Technical 340 in a 1937 Dodge truck

I put a new cam in my 340 and i believe i should have more vacuum. I have aluminum heads 750 dp rejetted. I have 9.5 inches and not sure how that will work with my power brakes When i bought it crane said 12 to 15 would be very possible but i am looking for more tuning ideas. Lets here some ideas.

Thanks Tony

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9.5 " @ what rpm ?

 

If it is really best at 3200 to 6800 rpm, I'd be shocked if it had even 9.5 inches of mercury vacuum at 900 to 1000 rpm. That is a fairly aggressive camshaft, at least compared to the pretty big stock 340 cam.

 

I would think that 12" at idle would be a ton and you need 14 for your brakes. Get yourself a vac cannister.

 

9.5 is at 750 rpm. As far as brakes go I would put manual in it if there is a problem.
Tony

 

I didn’t see the lobe separation on your cam card, but 232/240 at .050 is extremely healthy. I’m surprised it idles at 750 and makes 9.5. Is this in front of a Torqueflite or stick? If it’s automatic, you will need a 3500 RPM stall speed on your converter. Personally, I would not worry too much about power brakes, or like Beaner mentioned, a vacuum can.


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How much initial timing? That will affect idle vacuum in a big way.

So will late valve timing. Did you degree the cam in?

 

It has a dual point distributor set at 27 degrees and has a 5 spd behind it. It seems to have good manners but i just broke it in yesterday and tried to tune it today. I have learned alot about carbs latley but i know i have a long way to go. It has not been driven yet but pretty good throttle response.
Tony

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Is that total timing or the amount of advance? What's your intial timing?
What's your total advance?
Did you verify tdc with a piston stop vs. The balancer tdc?
How is the balancer marked for total timing?

 

As an example , the cam in my 388" SBC is 232-234 @ 050 108°. Lobe separation , .490 lift . timing is 14° initial, 20°centrifugal all in by 2500 . I have the idle set at 900 , idle vacuum is 13" . this is a manual trans car .

 

Andy, why do you say that it is extremely healthy? If someone understands this stuff better than me, lets break it down for everyone. I got the basics but I know there are more knowledgeable people than me. I spent a lot of time talking to cam manufactures and explaining what I wanted. It is my daily driver and it has to have good driving manners. I have been told different rpms but they were within 300 rpms of it

I did not degree the cam or use a piston stop. I set it with the balancer , and put it at 10 degrees. There is no vacuum advance on my distributor, so I then set my timing light to 27 and adjusted from there. I have been doing it this way and it works, but I believe I could get a few more ponies out of it.

Tony

 

I am definitely no cam guru, but the problem with installing a camshaft without degreeing it, is there is no way to know that it is installed correctly. If the cam has 2° of advance ground in, the dots on the timing chain set are lined up, in reality it could still be several degrees "late" or retarded.

Manufacturing tolerances in crank and cam keyways, the timing sets, and the cam itself will tend to cancel each other out, but they can sometimes stack up against you.

Iskenderian has a pretty good page on how to degree a cam, and some easy checks to make to at least determine what side of the fence it is at right now. Make sure TDC on the balancer is verified and marked and go from there.

 

Thanks, there is a lot of good info there. I do have a adjustable timing chain but was told by crane to set it up at like a stock one.

https://www.iskycams.com/cam-degreeing.html

Tony

 

Yes, but the point being, there is no way to tell where that cam is actually installed, without degreeing it. Or at least check for relative position.

Verify with a piston stop tool that the "0" mark on the balancer is accurate.

Then see where the "0" mark lays relative to the pointer when the the #1 cylinder valves are exactly at overlap. Maybe that camshaft isn't going to allow for more vacuum than you have right now, but without any idea where the cam is installed you're just guessin'. Measure, don't guess!

 

Verfied that the zero on the balancer is at top dead center. Then made sure that the distributor pointed at cylinder 1 on cap. Started truck let it warm up checked and timing is 12 degree at 900 rpm. It tops out at 28 degrees around 3000 rpm. Last year it was happiest around 35 all in but it has a new cam this year.

Tony

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Part of it just clicked in my head. The more valve overlap the less vacuum i can get because both valves are open at the same time. Please correct me if i am looking at it wrong.

Tony

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I don't know nothin' bout those fancy pants cams.

Has to do with symmetrical lobes or somesuch.

Anyhoo. The first thing (generally) before any tuning or timing van be done is to check or verify the "0" mark on the balancer. Using a piston stop tool. They are inexpensive or can make one. Remove #1 spark plug. Just because the "0" mark is lined up with pointer doesn't necessarily mean it is even close to TDC. See this all the time with old ratty balancers, and sometimes they are defective. It pays to check. One and done.

Install the piston stop. By hand, rotate the engine around clockwise till the #1 piston stops. Mark the balancer. Rotate the engine counterclockwise till the piston stops. Mark the balancer again. Halfway between the two marks is the true TDC regardless of where the "0" mark and pointer lay on the balancer. Of course it "should" coincide exactly, but this is why it's important to check.

Remove the piston stop tool. Don't forget.

Now you can set the initial timing, and check the total timing, with confidence that the #s you are seeing are correct and accurate.

As far as the camshaft is concerned, at least a stock camshaft without any jiggery-pokery, when the #1 piston valves are exactly at overlap (excepting Buick Nailheads) the "0" mark on the balancer will indicate the rough position of the camshaft, whether it is retarded, "straight up", or advanced. It is only a rough measurement, but it's more than enough info to tell if this is a factor in low manifold vacuum.

