What makes a cam swop cause some cars to be chopppy at idle and can it be prevented. I am ooking for options that will have a normal idle and driving experience with more power and low end torq if possilbe.

Recomendations on spec's would be great.

How much power and torq would just porting the heads add if I didn't swop the cam?

I have an LS2 engine, A6, ported TB/Intake, CAI, Headers, and 1.8 rockers if that makes a difference.

 

If u dont want that choppy idle.leave as is.but if ur gonna port ur heads go ahead in swap out ur cam. With the porting of ur heads ur increasing the fuel flow so u want the intake and exhaust vavles to detonate that more fuel by higher intake and exhaust duration by which u need a custom camshaft which makes that choppy idle..I think it sounds good. CompCam Specs..duration intake224 exhaust224, lift .610,.610

 

A lot of choppyness is caused by your LSA. The wider sep you go the less chop you have. If you go around 115 to 116 it should be pretty mild with the above mentioned intake and exhuast durations. You may even be able to go a little bigger on the duration say 228 or so and still be good.

 

112 on the lobe seperation. sounds pretty nasty.

 

Pretty much all the above are correct. the lower the LSA ( Lobe seperation angle) generally the more choppy. However the split between the intake and exhaust duration also plays a factory as well as the valve overlap ( time the intake and exhaust valve are open simulateneously for a split second but it matters) Like most said stick with a 115 or 116 LSA and keep the duration below like 228 and you should have a pretty tame car.

 

If you want to keep the "normal" idle you might consider a rear gear swap. It will increase the overall performance of the vehicle without cracking open the engine.

 

LSA means nothing. Focus on valve events and overlap. Overlap has the biggest effect on idle quality, probably followed by IVC. A lot of overlap at low rpm results in a polluted mixture. A 212/218 111 cam will idle at 700 much more easily than 232/232 114.

Keep your overlap at .05 to about 0, and you can spec a cam that will idle well and have good low end and midrange tq. You can raise the idle a bit to help, although with an A6 you have less flexibility than an M6.

 

I put in a 228/228 114LSA and it is right on the border of chugging at the stop lights (also automatic). My tuner was able to get the idle down to 700 and still run well. I would not have wanted to go any further.

You'll need to be careful, since you already have high-lift rockers. A cam that would be safe in other cars might not work so well in yours. You'll want to be sure you don't go too far on the lift unless you plan to fly-cut the pistons. My 228 would be like a 220 or 224 for you.

 

The rockers create a valve control issue if the lift is too high. Too much lift and the springs won't control it. PTV is a function of duration, ie. How much are the valves open near TDC. It is not a lot relative to peak lift. Peak lift occurs when the piston is not near TDC.

 

IVC has no affect on idle quality...overlap is the controlling factor. Overlap can also be determined from duration and LSA, so LSA does mean something. For a given cam, narrower LSAs will result in a choppier idle.

 

I agree that overlap is the main factor, but for a given amount of overlap, earlier IVC can help a bit. In my experience a 220/220 110 110 cam is easier to get to idle at a low rpm then say a 230/230 114 114, yet both have the same overlap.
If the duration is fixed, that's true, but a blanket statements of just pick a high LSA or 112 LSA cams have choppy idles are not correct. As you pointed out above, overlap is the controlling factor and low LSA cams should not be rejected just because you want a good idle.

 

As usual, Glass Slipper is right on the money. A blanket statement that "LSA means nothing" is just dead wrong, as is a blanket statement to "just pick a high LSA". Earlier IVC may help low-RPM throttle response, but by itself it does not affect overlap.

Instead of arguing amongst ourselves, let's focus on the question that was posed. For best street driveability and a stock sounding smooth idle, a cam with a combination of mild duration (say 220 to 224) and relatively wide LSA (114 or higher) will usually work best. The OEM cams for LSx engines usually have VERY wide LSA's, up around 117. There are also several "stealth" cams available from vendors that feature wide LSA's.

Another factor in idle quality is valvetrain noise. High lift cams (lift above .575" or so) usually have very fast ramp speeds, and can be noisy --- also wear out springs faster.

My advice is to consult the vendor where you plan to buy your cam, take their advice.

 

What is wrong? Can you point to anything that says "Low LSA is always bad idle"? You can't without first holding the duration constant and thus forcing overlap to change. Since we're all in agreement that overlap is the number one factor, lets break the myth that LSA drives idle quality.

