[CHTA]

Rebuilding original '70 340 T/A block
Mopar performance Purple Shaft 284°/.484°-68°, 108°was installed by previous owner. Low end torque was not bad, and 6500rpm was immediate. Old style rumble exiting...

Main problem was original six pack tuning, probabely related to low engine vacuum (7 ''Hg @ idle). But I'm not sure it's only related to the cam: general low compression test may also be responsible for that
(a friend has a fresh rebuilt 340: same cam but 13'' @ idle).


Should I stay with this cam or go for a more "modern" cam design ?
Any opinion or experience in the same application will be very appreciated.

BTW: M/T, 3:91 rear, performance street only, stock original 3x2 holleys

[dave-r]

I would always go for a more modern cam. Fast rate of lift means more power for less duration.

But ANY cam larger duration than stock will require you to make adjustments and modifications to your six pack. Usually a small hole in each throttle blade to allow more air in at idle. Easy to tell if you need this. Put a vacuum gauge on the spark advance port on the center carb metering block. You need th ethrottle blades to close enough to loose all vacuum on this port at idle. That way the idle mixture adjustmenst will work correctly.

[dave-r]

Oh. And non of what I just said will work if the torque converter is not a high enough stall speed for the cam.

One more small point. A modern fast rate of lift cam with the same duration as the stock cam will give a little more power at all rpms.

[dave-r]

Let me try to put this into a nutshell for you.

The cams to look at are those made by Lunati, Engle and those sold by Hughes Engines (although they don't actually make them). I would not use ANY other make due to quality issues as well as bad design.

Get the the ignition timing right before you start playing with the carbs. You need at least 14 degrees initial. Usually more like 18 degrees works even better. But no more than 35 degrees total at 3000rpm. Half in by 2000rpm. So mods to the timing curve are needed.

Once timing is right you will find tuning the carbs easier.

Close the throttle on the carbs until you loose vacuum on that advance port. Idle mixture adjustments will not work unless the throttles are this far closed. If the rpm drops below 850rpm drill a 1mm hole in each throttle blade on the center carb.

This will allow more air in (increasing rpm) and also lean out the mixture slightly so idle mixture adjustments will be needed.

If rpm is still not high enough go to a 1.5mm hole and adjust everything again.

If push comes to shove you can drill holes in the outer carbs too but as long as you don't go too stupid with the cam size you shouldn't have to.

Make your final idle mixture adjustments with the car in drive (if automatic) and if the rpm drops more then 50rpm when put in gear you should fit a higher stall torque converter.

If I were you I would only go one step bigger than stock with a more modern design. That way it will feel really strong without moving too far away from stock.

[CHTA]

Thanks a lot Dave.

It will be anyway a few month before break-in and tuning.

Have you heard anything against Comp Cams Xtreme Energy ?

[dave-r]

Nothing bad about them. You will just find the makes I listed will work that bit better.

[Jon]

Mine has the same old cam but with a holley 650 DP four barrel generates 13 vacuum also.

I have read elsewhere that Lunati makes a pretty good cam for this application. When the time comes, I plan to scrap the purple shaft for a newer design. Keep us posted which ever you decide.

[CHTA]

Jon

You have 13''Hg vacuum. You seem satisfied with the 284/484 Purple Shaft. Why scrap it?

It is exactely what you can read in other forums. Some criticize purple shaft "old design", but they don't clearly explain what are the real benefits of "modern" cam technology in performance-street applications. Others are very satisfied: the only relevant point seems the poor vacuum, probabely related to high cam overlap. But like you, most of the 284/484 purple shaft users don't experience low vacuum.

Just would like to understand what's wrong with purple shafts and if "old design" means "not appropriated anymore". Again, I'm talking about performance street, not race.

[Jon]

Yes the 13" vacuum works fine with the power brakes as a benifit. I tuned the carb with aguage a few weeks ago and managed to go from 12 to 14" but it changed the next startup. Could be the weather or engine temp?

The engine runs great, don't get me wrong. I just have heard the new cam designs are better. That's why I said "keep us posted"

[dave-r]

Power is made by getting more fuel and air into the engine.

There is only one way to do this and that is to open the valve as wide as possible (lift) for as long as possible (duration).

But in general, the wider you want the valve to open, the earlier you have to start opening the valves and the later you close them.

But if you open/close the valve at a faster rate you can open it wider, have it open a usable amount longer, and close it later, without increasing duration.

This is important on a street driven engine.

The longer the duration of the cam;

1. The higher the rpm you make power.
2. The less bottom end torque you have.
3. The more static compression you need.
4. The higher the stall speed needed.
5. The lower (higher numerically) the gears you need in the axle.
6. The bigger the head ports need to be.
7. The less vacuum you have for power brakes.
etc.

So if you can open the valve faster you can make more power without increasing duration and getting too far into the list above.

The Chrysler cams were designed in the 1960s with slide rules and head scratching.
With the advent of computers and engineering advances it was found that most cams were limited by the diameter of the lifter and that all manufactures assumed the same lift rates.
However Chrysler engines have wider lifters than other makes. This means you can have much faster rates of lift than most cams were originally designed for.
Take advantage of that.

[Jon]

Very nice synopsis Dave.

How does the quicker rise benefit a stock small block which is some what limited by the head w/single springs to a max lift of around .5" ? Also any idea what level of reduced duration would be too restrictive to make power at say 6500 RPM's.

[dave-r]

Getting a bit too technical for me now Jon.

Say, just for example, your intake port flows 200cfm of air at 0.5" valve lift.

If your valve only just reaches that height at max lift it is only going to flow that much for a tiny fraction of a second. The rest of the time it will flow a lot less.

But it you could open that valve faster you can hold it at that lift for longer before it needs to close again. Therefore get more air and fuel in.

There is also a benifit from opening the valve mush more than you need for max flow. This is because you then increase the average time that the head flows at max.

The air/fuel mixture is actually quite heavy. At least it behaves that way in an intake runner. It has inertia. So it is hard to get it going but also hard to stop.

If you get a big flow of air/fuel going into the head then you can still get something near the max flow happening even after the valve has closes some. The "RAM" effect. So getting that air moving well is vital to filling that cylinder with a decent pressure of mixture.

[dave-r]

Oh and you must always match the springs to the cam.

Don't try to reduce duration with your cam selection unless you want it to pull more like a truck. Pick a can with the same or only very slightly more duration to the stock cam to make a little more torque and HP over the whole rpm range.

[fbernard]

By the way, I just noticed someone selling Engle cams on this thread on moparts :
http://board.moparts.org/ubbthreads/sho ... art=2&vc=1

Look for "Rapid340".

[CHTA]

Thanks to all.

Lunati recommended hydraulic cam 60303 268/276 advertised duration, 226/234
duration @ .050,.494/.513 lift 110/106 LSA/CL for my application.

With .513 lift, should I expect clearance problems with the original cast valve guide & Teflon seals, original valvetrain ( 1.5 adjustable T/A rockers) and original pistons ?

[Jon]

I had run a .510 lift with no problems except broke a few valve springs. Mine is not cut for duals which may have been the problem. The single springs were probably at the limit for wire size thus stressed too far. What springs do they recommend with the cam you mentioned?

Does the T/A have adjustable rocker arms? If not you might want to look into them to be sure of proper preload on the lifters.

[CHTA]

T/A valvetrain has adjustable rockers ( This was specific to T/A 340).

Will inform as soon as possible when Lunati answers to the same questions.

[dave-r]

Hughes Engines say on their website "Maximum lift with stock guide height is 0.450", more lift requires shortening the valve guides."

[dave-r]

This page on the Hughes Engines website explains the effects of duration and fast lift quite well.

http://www.hughesengines.com/TechArticl ... lercam.php

[CHTA]

Thanks Dave.

Until now, I had no guide glearance problem with the .484 lift purple shaft.
BUT: my valves seem not to be original (5.000 lenght) and I'm not sure if my valve guides have been already shortened (see pic above).

Lunati answered that with .513 lift I should not have guide or valve-to- piston clearance. What do you think.