It doesn't necessarily matter where the camshaft was installed, whether the dots were lined up or not, or advanced, because of manufacturing tolerances or a defective grind or whatever. It should be, but this is why it's important to check.

Not saying this is your problem, I'm just suggesting if you check these things it puts you farther along the troubleshooting road by process of elimination. Find out what the problem ain't, and eventually the only thing left will be what the problem is. Nome sayin'?

 

Why is your mech advance limited to 14° ? Is this a Mopar thing ?

 

Tony, if your full ignition timing is only 28 degrees, you are leaving a lot of performance on the table. The full timing should be 32-35 degree range. I believe that is an ignition problem, not a cam problem unless the cam degree is way off.

Yes, you are correct. The valve overlap will have an impact on idle vacuum. The big overlap helps get more fresh fuel into the cylinders at high rpm. Its also part of what gives the motor that radical idle sound. With the high lift, the long duration, and the big valve overlap, with the proper total ignition timing, it should run pretty strong. Assuming you have the compression and head port flow this cam will want to have. Gene

 

I can adjust it further, but that is where it was at when I checked it. I am not sure if there should be more. I took it for a ride last night and it went good. I had plenty of brakes, I just have to fine tune a few things. I will get my total timing set today play with the carb read the plugs and go from there. As usual I have learned a lot during this install.

Gene I had it at 35 degrees last year and it liked it so that is my goal but I want to start somewhere.

Thanks Tony

 

I have it set now around 20 degrees at idle, and 35 at 3000 rpms. I got about 11 inches of vacuum and it has good throttle response. I have spun it to 7200 rpms. It seems really happy, but every now and them when I shut it down it does a little dieseling. I will keep trying to fine tune this but if anyone has more ideas I will take them, remember I have a Mallory dual point distributor. I might take it out and check all of it to make sure it is working properly.

I have been talking to racerx and he keeps pointing me to a 6al box and msd pro billet distributor. I have looked into it, and found good deals on them. My problem is I am stuck on the point setup just because that is what I want, but I might be leaning towards this new msd stuff, if I cant get this solved.

Thanks Tony

 

If you can lower your idle rpm or remove some initial timing and increase the centrifugal timing that could help resolve the dieseling ...

 

Yes too much initial timing. Back off on initial timing and replace it with advance to arrive at 35 degrees.

 

I got to go to Rockfalls raceway to run my truck with a few new parts. I put in a bigger cam and a MSD ignition. my best time was 12.2 @ 111mph. Is was spinning out off the line but pulling hard. I know that there is an 11 in there. For some reason it would start missing at 5800 rpms, and I couldn't get that to go away, and still have the problem. My cam builds power til 6800, so I am leaving a lot of power behind, and have no clue what is going on. I am stuck.

I kept playing with the timing, but couldn't get it to stop dieseling. I have gone from 8 degrees initial to 17 and it still does it. All in was from 34 to 43 and it still does it. I then went to the carb to get the idle down and still does it. I have idle from 650 to 1100 rpms. I believe its in the carb I just don't know where go from here. I believe the throttle blades are open just a bit but I don't know why. Sometimes it don't idle down unless I tap the throttle, then it goes down to 750rpms, where I have it set at. I have a lot of learning/tuning before the Out of site drags.

Tony

 

First off 43 degrees is way too much. Set initial at 10-12. Then get the total around 36 and leave it alone.
With the new cam are the springs matched to it?
Just because the cam card says power to 6800 doesn't mean you engine will rev up to that.
The only tach I have ever seen that was reliable was a cable drive. They are accurate. Forget about the top end of the track for now. Work on the launch to cut your et.
As mom used to say first things first.

 

Mcmopar,
Listen to Brian,,,he’s pretty sharp on this stuff in your thread .

And Brian ,
I wonder why more don’t use cable drive tachs,,,,,the added trouble of cable routing ,,,,or the expense of the tach ?
Although,,,it always seemed that a quality electronic tach was just as high as a mechanical .
I guess the added expense of a cable drive capable distributor is one factor ?

I’m old school I guess,,,,,mechanical for me .
I love them,,,I have three of them .

Tommy

 

Your idle problem sounds like a carburetor and timing combination. If the mechanical advance is not coming back to zero at idle, the engine will not idle down. I suspect the springs in the distributor are a little loose keeping it from pulling back in. It also sounds like you need the distributor spun on a machine to get the advance right. 20 degrees @ idle with 35 @ 3000 is not ideal, you need 8-10 @ idle with 34-36 @ 3000.

On to the carburetor, if the throttle stop is screwed in a lot to get the correct idle rpm, you are likely into the run circuit and out of the idle circuit. You may need to drill some air bleeds in the primary throttle blades, this will let you adjust the throttle stop correctly so the engine will idle on the idle circuit of the carburetor. Get the timing correct before working on the carburetor, but check the throttle blade location so we have a reverence point to help further.

What carburetor do you have?
What model distributor do you have?

When you blip the throttle, the distributor snaps back, thats why it idles down.

 

Hydroboost

 

My timing was left at 12 and 35 all in at 2100. When I have these problems, I usually end up talking to Brian or some other people til I get them right. This is why I say its time for learning.

I am running a 750 double pumper and a msd pro billet distributor.

I believe that dieseling comes from timing, but after a lot or reading and research. i am leaning towards other things. There is a lot of info out there debating either side of this. I will be starting at the beginning and double check things. I know I will get it just a matter of time and questions.

Tony

 

I would lower the idle rpm to remove the dieseling, leave the timing alone, 20 initial should be perfect for this set up...

Make sure the brake pedal is on the 6:1 manual ratio and not the power brake ratio.. It would be the hole closest to the top..