Thumpr cam and valve spring question 340 dart

[Raker43]

Hello All,

By way of introduction, my name is Cary and I Just picked up a 1970 Dodge Dart Swinger 340, 4 speed! She's a runner but needs the new heads that came with it due to three burnt exhaust valves. Have it torn down and heads being cleaned up a bit and new valves and springs being installed on NOS J heads cast 3769974 from around 1975. 2.02" intake and 1.60" exhaust. Received a Comp Cam today, Thumpr CL-20-600-4. 268/276 duration, .486/473 lift. Running the stock Carter AVS 625 Carb and stock intake manifold and cast iron exhaust. I'm throwing in some pics because we all love Pic's right!? I will try not to be a mooch and will always try to throw down some pics!

so I have a question and I hope the pics serve as some bait:

I have the Stock Mopar HP springs(part #P4120249) lift from .430-.500. I was wondering if any you out there have had experience with a similar set-up, i.e.,springs and cam with similar characteristics. I'm worried about the lift being too much and hitting a valve. Comp/Summit recommends their valve spring kit(995-16)but I would prefer to use the springs that I have if it will work.. Is this cam too aggressive or ok for the 340? Will it be hard on the old bottome end? I'm New to the Mopar game so bear with me.

This baby kind of fell in my lap and I'm loving every minute of it!

Note: so I'm on my iPad and it's not letting me post pics right now. I have a similar thread with pics on the welcome wagon thread if you want to see the old girl. Sorry.

Cary

 

[69_340_GTS]

I wouldn't worry about a thing. That isn't a very radical cam.

Also, if you are worried about coil bind, that's easily verified when you put it together.

 

[Raker43]

I wouldn't worry about a thing. That isn't a very radical cam.

Also, if you are worried about coil bind, that's easily verified when you put it together.

Thanks! I'm starting to figure that out slowly. Built a few VW motors and wrenched on some GM products but this foray into Mopar territory is new to me. I'm wanting to make sure I do it right.

Cary

 

[Raker43]

Plenty of room in the engine compartment to work. Just like I remember doing with my old man! Kinda brought a tear to my eye...

 

[69_340_GTS]

Plenty of room in the engine compartment to work. Just like I remember doing with my old man! Kinda brought a tear to my eye...

I've done that a few times, and I'm a little older, and a lot bigger than that little guy!

Strange that nobody else has chimed in on this. I'm far from an "expert" !

 

[BlueDream]

I think you'll be fine. I have the same cam in my 340 and I don't have 995 springs. I think part of the reason Comp recommends those springs is so they can sell em to people even if they don't really need them. If you want a cam that turns heads I think you'll be pleased with the Thumpr. It's definitely got a mean idle. I broke the engine in a few hours ago and haven't had the chance to drive it yet, but it sure sounds angry.
[ame="https://www.youtube.com/watch?v=JQPWXD7mOdc"]https://www.youtube.com/watch?v=JQPWXD7mOdc[/ame]

 

[perfacar]

Hello All,

By way of introduction, my name is Cary and I Just picked up a 1970 Dodge Dart Swinger 340, 4 speed! She's a runner but needs the new heads that came with it due to three burnt exhaust valves. Have it torn down and heads being cleaned up a bit and new valves and springs being installed on NOS J heads cast 3769974 from around 1975. 2.02" intake and 1.60" exhaust. Received a Comp Cam today, Thumpr CL-20-600-4. 268/276 duration, .486/473 lift. Running the stock Carter AVS 625 Carb and stock intake manifold and cast iron exhaust. I'm throwing in some pics because we all love Pic's right!? I will try not to be a mooch and will always try to throw down some pics!

so I have a question and I hope the pics serve as some bait:

I have the Stock Mopar HP springs(part #P4120249) lift from .430-.500. I was wondering if any you out there have had experience with a similar set-up, i.e.,springs and cam with similar characteristics. I'm worried about the lift being too much and hitting a valve. Comp/Summit recommends their valve spring kit(995-16)but I would prefer to use the springs that I have if it will work.. Is this cam too aggressive or ok for the 340? Will it be hard on the old bottome end? I'm New to the Mopar game so bear with me.