FWIW, from some of the published books I've seen, many designers pick the valve events and overlap and then let LSA fall out from the result. LSA is rarely, if ever, a starting point. I agree and didn't say that an early IVC effects overlap. I said that after overlap, the next variable that could be considered is IVC. As you note, IVC can impact low rpm throttle response and idle is low rpm operation.

 

I think we're all pretty much in agreement and have demonstrated to the OP that there are many variables to picking a cam which is why I usually give the same advice as HITMAN99 above: consult a vendor, be honest in your goals/expectations, and take his advice. He knows what works and what doesn't for your particular application and you'll be much happier than taking advice from any forum.

I design a cam for the most part the same way as in the bolded part of your post. But I usually do have an idea of the range of LSA I'm looking for because that affects the shape of the torque curve, so to say the LSA is rarely a starting point is not exactly correct. I'm limited in the amount of peak torque I can run through the drives on my boat so I wanted a wide LSA to flatten the torque curve so I could reduce peak torque in the midrange, increase it at low RPM to get a 9000 pound boat up on plane, and increase VE at high RPM to make peak HP at 6000 RPM for those 80+ MPH runs out in the ocean. I also needed low overlap as I had a 900 RPM max idle speed for my drives. For my boat engines, I decked the block to get the quench/squish I wanted and let the compression ratio come out to whatever it was because I wasn't buying another set of forged pistons. From there, I picked the IVC point to get the DCR back down to where I could run the 89 octane available at marinas. I knew the amount of duration I needed to run to make peak HP at ~6000 RPM...it seems low but these are boat engines that can run at high RPMs for miles unlike a drag race engine that sees peak RPM for just a second and you grab the next gear. It's also why lift is conservative in the .575"-.600" range even though I'm using solid roller cams. After duration, I used LSA to juggle the curves to get them where I wanted. I use carbs so I can deal with overlap easier...I could idle a 241/252 on a 114 LSA flat hydraulic cam down to 500 RPM where it sounded wicked. I guess LSA is another one of those "variables" that we can discuss like which came first, the chicken or the egg...there's really no answer. The only blanket statement we can make is overlap has the biggest impact on idle quality, so now we've come full circle to what you said in your first post.

 

That's seems to be the benefit of opening the exhaust valve early is a little flatter curve and a nice high end. I'll admit that I tend to have an LSA in mind too, but mainly because I've gone the opposite way in my street cars. The later EVO to try to be a bit more peaky since I don't have many opportunities to redline the motor in multiple gears, whereas on the commute i may get a number of 3000 - 5500 punches in various stretches.

Boat are fun. I wish I were in the neighborhood to offer to help you burn some gas on warm sunny afternoon.

 

rjung,
Not sure I understand your goal?
If your goal is more low end torque and smooth idle, you probably want to avoid most aftermarket cams. Regardless of all the terms being thrown out, most aftermarket cams increase power above 3000rpm at the sacrifice of torque and horsepower below 3000 rpm. The old saying you never get something for nothing hold true here. You are reducing low RPM performance in order to gain higher RPM performance.

In a race car, this is a perfect trade off since the engine will not even operate at low RPM. In a C6 with a 6 speed auto or manual and tall rear axle gears, 90% of your driving is below 2000 RPM. Why then would we sacrifice power below 2000RPM in order to gain power above 3000RPM where we rarely operate the engine???

The choppy “rump rump” idle sound is something some of us like to hear. This is mostly nostalgic as most of the fast cars of the 60’s and 70’s had high overlap camshafts and if they didn’t, we installed one ourselves. It is just a cool factor.

Download the Comp Cams Cam Quest cam selector and spend some time reviewing the LS cams. You will get a good feel for camshaft duration figures and how it affects the power range.

 

I have a similar set up for my C6 A6. I drive it everyday on the streets in Vegas. I utilized the help of Patrick G @ www.guerragroup.com Cam specs are 220/228 .564"/.571" 114LSA +2 advance. It is a Comp Cams custom grind with XE lobes (lobe numbers 3713R and 3717R.) I also wanted a relatively quiet valvetrain. The engine has a nice lope, lots of power, and no driveability problems. The 1.8 rockers may be a problem with some of the more aggressive lobes that are utilized on todays modern cams. With all of the many choices and opinions out there it can be somewhat confusing. Hope this helps.