Mopar Gen III Hemi COMP CAMS Thumper Cam Specs Incorrect?

[thrustbucket]

I've been working on a 1998 Dodge Dakota Gen III swap for about a year now. Its a street-truck build on a budget. 2007 5.7L, fully rebuilt with stock 3.92 bore, MDS delete, Mancini drag-pack style intake, 750 demon vac secondary carb, Hemi 6, and 44RE, toughened up with a shift kit, HP clutches and steels, 8.25" rear rebuilt with 4.56 gears and track-loc) Goal is around 400hp at the crank.

I apologize for the insane length of this. But, it's a long story.

I purchased and installed a Comp-Cams Thumper Cam. (112-700-11)

First, I know the Thumper probably has a little too much overlap for the single plane. But, the specs looked more attractive to me than the Comp-Cams Stage 1 NSR cam, It is a budget build and I was focused on reusing as much as possible including the valve springs. I thought the Thumper the better choice of the two Comp' 'no springs required' cams available for my build. I'm sure there was a better choice out there elsewhere... But, that's now beside the point.

My problem:

The cam card for this cam is not correct. Also, the intake closing point was/is not listed on their website specs for this cam. The card states an intake closing event of 81 degrees ABDC?!

When I degreed the cam I was getting 66 degrees, which makes sense for a 'street' oriented cam. I am using a stock chain setup and it installed at 112 degrees. (the 2 degree advance could just be bad resolution from the small degree wheel) 66-2=64 degrees (see below)

I called COMP twice and two separate associates told me: 'The cam card is correct.'

Look: intake open + 180 degrees + intake close = duration (as long as all numbers are referenced at the same valve lift)

24+180+64=268 This IS the advertised duration @ .006" on the card.

If you do the math @ .050" you get 37 degrees Intake close. That also makes sense.

24+180+81=285? That duration isn't even listed on the card.

I am confident the cam will work fine because I trust the measurements I took when degreeing the cam. But, the fact they won't admit their numbers are possibly incorrect is driving me nuts!

Either the 81 degree intake closing event is wrong: (waaaay too late for a street cam)
Or, some other number(s) is/are incorrect.

I explained the the two 'experts' at COMP that I thought it was a typo, and they insisted once again it was correct. One of them also tried to tell me it was because 81 degrees was measured at .050"?!!

I then explained to him that I had based my build, including head gasket thickness, raised shift points, tire size, and gearing based on the power-band of this cam which I chose first, (and calculated the intake closing point before purchase) and I was concerned that if 81 degrees IC were correct, I would have no dynamic compression.

I was then told by the associate at COMP that I was "OVERTHINKING IT..."

I have since emailed COMPS 'cam selection experts' about the issue and have not received a response in over 14 days.

Does anything they told me make any sense?

PLEASE LOOK AT MY MATH and let me know if I messed something up.

I am getting the feeling if you buy from COMP CAMS you are probably on your own as far as legitimate technical help.

Please help and let me know your thoughts.

 

[turbofreek]

you realize that is after bdc right? which means nothing about compression building. that is allowing last bit of exhaust cylinder debris to exit. for clean intake stroke atdc. also makes it easier for piston to travel across crank throw without stalling engime with compression before it passes bdc.

 

[thrustbucket]

turbofreek,

I am aware the observation is ABDC. If intake valve closing point ABDC had nothing to do with building compression, it would not try to 'stall' the piston with opposing pressure by nature of an earlier closing event closer to bottom dead center as you have stated. (BTW I totally agree with you on that) Unless of course, there is no relation in 'compression' to low speed cylinder pressure? I feel like you have to decide that the intake valve closing event does affect compression, or, it does not. Unfortunately, we can't have both.

Overlap scavenging happens primarily when the piston is hovering around TDC, and draws IN the intake charge using the temperature differential, and exhaust waves from the open exhaust valve in overlap. If we let the intake open early during the end of the exhaust stroke (the only other time those two open valve events can coincide) We do get a similar effect from overlap as the exhaust pulse pulls 'cool' intake charge 'across' the chamber at that point as well. Too early, we just blow exhaust charge back up into the intake manifold and suck it back in again as soon as the exhaust valve closes.

