Thumping Cam not Thumper

[340inabbody]

Is the thump of a engine proportional to vacuum?

ie the thumpier the sound the lower the vacuum? Does the thump and vacuum directly relate to valve timing in overlap?

Also how does lobe separation affect valve timing and overlap?

 

[PRH]

In a nutshell……… yes.
More overlap = less vacuum and a rougher idle.

 

[srduster340]

Tighter lobe separation = more overlap. Generally, plus more thump. Listen to a cam ground on a .106 vs .110.

 

[AJ/FormS]

Also how does lobe separation affect valve timing and overlap?

Lobe separation; Lets talk with a given cam, in a given engine, and installed at or near Split overlap.
the farther apart the centerline is, the later the intake closes. This reduces your Cylinder Pressure, and intake vacuum, which kills bottom end, and reduces your power.
On the Exhaust side, an early opening exhaust sends pressure into the tailpipes that could have been used to spin the flywheel.
When both are done, equally, the overlap is reduced. This reduces specific power output but does broaden the curve some.
Thus
cams with wide lobe centers are generally stuffed into grocery getters, etcetera.

For performance, we want longer intake/exhaust durations. But at a given Lsa, the durations start doing what the Wide Lsa cam does, so, we tighten up the Lsa, to reduce the crappy part.
This has the happy result of increasing overlap, which, if you have well-sealed headers, results in increasing horsepower leading up to and over the nose of the power curve. With the intake valve closing at an earlier angle, this keeps the pressure up, which produces low-rpm torque.
When going for performance, and chasing power, we don't much care about losing the bottom end, so not having an optimum exhaust opening event is of less importance than getting rid of the spent gasses.

Think of the Factory 340 cam. People worship that thing, but lemmee tell ya I got 4 of those, used but nice, sitting on the shelf here and have no intention of ever using them.
By the numbers, they look pretty good. 268/276/114, 44* overlap. What that means is installed at split overlap, the intake closes very late. Those cams worked in the factory 340s , cuz the were "high compression". But when you slide one into a smog-era 318, the cylinder pressure drops into never-never land.
But here's the thing, 340s never came with headers so the very modest 44 degrees of overlap was dead dead dead. Which resulted in a long broad power curve.
Finally, the exhaust opens very early so forget about fuel economy.
The thing is, split overlap occurs at an install of 112*, making the intake close, with a new chain, at 64*, a reasonable number, but this changes the exhaust opening to 104 ATDC, and the fuel economy was even worse. Plus that shifts the overlap centerline, reducing it's effectiveness, which, because there were no headers on that car, I guess was no big deal.

Now, for performance with headers, tighten up the Lsa to 108 with that cam, and now you could have a powerhouse, if the valves don't float first, which IMO, was the Achilles heel of the 340s, namely insufficient spring pressure, and subsequent lifter pump up.

Well, the thing is those cams were designed to last forever, so the lift was pretty modest and the intensity of lobes was off the wall slow.
Not a chance would I ever install one of those in something I want to flog.

Listen, I'm not real good at explaining stuff, so if I messed you up, I'm sorry.
The thing is that the cam manufacturers these days are putting out some excellent products compared to 55 years ago. You can always call one of them and explain what you are trying to do. To learn it all to a rudimentary understanding, could take maybe dozens of hours. and if you get it wrong, there you are with a used bumpstick and a used set of lifters, trying to sell it, probably here. I went thru that . I'm on my third cam. As for the first one, I pulled it out before the first summer was over. The Second one dropped lobes, but that was a real sweetheart. Now I'm on a third cam, only one size bigger than the sweetheart, and I've been waiting since 2004 for it to die so I can go back a size,....... but doggone it, wouldn't you know it, she's got nearly 100,000 miles on her now, and she just keeps on going.
Happy HotRodding

 

[340inabbody]

Lobe separation; Lets talk with a given cam, in a given engine, and installed at or near Split overlap.
the farther apart the centerline is, the later the intake closes. This reduces your Cylinder Pressure, and intake vacuum, which kills bottom end, and reduces your power.
On the Exhaust side, an early opening exhaust sends pressure into the tailpipes that could have been used to spin the flywheel.
When both are done, equally, the overlap is reduced. This reduces specific power output but does broaden the curve some.
Thus
cams with wide lobe centers are generally stuffed into grocery getters, etcetera.

