Need a plan - component selection for restoring 68 340 that runs on 91 oct

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And that is why I pulled the MOPAR 292/292/508 cam outta my engine.
It looked good in the calculator at 11.3 Scr, But the low-rpm pull was just not there with my starter gear of 2.66x3.55=9.44.
Here's what I thought I was getting,@900ft
1) Static compression ratio of 11.3:1.
in at 104, for an ICA of 70* using advertised
Effective stroke is 2.64 inches.
Your dynamic compression ratio is 8.60:1 .
Your dynamic cranking pressure is 174.84 PSI.
V/P (Volume to Pressure Index) is 142VP ................................................... 142VP
For aluminum heads that looks reasonable. With a clutch, I can work with that meager 142VP.
But .008 tappet lift, with my 1.6 arms is about .0128 valve lift. So we got a long way to go, before that intake seals up. Say it takes 8* to get there, And this is what the Wallace spits out;

2) Static compression ratio of 11.3:1.
True ICA is 78*
Effective stroke is 2.42 inches.
Your dynamic compression ratio is 7.96:1 .
Your dynamic cranking pressure is 157.68 PSI.
V/P (Volume to Pressure Index) is 117VP...................................................... 117
Well that Sux

Now,my actual measured pressure was not that low, So let's assume there was for a moment that 8* was too much compensation. Lets try it at 6*, and advance the cam to 100, and that would get us an ICA of 72* valve fully seated.
3) Static compression ratio of 11.3:1.
True ICA is 72*
Effective stroke is 2.58 inches.
Your dynamic compression ratio is 8.42:1 .
Your dynamic cranking pressure is 169.98 PSI.
V/P (Volume to Pressure Index) is 135VP .................................................. 135
Still soggy, almost a waste of aluminum heads, inas-much as I ran this with 87E10. I mightabin able to run iron and 91 with a gasket swap, and saved a ton of money..

So I pulled that cam and sold ASAP.Actually I sold it first, then pulled it

So then I got me a Hughes HE2430AL, with specs of 223/230/110 I figured that's a long way from the 248* of the previous cam. I still didn't know why that 292 had been so soggy, I just knew, I didn't want anything more to do with it.At the time (year 2000 or 2001) Hughes did not publish the advertised on this 223 cam. Now they call it a 270/276/[email protected] lift..... but they don't say .008 tappet or .008 valve. So, to be consistent, Lets go with tappet, and that gets me the same .0128 valve lift with 1.6 arms.
So this cam went in at 106, so the ICA was 61*, and the Wallace reads

4) Static compression ratio of 11.3:1.
ICA of 61*, using advertised
Effective stroke is 2.86 inches.
Your dynamic compression ratio is 9.23:1 .
Your dynamic cranking pressure is 191.98 PSI.
V/P (Volume to Pressure Index) is 169VP ............................................ 169
Which looks real nice on paper, but being a newbe,I deemed the pressure too much, and swapped in a different gasket, and changed the pushrods. And then on paper I got;

5) Static compression ratio of 10.9:1.
ICA still 61*still advertised
Effective stroke is 2.86 inches.
Your dynamic compression ratio is 8.91:1 .
Your dynamic cranking pressure is 183.24 PSI.
V/P (Volume to Pressure Index) is 161VP ........................................... 161
So I slammed it together, and this combo flew. It was just what I wanted.Oddly enough 185psi is what I measured.
And I drove it that way for 4 years until the lobes began to fall off.
But now this intake duration is also not with the valve closed. Hughes calls it a fast-rate, but very likely this does not include the clearance ramps, so to compare this cam to the Mopar, lets add 6* to get the intake valve sealed,as before. And I get a TRUE ICA of 67*
Aaaaaand Mr. Wallace says;

6) Static compression ratio of 10.9:1.
Now;True ICA of 67*
Effective stroke is 2.72 inches.
Your dynamic compression ratio is 8.52:1 .
Your dynamic cranking pressure is 172.68 PSI.
V/P (Volume to Pressure Index) is 145VP .............................................. 145
Well That's a little disappointing......... on paper.... Nevertheless this combo made huge torque.That combo was awesome at the line!!
So now to be fair to the Mopar, lets compare #6 here with #3 above...... Hah only 3 psi more here, yet the VP is 7.4% higher.
Who knew 7.4% could make such a HUGE difference?!
Well, that is akin to swapping rear gears from 3.55s to 3.81s

The 6*and 8* degrees of compensation above , I plucked from a hat. I have no idea what the real world numbers might be.