That sounds like something the EPA would recommend... Both usually happen.

But we are talking about the intake closing here. Not opening.

I'm not sure I follow the 'exhaust debris' explanation in relation to intake closing? I may have just misunderstand you.

Once again, I am not talking about 'static' compression here. I am talking about dynamic compression which is more or less, a loose assessment of average/peak cylinder pressures.

The cylinder isn't going to start building compression (cylinder pressure) until the intake valve closes. Period.

Ever needed a valve job? No squeeze = No dynamic compression. It's not like someone went into the engine and tripled your head gasket thickness, or shortened your rods. That's mechanical compression ratio. That never changed. We just have a valve that won't close.

Correctly, the intake closing point would be as the piston is coming up from BDC. More low speed cylinder pressure (closer to to ABDC) intake closing makes torque, and detonation (preignition) in the right (non-ideal) conditions. A later, (farther from ABDC) intake closing makes horsepower via greater piston speed as a result of a shorter 'mean' cylinder pressure duration. Greater piston speed will increase velocity of the incoming intake charge. Given a constant N/A intake tract, charge velocity is relative to piston speed.

I say this as you CAN have the same cylinder pressure with a later IVC assuming you have the velocity to cram the charge inducing the ever-exponentially shortening window between BDC, 90 degree crank angle, and/or intake close. Boost works. Other than that, you are relying on revs (exponential gained piston speed as a result of inertia from the prior cycle) to build 'power' Better hope you have an exceptional valve-train to capitalize on that pressure that is built by the 8000rpm intake velocities needed, especially if you have a heavy chassis.

So, yes. Even N/A you probably CAN theoretically make the same average cylinder pressure with two different intake closing points, at two different intake durations. But, your pressure peak would move. The pressure peak, combined with other factors, (usually low speed cylinder pressure) is typically what causes detonation. This is why we 'degree' a cam. To fine tune that relationship.

I believe this is also why V-Tec was invented. So you can have a low, and a high- piston speed cam shaft. V-Tec does't 'kick-in' it 'kicks-out' the low speed cylinder pressure event to help carry gained piston speed.(As you stated) Putting a cam with a large IVC number in a single cam V8 (81 degrees for example..) Is like putting two 'secondary' top-end cam's in your V-Tec Honda. No Bueno. That's for Bonneville. Not Normal-ville.

It's finding the balance of low speed cylinder pressure and mean/peak piston speeds that give you the broadest usable 'power band' during component operation.

We old school N/A guys don't just adjust cylinder pressure events with a knob or a ro-box and a computerator.

We use camshafts.

 

[dusterbing the peace]

I think the numbers are pretty close for advertised as they have shown, just in the wrong slots, and depending on advance. Go plug the numbers in here:
Camshaft Calculator & Valve Overlap Profiles | MGI SpeedWare

 

[thrustbucket]

Yes, they have the info incorrect. Go plug the numbers in here:
Camshaft Calculator & Valve Overlap Profiles | MGI SpeedWare

This calculator is telling me that intake close occurs at 37 degrees ABDC @ .050" That is my calculation as well. That is equivalent to 64 degrees ABDC @ .006". COMP CAMS states 81 degrees at ABDC @ .006" on the card.

This calculator is also telling me that COMP is incorrect.

What numbers did you plug in? Can you screen shot?
Whatever you did got what COMP was getting.

Could be the answer?

This was truly helpful.

Thanks!

 

[dusterbing the peace]

This calculator is telling me that intake close occurs at 37 degrees ABDC @ .050" That is my calculation as well. That is equivalent to 64 degrees ABDC @ .006". COMP CAMS states 81 degrees at ABDC @ .006" on the card.

This calculator is also telling me that COMP is incorrect.

What numbers did you plug in? Can you screen shot?
Whatever you did got what COMP was getting.

Could be the answer?

