Help with 340 compression + cam change

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Yes.... look to find where the valves are .010" from opening or closing to see if the advertised duration works out to what is expected, and measure peak lift too.

And for interest, check at .075"lift . Note those angles on valves for #1 and you can get a fair measurement on ICL (intake center line) to check cam timing. (For cam timing, you normally measure at the lifter body and look for the .050" lift points on both flanks of the intake lobe. Measuring at the valve is less accurate but will give you an estimate of timing.)

Again, make sure the engine has been run to pump up the lifters.

None of this may change anything in your thinking, though the cam timing might be of interest.
 
OK, I warmed the engine, and tried to get a reading on the valve, but the valve spring was bleeding down the lifter. (needle slowly dropping) I pulled the rocker arms and set up on the pushrod, and find the my ICL is indeed correct at 105*. So far Good!

Question: Even if the lifter was bled down, wouldn't it still give me a lift reading from base to lobe max?? I can't figure out why it wouldn't, but I only got a lift of .292" on both intake and exhaust? Even multiplied by a full 1.5 this is still only .438" not .458" that the cam is supposed to be. Am I missing something here??

I may simply have lobe wear happening after 65,000 miles. Would be no shame as the motor has seen 6500 quite a few times over the years, and even 7000 a few times. I was always surprised that this cam would happily wind that far. Leaning toward replacing it at this time.
 
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Before you said it, I was gonna say you would lose around .015" or a bit more (at the valves). The pushrods are angled approxiately 14* from the axis of the lifters, and that is enough to cost that amount of lift. It's the cosine of the angle, which is .970 for 14*, and that would make a .305" lobe lift become .296" at the top end of the pushrod. And any angle of the dial indicator from the push rod axis would lose a bit more. So I would not automatically suspect lobe or lifter wear just from that. OK on removing the rocker to get around the leakdown issue... that's good sense!

With the lifters leaking down as now seems to be obvious, then that would effect the valve openings during cranking..... durations would be shorter and that would push up the compression psi readings. So this seems likely to be the source of the mismatch in numbers from earlier. 'Good' lifters would not bleed down. So this exercise revealed that.
 
So this is making more sense!!! Thanks so much for the guidance and info, and for sticking with this!! I now have better info so picking a new cam (and lifters!) will be less of a risk. And i believe I’ve got a Spring project!
 
No you're not missing a thing. As our friend said above, you lose a small amount.......it varies because of this and that, but mostly from the pushrod angle, but generally it's between .015"-.020" "give or take". Sounds to me like you got it under control.
 
Here's some more info. I farted. It was a squeaker.
 
Gotta admit, I would never have thought bad lifters could give higher a higher cranking pressure. Makes sense once explained.
I’ve learned a lot here, and hope the info might help others too!
 
Its the same as the Rhodes lifters which intentionally have a 'controlled leakiness' so to speak. They make the street manners of a rough cam become milder at lower RPM's by leaking off lift in the lifter and reducing the duration.
 
nm9stheham said:
Its the same as the Rhodes lifters which intentionally have a 'controlled leakiness' so to speak. They make the street manners of a rough cam become milder at lower RPM's by leaking off lift in the lifter and reducing the duration.
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Is it possible the OP had Rhodes lifters?
 
No, my receipts say it was the standard Lunati Bracket Master II cam/lifter kit.

If they do leak down when cranking, would their condition affect performance at higher rpms? Seems like it would...
 
The fact that some are leaking says that some may leak very fast, and that would effect it. My son's 340 had 2 lifters go in <1000 miles from new, and you could plainly hear the worst one (#4, exhaust) in the exhaust note on the right side. It was a definite miss at idle, and even was felt at cruise RPMs as a bit more roughness in the engine.
 
Yeah, that makes sense to me as I’ve been hearing (for a while now) a roughness or unsteadiness at the pipes at idle.

This is good, as knowing why something might be happening is much better than just “a feeling that something isn't right”!!! :)
 
I was trying to find in my records or notes an indication of what the compression used test at. I think I remember (?) it used to be more like 145-150# though can’t say for sure. If so, my 160# doesn’t mean my lower numbers now are a sign of a wear, ...rather that those lifters are still working better than the ones that are bleeding down and pushing the cranking compression up.

Does this make sense?
 