By the next cam,I already had the GVOD, so I gave up and just put in 4.30s. Maybe I shouldda done that with the Mopar, but..... nobody said I was the sharpest knife in the drawer.
I gotta hand it to the Mopar tho, once the engine hit 5000, it just zinged to the top, 7000/7200 was BAM!!!





 
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AJ/FormS said:
And you might as well throw in some extra, to compensate for the time from advertised to no-longer bleeding-pressure, cuz .008, or .006 or .004 ain't closed yet. And few there are that know how much to add. I sure don't.........
I thought I was doing pretty good until Wyrmrider showed up. Of course I knew about that ramp, but I had no idea it was so large, nor varied so much. Yeah that was a humbling moment.
I first noticed the discrepency in 1999,by accident,during a LeakDown test. When the TDC piston flipped backwards and the pressure blew off,and the Leakpoint was way off scale. I reduced the test-pressure and snuck up on the advertised ICA. Crimminy, I thought, My cam is in wrong; how is that possible? It wasn't off.
Click to expand...
What you care about is when it is effectively closed.... i.e, does not bleed off any significant additional pressure. That probably is someplace in the .005 to .010" valve lift range.... as a guess. Taking the info statically (like in your last test) does not have any real meaning for a running engine. What you get from cranking compression is a more realistic measure.
 
Hank'sFs
were you thinking of this comp cam
20-309-4 - Magnum Muscle These cams follow the tradition of COMP Cams® excellence and offer a step up from original equipment cam designs.
or this one
20-223-3 - Xtreme Energy™ Hydraulic Flat Tappet Camshafts
both these cams are shorter on the seat than the 340 cam and should work with slightly less compression
However they still leave a lot of torque on the table- especially the XE-268H which is a 1977 Harold Berkshire profile for chevys - pretty big spread on the exhaust
here is a 268 Voodoo
494/513 268/276 226/234 108LCA 110/106 1800-5600 Voodoo remember .006 268/276 is not the same as MOPAR 268/276
474/494 265/271 220/226 110LCA 112/108 1400-5800 Lunati Voodoo

here is a quote from well respected JRobb (264 HR but that's not really relevant here where we are trying to determine duration and compression- some realistic numbers
.485/.485 264/270 213/219 112/106 Lunati SBM HR 1200-5500 headers 3:55 RAMM's cam

“371-374 hp @ 5100-5200 rpm to 5500 rpm where it was down about 1-2 hp from peak.

428 ft/lbs @ 3900 rpm. 423.8 ft/lbs @3250 376 ft/lbs @2500

. Idle vacuum was a steady and smooth 15-16" @ 750-800 rpm.

Total timing was wound up at 37-38 total likely due to the low compression and no real quench to speak of. A half inch spacer hurt power everywhere.

I feel that a 750 dbl or even a 650 dbl would likely crest the 380 hp mark and then some.

Manifold vacuum was registering 1.1-1.3" in the upper third of the pull indicating the carb is on the small side and presenting a restriction.
These heads are ported. This engine is about as tame as it gets.

