Manifold Vacuum Project

malex

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Figured that I would share something that I’ve been working on for some time on the Hemi with the group. When I built the hemi it initially had a Barton single plane intake and dominator 1250. Worked great and laid down nice numbers. Then along came a Mopar Performance Cross Ram that I grabbed. I spent a lot of time inside of it along with runner work and port matching to the Victor Heads. Holley 770 Hemi carbs went on top and the package was bolted onto the Hemi.

The cross ram was a real nice addition to the hemi and it was exactly what was missing. As far as performance, I’m confident that no power was lost but what did come along was a nasty surge at idle. I worked on those carbs trying different mixture settings, jet changes etc, but the surge was always there at idle. (1500). The surge was kind of cool, very radical sounding but I was determined to beat it. Timing is locked at 32 degrees, which was what the Hemi liked on the dyno. At idle the Hemi has 9” of ported vacuum. So this is what I did. Big cam, about .735 lift and lots of duration.

I purchased another distributor for the hemi. This one has mechanical advance as well as vacuum advance. I locked the mechanical advance out. I removed the vacuum pot and replaced it with an old echlin pot that I’ve had around here forever. This old soft vacuum pot starts pulling in at 3” and all in full by 8”. It was not a direct bolt on but with a little bit of work it fit perfect.

I replaced the locked distributor with the new vacuum distributor. The cross ram has a plug between the carbs at the rear that I ran a vacuum line from. Fired the hemi up and set the timing with no advance at 32 degrees and locked it down. Then with the hemi running I plugged the vacuum line onto the pot. As expected, the idle picked up and smoothened out. A recheck with the timing light. 52 degrees, idling at 1500. With any acceleration the timing fell back to 32 degrees. Some air bleed adjustments and the surge is now pretty much completely gone. Hemi idles nice, instant acceleration no run on when shutting off and the temp gauge in a half hour of running never got to 160. That was real nice to see, as I always had heat creep before. I’ve yet to go for a drive but so far, this is looking to be exactly what this hemi needed for street operation. I’ve got a couple of videos of idling before and after distributor swap that I’ll add to this thread, once I get it onto You Tube, then I can add it on here. Anyways, that is what I’ve been up to.

Video, hemi surge with locked timing

Video, hemi idle with manifold vacuum
 
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wagonman

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Sounds like it like 52 at idle, 9 inch at idle ported, you must have the throttle blades cracked open a bit. If A/T how is it in gear? Stick it dont matter. Sounds Bitchen either way. Congrats.
 

Geoff 2

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Malex,
Congrats on what you did. Smart move. None of the results surprise me, being doing the same for decades.
 

malex

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Sounds like it like 52 at idle, 9 inch at idle ported, you must have the throttle blades cracked open a bit. If A/T how is it in gear? Stick it dont matter. Sounds Bitchen either way. Congrats.
Yep, Manifold vacuum only and yes, it’s 52 degrees. The pot pulls 20 degrees. The 770’s have no ported vacuum provisions.
 

biomedtechguy

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Figured that I would share something that I’ve been working on for some time on the Hemi with the group. When I built the hemi it initially had a Barton single plane intake and dominator 1250. Worked great and laid down nice numbers. Then along came a Mopar Performance Cross Ram that I grabbed. I spent a lot of time inside of it along with runner work and port matching to the Victor Heads. Holley 770 Hemi carbs went on top and the package was bolted onto the Hemi.

The cross ram was a real nice addition to the hemi and it was exactly what was missing. As far as performance, I’m confident that no power was lost but what did come along was a nasty surge at idle. I worked on those carbs trying different mixture settings, jet changes etc, but the surge was always there at idle. (1500). The surge was kind of cool, very radical sounding but I was determined to beat it. Timing is locked at 32 degrees, which was what the Hemi liked on the dyno. At idle the Hemi has 9” of ported vacuum. So this is what I did. Big cam, about .735 lift and lots of duration.

I purchased another distributor for the hemi. This one has mechanical advance as well as vacuum advance. I locked the mechanical advance out. I removed the vacuum pot and replaced it with an old echlin pot that I’ve had around here forever. This old soft vacuum pot starts pulling in at 3” and all in full by 8”. It was not a direct bolt on but with a little bit of work it fit perfect.

I replaced the locked distributor with the new vacuum distributor. The cross ram has a plug between the carbs at the rear that I ran a vacuum line from. Fired the hemi up and set the timing with no advance at 32 degrees and locked it down. Then with the hemi running I plugged the vacuum line onto the pot. As expected, the idle picked up and smoothened out. A recheck with the timing light. 52 degrees, idling at 1500. With any acceleration the timing fell back to 32 degrees. Some air bleed adjustments and the surge is now pretty much completely gone. Hemi idles nice, instant acceleration no run on when shutting off and the temp gauge in a half hour of running never got to 160. That was real nice to see, as I always had heat creep before. I’ve yet to go for a drive but so far, this is looking to be exactly what this hemi needed for street operation. I’ve got a couple of videos of idling before and after distributor swap that I’ll add to this thread, once I get it onto You Tube, then I can add it on here. Anyways, that is what I’ve been up to.

Video, hemi surge with locked timing

Video, hemi idle with manifold vacuum

That is one of the benefits I have/got with my FBO ignition system from Don at FourSecondsFlat.
It's been over 4 years, but I was having a number of problems with idling my 6bbl with the 292/509 cam, etc around 12 in of vacuum.
The extra advance at idle, and switching to manifold vacuum (the Promax center carb metering block doesn't have ported vacuum) with the mechanical limiter to keep the advance from going too high really helped all around.
My next ignition system is the Progression Ignition system, so I will have even more control, and the idle will be compensated w/more advance when the air conditioning compressor kicks on and the RPMs drop, to return the idle RPMs back to "regular" idle speed, which I like to keep as low as practical.
 

Budnicks

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Sounds & looks great too :thumbsup: clever cleaver

almost had the old Roots Blower Surge 'sound of old'
 

malex

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Thats a fantastic fix. What gave you the idea to do that?
I’ve been working at fine tuning this Dart off and on since I built it. Surging and the heat creep were the main issues and timing advance would benefit both issues. I’m happy how the coolant temperature responded to the additional advance. I’m still monkeying with the idle jets a little. And there is still some surge that I’m able to dig up at will. And there’s no one that isn’t looking to see what the hell’s coming.
Another problem that I came upon is that there is a momentary bog as the secondaries open, so I’ve got to see what diaphragm springs are in the pots. I will say that testing the secondaries is quite a ride. I’m going to have to bolt the slicks on because cracking the secondaries with the street tires on is instantly unpredictable. In any gear.
Last weekend I had the Dart out to a outdoor show over in Moose Jaw. A small city 45 minutes to the west. There’s just something about Moose Jaw..
A64420C0-51B1-41B5-B77F-9113B3E8340E.jpeg
 

malex

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:thumbsup:

:lowdown: Stroker HEMI+Lightest Mopar Body=
A lesson LEGEND in power to weight ratio!

MORE MOOSE JAW!!:bananadance::bananaweed::rofl:
Moose Jaw is a great little City. Laid back car town. A small downtown district with underground Tunnels linked to the mafia, going back close to a century now, train tracks running bee-line to Chicago. Excellent hockey town, took in lots of games back in the 80’s/90’s at the Crushed Can! Great hockey, major junior. Players like Theo Fleury, Mike Kean, Ken Baumgartner, Lyle Odelein, Reed Low to name a few, same Moose Jaw team, same time. Crazy rivalries too. Especially when the likes of Todd Fedoruk, Scott Daniels, Colton Orr, Barrett Jackman and others who were dressing for my home town team would roll in. Old time hockey at it’s best. Even Puckhead the Moose Jaw Mascot got roughed up one game. Lol. True! Yes there’s something about Moose Jaw!
785AF119-16D7-4072-99D9-3ED9E13E03A4.jpeg
 

beanhead

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Ahh Moosejaw! One of my nephews played part of his semi-pro baseball there. I've been hooked on dill pickle chips ever since (because no one sells ketchup chips down here!!)
 

2blader

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Yup just did a small job doing that to a Barton Cross ram that only had two breathers and no PCV system. Bought 3 vacuum ports that would be used for a base plate and drilled out the 1/2 spacer under the 770’s. Ran the lines to a plastic Y connector and then to a M/E Wagner pcv valve. Just did as part of fixing a friggin annoying rear main seal leak. That is done (at idle for 20 minutes) but we will see how it is when I drive it. Then onto the O2’s and afr guage.
 

biomedtechguy

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M/E Wagner pcv valve. Just did as part of fixing a friggin annoying rear main seal leak.
The BEST PCV valve! I don't know how familiar you are with them, but taking the time to "tune" it is worth it.
By imparting vacuum through it, that would help prevent leaks too I'd believe.
 

2blader

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Yup, that is what I read. Besides it took me many tries before I got it as the Hemi is in the car. Luckily climate control and a lift makes it tolerable.
 
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