For performance, we want longer intake/exhaust durations. But at a given Lsa, the durations start doing what the Wide Lsa cam does, so, we tighten up the Lsa, to reduce the crappy part.
This has the happy result of increasing overlap, which, if you have well-sealed headers, results in increasing horsepower leading up to and over the nose of the power curve. With the intake valve closing at an earlier angle, this keeps the pressure up, which produces low-rpm torque.
When going for performance, and chasing power, we don't much care about losing the bottom end, so not having an optimum exhaust opening event is of less importance than getting rid of the spent gasses.

Think of the Factory 340 cam. People worship that thing, but lemmee tell ya I got 4 of those, used but nice, sitting on the shelf here and have no intention of ever using them.
By the numbers, they look pretty good. 268/276/114, 44* overlap. What that means is installed at split overlap, the intake closes very late. Those cams worked in the factory 340s , cuz the were "high compression". But when you slide one into a smog-era 318, the cylinder pressure drops into never-never land.
But here's the thing, 340s never came with headers so the very modest 44 degrees of overlap was dead dead dead. Which resulted in a long broad power curve.
Finally, the exhaust opens very early so forget about fuel economy.
The thing is, split overlap occurs at an install of 112*, making the intake close, with a new chain, at 64*, a reasonable number, but this changes the exhaust opening to 104 ATDC, and the fuel economy was even worse. Plus that shifts the overlap centerline, reducing it's effectiveness, which, because there were no headers on that car, I guess was no big deal.

Now, for performance with headers, tighten up the Lsa to 108 with that cam, and now you could have a powerhouse, if the valves don't float first, which IMO, was the Achilles heel of the 340s, namely insufficient spring pressure, and subsequent lifter pump up.

Well, the thing is those cams were designed to last forever, so the lift was pretty modest and the intensity of lobes was off the wall slow.
Not a chance would I ever install one of those in something I want to flog.

Listen, I'm not real good at explaining stuff, so if I messed you up, I'm sorry.
The thing is that the cam manufacturers these days are putting out some excellent products compared to 55 years ago. You can always call one of them and explain what you are trying to do. To learn it all to a rudimentary understanding, could take maybe dozens of hours. and if you get it wrong, there you are with a used bumpstick and a used set of lifters, trying to sell it, probably here. I went thru that . I'm on my third cam. As for the first one, I pulled it out before the first summer was over. The Second one dropped lobes, but that was a real sweetheart. Now I'm on a third cam, only one size bigger than the sweetheart, and I've been waiting since 2004 for it to die so I can go back a size,....... but doggone it, wouldn't you know it, she's got nearly 100,000 miles on her now, and she just keeps on going.
Happy HotRodding

Thanks for taking the time to try and explain this. I don’t rightly understand much of what you’re saying initially but my current engine has solid lifters perhaps helping address that weakness of the floating valves. Yet I rearly in fact have never seen another solid lifter 340 personally. I have seen a solid lifter LT1 before.

Looks like there.needs to be a coordinated effort on LSA and timing of events and thats the crux of it?

Am I asking too much to have a relatively flat torque curve from a 340? I don’t think so. Nicks Garage has at least two videos at the dyno that have relatively mild mannered cams in a relatively stock motor showing a nice broad relatively broad torque curve.

If I wanted to drive this on the street with headers and a 3.55:1 rear which I am and not for the strip then this is my goal. I’ve been told I’m crazy because I put a 340 in a Bbody. Originally, I had a 318 in it so I was set up for small block already.

