Changing Cams

-
bobscuda67

bobscuda67

Well-Known Member
Joined
Aug 27, 2005
Messages
2,117
Reaction score
392
Location
Michigan
After reading Rusty's guide to hot rod bliss I'm going to remove my Comp XE274 cam. I can't get it to idle in gear and is way to much cam for my car. Of course my really tight converter has a lot to do with that.
I plan on buying a Lunati 262/268 cam this winter along with a better converter.
Will there be a problem running pump gas? With the Comp I have no spark knock at all with my 340's 10:1 compression and iron Magnum heads. The XE 274 has 60* of overlap so it bleeds off compression. But the Lunati only has 41* of overlap. Anyone see a problem there?
 
Smarter than me will chime in but I would take a look at your timing advance curve.It may need to be slowed down a bit.

Why not just put a better/looser converter in your setup?
 
I would do the converter first and try some tuning.

Is the "way too much cam for my car" due to rear axle gear?
 
Keep the comp and converter, and throw a set of Rhoades lifters in there... They will tame it out a bit...
 
Agree, they should work out nicely also pay attention to the idle mix circut on carb (needs to be richer)
 
after reading into tuning Holleys, I think there is about nothing that a wideband and a set of pin drills cant do to a Holley to make it run right. Of course your other parts have an effect. I ran a 284/.484 MP cam and had great results on a 68 340. 800 idle in gear, 3.91 rear MP 2500 convertor. If you pull the cam, keep the lifters in order...just in case.
 
Right now my advance curve is 16*/34* at 3400 and seems to work well. And the gear ratio is 3:55 with 26 inch tall tires.
I called Comp 2 years ago on using the XE 268 cam, if it would work with my existing valve springs. They have 120# seat pressure and 300# at lift. The Comp tech that the XE274 cam would be a better fit and I went with it.
I have it tuned as well as I can, the carb transition slot is square and the idle is at 850 rpm with 15 inches of vacuum.
The timing was setup by FBO for the cam.
 
I read that you could not get it to idle right. Is the 850 in gear w/15" bad? are you rich or lean at idle?
 
bobscuda67 said:
After reading Rusty's guide to hot rod bliss I'm going to remove my Comp XE274 cam. I can't get it to idle in gear and is way to much cam for my car. Of course my really tight converter has a lot to do with that.
I plan on buying a Lunati 262/268 cam this winter along with a better converter.
Will there be a problem running pump gas? With the Comp I have no spark knock at all with my 340's 10:1 compression and iron Magnum heads. The XE 274 has 60* of overlap so it bleeds off compression. But the Lunati only has 41* of overlap. Anyone see a problem there?
Click to expand...
Once again, overlap doesn't "bleed off compression". Overlap occurs during the intake opening cycle. Cylinder pressure begins building when the intake valve closes. Installed per manufacture's directions, the intake closes about 3 degrees sooner with the 262/268 so you'll gain about .2 points of dynamic compression. Will it cause detonation? Guess you'll find out. My thinking is it depends if you really have 10.1 compression.
 
A well known carb guy once told me that he can get a dominator to work on a pinto its all about getting the timing/idle richness correct.Are you sure the idle circuit is rich enough? Sounds like your timing is ok
 
Yea, it might be a touch lean at idle. Having stock exhaust manifolds doesn't help either.
The 850 rpm idle is in neutral, as it barely will idle in gear.
The compression was measured with 0 deck piston with a 19cc dish, 57 cc heads and a 4.140 bore head gasket at .038 compressed thickness. The only thing that was not measured is the cc around the piston to the first ring. The stroke is 3.58 and the bore is 4.070 with a stock rod.
I thought about going to Rhoads lifters too, but just seemed like a band aid to me.
 
Okay, I think you are good to go with the new cam.
 
Before you change it, you might try more initial timing. I don't think you have enough at 16. You probably need on the order of at least 20. That will clean the idle up and also increase the vacuum signal. Give it more initial timing until the engine stops idling up. That's where it wants to be. May end up as high as 22-24. I would try that first and then if you don't get the results you're after, think about a different cam.

......but as inexpensive as custom grinds have become, if you do decide to recam it, I sure as heck wouldn't go with something off the shelf........well......maybe one of the Lunati Voodoo cams. Harold (RIP) really got those right.
 
