Does advancing a cam affect idle?

IE: does a 224/228 on a 114 idle better than a 224/228 on a 114+2?

From another thread:

 

Considering the authoritative advice you throw out on various camshafts and camshaft head combinations I figure how could you not know the answer?

Well, here's the data that may enlighten you and then again, maybe not.

Read the cam events -that will tell you which LSA and which ILC you are comparing.

 

Thats an aweful lot of distain built into that response sir.

I dont know much at all about cams. Truth is the advice I give is observed performance on repeatable builds that people can and do get. I know the basics about certain aspects from reading and asking questions. Every cam I ever commented on, I had used and was stating my personal experience. Sometimes I need a little info from the pro's. When I saw that you responded to this thread, I was happy since I thought it would be a nice response with a yes/no and an explanation, instead I feel pretty bad that aftert having applogzed to you in a PM and in a phone call. I ask a simple question and get that type of answer.

I differed in opinion on L92 heads and now after no changes but the heads themselves with a given cam, I have a slower car. I still have that opinion but it seems to bother you regardless of positive remarks and PM's to you to applogize and ask advice. In honest humility, there doesnt seem to be a way to get out from under when you make a bad post or come off as a know it all.

I asked you in a PM if L92's shaved .030 with stock gaskets fits a 228/232 114lsa cam and you said yes. You still stand by that?

It appears that since they have the same overlap they would have the same idle quality. Wouldnt it be better to have a wider LSA and advance the cam to mave the power band back down? I know it hurts the intake valve clearance.

 

Just giving back to you what you asked for.

 

Very little of what I learned on this thread had to do with cams.

 

its the in'rnet, dont take it personal

 

Don't give up on him Spin, you and me had our differences and you kept trying until I came around, and I'm pretty stubborn...at least that's what I hear about engineers. Don't give up on him.

I got your PM but figured this would be a better place to answer it since Charlie was kind enough to post some cam info with some relevant data if you pick through all the numbers there. I also thought it would be beneficial to others. Looking at his data sheets, he has two cams with the same duration/lift but different LSAs (114 for the first one and 112 for the second one). You can see the idle vacuum of the narrower LSA is lower indicating poorer idle quality as expected and predicted by HITMAN99's formula. Then you'll notice each cam is installed one way and then 2 degrees retarded for the 114 LSA cam and 4 degrees advanced for the 112 LSA cam. In both circumstances, the idle vacuum stayed the same indicating idle quality was unaffected by changing the ICL/ECL (retarding or advancing). This is consistant with what would be expected since overlap is what affects idle quality...remember duration and LSA affect overlap so while cams with lots of duration can be expected to have poor idle quality, changing LSA can improve/worsen the condition by changing the amount of overlap leaving overlap the controlling factor for idle quality. HITMAN99's formula is a quick way of finding the relative idle quality for cams with different duration/same LSA, same duration/different LSA, and different duration/LSA by telling you the relative amount of overlap added/reduced without having to go through all the gyrations of figuring the new valve opening/closing points and then looking at the intake open/exhaust closing points to figure the new overlap...pretty slick formula I'd say.

You can read through this post from earlier this month in a different cam thread.
http://forums.corvetteforum.com/show...4&postcount=15
You'll have to wade through some stuff, but you'll find an explanation of idle quality as related to overlap...then you'll see why advancing/retarding the cam won't affect idle quality. If you open the intake/close the exhaust sooner, more burned gases (exhaust) goes into the intake manifold only to be brought back in to the cylinder. If you open the intake/close the exhaust later, less burned gases go into the intake manifold but more gets brought in to the cylinder through the later closing exhaust valve. Either way, the net result is no change in exhaust dilution of the incoming charge. There is also no change in the period of time (degrees and/or milliseconds) the intake manifold is exposed to the pressure of the exhaust manifold.

As far as advancing the cam to improve low end power, it'll only help if you're bogging off the line when drag racing, but I don't have to tell you given the "toy" you have. If you're talking about on the street, then it'll improve the low RPM characteristics and general drivability of the car. Finally, with respect to decreasing the intake valve-piston clearance, you are correct. But what you lose on the intake, you gain on the exhaust valve-piston clearance. Since exhaust lift is usually more than intake lift and the exhaust valve usually closes more degrees ATDC than the number of degrees the intake valve opens BTDC, the exhaust valve-piston clearance is the more critical of the two assuming the piston has notches for the respective valve diameters. If not, the added diameter of the intake valve will have to be factored in.

I know I said some things here you already knew, that was for the uninitiated and not intended to mean you didn't know. I hope it doesn't confuse the issue nor too long/boring.

 

You dont need the kid gloves, I'm ok. Good info and thanks for sharing. The thing I left out of the original post was that many cams probably have an advance to move a powerband down while maintaining a better idle quality with a wide LSA. I have a 228/232 cam that has a very rough idle compared to a friends car that has an advance on what appears to be the exact same cam.

If there is one thing I learned from the L92 head swap which is to be undone this week: It is better to have 30 more TQ at 3500rpm than +20HP at 6000rpm. The L92's may look super inviting with hyped up top end power but if your car gives up 20-30rwtq at 2.8k-4k you will feel it. Missing 20hp up at peak is not readily noticed even by guys that actually take their cars up that high. Never dismiss the power under the curve and certainly dont get sold on hyped up peak rwhp numbers. The next time you are tooling around town and enjoying the power, look down at the tach and take note that the power you are feeling is 2500-4k where you do 90% of your driving. Advancing a cam can give that exact trade off.

 

Kid gloves are pretty easy to take, compared to a sharp tongue and a snotty attitude.

Sorry your L92 experiment didn't work out for you. I applaud your willingness to give it a try, despite your well-founded doubts. That's old school, and what hot rodding is all about!

 

SPIN has offered alot of good advice to many people on this forum. THANX.

 

I will still play with it before moving on. It sux to have dropped 1500 with a loss in the fun band of street torque. No other changes were made but the heads and intake over my heads and FAST. At least I can swap back for the cost of gaskets and valve stem oil seals. There is always the tune and I will do a speed density tune and advance timing with this low 11:1 compression.

High rpm is probably still the same or even better but the soft low end takes the fun out. Dont dismiss the dip you see on the happy dyno's sporting silly peak numbers. You will definitely feel a 20-30rwtq loss at 3500rpm.

 

More good info, thanks guys. Charlie and Guy...group hug?

 

I found he wasn't being receptive but he is still a heck of a HPtuners man. No problems with me.

 

 

How about posting a dyno sheet showing the comparision? Did you finish the tune? You also dropped the compression ratio?

 

Posting actual dyno charts would show a 6-9 RWT torque loss in the sub 3500 range not 30 foot lbs.....and simply put, that would not fit the agenda here.

Having done real development and testing I've seen as little as 4 foot lbs loss and as high as 11 foot lbs loss in that range.

Attached is a graph showing the typical differences and most will agree that the loss is worth the gain. Bear in mind that you are looking at a stock 6.0 liter with the only changes being the L92 Stock heads and intake.

 

Here's a graph showing a stock 6.0 and stock L92 heads with a complimentary camshaft vs. stock 6.0 and stock LS6 heads with a 228/232 EXR camshaft..

note the same, narrow difference in low end grunt.

 

A 30rwtq loss is quite a bit.

 

OK....here's a graph showing a stock 6.0 and stock L92 heads vs. a stock 6.0 and stock LS6 heads . BOTH using the 228/232 XER camshaft..

note the larger difference in low end grunt.....

My point.....matching the components makes the difference and that overall evaluations based on limited data and incorrect part matching is mis-information.

 

Where do you see a 30 RWTQ loss?