cam timing: Advancing overall cam timing is better for low-RPM power. This is because the valves are closing a little sooner. The intake valve is closing AT or NEAR when the piston is at the bottom of the bore for the start of the compression stroke. This is great at low RPM because the intake air velocity is low and easily affected by changes in the direction of piston movement in the engine. Almost as soon as the piston gets to the bottom of the bore on the intake stroke, the valve gets slammed shut so no air can escape as the piston begins to travel back up the cylinder on the compression cycle. At high-RPM, this may become a restriction since the air has inertia and responds a little slower to pressure changes, potentially choking the air flow to the engine a little. Conclusion: This information is aimed at allowing tuners to understand what happens when cam timing is altered. When a larger duration camshaft is being installed, unless the lobe centerlines have been changed, the overlap will be increased. If installing larger camshafts in a turbo application, advancing the exhaust and retarding the intake will reduce the inherent increase in overlap caused by upgrading to a larger profile. Most cam grinders, especially regrinders, put the new profile in the same position as the old profile because it is easier, or the only way possible. This has to be changed when the cams are installed in an engine to attain the desired result. A forced-induction engine should idle smooth when properly tuned. If a forced induction engine is loping at idle, fuel is being wasted, turbo spool time is being increased, and power is being lost. I'm genuinely interested if anyone here has any good info to share or perhaps links to really well put together info on when you run into limitations of each scenario of advance/retard combinations. Dilution, reversion, etc. mostly with respect to turbocharged engines. School me :) |
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Originally Posted by hraday93
(Post 1424255)
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Yah I don't buy the "More overlap => higher EGTs, slower turbo spool, more fuel"
That's completely contradictory. Higher EGT's mean more exhaust energy for the turbo to harness, meaning faster spool. Fact: At the torque intercept if you retarded the intake by more than about 5 degrees from it's best torque setting you will see boost pressure plummet. Fact: EGT is directly related to turbo speed. It's common to increase torque around the torque intercept by retarding IG, which increases EGT and drives the turbo harder. |
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I could see this discussion making a lot of sense if we all had the same turbos and turbo manifolds, but that giant obstruction after the exhaust valves will have a huge impact on what gains you make with what cams. IMO.
My mild steel schedule 40 master race log manifold will probably not see the same gains from a turbo cam that a bundle of snakes manifold might see. Effects on spoolup will be different as well. A bundle of snakes has several times more volume in each runner than a log has overall. Effects will vary depending upon how much boost is being run and from what turbo with what piping and what intake manifold. Logs have more backpressure but less volume. The exhaust pulses from a log will interfere with each other more than a bundle of snakes but this effect isn't the same at every RPM and it will be different depending upon the placement of the turbo on the log. I would expect. And so on, ad infinitum. I suspect the answer is "experiment with different grinds and see what works on your setup" |
I know you said no DI engines, but maybe a good primer would be to look at the maps Shiv does for N/A vs the SBD kit (both should easily available) and compare how the cam timing changes between the two? Not a whole lot, but it could help be a start.
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Also an interesting trend seen in the industry is usage of variable lift and timing on the exhaust cam rather than the intake cam.
The FK2 / FK8 Civic Type-R and the Golf R EA888 engine both have this. I am amazed every time by how well these turbo engines produce top-end power above 5000 RPM with such a relatively small exhaust turbine. |
Originally Posted by engineered2win
(Post 1424309)
Yah I don't buy the "More overlap => higher EGTs, slower turbo spool, more fuel"
That's completely contradictory. Higher EGT's mean more exhaust energy for the turbo to harness, meaning faster spool. Fact: At the torque intercept if you retarded the intake by more than about 5 degrees from it's best torque setting you will see boost pressure plummet. Fact: EGT is directly related to turbo speed. It's common to increase torque around the torque intercept by retarding IG, which increases EGT and drives the turbo harder. Generally speaking: you'd want as much advance as possible down low without diluting it or effectively creating an egr effect, and then gradually back down intake advance in the middle while gradually starting to retard exhaust cam.......right?
Originally Posted by Art
(Post 1424315)
"Turbo cams" that are marketed as upgrades for turbo engines with higher lift but shorter duration and/or overlap often suck and don't deliver as promised. Many of these DOHC 4 cylinder engines especially with variable timing have quite optimized cams for the designed rpm range from the factory. High end racers have shown big gains by going with big cams, but this is when coupled with an efficient boost to exhaust pressure ratio by way of large free flowing exhaust turbine. The turbo Miata world seems to tend toward the opposite direction with tiny exhaust sides for early spool. I'm not sure any other popular turbo 4 cylinder favors such small exhaust sides as the common Miata choices.
But comparing to OEM? No way. Most modern oem's use downright microscopic exhaust housings for that 1500whp boost threshold.
Originally Posted by AlwaysBroken
(Post 1424408)
I could see this discussion making a lot of sense if we all had the same turbos and turbo manifolds, but that giant obstruction after the exhaust valves will have a huge impact on what gains you make with what cams. IMO.
My mild steel schedule 40 master race log manifold will probably not see the same gains from a turbo cam that a bundle of snakes manifold might see. Effects on spoolup will be different as well. A bundle of snakes has several times more volume in each runner than a log has overall. Effects will vary depending upon how much boost is being run and from what turbo with what piping and what intake manifold. Logs have more backpressure but less volume. The exhaust pulses from a log will interfere with each other more than a bundle of snakes but this effect isn't the same at every RPM and it will be different depending upon the placement of the turbo on the log. I would expect. And so on, ad infinitum. I suspect the answer is "experiment with different grinds and see what works on your setup"
Originally Posted by z31maniac
(Post 1424412)
I know you said no DI engines, but maybe a good primer would be to look at the maps Shiv does for N/A vs the SBD kit (both should easily available) and compare how the cam timing changes between the two? Not a whole lot, but it could help be a start.
Originally Posted by DaWaN
(Post 1424414)
Also an interesting trend seen in the industry is usage of variable lift and timing on the exhaust cam rather than the intake cam.
The FK2 / FK8 Civic Type-R and the Golf R EA888 engine both have this. I am amazed every time by how well these turbo engines produce top-end power above 5000 RPM with such a relatively small exhaust turbine. Do you know what they do with that cam up top? advance/retard/lift/lower? |
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id only like to say that vvt is only useful for idling big cams because you could electronically lower the overlap at idle and improve idle quality. too bad that such cams aren't even available ots. For an all out race build vvt wouldnt matter and if anything would likely give you issues when trying to spin it to the moon while trying to make big power
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This discussion isn't about just miatas (even thought the ND uses this technology)
This is about how to make power with variable cam timing and boost. Just about every modern performance car I can think of uses variable cam timing, and uses it to increase power/torque everywhere. We need arghx7 up in here :) |
Originally Posted by Twibs415
(Post 1424543)
id only like to say that vvt is only useful for idling big cams because you could electronically lower the overlap at idle and improve idle quality. too bad that such cams aren't even available ots. For an all out race build vvt wouldnt matter and if anything would likely give you issues when trying to spin it to the moon while trying to make big power
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This is starting to make me wonder if the new cams released by Corksport for the Mazdaspeed 2.3 platform have fallen prey to that last bit of info. Supposedly they're the bee's knees, but in real world application the cars are seeing very minimum HP gains. I haven't gotten a chance to look at the cams compared to the stock ones, but considering the fact the stock Mazdaspeed cams are pushing those DI 2.3 engines with over 600hp to the wheels... and the Corksport ones are only adding 20 or so hp to a setup already pushing 600-700, I imagine that the centerlines of the lobes are exactly in the same spot. It never occurred to me until after reading this that you'd have to do a lot more than just change top end of the lobe itself. You basically have to start from square one to properly attack the concept of making more power, or becoming more efficient with the power delivered
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Yes. Simply increasing or tweaking lift/duration aint gonna do a whole lot.
Hence this thread. I'm starting to see that this is gonna be a really hard topic to stay focused on because every other post seems to want to derail to irrelevant discussion of BP specific stuff |
FWIW the Mazdaspeed 2.3s are basically a revitalized SVO Mustang motors, that turned into the Focus ST block, then the block was thrown into the Ecoboost Mustang and now the Focus RS. I try to talk about anything besides BPs whenever I get the chance and this thread was the perfect avenue for that. hahaha.
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Sorry vlad, the OFT is gone. But you should be able to find some freely available maps where people have taken the OFT tunes and used ROMraider.
Originally Posted by hraday93
(Post 1424601)
FWIW the Mazdaspeed 2.3s are basically a revitalized SVO Mustang motors, that turned into the Focus ST block, then the block was thrown into the Ecoboost Mustang and now the Focus RS. I try to talk about anything besides BPs whenever I get the chance and this thread was the perfect avenue for that. hahaha.
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Nope. The only true thing that is changing over time were the heads, and the taps to allow direct injection.
Source: There are dozens of these blocks lying around Kotzur Racing, everything from SVOs to Focus STs and Ecoboost engines. Take all the accessories off, take the paint off, and the blocks themselves are almost all identical. Even the Mazdaspeed6, Mazdaspeed3, and CX-7 blocks have "FoMoCo" stamped in the exact same spots as the Mustang and Ford Focus blocks he has lying around. Yes, the rotating assemblies and heads will vary, for example: the effective displacement of the Focus ST is only 2.0. Same block nonetheless. The front main seals are all the same too... Not sure what else you'd be looking for to differentiate them beyond that. Everything that separates them from the next is all bolted on and easily removable. |
Isn't the point of this discussion to result in information that we can apply to affordably make more power from existing setups? Is off the shelf head/intake really that much of a limitation with an appropriately sized turbo? What percentage of us are running boost levels that approach the upper limits of flow as opposed to the upper limits of transmission durability? How many of the amazing builds on here (like soviet for example) are running boring configurations (in terms of block/head/cams) but making tons of power through good manifold/turbo selection and decent tuning?
If you're not going to confine this to BP-specific VVT stuff, you're talking about engine/transmission swaps, right? So why not swap in an LS and vary some pushrods instead of playing with vtec? |
Ok I give up. I'll go ask a different audience
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