I do know the faster the event timing/ramping the better the scavenging and a solid lifter gets you that. I just have zero feel the numbers for the specs on the cam. But I definitely hear what you’re saying. I should talk to Comm and powered and Edelbrock cough cough. I tried to look at their curves, but there’s a lot of variables.

 

[273]

Closing intake valve later can increase VE% which increases Torque even if it lowers calculated DCR.

 

[George Jets]

Here is a great baseline for the "340" camshaft grind numbers that you can use as comparison.

'68-'73 340
.430/.444 lift
268/276 duration

114 LCA

When a person starts adding more lift and duration and less LCA, it pushes the power band up the rpm range causing the engine to choke (thump) at idle.

For comparison here is a 340 manual trans camshaft idling at 750 rpm.




.

 

[mopowers]

I saw this a while back and thought it was interesting:

 

[340inabbody]

I saw this a while back and thought it was interesting:

I’ve watched this and some other f his other videos. He seems very knowledgeable. The more I learn about cM shafts the more questions I have. It’s like a never ending circle due to the fact that there are so many related variables. I really respect guys that understand this well along with guys that know how to match torque converters!

 

[340inabbody]

I wonder why so few people use both (I know they are unrelated) a solid roller lifter set up AND gear driven CAM. Seems like that would mechanically be the most accurate and fastest way to control valve event timing in our engines (SBM).

 

[AJ/FormS]

Thanks for taking the time to try and explain this. I don’t rightly understand much of what you’re saying initially but my current engine has solid lifters perhaps helping address that weakness of the floating valves. Yet I rearly in fact have never seen another solid lifter 340 personally. I have seen a solid lifter LT1 before.

Looks like there.needs to be a coordinated effort on LSA and timing of events and thats the crux of it?

Am I asking too much to have a relatively flat torque curve from a 340? I don’t think so. Nicks Garage has at least two videos at the dyno that have relatively mild mannered cams in a relatively stock motor showing a nice broad relatively broad torque curve.

If I wanted to drive this on the street with headers and a 3.55:1 rear which I am and not for the strip then this is my goal. I’ve been told I’m crazy because I put a 340 in a Bbody. Originally, I had a 318 in it so I was set up for small block already.

I do know the faster the event timing/ramping the better the scavenging and a solid lifter gets you that. I just have zero feel the numbers for the specs on the cam. But I definitely hear what you’re saying. I should talk to Comm and powered and Edelbrock cough cough. I tried to look at their curves, but there’s a lot of variables.

Whether B or A is not the deal-breaker; weight and aero is.
the early factory 340s were rated at the same torque as the early 318s.... but at 800rpm higher, 3200 versus 2400. That should tell you something.
It tells me that the 340 has traded away bottom-end torque to get top-end power, and that is exactly what happens with a bigger cam .
lets look at ALL the numbers of these engines, first up is the 340, with the cam on split overlap;
268 intake/114 compression/106 power/276 exhaust/44 overlap, 318next
240 intake/130 compression/122 power/248 exhaust/20 overlap

Plugging the numbers into he Wallace Calculator, at their rated Compression ratios, here's what we get;
First the 340,
Scr of 10.5, Dcr of 8/1, pressure of 165, and a V/P of 128
Scr of 9.0, a Dcr of 7.9 , pressure of 156, and a V/P of 129
Now first up, check out the near identical Dcrs, and the near identical V/Ps . This brings credence to the torque numbers being so similar. in spite of the 318s lower pressure. This is why these 318s are so loved cuz below 3200, they have more torque than the early 340s.
In other words, your B will perform just like a 318, below about 30mph. Well hang on, that's not exactly true;
340 ftlbs at 2400rpm is 155horsepower, whereas
340 ftlbs at 3200rpm is 207hp.
The problem is gearing.
the 318 mighta had 2.76 gears so 2400 in 1st is 28.5mph at zero-slip, whereas
the 340 car is sure to have at least 3.23s so, 3200 in 1st, is 32.5mph;
so then, with identical cars, in a race to 30mph, it could be a toss-up.