This baby kind of fell in my lap and I'm loving every minute of it!

Note: so I'm on my iPad and it's not letting me post pics right now. I have a similar thread with pics on the welcome wagon thread if you want to see the old girl. Sorry.

Cary

hi, you can check where the coil bind is on spring. installed height, minus lift. if its too close change springs. the mopar springs have thick coils. comp cams has springs that fit.
stock retainers will give you short heights, I use comp retainers, gains room. then can use a single spring. the 942 or 926 would be good choice.

 

[RustyRatRod]

Like I told you in your other post, you've quoted the wrong specs for that thumpr cam. I recommend using the spring the cam calls for. It's a dual spring. They are not that expensive. It's not just the lift you have to take into consideration, it's the ramp rate of the cam as well. Those springs are designed to work with that cam. The MP springs you have are not.

 

[Raker43]

Like I told you in your other post, you've quoted the wrong specs for that thumpr cam. I recommend using the spring the cam calls for. It's a dual spring. They are not that expensive. It's not just the lift you have to take into consideration, it's the ramp rate of the cam as well. Those springs are designed to work with that cam. The MP springs you have are not.

You are correct on the duration numbers I originally posted. I think I was looking at the stock cam specs. and mixed the numbers up. Anyway, there are two different numbers regarding duration for that cam.

Advertised duration of 279/296
And lift at .050" of 227/241

I think the actual .050" numbers are the ones to go by if I'm not mistaken.
I am interested in your opinion as you are the only person who is counter to the idea of using the Stock Mopar HP springs. So you think the springs don't have enough spring rate to work properly. Is that what your saying? My builder is going to call me back shortly this morning and I will revisit the issue with him based on what you are saying. Heads are supposed to come out of the shop tomorrow! WhOop whoop!

I appreciate your input. Thanks again!

Cary

 

[Raker43]

I think you'll be fine. I have the same cam in my 340 and I don't have 995 springs. I think part of the reason Comp recommends those springs is so they can sell em to people even if they don't really need them. If you want a cam that turns heads I think you'll be pleased with the Thumpr. It's definitely got a mean idle. I broke the engine in a few hours ago and haven't had the chance to drive it yet, but it sure sounds angry.
https://www.youtube.com/watch?v=JQPWXD7mOdc

That's the sound I'm looking for! Is that going through any pipe or straight out the manifolds?

Thanks for the sweet video. I'll post one when I get it running!

Cary

 

[BlueDream]

That's the sound I'm looking for! Is that going through any pipe or straight out the manifolds?

Thanks for the sweet video. I'll post one when I get it running!

Cary

It's through open headers. 1 5/8" primaries to 3" collectors. I still need to bolt my exhaust back on... last time I took the car out with open headers there were four people that called the cops. I look forward to hearing yours run :glasses7:

 

[Captainkirk]

Sounds much like my old cam.....450/.475, 298/308...232/.242@ .050. I ran stock shimmed springs at the recommendation of the engine shop, but that was a long time ago....

 

[RustyRatRod]

You are correct on the duration numbers I originally posted. I think I was looking at the stock cam specs. and mixed the numbers up. Anyway, there are two different numbers regarding duration for that cam.

Advertised duration of 279/296
And lift at .050" of 227/241

I think the actual numbers are the ones to go by if I'm not mistaken.
I am interested in your opinion as you are the only person who is counter to the idea of using the Stock Mopar HP springs. So you think the springs don't have enough spring rate to work properly. Is that what your saying? My builder is going to call me back shortly this morning and I will revisit the issue with him based on what you are saying. Heads are supposed to come out of the shop tomorrow! WhOop whoop!

I appreciate your input. Thanks again!