It almost matches the output of a Mopar 380 hp 360 crate but with FAR better manners/driveability and on cheap fuel and likely a bunch cheaper to acquire as well. J.Rob

here's one size smaller If you have no quench and the lower compression you go the shorter your cam needs to be to keep low end
.475 262 220 VD
want more at 262 @.006 duration
494/506 261/267 214/220 110/106 .904 Howard grind not hard to get more dynamic compression and more lift (breathing)
cheers
 
MP# P3412044 68 340 Man.-114 276/284 .444/.453" 112 icl 114 lca
52* Mopar degrees Overlap
2899205 OEM number
crackedback posted last year
All the ones I measured where the 276/284 variant, which was the 340 A/T exhaust lobe moved to intake and a 440 cam intake on the exhaust side.
That's the way I remember it- sort of an hermaphrodite
221in./227ex. @ .050"???


note that NHRA allowed the 440 Magnum 276 intake lobe to be used
giving .465 lift

 
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Wymrider, you clearly know a lot more about this art than I ever will. I get the gist of what you are saying but I have only just played with the Wallace calculator and don't really understand how to use the published data and that tool to get to my goal. That is why I started this thread. The goal is not perfection on the inside. I don't have to have an OEM cam or pistons or anything else you cannot see. My goal is the best performance given these basic constraints - a restored version of the 68 car that I posted the fender tag earlier in this post running on 91 gas. Sum that car up as 3.23 rear, std 4-speed, and must look completely original from the outside. So, X heads, AVS carb, HP exhaust manifold, et cetera.

With those goals and reading your post above, are you recommending a Voodoo 703 with something like the L2316 or KB243 pistons. Is that about right?
 
That's about right
I would not use FT pistons, but that's me I'd get custom pistons made or put a stroker in it but... just plan on working on your ignition curve
I built a one of a kind 383 4 seed Superbird with a 440 crank, 440 rods and custom Arias step quench pistons- custom crower cam- same duration as 440 HP cam but lots more lift 550 I think and exhaust was not stretched as Big as 440 HP is way to long on the closing ramps- as is the the 340 we ran dual Isky springs and broke in on the outers
maybe more of a PITA than you want to do
looked stock but very stealth I can post a pic of the piston we made it true 9.5
same problem- nothing off the shelf that works
FT pistons and open chamber heads work best with low gears where you are not working in the 2500-3500 rpm range- ie brackets or a heads up stock class and that's not you
we did what are now Dura Bond powdered metal exhaust seats- larger intake and exhaust valves Back then they were mfg in Washington State till Dura Bond bought them out
If you must do FT piston check you X heads for spark plug threads and polish the combustion chambers, do NOT polish the Intakes
see the DIY porting guidelines at the top of the forum valve job makes a big difference 45 60 about as wide as the 45 then 75 and 82 (will not be much if any) than blend the bottom cut into the bowl you will want a top cut or two and open the chamber around the intake back cut the intakes
stock intake works up to about the same rpm as where the exhaust starts to get restrictive so no use building for more rpm than that
so you end up building for a big fat flat "engine masters" type torque curve

Let AJ do the Wallace calculator
you will have to run half point less compression with the FT/X combination than with a tight step quench piston or closed chamber heads
can you get away with 308 heads? guess not
ask the forum about porting the exhaust- that's where the stock heads hurt the worst
323 gears- do not go nuts on the duration unless this is a freeway/ mountain road kind of build
you sticking with Hyd lifters? Voodoo is a safe choice I'll have to look up that 703
if you have great attention to detail the Howards have more potential
Iron rockers will work fine as I have posted I'd run Magnum lifters and oil through the pushrods hollow pushrods to oil the rocker balls
The iron rockers should have a squirt hole but it can miss with the later longer adjusters
let me see if I have any small block
if not rocker arm specialists in Redding is best people to work with
you can run stock rockers but getting the pushrod length exactly right is a PITA
If you do go roller tip rockers plan on the B3 fix and go 1.6 on the intakes (and maybe one size shorter cam depending on what CR you come up wit)
should be a nice build
 
Its like watching an artist paint

And to answer the questions
I'm not stuck on FT pistons but didn't know where else to look. It is not a highway cruiser, it is for having fun with friends around town with the top down. I was planning on sticking with stock hyd lifters.
703 is one of the ones you quoted I think
226/234 @.50, .494/.513, 110/106
 
The piston type that wyrm is probably referring to is one that has a small raised quench pad on the edge above the eyebrows. This pad will poke well above the deck into the 'open' part of the stock chamber. (The 'open' part is the shallow flat area of the chamber opposite from the spark plug side.) This quench pad will come up veery close to the flat area at TDC and form a quench/squish area, and that effect will help fight detonation, while raising CR to help low/mid RPM torque...good torque is what you want on street driving/cruising.