RustyRatRod said:
Before you change it, you might try more initial timing. I don't think you have enough at 16. You probably need on the order of at least 20. That will clean the idle up and also increase the vacuum signal. Give it more initial timing until the engine stops idling up. That's where it wants to be. May end up as high as 22-24. I would try that first and then if you don't get the results you're after, think about a different cam.

......but as inexpensive as custom grinds have become, if you do decide to recam it, I sure as heck wouldn't go with something off the shelf........well......maybe one of the Lunati Voodoo cams. Harold (RIP) really got those right.
Click to expand...

I have been concerning myself only with the mechanical aspects of the cam. The others, like Rob here, are correct in that the ignition timing should be toyed with before the cam change.
 
Running your numbers, I come up with 9.86 static CR. I am not finding a 262/268 Lunati cam, but will assume 1/2 step larger at 265/274 Lunati Voodoo and get just a 2.5 degree change in intake closing angle. So that is not much of a change in DCR.

Going 1/2 step smaller with the Lunati Voodoo 258/265 cam, that is a 6 degree earlier intake closure angle. DCR rises from 8.0 for the XE274 to 8.3. So you will have to take care with the timing but it is doable IMHO. Your timing curve sounds conservative so that is in your favor.

As for your present tuning, the secondary ports being square may not be what is needed; you need to adjust it a couple of times smaller or larger and see how it works. Also, your idle issue could simply be the PCV not closing down properly at moderate vacuum levels, allowing in too much air and leaning the idle out. Pull the PCV while idling and cover the port with your finger and see how it improves. What PCV do you have? The stock metal Mopar PCV's don't close down properly at moderate vacuum levels, so if that is what you have, it needs to be replaced with a more suitable PCV.

BTW, what do you mean by the present cam being way too much cam for your car? Too hairy and hot at the top end or??
 
Roger... the online catalog (that I downloaded a year ago....old??) lists that cam as 265/271 duration, with all other specs being the same. Not much difference with the ICA as you noted.
 
Wow, just had a tornado touch down 10 miles form here and the sirens are on.
The compression calculator I used says 10.045.
Yes, I do have the metal PCV valve, I could try and swap it out.
Forgot to mention I have my vacuum advance hose on the manifold port on the carb and with 16 initial has 24* of total timing at idle.
And I did intend to use the VooDoo 262 cam that Brian posted.
It's not too hairy, it's just too soft on the bottom.
My converter is what I had in the 273 engine and I cheaped out and reused it. It has a rated stall of 1600 to 2200 and when I shift it from neutral to drive it clunks into gear and it doesn't seem to stall at all when I stomp on it from a stop. It can spin the tires,but not like it should.
A new converter is on the list, I just didn't want to buy one using the Comp specs and then having to redo it again if I switched over to the VooDoo cam.
 
I have the same basic cam in my 73 duster...I got alot of intial timing in it...it idles in gear at 800 rpm,,,,,with stock converter...
hughes 230/236

As RRR more initial timing...
 
I plan on taking it to Milan Dragway this week and I can play with the timing there.
While checking on the static compression of my engine I plugged in the Specs for the XE274 I have now and the dynamic ratio is at 7.1:1. About a point or so low from where it should be. Maybe advancing the cam 4* would help the bottom end, any thoughts?
I can't find the intake closing specs for the Lunati VooDoo 262 cam online to check the dynamic ratio on it, but I ran the Hughes Engines SEH2024AL cam that is comparable to the Lunati and the Hughes has a dynamic of 8.70. To high for pump premium?
 
...so you have a 10.0:1 372
can anybody using the Voodoo 100200702 in a similar build help Bob out
performance,vacuum,idle and detonation are his questions
 
Last edited:
krazykuda said:
Keep the comp and converter, and throw a set of Rhoades lifters in there... They will tame it out a bit...
Click to expand...


Lol.... Not really.... I have a motor on the stand now that was a "hot" street 340 with a 509 Mopar Grind and those lifters really work. You could tell the difference between the motor being dead cold and getting temps in it. After a few minutes you would never know how big the stick was in it lol....

However it's a click, click, click kinda day when you use Rhoades lifters. Definitely unique.

JW
 
You must log in or register to reply here.
-

Back
Top