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So after getting a recommendation from Howards, talking with Mike Jones for a while, and having a long talk with Dwayne Porter, their thoughts are generally, yes, I could gain some with more lift but at some risk it wouldn't be much different.

The recommendations were pretty similar: Single pattern (which surprised me) 108* LC with about .220" (one was .224") at .050 and .495" to .506" lifts.

Mike Jones was pretty clear I should somehow deal with my compression first, and thinks I should change springs if I do any more lift. Dwayne suggested I look into something like the CH318B heads, as they'd actually do better than the ported ones I have.

And of course, I don't have a lot of converter either. So I'm on hold with any cam change unless or until I decide to tear it all down and do different pistons to get more compression and do a different converter, etc.

So for now, I ordered one of the new AVS2 650 carbs which came today. And I gotta tell you, after being used to my 750, looking at those 650 primary bores I can't believe they are enough!! All formulas and advice say it's enough carb. If I think of the size of Thermoquad primaries then maybe. And maybe set the AVS to open fairly quickly??

Anyone have specific experience with either the 800 or AVS 650 Edelbrocks on their 340's???????
 
Of interest regarding carb size:
I have the "Tuning To Win" book from Don at Four Seconds Flat, and he and others, remind us that, in a sense, we burn air, not fuel. And if we have a hard time getting enough fuel to the motor it means the carb is too big, not too small. It's the mixture we are going for, and a large carb will feed so much air that it's hard to get the right amount of fuel.

And also, someone here had posted a video talking about how a smaller carb helps with the velocity into the intake, giving more torque at the bottom end. And, besides street performance, the video talked about how a smaller carb will often win on the strip due to the influence off the line.

All of this says a smaller carb is good, but then I always hear how the 340's LOVE bigger carbs!!! Makes it damn hard to decide whether the 650 or the 800 would be best.
 
midnight340 said:
fuel to the motor it means the carb is too big, not too small. It's the mixture we are going for, and a large carb will feed so much air that it's hard to get the right amount of fuel.
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That's a BS explanation. On a larger carb it can be more difficult to get good response on the idle circuits and if all other things are equal, will be at a higher rpm before there is enough velocity through the boosters to get the main circuits going.
At wide open throttle, especially with mechanical secondaries, there's less manifold vacuum with a larger carb. This potentially can mean more power if fuel-air distribution to all cylinders is really good. There is a diminishing return because the vacuum is helping the fuel droplets stay well distributed and available to each cylinder.
midnight340 said:
And also, someone here had posted a video talking about how a smaller carb helps with the velocity into the intake, giving more torque at the bottom end. And, besides street performance, the video talked about how a smaller carb will often win on the strip due to the influence off the line.
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Sortof. More velocity at lower rpm gets the mains started earlier and the main circuits are generally more responsive and efficient than the "idle" circuit.

midnight340 said:
All of this says a smaller carb is good, but then I always hear how the 340's LOVE bigger carbs!!! Makes it damn hard to decide whether the 650 or the 800 would be best.
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Tough call! There's no absolute answer. If you shooting for a mph goal at the end of the 1/4, then the 800 has more potential (assuming the tires and gearing are such that the engine will be near its peak at the stripe). The 750 Eddys need some work to run to their potential. So on that series, the smaller carb is a better choice. For the AVS2, can't say, but I think for your situation you made a good choice. You can always go back to the 750.
I was running a 650 vacuum secondary Holley for a while. Just a little less cam than you have. It ran just as good as the 750 on the dyno. Don't recall taking any 1/4 mile passes with it, which would have been a better comparison.

my
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and some :)
 
I run a 800 AVS on my 410 stroker 5 speed manual. My brother came over with his brand new 600 AVS 2 and bolted it on for a try. It had a bit better light throttle response but lost a heap on full throttle. Just did not smoke the tyres when you flattened it in second gear as per the 800. I know this was out of the box but it did not have the seat of the pants feel.
 