The challenge in this with the stock type open heads is that:
  • All of the pistons' height relative to the block deck have to be measured (that can REALLY vary), then the block milled to equalize all the piston deck heights.
  • The open area's depths in the chambers vary some. That has to be measured on both heads, and them maybe heads milled to equalize those.
  • Now the quench pad has to be milled down, in conjunction with selecting a head gasket thickness, to make the top of the pad come to about .035-.045" from the open area in the head, at piston TDC. You need get in that clearance range to make the quench/squish effect work well but not get too close and have piston contact with the heads over the long term or at very high RPM's.
So, you can see it is several steps of work, machining, and cost with careful measurements; that kind of work may be done commonly for more serious engines, but it is a lot of work for a rebuild that is meant for more 'casual' use.

This is all a lot easier to get to work with closed chamber heads (like the AL Edelbrocks or the cast iron Indy heads) with flat top pistons. You get rid of the problem of the depth of the open part of the chamber and all the rigamarole of adjusting that quench pad height. (And you get larger quench pad areas.)

IMHO, for making life easier for your goals, I think you are best off with the 2 flat top piston types mentioned. However, this is not 'best performance' in terms of:
  • fighting any detonation tendencies with 91 octane; you will have to do that with tuning ignition timing as said
  • best possible low/mid RPM torque
So it is a choice you need to think about what 'best performance' means to you in terms of expense and effort versus results.

BTW, for a KB quench dome look at PN 373, it is listed for 360's; you just buy a big enough oversize to fit your 340 bores...if they are not too large.
 
Hang on there nm9, did I miss where he put a 3.58 crank in there? lol. A 360 piston might sit a bit low in a 340 with a 3.315 crank, on account of the stroke difference is is going on .133
 
Oh yeah.. duh! LOL Tnx for keeping me straight.

Forget that PN, OP. In that case, I don't rightly know of a piston for the 340 with a raised quench pad....I thought KB used to have one but I don't see it listed now.
 
AFIK there is nopop up piston for the 340 so I thought of custom forged but then realized that custom forged pistons would buy a 4" stroker and shelf KB's
however he is using stock stroke and heads so trying to make a FT work
Straw man cam is the Lunati Advertised Duration .006 (Int/Exh): 268/276 226/234 @.50, .494/.513, 110/106 RPM Range: 1800-6200 Intake closes 60ABDC @.006
compares well with the Comp 68 268-276 222-266 .464 lift 110
all of these 268 cams with 110lca would close the intake at 60 ABDC @.006 which is about 5-6 degrees earlier than the factory 340 cam
(actually more than that as the 340 cam really stretches the close ramps)
here Howards
711651-10 CL711651-10 dual pattern for stock exhaust
265 273 218 226 .502 .525 110 LCA 106 ICL 1,15 the 15 indicates for .904 lifter just a little shorter than the comp or lunati 268's more lift
1800-5600 Mild idle, Street/Street Performance. Excellent throttle response

Aj could you run the numbers on these? I think they are in the ballpark depending on what his final CR comes out to be (which piston, head cc's for his X heads
Do we have a target compression ratio with choice of FT pistons? I was using KB's

BTW some say they have a hard time getting through to Lunati and Howards- I 've never had a problem going in without a bunch of lookie loo questions
but if you do try Bullet real easy to work with (list resorted by seat duration @.006 duration at 200 shows power @.050 is worthless for picking cam- its' for install
duration/lobe .050 .200 valve lift
HC260/296 260 212 126 .2960 CTA for those cases where those buying a 268 cam are advancing 4 degrees this lobe is bigger than the HP cam @200 IC 56 ABDC
HC268/326 268 220 136 .3260 .489 CTA= asymetrical maybe (big maybe intake) Intake close 60 ABDC
HC270/335 270 226 142 .3350 .503 CTS symetrical
HC272/336 272 224 140 .3360 CTA maybe intake if higher compression than what I'm guessing ...62 ABDC
HC273/330 273 220 137 .3300 CTA maybe exhaust
etc.
never just order a cam off a list like this - you have no idea about spring or street vs higher maintenance
Bullet Cams Master List lots of other lobe choices but let's not go nuts till we get close, same with Howard or Lunati if shelf grind is not a fit
 