Ok but lets look at those numbers again;
268 intake/114 compression/106 power/276 exhaust/44 overlap, 318next
240 intake/130 compression/122 power/248 exhaust/20 overlap

You see how the 318 has 130 degrees of compression versus the 340 at just 114; in spite of having just a 9/1 Scr versus the 340 at 10.5.
What do you think might happen if we pumped the 318 up to 10.5"
answer; with no other changes, the pressure jumps to 191 psi, and the V/P to 158. Now that's a torque-monster!
But let's back up, and put the 340 cam into the 9/1 318.
Now the pressure falls to 135 and the V/P to 98; which is barely better than a 225 slanty (87 IIRC)

The point is that for a streeter, that operates almost exclusively way off the power curve, pressure is everything. and I can tell you that 195psi, which I have run (in a 367 mind you, with a 270 cam), is effing nuts. In my opinion, it was well worth swapping away the 292/292/108 cam that I took out.
******************
on a different note
a lot of performance cams are ground on a 110 Lsa, which for a streeter is a good balance for absolute power and and good street manners. For most of us, we're gunna pick a cam from around 262 to 284 advertised intake duration. (I mean at 292 I was out of the ballpark). this is a range of 22 degrees which is about 4 cam sizes, from mild to "wild".
But what I really want to talk about is "intensity" which is best revealed by the time, in degrees, from when the valves are actually tickled into opening called the advertised duration, and the 050 duration.
Advertised durations can be measured from wherever after the valve opens. Some grinders choose .008 tappet rise, some choose .006, and at least one chooses .001.... so you can't always tell much from the advertised unless you know the grinders history. But for hydros everybody has to be accurate at 050, cuz if you lie and your cam is a flop, that's gunna hurt your business.
Generally, for hydros, I see intensities of 44* to 58*. Obviously a slow cam (58*), should last a long time, or at least longer than one at 44*.
But that's just part of the story. Two cams with identical 050 numbers and polar opposite intensities will push the intake opening/closing points some 7 degrees each way. Opening earlier increases overlap, which we can deal with but the later closing-end has repercussions of reducing cylinder pressure, reducing idle-vacuum, and reducing bottom end torque, none of which make for a good streeter. I mean there are ways of dealing with that after the fact, but right now for this discussion, I'm talking about intensity.
At the other end of intensity, 44*, to keep the valve gear together with increasing rpms, spring pressure has to be increased, and that is never good for the lobes. So, high intensity requires special considerations, such as oil chemistry, and oil-change intervals.
Thus choosing a cam based on intensity is a bit of risk.

Now, when we get to solids, intensity is signalled from a different point of the lobe, after the lash has been taken up. The chosen number is usually 020. and 050 is still where we're going, but now the intensity is between those two, and the number gets pretty small. which means, now you are choosing a solid cam by it's 050 numbers, knowing that you can't buy a higher intensity other style of cam for your LA engine, except maybe a mushroom lifter......
Of course this opens the door to installing a bigger solid, for the same specs of a hydro at advertised. and if you close the LSA, even bigger.
The downside is constantly checking the lash so you don't wear the cam out prematurely, and so you can always be sure the valves are closing, etcetera; and you know, streeters are not particularly fond of all the clatter going on under the hood.
Another thing about solids is, for a given modest 050 cam, that cam is guaranteed to offer better fuel economy over a same 050-sized hydro , in a same spec engine........ because the intake is closing a good two sizes earlier, say about 7 degrees. But it doesn't stop there. Knowing this, if you have enough pressure, you can increase the solid cam size at least one size and probably two if you run a tighter Lsa, without affecting fuel economy, from what the current hydro might be getting. ....
But another thing is that, on the street, yur actully, IMHO, just playing a game of numbers. The truth is that there is a hydro available to suit just about every streeter. I mean once you get to 230 to 235@050, 90% of the time, or more, yur just lugging that bad boy around for bragging rights. It doesn't really show up in your zero to 60mph time trial, unless you car 60fts really strong. and if it ain't there, then it's just spinning, and costing you gas money with every mile it turns. and when the cost of gas goes up, there you are in the que, waiting for your turn at the gas-bowser.