Boy
one of the first threads I've read with consistent good advice AND responses from OP
What I'd do
what Rusty said- leave it alone
but get tune perfect
then try 4 degree advance and see how you like it - if you like it better then maybe a shorter cam with more lift like Voodoo, Howards -
and 4 degrees retard and see how you like it if better you could go to a slightly bigger cam say the Comp XE274- even though the duration is the same as now it is a bigger cam- or .904 grinds from Lunati, Howards, Bullett, Engle
Actually if you want to make a meaningful difference go solid and work with Jones
IDK if Jones has any Mopar hyd grinds except for his excellent "low compression" grind which is much smaller than what you are working with 256 .305 cam lift but 50% larger at .275 than The DC 260 grind
single pattern/ dual depends on intake/exhaust ratio from carb to exhaust
forget roller tip rockers as stated above for your lift needs B3 kit is a must etc
I do recommend oil through the pushrod lifters and pushrods to help the rocker cup live
8.5 motors can be made to run well
anything else is going to require heads, cam and lots of $$$
cheers
 
Have decided to try the 650 AVS2, the size of the primaries is right between those of the smaller and larger TQ’s.

And for now this cam stays. It’s working fairly well. But I am interested in optimizing what I have.

I was actually going to ask advice about the possibility of advancing the current cam a few degrees?? (Since I need to install a new timing cover anyway.)

Would this maybe build more cylinder pressure? If the torque curve moved down a bit I’d be fine with that.

Wyrmrider, I’m not sure what you were saying in regards to 1.6 roller rockers. Were you advising against? (I do understand the necessity of getting the geometry correct if used, and saw info on the B3 Kit.)

Again, I really appreciate the advice, I continue to learn a lot as I work through these possibilities.
 
I think your trap-speed is telling you something;
98 mph is a W/P ratio of 13.0, which at 3200 plus driver is say; 3380/13=260hp
but even if you meant 3200 race weight; then 3200/13=246hp
My factory stock 1970 340 Swinger @3310 me in it pulled that 98mph trapspeed...... with 3.55s, (255hp)
So My opinion does not agree with others here.

If it was mine, I'd be tearing it apart, looking for the problem in those low-pressure holes. And that 284/218 cam would not be going back in; not ever.
If you are trapping at 98@~5300rpm you need a cam that matches that, and 218@.050 ain't it. That 218*, coupled with 160psi is a good street pairing, but, IMO, your 4.10s/hi-stall, are giving you the impression of good performance, by multiplying the torque you have, to a higher feel-good number. This is my opinion. Obviously this combo is not optimized for the track. The 284*, with it's late-closing intake, is what is reducing the cylinder pressure so you can run pumpgas. The proper,IMO, advertised duration for a 218@.050 would be ~264 advertised@.008. But, your cylinder pressure would probably rise into the detonation zone. So the 284/218 cam is doing what it is intended to do, but certainly doesn't or shouldn't need 4.10s nor a hi-stall.

At 160 psi with iron open-chamber heads,in all 8 holes; you can't go much larger than the 284 cam, but you can certainly increase the .050 numbers from 218* to something faster, like up to ~240*. That would be 3 cam sizes and ~600rpm higher power peak; so now ~5600. With the new-found power, the trapspeed will rise to maybe 104@5600 so now your components are better matched, and your ET is guaranteed to fall; maybe to as low as to 13.4. Which is still off the mark for that combo.

I agree with the XE275HL. At 275/287/110+4, 231/237@.050, .525/.525 lift this is a great street cam, especially for factory X/J heads.About as big as I would ever go. Oh wait, that is as big as I ended up at on the third try,lol.
You will pick up both low-rpm torque, and at peak,2 cam sizes worth of power over the 218, and it will run pretty good with the 4.10s. But the 275HL is gonna require a higher octane gas, cuz the cylinder pressure is gonna rise. Maybe to 165 which is generally considered border line too much for open chamber iron heads. You may have to retard the cam up to 4 degrees to bleed the pressure. This is OK cuz on the track it will better match your trap speed, and with 4.10s you won't notice the low-rpm torque loss, because you are already used to 160psi, and the 4.10s/hi-stall..
But before I would do anything,I would, as I say, chase after the why of those low-pressure holes
 
AJ, I always appreciate your posts. And they’ve certainly challenged me to learn more. And since I’m not a young guy anymore they tell me that’s a good thing!!! :D

I’ll ponder this a bit and respond later.
 
nice analysis AJ that old school cam long duration low lift - he could do better
but as was said above- is it worth it- you evidently think not
and I'd change it too as long as that comp 275xl is not too much duration
you can get similar cams with different LCAs from Howards, Lunati, Bullet etc if you need to move the events around
AJ can guide OP on that
 
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