Cary

Actually, with cams other than Mopar Performance, the duration @ .050" is the standard by which to compare.

 

[RustyRatRod]

That's the sound I'm looking for! Is that going through any pipe or straight out the manifolds?

Thanks for the sweet video. I'll post one when I get it running!

Cary

Be advised, if you cam for sound that your power results may not be what you hope for. It's a delicate balance between a cam that provides a good broad torque band on the street and one that sounds good but does nothing between 2000-3000 which is where 90% of your driving will be, so be careful making your choice.

 

[Captainkirk]

Be advised, if you cam for sound that your power results may not be what you hope for. It's a delicate balance between a cam that provides a good broad torque band on the street and one that sounds good but does nothing between 2000-3000 which is where 90% of your driving will be, so be careful making your choice.

^^^^^^^^^^
This is true. Also, your vacuum could be in the toilet at idle. With my cam I was running around 9 at idle...made the power brakes kinda suck at lower speeds.

 

[Raker43]

Thought about the vacuum issue. 4-speed. No power brakes. No A/C. Electronic choke. Am I missing anything that needs idle vac?

 

[RustyRatRod]

Thought about the vacuum issue. 4-speed. No power brakes. No A/C. Electronic choke. Am I missing anything that needs idle vac?

No, you're not missing anything. The only thing that will suffer from a cam too large is idle quality and loss of some bottom end torque. The 4 speed may help make up for some of that.

 

[Marcohotrod]

the 995-16 comp springs are listed as 115 pounds at 1.700" and 320 at 1.200" . any springs with the same specs will work fine. stock 340 installed height is around 1.688", and the 4120249 spring is good to .500" lift plus a safety margin before coil bind. are U assembling the heads? with a .015" shim the 4120249 springs and stock retainers will be fine, but so we can sleep at night things should be measured. this cam will not be hard on the old bottom end, and V to P will be fine

 

[Raker43]

the 995-16 comp springs are listed as 115 pounds at 1.700" and 320 at 1.200" . any springs with the same specs will work fine. stock 340 installed height is around 1.688", and the 4120249 spring is good to .500" lift plus a safety margin before coil bind. are U assembling the heads? with a .015" shim the 4120249 springs and stock retainers will be fine, but so we can sleep at night things should be measured. this cam will not be hard on the old bottom end, and V to P will be fine

I'm not doing the assembly. My machine shop is based on the new cam specs We spoke about it today and he's going to match springs and cam. It's a no brainier now as he has any of the necessary parts for the valve train and will build accordingly.

Thanks for the specs on those springs!

Cary

 

[RustyRatRod]

the 995-16 comp springs are listed as 115 pounds at 1.700" and 320 at 1.200" . any springs with the same specs will work fine. stock 340 installed height is around 1.688", and the 4120249 spring is good to .500" lift plus a safety margin before coil bind. are U assembling the heads? with a .015" shim the 4120249 springs and stock retainers will be fine, but so we can sleep at night things should be measured. this cam will not be hard on the old bottom end, and V to P will be fine

Please don't tell him that. It's not true. The springs have the RATE designed to work with that cam. That's why they are the recommended spring. There are several things to take into consideration besides seat height and open pressure. Those are only two parts of the equation. If you want to cut corners on your own stuff, fine. But are you ready to take responsibility when your wrong information costs him a camshaft failure? The best advice here is use the correct spring.

 

[Marcohotrod]

If a Mopar or Elgin or Crane or whatever spring has 115 at 1.700" and 320 at 1.200" it has the same 409 lbs/in rate as the 995-16 spring. The 995-16 Comp springs have been around a lot longer than the Thumpr cams, so the springs were designed before the cam. Comp designed a new cam and picked a spring of the shelf, any competent engine builder can find a spring to complete the complete eqaution, and that is NOT cutting corners. A few companies manufacture engine parts, many make boxes to sell parts. This info will NOT cost cam failure