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
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
Last edited: