408 Stroker vacuum fluctuation at idle

[Jimmmmmay]

Hey everyone,

I've got a blueprint engines 408 stroker in my '71. My vacuum gauge hooked up to manifold vacuum bounces erratically between 10-15" Hg at idle. It's got a Mopar Performance Purple cam in it (from blueprint's website):

Cam Type: Flat Tappet
.474 Intake .474 Exhaust
238 Intake / 238 Exhaust duration
@ .050 - 110 degree lobe separation

I'm not very well versed on camshafts so I was wondering would a cam with those specs be enough to cause a vacuum fluctuation at idle between 700-1000 RPM? It does smooth out at higher RPM. I know this can be caused by a huge number of things, but I just want to be sure I'm not chasing my tail if the vacuum fluctuation is normal for a cam of that size.

 

[Buckminster]

Jim. i can share with you my experience. I have a fairly well warmed over 340. It has the Edelbrock Victor manifold, heads and a large camshaft (I did not get spec's with the car, but it lopes pretty aggressively and peak HP is around 6,500) and a very large carb for the application. I purchased the car with this combination and have challenged myself to figure it out. So. Having said that, here is what I've done.

First of all, throttle response and idle quality have a lot to do with cam duration and timing, but also ignition timing and carb setup. I ended up having to put a lot of advance into the engine at idle. I did this because of information I read by reading this forum, many others, Edelbrock's website and watching video after video, my experience as a former racer and a lot of trial and error. I ended up setting the ignition timing by using a vacuum gage. the vacuum gage peaked at 9-10" of vacuum with 21 degrees of advance. Keep in mind, we are way out of the stock world here. This of course seems like a lot, but the car pops to life and the starter has no problem cranking. I have 10:1 compression as well, but because of the camshaft effective cranking pressure is only 180psi. fairly normal. The car starts up and idles well once warmed up. I get roughly 9-10" of vacuum at idle. it is not a steady vacuum signal, but it stays between the range listed. If the car (any car) has a burble at idle, it is because it does not hit on every power stroke. this is a byproduct of big cams. If you have trouble cranking you will need to back off a few degrees until it cranks smoothly. This of course precipitated limiting top end advance. I have 35 degrees advance at 3,500 all the way to 6,500 rpm. I made a bushing for the MSD distributor that I have to limit total timing advance to 14 degrees. You can buy them, but making them is just as easy. I've also had to drill holes in the primary throttle plates to get them to close down the throttles enough at idle to ensure that the idle circuits have control authority and the transition ports work as they are supposed to. Just to provide all the information, I do NOT have vacuum advance on the distributor, it is an MSD billet distributor with only springs and cams to change the curve.

So the short version is that I would try to add some advance at idle and see if you can improve the idle. if so, then that will lead to other changes. I hope that helps.

 

[Adam]

Hey everyone,

I've got a blueprint engines 408 stroker in my '71. My vacuum gauge hooked up to manifold vacuum bounces erratically between 10-15" Hg at idle. It's got a Mopar Performance Purple cam in it (from blueprint's website):

Cam Type: Flat Tappet
.474 Intake .474 Exhaust
238 Intake / 238 Exhaust duration
@ .050 - 110 degree lobe separation

I'm not very well versed on camshafts so I was wondering would a cam with those specs be enough to cause a vacuum fluctuation at idle between 700-1000 RPM? It does smooth out at higher RPM. I know this can be caused by a huge number of things, but I just want to be sure I'm not chasing my tail if the vacuum fluctuation is normal for a cam of that size.

That is a big cam, but not too big. A larger displacement engine (408) should be fine with that.

I would look for a vacuum leak, check your pcv is operating correctly. And ensure your valve train rockers and pushrods are properly adjusted, not bent, etc.

Steve340 recently posted a link to a good article on PCV valves. You can find the thread about adjustable Wagoner PCV valves.

 

[Rob C]

?????

View attachment 7

View attachment 70429

 

[fasjac]

That is a big cam, but not too big. A larger displacement engine (408) should be fine with that.

