BACNBLK

Cant wait to see how this works out for you, please keep me posted as you defintely have done your research. If all checks out I may be on board for the same swap, Good Luck!

VegasJen

i don't think there's any way the stock springs will hold up to that cam. if you have some aftermarket springs that aren't 25 years old that i missed somewhere, you may be fine.
as for the 1.6 rockers, even if the springs will hold them you have to notch the guide in the head to clear. the way they make those, they machine the pushrod cup closer to the pivot point. on stock non-vortec heads, L-81 or not, the pushrods will hit the inner casting. jegs and summit sell a tool that you can do it yourself at home. i did it to a set of heads some years ago and they worked fine. definately something you really want to do with the heads off.
rockers are one of those things that you can use as a tuning aid too. at one point, i had a cam i wasn't entirely happy with so i tried a couple different variations. i ended up running 1.5 rockers on the intake and 1.6 on the exhaust. you can effectively change either intake or exhaust or both by mixing the rockers and seeing how your engine responds.

Angel303

This is what the summit representative told me so I decided to just get the 1.52. I didn’t know that you could have different rocker ratio between the intake and exhaust! Pretty smart! I guess that's a good thing that this cam is symmetrical then.

This is still a little confusing for me but what will be the effect on the engine if the lift is increased (with 1.6 ratio rocker arm) on the intake? Would that help getting more power/torque on low end rpm and give up some at high rpm? And vice versa if the lift is increased on the exhaust?

Worked on the shark today and took the hood off, all the accessories and the radiators . Tomorrow I will remove carb, Intake, water pump.... Lookin real good so far!

Thanks guys!

VegasJen

honestly, i don't think the stock springs are up to the job even with just the 1.52 rockers. if you're putting in a fresh cam and lifters, i'd seriously suggest a new set of springs. you can get a reasonably priced set from summit or jegs but even NOS L-82 springs would be far better than the stockers.
as for changing the rockers, the change will be minimal but typically more ratio, i.e. 1.6, will like a little more RPM than a lower ratio.
don't expect much though as the specs for the comp cams xe268 are .477 lift on the intake and .480 on the exhaust. this is the "advertised lift", meaning they figure you're using 1.5 rockers. the actual lift on the cam lobe is only .318/intake and .320/exhaust. now, if you then factor that by 1.6 instead of 1.5 (or 1.52) you'll net an intake lift of .508 on the intake and .512 on the exhaust. while that is a pretty decent jump in lift, there's no increase in duration.
check the cam card yourself http://www.compcams.com/Technical/Search/CamDetails.asp?PartNumber=12-242-2.
while i kind of know what i'm talking about as do many of the other people giving advice, i'd call comps tech line for more advice. just don't mention the computer controls as it's such old technology, most of their techs weren't even old enough to drive when your car was made (kind of scary, huh?).

Angel303

Thanks Clutchdust! I actually got a set of spring (+ retainer and locks) today (comp P/N: 981). I wish I had done it with my first order and bought the set that includes the springs, it would have saved me $50 . I just thought that I would get new springs with a new set of head. Oh well that’s how you learn right!!!
Anyway, I got it, it's best to get the springs that match the cam in order for it to work at its best performance. The cam I got is the 268H (comp P/N: 2-210-2) not the XE268H. It's a little milder and more conservative than the XE. I read a lot of good things about it and I figured that I would go easy for my first one. Here is the link:

http://www.compcams.com/Technical/Se...umber=12-210-2

I have a question: How do you change the springs without using an air compressor while keeping the heads in place on the block? I guess I already know the answer but maybe someone has an old trick?

TheSkunkWorks

Rotate each piston to TDC, then fill the chamber with small rope thru the plug hole (leave plenty sticking out to retrieve it by) to keep the valve seated, and get one of these...

http://www.moroso.com/catalog/catego...?CatCode=27035

Angel303

Finally finished today! The cam is in and sounds great. Reminds me the sound of an L82 I love it!
The brake-in went pretty smoothly with no problem: ran it at various speeds between 1800 and 2200 Rpm for 30 minutes, no leaks/unusual noises.

Now I have a terrible off idle throttle response; I lowered the idle speed to 650 Rpm the timing didn’t seems to help and it’s set at 10* BTDC right now. I had this problem before the cam swap but now it is worse. I think maybe the carb needs a serious overhaul.

