FI Plenums

[Steven B.]

Were there any internal differences (casting, machining, etc.) in the '57-'62 FI plenums?

Thanks,

Steve

[Don H.]

Re: FI Plenums

Great question. I have two "flat top" (7320) & one "ribbed" (4900), I can't see any difference but that is just a casual inspection. Hopefully someone will have a more sceintific answer. Don H.

[Bill M.]

Re: FI Plenums

Were there any internal differences (casting, machining, etc.) in the '57-'62 FI plenums?

Thanks,

Steve

Steve:

Yes. Early (7300) to the left, Late (7360) to the right.



Bill

[Mike M.]

Re: FI Plenums

i seem to recall changes in plenum runner lengths between 57 and 62 but haven't ever measured them. mike

[Tom P.]

Re: FI Plenums

ALL of the early plenums that I've seen (57-62) have the runners coming up into the plenum chamber about half way. I've never made actual measurements, but my eyeball gauge comes up with the same amount of height inside the plenum, as shown below.


Also, I've never made actual measurements of the lower end of the runners, but all finned top plenums APPEAR to have the same size runners (I refer to them as the small runners) as seen in these pictures.




All of the smooth top 62 units (7355-7360) certainly had the larger plenum runners. Although, I must admit that I will need to pay much closer attention to the 60-61 smooth top plenum runners to see if they are the smaller or larger version.
This should narrow down the answer to your question as to the size of the 57-62 runners, with the exception of the 60-61 smooth top plenums.



And then on those ultra rare ocassions you may run across a finned top anomaly such as this.

[John D.]

Re: FI Plenums

Nice pics Tom Parsons. I have always preached that the 62 Fi is a real sleeper. As Tom illustrated the 62 plenum is hogged out from the factory. More CFM's. Bigger ports. Learned this back in the summer of 62 at the GM training center in the Pittsburgh, PA area. A student in the class was complaining that his new 62 was perculating in the hot weather. Remember this was in 1962. So the instructor named Paul Glagola had him bring his 62 into the classroom. Paul and the old guys in class removed the 62 FI unit and put a one piece base gasket between the FI unit and the baseplate. Paul showed us that the ports did not line up as the 62 had bigger ports. I never forgot that.

Now if you have a 60-61 FI that you want to wake up a tad try usings a 62 plenum. You have to weld one hole up and drill one hole for the cranking signal valve.

[Duke W.]

Re: FI Plenums

I always assumed that the 275/315 HP FI units had bigger runners - same at the '63-'65 plenums - to match the larger 461 inlet ports, but from the above that doesn't appear to be the case, correct?

Duke

[Tom P.]

Re: FI Plenums

I always assumed that the 275/315 HP FI units had bigger runners - same at the '63-'65 plenums - to match the larger 461 inlet ports, but from the above that doesn't appear to be the case, correct?

Duke

In Bill Mashinter's picture above (#3), an excellent comparrison shows the difference in runner size between the early plenum legs (finned top plenums) and the 62 style plenum runners. UNFORTUNATELY, I've never taken pictures of the underside of SMOOTH top 60-61 plenum runners. So, I can't show a comparrison of the runner size of 60-61 runner size, but absolutely the runners of the 62 7355 and 7360 were noticeably larger than any of the finned top plenum runners (oh ya, I PRESUME that those of you who know FI units, know that finned top plenums WERE USED in 60-61 on some hyd cam engines).
Also, I've never measured the the lower ends of the 62 runners and the lower ends of the 63-65 runners to compare the area of the openings. The 62 runner openings and the 63-65 runner openings are totally different shapes. But possibly when measured, the LxW of each may equal the same area. And if so, then possibly the flow rate between the two may be different because shape of one may flow more efficiently than the other.

[Bill M.]

Re: FI Plenums

In Bill Mashinter's picture above (#3), an excellent comparrison shows the difference in runner size between the early plenum legs (finned top plenums) and the 62 style plenum runners. UNFORTUNATELY, I've never taken pictures of the underside of SMOOTH top 60-61 plenum runners. So, I can't show a comparrison of the runner size of 60-61 runner size, but absolutely the runners of the 62 7355 and 7360 were noticeably larger than any of the finned top plenum runners (oh ya, I PRESUME that those of you who know FI units, know that finned top plenums WERE USED in 60-61 on some hyd cam engines).

My 7360 has a base plate 3768233 dated 8/14/61. This date makes sense for a '62. I have another 3768233 base plate dated 10/20/60 that I assume would have been used on a '61 7320. It has the same large ports as the 7360 3768233 base plate.

[John D.]

Re: FI Plenums

Bill, That's a good post you made on 62 baseplates compared to the older '233's. I have a late 62 baseplate that has the huge winterleaf snowflake in the center of it. I will unbury it and compare it with one from a 283.

I doubt if I am going to see a difference in port size but you never know. JD

[Joe C.]

Re: FI Plenums

The older style runners appear to be a better design, at least for smaller (283 and 327) engines with small valves. The larger cross sectional areas seem to correspond to the roof of the intake ports in the manifold (base plate) and heads (I am no expert on Rochester units). If that is the case, then enlarging the smaller cross sectional area (port floor) will do little to increase port flow. It will, in fact, most likely result in a torque loss at low to mid RPM range, with modest (at best) increase only in upper rev range.

The best performance increase would likely be achieved by further enlarging the port roof cross sectional areas.

[Clem Z.]

Re: FI Plenums

for racing we sawed the dog house down the middle to be able to shorten the runners inside the plenum to add HP to the upper RPMs. longer runners more torque,shorter runner more top end HP

[Tom P.]

Re: FI Plenums

And that's what's on the SB400 in my 56. Notice where it's been welded back together. But yet on the ends and between the center fins, the welds have been finished down and are almost undetectable.
Although, with 400+ cubic inches sucking through the enlarged, shorter runners of my plenum, the low-mid range is very adequate.













Although, some units never had the welds finished down.

[Bill B.]

Re: FI Plenums

My 7360 has a base plate 3768233 dated 8/14/61. This date makes sense for a '62. I have another 3768233 base plate dated 10/20/60 that I assume would have been used on a '61 7320. It has the same large ports as the 7360 3768233 base plate.

I also have a 7320 and a 7360 FI units and find the same results, so I concur!
Does anyone know the differences in cfm between the years? John mentioned the 62 had increased cfm does anyone know the measurements?

Bill

1961 Black/Silver/Red Int. 283/315 FI
power windows. Frame off 65% completed.
1989 White/hard top/Grey Int. daily driver 167,000 miles

"The problem with Liberalism is that eventually you run out of other people's money" Margaret Thatcher

[John D.]

Re: FI Plenums

for racing we sawed the dog house down the middle to be able to shorten the runners inside the plenum to add HP to the upper RPMs. longer runners more torque,shorter runner more top end HP

"We" sawed the dog house the middle Clem? Just who around here did that? Maybe Grady Davis did but none of us did that in the old days. We just boycotted the 57 to 62 FI's and ran midyear FI's as I recall. That's what Timmy did anyhow. JD