I would look for a vacuum leak, check your pcv is operating correctly. And ensure your valve train rockers and pushrods are properly adjusted, not bent, etc.

Steve340 recently posted a link to a good article on PCV valves. You can find the thread about adjustable Wagoner PCV valves.

I did pull the trigger yesterday and ordered the valve. I’ve been selling parts for 34 years and never seen a PCV valve for $129. But, it has my interest and I have to give it a try. Gene (M E Wagoner) says they stand behind there products. I will let you guys know how it does for mine.

 

[Jimmmmmay]

Thanks guys. I've been starting to look at the usual suspects to see what could be up with this. The one thing I wasn't familiar with was if my cam would be considered aggressive, mild, etc - and I didn't know how far I should go with trying to solve this if the vacuum fluctuation is just a characteristic of the cam I'm running. I've always been suspicious that the vacuum shouldn't be bouncing like that at idle, so expert opinions are definitely welcome.

The car starts/drives/cruises around with no issues, has really good throttle response with no stumbles or anything but my idle quality isn't that great. I have the typical thing where I can't accurately adjust the idle mixture screws because of the bouncing needle. So far I've replaced a lot of the ignition parts with the exception of the under dash wiring harness (ignition switch, coil, distributor, voltage regulator, ecu, plugs, wires), new carb, and fuel pressure regulator. I did check the PCV valve and it looked to be working correctly. So far no luck on finding a vacuum leak by spraying around either but I'm sort of a rookie at that. I haven't got into the valve train yet to check there but my next steps are probably a compression test to get more details and like Adam mentioned take a look at the rockers and pushrods. I haven't taken the valve covers off yet.

 

[moparleo]

Overlap will cause vacuum fluctuation at idle. That is the lumpity lump you get with the more aggressive cam timing. It will only be solid with a stock ,mild cam. You are just replacing a lot of parts. Unfortunately that is the way most people have learned to diagnose a problem. Replace everything until it is fixed or you run out of money.

 

[Chryco Psycho]

The Purple shafts are good but old designs for only .474 lift I would use a 112* CL not 110 ,
If you look at similar Lunati Voodoo grinds you can see the difference & I have had a ton of success with Voodoo grinds .
16 -18* initial timing will help but the problem is really with the cam , you can tune it out a bit . & iif it preforms ok you may just want to live with it as is

 

[Jimmmmmay]

Ok, that makes sense. I picked this car up late last summer and dont know much of the history or service records. I know this engine was put in back in 2012 and has about 10k miles on it, but that's the extent of the history that I know of it. I spent the winter going through a lot of the electrical system and testing or fixing stuff as I went. Honestly I can live with how it's behaving right now because it's not that bad. This is my first classic car and I just dont want to do anything that could prematurely hurt the engine at this point. Thanks again to everyone for all the help.

 

[Steve340]

When I was an apprentice a bouncing vacuum gauge at idle would indicate a cylinder or two that are not operating correctly.
Check how the plugs look and do a compression check.
Things to look for would be valve not sealing or worn cam lobe would do that.

 

[Robert Franke]

NORMAL...Let it eat!

 

[bossmike]

Here is a great video.

I’m not an expert tuner still learning but I suspect there is nothing wrong except that the initial timing and idle setting is off. First set the idle and timing to run smooth and consistent. Then check and adjust the advance from there. I ran that cam before and it is a mild cam with not a ton of overlap (more overlap = less vacuum), and it should run fine with the right tune.

 

[Robert Franke]

Great video to make a novice head explode! Ported vacuum increases as throttle and rpms increase...manifold vacuum is just that..sucking at idle..drops as throttle applyed..meaning timing will retard on throttle application then return to higher vaccum at light or crusing rpms...this was used mostly for emmissions and fuel mileage..depending on many factors!! If it starts..idles..runs out good..leave it alone! I would suggest looking at blueprint website and find that package specs and tune to that..they also have a great tech support dept. To answer questions. Remember..if it ain't broke...dont fix it! Tuning best left to experienced mechanics...p.s. I've love the afr guage