Do you guys think that this can be a carb problem? What can cause a bad throttle responce?

UKPaul

It could be something simple like a vacuum leak, although tuning the carb wouldn't be a bad idea, 'specially after swapping out the cam. Have you got the workshop manual for the L81 (not the Haynes, etc type, the (expensive) factory one)? There's pretty good info on setting up the carb in it. Doug Roe's book on Q-jets also contains a lot of good info on it as well.
I'd disconnect entire vac systems (& make sure the ports on the carb are blanked off) first just to check that it's nothing silly. Mine had a bad off idle response due to a vac leak at the EGR bleed solenoid, or whatever it's called. It also suffered a vac leak from the valves on the charcoal cannister. Disconnecting the vac pipes from the carb (and blanking off the ports!) resulted in much better running & gave a big clue of where to look for leaks.
If you do start playing with the carb, make a note of the orignal setting of everything you change eg. number of turns out from fully seated, etc, so that you can go back to the original settings if it all goes horribly wrong

Angel303

Thanks UK Paul I'll check for vacuum leak...
I do not have the workwshop manual but I have a book from Cliff Ruggles on how to rebuilt and modify Q-jet. The carb rebuilt is next on the list.

BACNBLK

Hey Angel, checking in to see how the set-up is going. Did the Idle issue get worked out, what was it vacuum or Carb? Curious to how much difference the cam swap made, have you tested any before and after numbers on it? Always nice to get some real world opinions before any $$ changes are made. Keep us posted f you get a chance!

cv67

IT may be getting a little lean now that you have more cam, just have to spend time with it. Man I miss working on stuff with a carb on it.
Good move on not getting the XE, it is fairly lopey even in a 383, in your app I think it would have given you more headache than smiles.

Angel303

Well, after some research and lots of reading , I think that the problem I'm having is simply due to the fact that I removed the CAT:

When I got the vette (completely stock) in CO, I lived in NM (high altitude) and it worked great. Then, I moved to NY for a year. I removed the CAT and installed a pair of Magnaflow mufflers without doing anything to the carb. I felt a nice improvement by doing this but I was almost at sea level. Now I'm back in the Rockies in UT. Ever since I drove the vette in UT, I noticed this off idle bog even before the cam swap. I believe that removing the CAT lowered the back pressure in the exhaust, which made the A/F mixture leaner in the combustion chamber. I didn’t notice it in NY but now since I’m back in a higher altitude, the A/F should be even leaner (please tell me if I'm wrong here). I just thought that the E4ME would adjust the A/F automatically without having to do anything to the carb but apparently not. The next step would be to put the CAT back on and see if it gets rid of this bog which would confirm this theory.
Other than that, the cam made a noticeable difference. The vette has definitely more torque than before and sound pretty damn good with the Magnaflows. I just haven't driven it too much recently because I don't want to put holes in my cylinders (which would be the result of running too lean). If the missing CAT is causing the bog, then I would need to get the shop manual and re-adjust the carb.

UKPaul

Angel,
Does it run OK when cruising, ie. running on the primaries? Is the bog only when the throttle is just opened from closed? If so, is the bog worse if it's been idling for a while, or is it the same however long the throttle has been closed?
It could be something as simple as needing adjustment of the idle mixture screws & the idle air bleed (the large adjuster in the tower at the top of the carb on the front). As you say, the computer should take care of the primary mixture (mine certainly has after all the changes I've done), but maybe the idle (& IAB) mixture is just set wrong (computer doesn't control the idle mixture)? After fitting dual exhausts & different "muflers" to mine I did have to tweak the idle mixture screws, & IAB setting, to stop the computer complaining. Maybe you've just got the same thing, but accentuated by the high altitude? Does the computer throw any faults, or flash the Check Engine light at all?

I've no experience of running these things at high altitude, but I know a man that does (who lives in Utah ), so I'll give him a "nudge".

Something you really need to do, if you're going to fiddle with the carb, is get hold of an analogue AVO with a 6 cylinder dwell scale (NOT a digital one, they won't work). You'll need this to accurately set up the IAB (as described in the shop manual - which I really recommend getting as they're invaluable when you're having problems). It is possible to roughly adjust the IAB by ear, but using a (cheap) dwell meter should help you find out some info about what's going on with the mixture.