Well there you go switch to a 3.58 stroke and you got whole new world of pistons to choose from, and, cam for cam, the 3.58 should be worth one rear gear ratio, on the street.

Here is 3.31 stroker at 60*ICA
Static compression ratio of 9.8:1.
Effective stroke is 2.66 inches.
Your dynamic compression ratio is 8.06:1 .
Your dynamic cranking pressure is 160.34 PSI.
V/P (Volume to Pressure Index) is 131VP........................................... 131

And here it is at 3.58stroke
Static compression ratio of 9.8:1.
Effective stroke is 2.88 inches.
Your dynamic compression ratio is 8.08:1 .
Your dynamic cranking pressure is 160.88 PSI.
V/P (Volume to Pressure Index) is 143VP ........................................... 143

Notice the VP is up by...... 9.2%,and
from 3.55s to 3.23s is..... 9.1% I rest my case.

So if you're gonna screw up on the cam choice, I'd rather it be with the 3.58 stroker.
Quick test OP; Installed and ready to run,in your car; how do you tell a 360 block from a 340 block, from a cursory look-see from the top?....... once they are both painted the same color, lol.
Shhhhhhhh, nobody help him.
 
Starting back with NM comments on the custom pistons. Thank you, I did not realize what went into that choice. I will stick with one of the two flat tops. Actually I am going with the KBs.

Not going to stroke the motor.
So how do the numbers stack up with KB's FTs using either Lunati or the Howards as Wym has suggested as an alternative?

As for AJ's quiz. Never had a 360 and never worked on one. I would recognize my intake manifold in a moment but I don't think that was the test you intended or was it? If not, then I'm really not sure, maybe water pump was different or ah, perhaps the harmonic dampner... is the 360 externally balanced?
 
You got it hank; the balancer.
And I didn't actually mean for you to stroke your 340 to a measly 360. But rather to illustrate the point of what stroking can do for you.
V/P Index Calculation
It's not about the extra cubes. It is about the cam-for-cam stronger low rpm power, that allows you to run less gear,less TC and makes the car more fun in the city.
And face it everybody loves to ride in a convertible, so most of the time you might be several hundred pounds heavier than if you were a notchcie. Therefore, you have some choices;
To get off the line you will need to compensate for those maybe 300 to 500 extra pounds. That means a bigger TC, more gear and or a lower first gear; or more cubes.
But when you hit second gear, that split is gonna hurt you, so you are left with gear or more cubes. Top of second will be close to the speed limit, and to continue briskly will require a bigger cam or more cubes.
Do you see the common theme running thru here?
The 360 is about 40 cubes bigger, just right to compensate for 400 pounds; cam-for-cam and gear for gear.And to build a proper hi-compression 360 will cost you a lil less than a similar 340 in terms of machining, and also in terms of parts. And you can upsize to the next bigger cam without losing as much low-rpm power.

And you never have to worry about blowing up your numbers matching 340. Jus saying.
 
KB243's with a .030" overbore and a Felpro 1008 head gasket will give you right around 9.25 static CR. You can get up to a 9.5 static CR by changing the head gasket to a Mr Gasket 1121G.

It'll vary up and down a bit with the chambers and the particular parts... sometimes rod length vary a bit, deck heights are uneven, crank rod journals can be off a bit.

Remember.. with the KB243's you need to get the crank rebalanced. There are 4 balance options:
  • You can get by with a 'simple' rebalance where the machinist takes the average weights of the pistons, pins, rods, and rings of the old parts versus the new and uses that in the bobweight computation; that can save a fair amount of labor cost in the re-balance. If you do that, make sure those original parts are all saved when the pistons, rings, and pins are taken off of the rods.
  • Or,you can pay more for a full re-balance job, where the rods are all balanced against each other; that is more labor intensive and time consuming.
  • OR, you can just buy a new set of SCAT I-beam rods for around $300, which are already balanced against each other, and with the new KB pistons, will nicely lighten up the bottom end and make the balance job very simple. (That is way we went with my son's 340.)
  • The L2316F pistons will give you a 9.0 static CR without having to re-balance.

Next is the question of what cam to use and what computed DCR to target for your fuel, head type, chamber type, etc. The 'computed' DCR really only depends on one cam parameter, the ICA (intake closing angle) which is set with the advertised duration, LSA, and the setting of the cam timing. For being safe, in my personal view and experience, and with the 91 octane, let's keep the computed DCR down in the 7.5-7.7 or so range. IMHO, you can tune around that level but not give away too much low end. My read of this situation is that you don't want to do a ton of tuning, but get it to work decently well, not detonate, then drive it!

OP, do you have a favorite cam at this point to look at? Or one to start with? Oh, OK, I see you have answered that.... next post....
 
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Starting with the KB 243's combined with the Felpro 1008 head gaskets as above.

Wyrmrider said:
Straw man cam is the Lunati Advertised Duration .006 (Int/Exh): 268/276 226/234 @.50, .494/.513, 110/106 RPM Range: 1800-6200 Intake closes 60ABDC @.006
Click to expand...

DCR works out 7.6 with the Lunati cam above, installed with ICL at 106 degrees.

Wyrmrider said:
here Howards
711651-10 CL711651-10 dual pattern for stock exhaust
265 273 218 226 .502 .525 110 LCA 106 ICL 1,15 the 15 indicates for .904 lifter just a little shorter than the comp or lunati 268's more lift
1800-5600 Mild idle, Street/Street Performance. Excellent throttle response
Click to expand...

DCR works out to 7.7 with the Howard's cam, installed with ICL at 106 degrees.

Not enough difference to care about between these 2 cams as far as DCR goes. Computed DCR is to get you in the right operating ballpark, not a 100% definitive number. The only thing I do to vary from this is to look at elevation, but you are at sea level so your engines will have the full DCR number as computed above.

As you can probably see.. this will put you in a DCR range where you can pretty easily tune ignition timing to run the 91 octane. Just keep in mind that your worst case days for detonation will be cool dry days...like 50F and very clear and dry. So that is your 'test day' for ignition timing versus detonation.

And you have 'outs'... just change to a thicker the head gasket with a common Felpro 8553PT head gasket and you drop the SCR and DCR further down about .2-.25 points. Easy....
 
oops forgot to post this one some time agoe
I'd rather screw up with a 4" stroke which is why you/ and WE are going to all the trouble to get a 340 stroke right :)

Have you cc'd your X heads? what are you going to do with them which valves
there are others better than I that can advise on valve sizes for this build
b 4 picking cam
get the compression nailed down with the f91 uel you plan on using
then get the cam that works with your gear and driving style see the rpm ranges in my post above
with your heads they are going to run out of air at not a lot of lift
so you want to get to that lift as quickly as possibly and keep it there as long as possible
it does no good to go to too much duration as your heads will not support the rpm ( this is not a race car)
really easy to not be able to get to the top end and loose the low end at the same time
so build for a big fat torque curve that when you shift your are still on a good part of the torque curve, and you step on the gas a little it does not go flat
note that Aj used 60 as the intake closed point- he can modify that once he gets the exact compression data - am I right Aj?
ps a 256 cam at .006 is about 54 ABDC depending on LCA and intake centerline
 
I used the 60 for demonstration only purposes, to illustrate the 340/360 conundrum as to VP. And to show that 160psi is easy to achieve at a 9.8 Scr, that was all.
Well that, and almost any warm street cam you put in there, can be installed at or very near 60 degrees;the 268 in at 106 for sure; a 262 in at 109, a 276 in at 102
But I would pick the LSA first for the application.Then the intake duration And finally the Exhaust duration for the head . If the overlap gets crossed up, back up the bus.
What do I mean?
For this app I would pick a 108LS. Why?
Well normally the autos want a 112/114 right? Why is that? Cuz when you take 'em to the track and wind 'em out, the 1-2 shift drops the rpm to 1.45/2.45 =59% and the 2-3 is 1/1.45=69% So a 6000 shift, will drop to 3540 and 4140. So the powerband requirement of that trans is 2460 and 1860.......averages to 2160. Whereas with a 4 speed the shift rpm will be about 150 rpm lower and the requirement is about 1640 in all gears. So the auto needs a wider powerband and that is why we use a 112/114.
But think about it, this vert is not a track car so we don't care about that. And a 108LSA cam, all other things being equal, should make a bit more absolute power, which we can take advantage of in second gear. What do I mean?
Well as a street cruiser you are sorta limited to 3.55s, right? An 3.55s will trap at 60~4030 rpm. Would you rather have 260 hp there, or 280hp? I bet you didn't say 260! See, I thought so. And that is why I would choose a 108 or even a 106; namely, to pump up the power at an impossibly low rpm. ( this is also why I would choose more cubes). In first gear we don't care cuz with street suspension, the tires are gonna smoke right thru it.
Then like Wyrmrider mentions, rather implied, easy on the duration with those heads.
Because of the 3.55s, I feel the 268 is too much cam. You will have more power at 4030 with the next smaller cam. Remember first gear is a throw-away gear with stock street suspension.
So then I get a 262/108, as might be the way to go. Now they say and I don't know this, only heard it, that those heads in this application will need additional degrees on the exhaust side. And that could then get us a 262/270/108 cam..... and that would get us 50* overlap. and to go in at ICA of 60* would require a 109install, which would skew the overlap to 22 intake/28 exhaust. Which is messed up. So we back up the bus and re-time the cam to 106* and get 25/25. And the bonus of that is the ICA comes in at 57* for a tad more pressure and a lil more low-rpm performance. Of course to take advantage of that 108LSA, headers are required.
This is an exercise only; no animals were blood-letted.
Also to make best use of this strategy, you want a fast rate cam from .050 to advertised , but even more importantly from advertised to intake-actually-sealed ........ because this will get the pressure up sooner, making the 340 pull like a 360 with a lazy 262. And to further make power a max lift will give you more dwell-time to better fill the cylinders and so make the 262 perform like a lazy 268...... at least in second gear to 60mph/4030 rpm.
Now if you swap in 391s, before you build the engine, well then you would need a new exercise.

Now you may have an understanding of the street gear/big cam conundrum. For a trapspeed of 60 mph, and in 1.45-2nd;
3.23s need power below 3670......3.55s to 4030.
3.73s need power below 4230......3.91s to 4440
4.10s to 4650,.. 4.30s to 4880,.. 4.56s to 5180
 
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Well guys looks like we are in the ballpark
The extra lobe size of the Lunati or Howard or Bullet vs Comp will really help the torque even if the Intake close is at the same .006 ABDC
now some are suggesting Lobe Centers or lCA as in INPUT parameter
let's do it the other way 3.25 gears and 4 speed given
340 with 6.125 rod is a "long rod" motor compared to a 327 or 350 chevy which is what most camgrinders know best- and chevy guys would die for a .904 lifter and a longer rod ratio
The long rod is much more sensitive around BDC than a short rod chevy where it does not make much difference if you are off on the Intake Close or Exhaust open as the piston in the short rod motor is going to "dwell" much longer
and we are postulating that 4 degrees makes a meaningful difference (all the advice to advance a big cam 4 degrees or in at 100 ICL yada yada yada)
we want to open the exhaust with a long rod motor as late as possible to capture POWER and have some gas mileage

Now around TDC a short rod chevy jerks the piston away from TDC much quicker than a MOPAR
We have to consider Crank angle> piston motion> flow Demand which is 0 at TDC except for exhaust pull on the intake at overlap
and we here are running stock exhaust not tuned open headers- which makes a BIG difference
A chevy HAS to start opening the intake earlier to get the valve open far enough to feed the flow demand with the quick after TDC moving piston
With a MOPAR you have much more time / crank angle so you can start opening the intake LATER and still get your valve open
Remember a short rod engine gets the crank to rod at tangent (fastest piston motion) several degrees earlier than a long rod motor- which also helps
if if the piston is starting to slow down after tangent the gas is still accelerating as it has mass
With a long rod engine you can shut the exhaust earlier in degrees and still have it open for more piston motion time (my words there must be a real word)
So a Mopar can live with less overlap than a chevy (everything else like head flow and intake and exhaust the same- which it never is :)
So we have spent pages figuring out a good intake close point based on compression and fuel
total duration based on rpm range and gear torque multiplication
we want lots of lift if the rod to crank tangent area to intake center-line area ie lots of duration at 200 where it matters to make some power
which is why we do not want a Chevy cam in our MOPAR
The LCA is a result of the above considerations- not an input parameter
would be interesting to run our build through rick Jones Controlled induction software package- he calculates all the variables
and you may have noticed it have not mentioned Mike Jones for a cam as IDK if he has any MOPAR grinds in this duration range-someone could ask
his excellent one is shorter [email protected] .305 lobe lift and 50% bigger than the DC 260 cam at 200 -real compression around 9 and an AT and I'd use it in a minuite
He does have a complete range of .904 Solid lifter profiles
did you say what rockers?
 
Lots a computations...great theory to get into details.... but maybe veering of the given constraints.

Small carb that has little adjustments to tune with. 68 AVS carbs have lead plug in idle screws unlike all other years. I took the plugs out, idle screws don’t move. There is one adjustment screw.

Do the cams suggested have adaquate vacuum signal for THAT specific carb? Is that carb going to transition well? You can’t adjust throttle blades, idle bleeds, even fine tuning the float level is a chore.

I think something like a hyd comp XE268 range with 4” stroker kit with 9.0-9.5 max compression would make a fun all around torquey street cruiser. Similar cams in a stone stock 340 sounds tougher than stock. But not much in a stroker. Visually outside you can’t see stroke. Can run your stock rocker arms.
 
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Good points on the carb autocuda
cams suggested are all shorter on the seat due to the lower compression so should have good vacuum
stroker is out for this build but we all agree except the owner so we are trying to optimize for his givens
A stocker 340 with less compression- but not as low as a late 340 which I do have some experience with
too bad comp does not make a XE268XL or XE262XL but the do not, and they make no FT cams special for AMC/MOPAR. The comp 268 is similar to a Mopar 260
Cams suggested should idle along in low gear with 3.25s in stop and go traffic then step on the gas and go up till the heads run out of air
not jeep rockcrawler low but City low should be fun should pull in top gear
valve job on stock heads makes a big difference and IDK about his head work
some said that low lift flow does not make any difference
here it does for a couple of reasons
First with stock exhaust manifolds you do not get a lot of exhaust pull on the intake/ scavenging and you have to watch reversion so not a lot of overlap so not a lot of squeeze on the LCA- we are setting the Intake Close first- not with lobe centers
Second without the exhaust pull we have to get the intake flowing with piston motion so low lift flow starts accelerating the intake mass and the velocity increases as the piston speed increases and the cam lift increases. No use starting the intake open early-exhaust close late-
it's counterproductive in this street car- you can ignore it in a street-strip or bracket car but not here with 3.25 gears and no torque converter
any other 68 AVS experts around?
 
The problem, from the hot rodder's perspective, of the '68 and up factory carbs would be the idle circuit was designed for a target 14.2 AFR ratio at idle speed and relatively retarded timing. Bumping the timing at idle will help make it richer, but yes, I think for any hot rodding the idle mixture screws will need to be free'd up so that or richer can be obtained with cam that makes less vacuum at idle. Some of the guys on RFS forum have done that and can provide suggestions. Sometimes even for a 100% restoration they need to be freed so they can be cleaned thoroughly.
 
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