340 Cam, rocker, valve spring questions

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nick455440

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The build is a mild 340, Speed pro 10.1 pistons 60 over, stock rods, 65 ish CC mildly ported 202 jhead, performer RPM, no carb yet but leaning 750 DP,

Q1- picked up this and a few 340s in a lot it came with few sets of 273 rockers, other than cupped push rods are there any other issues running these?

Q2- the heads had been rebuilt when I got them but didn’t know the history on them so I had a sho check them out, was going to replace the springs but the shop said they checked the springs and there good, if u get a spring cam lifter combo is it still nessesary to check or “set up” new springs?

Q3- the cam..... the shop said the max lift was around 490, looking at the comp x 274/286 .488/.491, but had heard good things about the purple or comparable spec half the price comp nostalgia 284/291 .484/.484, or the thumper 279/296 .486/.473, is it better to get max lift like the compx or sacerfice a little lift for more int/ex like the other two, or am I splitting hairs and these r close to the point where they would be about the same?


Any and all info is appreciated, thanks
 
Those rockers work fine...my dirt track motor had them with zero issues.

If you trust the shop you are good to go.

I don't like the 40 year old grinds the Mopar cams have but they are OK for a street car.

I really don't like the thumper cams...all noise with no performance advantage IMHO.
 
calculate your compression

someone else just went with this low lift selection investigation- went with engle with stock rockers you can run more lift if guides cut with iron rockers
engle, crower, racer brown
then
mopar purple cams or comp cams purple plus (but it's bigger)
i stay away from the comp xe line xcept for the HL which is not U unless more pro style attention to details Lunati may make the cut or may have too much lift most likely not howards same reason
check spring requirements before you buy cam and you need compression and estimate your head flows, intake, exhaust etc chassis converter, gears
how do youwant to drive this
Don't put a cam designed for a chevie in your mopar- there are many other better choices

get the right shafts for those rockers or plan on drlllng some holes
see YR yellow rose procedure or the stickie
or running oil through the pushrods to lube your balls
are your heads cut for viton seals
solid lifters is really way to go as you have the rockers and haave to buy pushrods and lifters in any case- EDM lifters are more reliable than yhd with above stock spring pressure
q 2 yes i check everything springs may or may not for your new cam
do you have the specs?
 
Last edited:
Wyrm, proof read your reply please...


Wyrmrider said:
calculate your compression

someone else just went this low lift selection- went with engle with stock rockers you can ron more lift if guides cut...
i stay away from the comp xe line xcept for the HL which is not U Dpn't but acam designed for a chevie in your mopar

plan on dtillng some holes or running oil through the pushrods to your balls...
q 2yes i check everything
Click to expand...
 
It will be street 98 percent of the time. But want it drivable but as hot as I can get it within reason and be drivable, don’t care about gas milage ect as it’s not going to be a main mode of transportation, thinking a manual VB 904, not sure on stall yet as I have a line on a converter but may be to hot for a street car, think it was 4500/4900 something like that, and the gears I will run for now are to hot and will prob try to trade center sections at some point, can’t remember exactly but they were high 4’s low 5’s, what do most cut iron heads to eslitg 500lift?
 
Where to start and in what order.

First you need to calculate your actual static compression ratio. Assuming 10:1 is correct, you are going to want a cam around 221 duration at .050. Advertised duration will not really tell you how the camshaft will act. You need to concentrate on duration at .050 on the intake side. If the head hasn’t been altered around .500 lift is where you will be safe at with a true 1.5:1 rocker ratio. Also, J heads, unless they are heavily ported, will flatten and lose flow after about .500. Lift. 550-.570 Lift if they are heavily ported, bowl worked, unshrouded, larger valves etc. Yata, yata.

The radical sound of the cam I believe you are wanting comes mostly from valve overlap/ valve separation. A tighter lobe separation will give it that drag car cammed sound, but at the loss of power curve. Like 107 for example, This is good for a drag car because properly set up you would be working within that narrow power curve, a 112 on the other hand would sound more mellow but give you a much more broad curve, better suited for street.

So I’m thinking you want a cam around 221 at .050 intake, and just short of 500 lift. Probably around 108-110 separation. Going over 500 lift won’t really help you with J heads. That is “IF” you verify that you are for sure 10:1 static compression. Truly is best to call a custom cam grinder, it won’t cost much more and will work better for what you want.

Once the engine is complete, the correct way to figure out stall speed is to break in and dyno the engine before install, find out where your torque curve comes in at, then select a converter that flashes about 200 rpm below that point. Again this would be for a strip oriented setup. For street I personally wouldn’t go over 2500-3500 stall for drivability reasons, not saying you couldn’t go higher, just depends on you and what your willing to put up with in stop and go traffic.

Gear size comes into play once all that is finished. If you want a max effort 1/8, or 1/4 car, you will need to select a gear that will have you at max horse power rpm when your going through the traps, that goes back to the combo and dyno results. Just a quick figure in my head, 10.1 static, with 221 duration at .050 and just shy of .500 lift at a true 1.5 should have you somewhere in the 5800-6000 rpm trap speed range with the correct gear. Probably 4.10 and up, depending on tire size and assuming 1.00 final drive ratio. Again strip setup. Street you would want the gear that will give you the best of both worlds, but emphasis on highway speeds if you intend to travel on the interstate. Just speculating that would probably fall around 3.55 to 3.73 for your combo, depending on you.

So to directly answer your question, the cam choices you are looking at will all work, but none will work as well as having the right cam for your combo custom ground. Factory Mopar rockers are around 1.45:1, I don’t remember if those 273 rockers are true 1.5 or not, someone else will chime in on that. A 4500/4900 stall converter for a street car is not something your going to like in stop and go traffic. But remember there are loose converters and tight ones. Some will flash to stall before they even move, some will gradually come in. But that’s another conversation. Long winded post but wanted you to have as much info as I could at this stage of your build so you could get the most out of it. A 750 double pump race carb would be a good choice for a hot street setup. Might also consider a street avenger 780 vac. Or if you can score a original 340 intake there is a 1000cfm thermoquad competition carb on eBay right now for less than 400 with the choke horn removed. Good luck buddy...
 
Also for the good of the group.

I see a lot of people just throwing high lift cams in their setup. Seems to be a general consensus that the larger the lift the better the flow and performance. Couldn’t be further from the truth. Over camming an engine will kill it. Some say to pick the cam first and build the combo around it. You could, but I choose to pick the heads and build around that. Reason is you are limited by what your heads will flow and at what lift before you get into port stall etc. What I meant above by .500 for your combo is, current flow bench results show that the J head flows best at .500 lift, un-ported. If you go over that you are actually stalling the port and reducing velocity, which reduces the over all flow. Now, let’s say you have a set of heads that flows 360cfm at 600, 310cfm at 500 or 340cfm at 700, then you can see you would want to be right at .600 lift, no more no less. Same with the J head, UN-ported flows best at .500, heavy port flows best around .550 to .570 depending on who did them.

I forgot to mention above that degreeing the cam can move your power band up or down. Example let’s say you went with that 107 LSA cam. You can move that narrow power band up and down. Adv 5 degree from centerline would make your torque come in early and run out early, Ret 5 degree from centerline would raise where the torque came in at and run out later. Some off the shelf cams have advance ground into them, so you must check the cam card and installed centerline when you install.
 
Thanks all for a lot of good info, definitely gives me a starting point to start digging,
 
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