 

[273]

You can't use factory gross hp and torque rating to try to prove a point, there's no way a stock 318 made 230hp and 340lbs-ft.

 

[273]

There's no reason a high cr smaller engine VS a lower cr larger engine can't have similar powerbands (hp @ TQ curves) therefor similar performance, eg.. High cr 302 VS low cr 318, same with 318 VS 340, 340 VS 360 etc..

 

[340inabbody]

Whether B or A is not the deal-breaker; weight and aero is.
the early factory 340s were rated at the same torque as the early 318s.... but at 800rpm higher, 3200 versus 2400. That should tell you something.
It tells me that the 340 has traded away bottom-end torque to get top-end power, and that is exactly what happens with a bigger cam .
lets look at ALL the numbers of these engines, first up is the 340, with the cam on split overlap;
268 intake/114 compression/106 power/276 exhaust/44 overlap, 318next
240 intake/130 compression/122 power/248 exhaust/20 overlap

Plugging the numbers into he Wallace Calculator, at their rated Compression ratios, here's what we get;
First the 340,
Scr of 10.5, Dcr of 8/1, pressure of 165, and a V/P of 128
Scr of 9.0, a Dcr of 7.9 , pressure of 156, and a V/P of 129
Now first up, check out the near identical Dcrs, and the near identical V/Ps . This brings credence to the torque numbers being so similar. in spite of the 318s lower pressure. This is why these 318s are so loved cuz below 3200, they have more torque than the early 340s.
In other words, your B will perform just like a 318, below about 30mph. Well hang on, that's not exactly true;
340 ftlbs at 2400rpm is 155horsepower, whereas
340 ftlbs at 3200rpm is 207hp.
The problem is gearing.
the 318 mighta had 2.76 gears so 2400 in 1st is 28.5mph at zero-slip, whereas
the 340 car is sure to have at least 3.23s so, 3200 in 1st, is 32.5mph;
so then, with identical cars, in a race to 30mph, it could be a toss-up.

Ok but lets look at those numbers again;
268 intake/114 compression/106 power/276 exhaust/44 overlap, 318next
240 intake/130 compression/122 power/248 exhaust/20 overlap

You see how the 318 has 130 degrees of compression versus the 340 at just 114; in spite of having just a 9/1 Scr versus the 340 at 10.5.
What do you think might happen if we pumped the 318 up to 10.5"
answer; with no other changes, the pressure jumps to 191 psi, and the V/P to 158. Now that's a torque-monster!
But let's back up, and put the 340 cam into the 9/1 318.
Now the pressure falls to 135 and the V/P to 98; which is barely better than a 225 slanty (87 IIRC)