[dave-r]

I haven't built enough engines to be sure and I don't do the machine work. You can only go by what the people selling the product say.

I would take the advice in writing from the company supplying your cam and valve springs. Then if there are any issues you have legal come back.

[Eddie]

It's easy to check with the heads off the engine. Just remove the spring. Place the retainer and keepers on the valve stem without the spring. Pull up all the way tight. Now measure the underside of the retainer at the the head surface with a snap gauge, small machinists rule or valve spring micrometer. Thats the installed height. Now measure the top of the guide to the bottom of the retainer. Subtract that number from the installed height number, it should be more than the lift at the valve. Seals that ride on the valve stem like stock umbrella seals have more clearance than seals that attach to the guide. Usually about .075 more. If you have had the top of the guides cut for PC seals you 'should' have adequate clearance but it's worth checking and not that hard to do especially if the head is removed form the shortblock.

[Eddie]

Here's a pic of the valve spring micrometer. Easy to use just expand it with the retainers and keepers in place and the valve on the seat. This gives you your valve spring installed height.

[CHTA]

Eddie: very useful. My heads are removed. Will check. I'm on the east side of the "River": better if I know before UPS delivers the stick...

Jon: sorry I didn't answer to your question: Lunati recommended Intro Series 73949 springs. These are hi perf chrome/silicon duals w/damper. For my application, they insisted on recommended 100-120lbs on the seat pressure and 265-290lbs open pressure. Of course they suggested "to check every time you make a cam change"...

[CHTA]

Spring shims: basic information please.
Removing valves, I discovered one shim under my valve springs.
If I use new valves,a new cam design with recommended new springs, how do I know if I need spring shims ?
Stock adjustable 340T/A rockers will be used.
I'm not sure this as been explained somewhere else in this forum.
Thanks

[Eddie]

First find out the recommended installed height of the spring you are ordering, the manufacturer will know this. Then with the above instructions on checking installed height, do this with the shim removed. Then write down the installed height of that particular valve with the shim. It should be taller than the rest. The shim is used to reduce the distance of the spring seat and under the valve retainer. This is done to correct the installed height due to the valve seat being cut, perhaps a valve job was done. Everytime you perform a valve job it places the valve higher in the chamber, this doesnt hurt that much but isnt desirable. The only way to bring the seat back down is to cut a valve seat for a larger valve diameter. Sunken seats from too many valve jobs may cause this. How thick is the shim and what condition is it in? If it's scratched and shardded it should be replaced with one of the same thickness, but this why you check everything out. This also collapses the spring slightly to have the seated/open pressure on spec. This also causes coil bind clearances to be reduced somewhat but shouldnt be a problem as long as the shim isnt too thick, normally .015-060 are used this can also be caused byoverzealous seat cutting. When they cut the guides for PC type seals, they shouldnt have removed any off the floor more than they have to. This area is above water and isnt too thick from the factory. Although they exist dont use alluminum shims, steel only!

[Jon]

As for max lift relating to valve stem/guide clearance it would seam conflicts in both the retainer base depth and spring coil bind should be considered as Eddie said. If my Teflon seals were getting hammered by the retainer at .510 lift though I think oil leakage would be obvious which was not present in my past situation. (Only leaks were form a stray rod sent thru the oil pan )

Dave and Hughes maybe correct though as the retainers in mine are aftermarket (aluminum) 70's vintage. Possibly shallower then stock, so I can't really relate that to your T/A heads. Sorry about that misinformation.

With regards to the springs, another thought was possibly binding at high RPMs had caused a couple of them to break. Again old stuff and hard to say since that was never checked either.

[Eddie]

I wouldnt deal with that 'other' guy. I would buy my parts from www.buyracingparts.com yeah the site sucks, but Scott will actually talk to you, not call you any names or accuse you of "fantasies" in his head? (E-mail them and ask for Scott Brown), or Lunati, Jeggies, Summit, Goodson ect.. Problems usually arise when dealing with paranoid incompetents

[dave-r]

Dave Hughes is as mad as a box of frogs.

[Eddie]

Dave Hughes is as mad as a box of frogs.