If setting the idle mixture (according to the shop manual) doesn't help, then perhaps the lean stop needs to be adjusted for the primaries (always take a note of the original setting before changing it so that you can set it back to original if (when?!) it all goes horribly wrong). I have a hazy memory that high altitude causes a richer mixture. I could be completely wrong there, & the result is a lean mixture (so the rich stop could need adjusting)! Hopefully somebody that knows what they're talking about will post because I've just realised how much I've forgotten!!!

Good luck!

BACNBLK

Sorry Angel, wish I could help, but unfortunately I live in the valley (sea level) and after changing out to set of Flowmasters and a new pair of Hi-Flow Cats this thing sounds great and idles/purrs like a kitten. UK sounds like he has forgotten more than I have ever known so you may give it all a try, he sounds right on. Good Luck!

Angel303

UKPaul, yes it runs fine when cruising and at WOT. The bog appears only when the engine is HOT (not warm), in neutral, and when the throttle is opened fast from closed to open. It remains consistent no mater how long it idles. If I blip the throttle (from closed) fast enough several times the engine stalls. I changed the O2 sensor, no improvement. I got to get the shop manual and check/adjust the carb's setting properly! I already have both Q-Jet related books from Doug Roe and Cliff Ruggles (which by the way I would recommend to any Q-jet owner) and they are an excellent reference.

Thank you for your help

mikep3

Angel, Did you change the cam while the engine was in the car, I would like a cam change to improve the sound with the new dual exhaust, next year it is Magnaflows and a cam. I would like to hear from someone that changed it in the car, for now the cam is the only thing I would like to change on the stock engine.

UKPaul

Aha! That should tell us something, but I'm darned if I can remember what it is! If it only does it when hot it would suggest to me that it's a rich condition. If I'm right about it running richer at high altitude then perhaps the accelerator pump is supplying a bit too much fuel when you ram open the throttle? That could explain how you can stall the engine after a few pumps (flooded with fuel).... does it start back up with a cloud of brown/black smoke from the exhausts after this has happened? Have a look in one of your books and see if changing the pump shot volume is an easy job, it might be worth reducing it to see if you get an improvement (& if you do, check that acceleration & normal driving is still good).
What we really need here is a carb guru, somebody like Lars, who can immediately say something like "Oh yes, that's due to such & such because of the altitude" (whether it be reduced cylinder pressure/temp/whatever). I know Lars doesn't deal with the E4ME, but it may be worth PMing him & asking what sort of things need to be done for high altitude running, or point him to this post?

Rich's'78

I have that cam in a 355, 1.6 rockers, with more compression than stock. 4 speed stik, like you. 17" vacuum when hot at 750 rpm idle. No problem launching and no low speed bog. if you haven't adjusted the carb i'm sure you are lean. you probably have a vacuum leak on the ported vacuum system that is causing the bog.

UKPaul

Just remembered something important! I've been trying to work out this lean/rich thing at altitude by thinking "OK, so there's less air pressure, probably giving less air velocity through the carb, maybe a lower pressure difference in the venturi, will this atomise less fuel (lean) or will the pressure difference be insignificant from the fuel supply point of view & the same amount of fuel will be delivered (rich)?". My head hurts. What I've just remembered is that Lars told me to change my secondary rods from the standard CH to DA to richen the secondary mixture to cope with headers. I'm pretty sure (& I'll have to check this) that he told a friend, living at high altitude in UT, to stay with the CH rods with headers fitted as it would be running richer due to the altitude anyway, and see if it was OK.

mikep3, I changed the cam with teh engine in the car. Pretty easy job, but you've got to get the rad out (along with the A/C condenser). This can lead to R&R of the rad support frame, & other snowball effects that take a lot longer than swapping the cam, so be careful while doing it! Make sure you get the shroud back in at the right time when you put it back together. Forgetting and finding out that you've got to remove a bunch of stuff you've just fitted to get it back in isn't a whole lotta fun

edit: rich, thanks for the vacuum info

Angel303

Yes, I replaced the cam with the engine still in the car. I also took the heads off to replace the springs and inspect my combustion chambers. It's a big job but fairly easy to do if you have the right tools. You will need to remove the hood and the radiator.