This was truly helpful. View attachment 1715975448

Thanks!

plug the advertised in as 050, and leave the optional advertised fields blank. Then, like I said I think the numbers are close, just in the wrong spots on the cam card.

 

[thrustbucket]

plug the advertised in as 050, and leave the optional advertised fields blank. Then, like I said I think the numbers are close, just in the wrong spots on the cam card.

My bad! I thought you had said COMP was correct, and you had arrived at their numbers... Thanks so much for posting that calc. It has made me feel slightly less insane! I bet it was either a calc field error, or maybe somebody forgot their glasses and thought 37 was 81.. haha!

I really appreciate you humoring my last post and plugging those numbers!

 

[dusterbing the peace]

My bad. thought you had said that they had it correct, and had got their numbers. It looks like the mystery is solved. B

My bad! I thought you had said COMP was correct, and you had arrived at their numbers... Thanks so much for posting that calc. It has made me feel slightly less insane! I bet it was either a calc field error, or maybe somebody forgot their glasses and thought 37 was 81.. haha!

I really appreciate you humoring my last post and plugging those numbers!

happy to help when I can, as this site and its members have helped me quite a bit with numerous things. Stumbled on the MGI calculator when searching “overlap calculator” a while back.

 

[thrustbucket]

happy to help when I can, as this site and its members have helped me quite a bit with numerous things. Stumbled on the MGI calculator when searching “overlap calculator” a while back.

You mentioning overlap got me thinking. I find interesting the -1 degree overlap result.. @ .050" I can see it is correct. However, overlap at .006" is 58 degrees!
It can be seen on the graph.

-1 degrees is really deceiving. This cam has significant overlap. This is a good example of why specs are important, and also why we should check them at multiple lift points to get the whole story. I suspect its also why different cam companies list specs at different lift points .006" .009", .050" etc.. They don't want us comparing other companies cams to theirs on an equal basis. They know it gets so confusing that most people will just stick with one manufacture because they understand how the specs worked in their last build.

If you look at the graph you have hella overlap from .006" lift to maybe about .040". This must be what creates the "thump" aspect..

 

[dusterbing the peace]

This cam has significant overlap. This must be what creates the "thump" aspect..

This cam has way more overlap than the factory cam for sure. Depending on which factory hemi cam we're talkin about, they can be -30* or even more negative. It should sound more lumpy at idle for sure as you have noted that the overlap will cause more intake tract reversion.

Here are some rough overlap numbers for real world comparison:
The 5.7 hemi: -33* (depending on vehicle).
The 6.4 hemi: -22* (charger/challenger).
The 6.2 house cat: -13*.
These are at idle before VVT has kicked in of course...but so you have an idea since you can hear those idling while out driving and such.
The Hughes whiplash: +15*.
The COMP Thumpr for older small blocks: +19*.

 

[thrustbucket]

This cam has way more overlap than the factory cam for sure. Depending on which factory hemi cam we're talkin about, they can be -30* or even more negative. It should sound more lumpy at idle for sure as you have noted that the overlap will cause more intake tract reversion.

Here are some rough overlap numbers for real world comparison:
The 5.7 hemi: -33* (depending on vehicle).
The 6.4 hemi: -22* (charger/challenger).
The 6.2 house cat: -13*.
These are at idle before VVT has kicked in of course...but so you have an idea since you can hear those idling while out driving and such.
The Hughes whiplash: +15*.
The COMP Thumpr for older small blocks: +19*.

This is great and useful info!

 

[racerjoe]

I think the numbers are pretty close for advertised as they have shown, just in the wrong slots, and depending on advance. Go plug the numbers in here:
Camshaft Calculator & Valve Overlap Profiles | MGI SpeedWare

Sorry to drag up such an old post, but I've been trying to decide what cam to use on a 5.7 that will ultimately have a supercharger on it. This tool helped visualize the differences. So many manufactures have specs that are close to one another. I feel like they are changing specs from the other guy by a degree or so and adjusting lift just to say its "their" design.