The point is that for a streeter, that operates almost exclusively way off the power curve, pressure is everything. and I can tell you that 195psi, which I have run (in a 367 mind you, with a 270 cam), is effing nuts. In my opinion, it was well worth swapping away the 292/292/108 cam that I took out.
******************
on a different note
a lot of performance cams are ground on a 110 Lsa, which for a streeter is a good balance for absolute power and and good street manners. For most of us, we're gunna pick a cam from around 262 to 284 advertised intake duration. (I mean at 292 I was out of the ballpark). this is a range of 22 degrees which is about 4 cam sizes, from mild to "wild".
But what I really want to talk about is "intensity" which is best revealed by the time, in degrees, from when the valves are actually tickled into opening called the advertised duration, and the 050 duration.
Advertised durations can be measured from wherever after the valve opens. Some grinders choose .008 tappet rise, some choose .006, and at least one chooses .001.... so you can't always tell much from the advertised unless you know the grinders history. But for hydros everybody has to be accurate at 050, cuz if you lie and your cam is a flop, that's gunna hurt your business.
Generally, for hydros, I see intensities of 44* to 58*. Obviously a slow cam (58*), should last a long time, or at least longer than one at 44*.
But that's just part of the story. Two cams with identical 050 numbers and polar opposite intensities will push the intake opening/closing points some 7 degrees each way. Opening earlier increases overlap, which we can deal with but the later closing-end has repercussions of reducing cylinder pressure, reducing idle-vacuum, and reducing bottom end torque, none of which make for a good streeter. I mean there are ways of dealing with that after the fact, but right now for this discussion, I'm talking about intensity.
At the other end of intensity, 44*, to keep the valve gear together with increasing rpms, spring pressure has to be increased, and that is never good for the lobes. So, high intensity requires special considerations, such as oil chemistry, and oil-change intervals.
Thus choosing a cam based on intensity is a bit of risk.

Now, when we get to solids, intensity is signalled from a different point of the lobe, after the lash has been taken up. The chosen number is usually 020. and 050 is still where we're going, but now the intensity is between those two, and the number gets pretty small. which means, now you are choosing a solid cam by it's 050 numbers, knowing that you can't buy a higher intensity other style of cam for your LA engine, except maybe a mushroom lifter......
Of course this opens the door to installing a bigger solid, for the same specs of a hydro at advertised. and if you close the LSA, even bigger.
The downside is constantly checking the lash so you don't wear the cam out prematurely, and so you can always be sure the valves are closing, etcetera; and you know, streeters are not particularly fond of all the clatter going on under the hood.
Another thing about solids is, for a given modest 050 cam, that cam is guaranteed to offer better fuel economy over a same 050-sized hydro , in a same spec engine........ because the intake is closing a good two sizes earlier, say about 7 degrees. But it doesn't stop there. Knowing this, if you have enough pressure, you can increase the solid cam size at least one size and probably two if you run a tighter Lsa, without affecting fuel economy, from what the current hydro might be getting. ....
But another thing is that, on the street, yur actully, IMHO, just playing a game of numbers. The truth is that there is a hydro available to suit just about every streeter. I mean once you get to 230 to 235@050, 90% of the time, or more, yur just lugging that bad boy around for bragging rights. It doesn't really show up in your zero to 60mph time trial, unless you car 60fts really strong. and if it ain't there, then it's just spinning, and costing you gas money with every mile it turns. and when the cost of gas goes up, there you are in the que, waiting for your turn at the gas-bowser.

I’m gonna need to read this two or three more times maybe even more to understand this better but I think I understand the gist of what you’re saying. People have been telling me this type of story for a while and it always makes me question is a 318 better for the street than a 340. I suppose it depends on how you set it up. This is why I always think of a flat broad torque curve.

 

[273]

I’m gonna need to read this two or three more times maybe even more to understand this better but I think I understand the gist of what you’re saying. People have been telling me this type of story for a while and it always makes me question is a 318 better for the street than a 340. I suppose it depends on how you set it up. This is why I always think of a flat broad torque curve.

VE% is more important than CR (V/P).

 

[340inabbody]

Great

VE% is more important than CR (V/P).

How does the average guy measure actual air volume? Or can you somehow back into it by using HP/cI on a dyno?

 

[273]

Great

How does the average guy measure actual air volume?

You can't, there's basically two ways to make more power, squeeze more out the fuel and air your already using like compression ratio and or more fuel and air which generally is greater VE%. If your making more power from 2,500-6,000 rpm with a cam change it's basically cause of an increased VE% across those rpms, Obviously compression is important too but you can't squeeze air that ain't there in the 1st place, like everything it's a balancing act.