HEI upgrade options - street car 350

G72Zed

Veteran Member
Sep 8, 2015
4,405
Canada
reason im asking is because ive read the hEI kinda falls off around 4-4.5k - thats what ive "read" -not experienced.. the 6a units are found cheap so was wondering - im trying to eliminate possible issues one at a time.

Can someone explain what valve float feels like? When i mash it from say 10-15mph.. it takes off but soon after it kinda starts floating, like the trans jumped into neutral (im talking pedal to the floor / WOT) with a 3k stall.. the carb seems to run well - i get 23mpg highway and it has some nice power but the top end issue is odd..

when i put in the hotcam i matched a set of single street springs, i didnt want to go dual springs cos the hotcam isnt THAT crazy on the ramps etc - or am i wrong? i used the comp tool and my micrometer to cut down the guide bosses almost equal../QUOTE]

If you go with an MSD 6a with the HEI, eliminate the module and convert to external coil, and use the HEI mag pick up only.

Valve float or spring surge generally happens higher in the RPM ranges, (unless the springs are totally wrong) and will not RPM anymore and can feel/sound like a fast mini jakebrake or like your hitting the limiter. On the dyno, it's sounds like you put a 45 gallon drum behind the engine, and your SCFM goes down and HP brutality falls of the cliff.

What was the recommended installed height and rate of the springs, what was your installed height? does the cam open pressure recommendations match your springs at the net lift your attaining?

If your ignition and valve sprigs are up to spec, you can look at the fuel delivery side of things, an inline pressure gauge during WOT would tell you this right away.

Biker and 1980RS make some great points based on experience, same here.
 

biker

Veteran Member
Gold Member
Dec 7, 2014
5,524
Canada
Best thing ever made for an HEI is a MSD 6AL, just make sure you put a low resistance button under the coil or it will burn through the rotor. Mine did. Make sure you get a good coil as it will help a lot. I can spin my HEI to 7K but my distributor now needs to be replaced as it has too much shaft side clearance.
New bushings and you are off to the races again! I never considered hooking one up to a CD box, but it would be an easy way to add a rev limiter too.
 

biker

Veteran Member
Gold Member
Dec 7, 2014
5,524
Canada
If you go with an MSD 6a with the HEI, eliminate the module and convert to external coil, and use the HEI mag pick up only.

Valve float or spring surge generally happens higher in the RPM ranges, (unless the springs are totally wrong) and will not RPM anymore and can feel/sound like a fast mini jakebrake or like your hitting the limiter. On the dyno, it's sounds like you put a 45 gallon drum behind the engine, and your SCFM goes down and HP brutality falls of the cliff.

What was the recommended installed height and rate of the springs, what was your installed height? does the cam open pressure recommendations match your springs at the net lift your attaining?

If your ignition and valve sprigs are up to spec, you can look at the fuel delivery side of things, an inline pressure gauge during WOT would tell you this right away.

Biker and 1980RS make some great points based on experience, same here.
Yes! Another great HEI idea. Could you use the small cap HEI for the same purpose?
 

G72Zed

Veteran Member
Sep 8, 2015
4,405
Canada
I'm not sure, don't see why not, but I've only used the large HEI, 2 of them, first one I got from the parts yard back in the 80's, and the last one was a DUI a few years ago, i most likely not see the end of that one.
 

Lowend

Administrator. .a car, a man, a maraca.
Staff member
Lifetime Gold Member
Mar 25, 1999
16,895
San Jose, CA, USA
The 6A is definitely an upgrade to a factory HEI, but there's a different issue at work here. Factory HEI's are dependable to 6000+ RPM without any gee-gaws

A few questions to consider:
  1. How old are your spark plug wires, and what kind of condition are they in? Do they have any burn marks on them (hint, if they do, you need to replace them)?
  2. How old are your spark plugs and what are they gapped to? If they are over .045 they need to be narrowed (0.035 is a good starting point). Good news, gapping plugs is free and the tool costs about $2
  3. Did you do any sort of advance curve tuning on the distributor, or did you just throw it in? Most people just throw them in and set the initial advance, which is a huge mistake. Setting a proper advance curve is one of the most simple ways to make power. The best way to do this is on a chassis dyno, but if you can find someone with a distributor tuning machines (like a Sun 404) locally they can do a lot. More info on this here https://www.racingjunk.com/news/2015/04/16/performance-tuning-your-distributor/
 

G72Zed

Veteran Member
Sep 8, 2015
4,405
Canada
FWIW, I have done A-B-A dyno testing on a complete MSD'd HEI vs a complete DUI HEI including the VIP 18v on the plug side.

Simplicity, ease of install and hookup, the DUI/HEI proved very well, and if starting from scratch, is a great option.

But the HP/TQ ave advantage went to the MSD'd HEI with remote coil, not to mention the advantages under constant 3k rpm driving with MPG & clean plugs on the cold side in mind, this is a key advantage in a "street" setup, especially with a high overlap cam.

These test were with a rebuilt "analogue" 6A unit converted to 6T almost a decade ago, still using the same box.
 

G72Zed

Veteran Member
Sep 8, 2015
4,405
Canada
It was in the 4-6 range, small, but clear advantage right off the hit and stayed right to the end, Graph printout lost unfortunately.

I was more interested in what I wanted to use and setup under the hood. MPG was one of my targets, and the goal was attained, 20+ MPG highway speeds @ 75mph with 535 hp.
 

80sz

sup
Feb 2, 2014
4,068
Buffalo,n.y.
Op not thread jumping jus adding to discussion and hope fully it helps you out. I’ve had the msd remote coil set up. I now run msd hei set up. I see no difference. Zed confirmed. Unless your turning 7000 rpm I see no need either. The extra spark occurs below 3000 rpm. It does clean things up with a overlap cam choice which I’ve had.
 




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