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Effects of retarding Exhaust cam on the turbocharged DOHC gasoline engine 1

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MRDAGERUS

Automotive
May 19, 2009
31
Let's consider a
2.5 L, 4cyl, 16v, DOHC,
turbocharged, Max boost = 10psig,
CIS fuel injected engine(Lotus 910),
7500 red line

Where: #1 is a OEM design configuration
#2 is a modified configuration,(exh. cam retarded from 104 MOP to 110 MOP), considered for implementation
#3 for comparison, as an afterthought, just to see how far the overlap can be reduced

.................. .......#1........ . ....#3...............#2
................ ....OEM Turbo...OEred/red....OEgreen/red
Cam ID ..............107/107.....107/107.....107/107
Cam Pulley MOP..104/104.....110/110.....104/110
Int. Total Lift.........0.378"......0.378”.......0.378”
In. Opens, BTDC........22°.........16°...........22°
In. Closes, ABDC.........50°.........56°..........50°
In. MOP ATDC...........104°.......110°.........104°
Int. Duration..............252°.......252°.........252°

Ex. Total Lift..........0.378”....0.378”.......0.378”
Ex Opens, BBDC.........50°........56°...........56°
Ex. Closes, ATDC.......22°........16°...........16°
Ex. MOP, BTDC.........104°.....110°..........110°
Ex. Duration.............252°.....252°..........252°
Overlap...................44°........32°............38°
Lobe separation.......104°.....110°...........107°
Intake C/line offset...0°...........0°...........+3°

Trying to determine:
When it comes to less propensity to detonation, is #2 better? Why? (I was told it would be better, but no explanation was offered.)
What is the effect of advancing Int. cam C/Line?
Less overlap is desirable, but isn't 38° too low?
What would be advantages of #3, if any?

Thank you for your input.
 
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"Less overlap is better" Is a completely uneducated statement about boosted engines.

One of the most important and yet least discussed aspects in turbocharging is the ratio of boost to (PTBP) Pre turbine back pressure.
If your ratio is 1:1 IE 10 PSIG boost and 10 PSIG PTBP then your optimal cam timing would be the same as a comparbly prepped NA motor.

On the other hand if you look at a supercharged motor making 10 PSIG boost and say 1 PSIG exhaust back pressure (since it doesn't have a turbine) then the pressure differential will pass more air and fuel through the exhaust valve and into the exhaust before the valve closes. This wastes air and fuel and can also throw off things like O2 sensors.

If you have a tiny turbo and have 10 PSI boost and 20 PSI PTBP then if there is too much overlap the PTBP can push exhaust into the intake manifold and reduce the amount of oxygen the engine can take in on the intake stroke.

Then there is RPM. It's not that big cams are bad or that overlap is bad it's that it is all relative to the rest of the system. I once heard as a general rule of thumb that for a supercharged motor cams will behave as 30 degree bigger cam on an NA motor. This is horribly oversimplified but portrays the idea. If 300 a 300 degree cam would make you peak power at 8000 RPM in an NA motor then a 270 cam might get you peak power around the same RPM. Again an oversimplified example.

The upside with a supercharged engine is that it will stay the same across the board so you can size cams accordingly.
A turbo changes the boost to PTBP ratio across the rev range. My DSM T25 on my 1.6 liter 4AGE at 7 PSI boost has about 6 PSI PTBP at 3000 RPM but has about 13 PSI PTBP at 7500 RPM. I love the turbo because of it's quick spool but that is a pretty horrible ratio.
A well balanced turbo shouldn't go much above 1:1.

So each of the setups I have described above would have different optimal valve timing. A high reving motor with a well balanced turbo could happliy run massive cams with a ton of overlap. Another setup may barely be able to run any overlap at all.

There are some fairly complex mathmatics you could do to get some fairly accurate predictions but generally you will be much better off answering these questions on your car on the dyno.
It's also useful to look at what other people have done and use that input to effect your decisions but you have to learn to tell the idiots from the intilligent and experienced before you can really benefit from others builds and thoughts.

Remember some people can blow up a motor at 5 PSI. It doesn't mean that's all that setup is capable of making. Same goes for dyno runs. Someone cann "prove" a set of cams make horrible power until you learn that they didn't monitor AFRs or didn't have a good fuel tune or spark tune or any number of other variables.
 
Thank you Yoshimitsu for addressing the overlap issue.
I'm wondering, what is the simplest way one can find out what PTBP is generated by his particular turbo set up?
 
Lol I think Brian gets a gold star for that one.

I have an EGR fitting right at the collector on my manifold. it's a 1/8 NPT fitting so it was real easy for me to take out the EGR thermocouple and install some tubing. I ran steel fittings and tubing about 10" to get it away from the turbo and then ran a plastic line to the cabin. You could also use vacuum line, copper, whatever is most convienent you will just want to make sure the first little bit can take the heat. Then hook the line up to a boost gauge or other air pressure gauge. Since the pressure will change a lot across the RPM range you will want to do a full pull from low RPM to redline and monitor it's variation.

If you don't already have a bung between the head and the turbo you could weld one in or even tap one into a flange or the turbine housing. I think I tapped mine into the turbine housing to header flange. I would reccomend doing this with the turbo off so you don't get any metal into the turbine housing or can at least clean it out before starting it back up.
 
Thank you for the excellent suggestions. Would the turbo manufacturer/builder have PTBP values? Is it a part of pre-production testing?

How about my other questions:
- Is setup #2 better for detonation? Why?

- What is the effect of advancing Int. cam C/Line?

- What would be advantages of #3, if any?

 
MRDAGERUS said:
Would the turbo manufacturer/builder have PTBP values? Is it a part of pre-production testing?
The short answers are Yes, and Yes.
However, if you have any expectation that (A), these measurements are publicly available (except as a turbine swallowing capacity map), or (B), that they are of any use unless you have the ability to calculate or model engine and turbocharger matching, you have another thing coming.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz
 
- Is setup #2 better for detonation? Why?

- What is the effect of advancing Int. cam C/Line?

 
Your question doesn't really make sense because there are still too many unanswered variables. I think the best thing you could do is start researching basic cam timing principles. Like I said much of the theory is the same between NA and boosted but you also have to be aware of the pressure differential between intake and exhaust.

Next you need to study detonation and pre ignition more and understand the causes and the difference between the two.

Detonation and preigntion can be caused by any number of different things. Sometimes they can happen together and sometimes seperate. The cause can be a hot spot in the combustion chamber, too much pressure in the combustion chamber, too hot of an intake charge,too low of octane or most often some combination of the above.

If the cause is a hot spot in the combustion chamber then it's unlikely cam timing will have any effect. If it does it might be from reducing VE enough or letting enough energy escape into the exhaust to allow the chamber to run a little cooler. Either of the above however will loose you power. You would be best off finding the hot spot and eliminating it.

Too much pressure in the combustion chamber.
If you are knocking at one point in the rev range you could change your cam timing to decrease the VE in that area. This will make it less likely to detonate there but may make it more likely to detonate somewhere else. Exactly what you should do to the cam timing would totally depend on where the knock was, how agressive it was, how much of the rev range it covered and how you chose to address the problem. Cam timing is only one option and not the first one I would think of. You could pull timing in that area, you could increase the octane, you could decrease the inlet temps.

If your intake charge is hot then addressing this will give you gains across the board. You might get rid of your detonation in that one spot and be able to throw more timing at it everywhere else.
Same goes for octane.
 
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