Baseline 858 head air flow test results

[Joe R.]

I had my heads air flow tested for baselining purposes before going to the 1.88 " exhaust valve. Here are the results (I have the data in Microsoft Excel format if anyone is interested):

-------------|---------|---------------------Lift ["]----------------------|
Valve-------| Number | 0.050 | 0.100 | 0.200 | 0.300 | 0.400 | 0.500 |
Intake (I)-- |------- 1 |---- 66 |--- 87 |--- 152 |-- 222 |--- 275 |-- 311 | Flow [cfm]
-------------|------- 3 |---- 48 |--- 78 |--- 152 |-- 221 |--- 279 |-- 323 | Flow [cfm]
Exhaust (E) |------- 1 |---- 45 |--- 70 |--- 118 |-- 166 |--- 196 |-- 212 | Flow [cfm]
-------------|------- 3 |---- 45 |--- 67 |--- 115 |-- 163 |--- 190 |-- 209 | Flow [cfm]
E / I [%]--- |------- 1 |---- 68 |--- 80 |---- 78 |--- 75 |---- 71 |---- 68 | Average = 73
-------------|------- 3 |---- 94 |--- 86 |---- 76 |--- 74 |---- 68 |---- 65 | Average = 77

Notes: 1. 3873858 heads from '66 L-72 (427 / 425) Corvette, 2.19 " intake valves, 1.72 " exhaust valves, 3 angle valve job completed.
2. Data taken at 28 " of Mercury.
3. Cylinder 1 is long port, cylinder 3 is short port.

Joe

[Joe R.]

Re: Baseline 858 head air flow test results

I also had the chambers CC'd when the air flow testing was done. Here are the results:

Cylinder -+- Chamber Volume [CC]
-------- 1 + ------------------ 106.3
-------- 2 + ------------------ 106.5
-------- 3 + ------------------ 106.2
-------- 4 + ------------------ 106.7
-------- 5 + ------------------ 106.4
-------- 6 + ------------------ 106.5
-------- 7 + ------------------ 106.6
-------- 8 + ------------------ 106.8


Joe

[Duke W.]

Re: Baseline 858 head air flow test results

Nice data set, but the test depression should be 28" H2O, not Hg.

In addition to the three angle valve work, has any pocket porting been done or is it planned?

Also the chamber volumes are close to the NHRA minimums and in a fairly narrow range. Have the heads been milled, and is there any evidence of chamber grinding? Untouched OE heads have broach marks similar to unmolested OE blocks. If they've been milled in the field there will be circular tool marks. OE chambers are "as cast", so any chamber grinding should be obvious.

Duke

[Joe R.]

Re: Baseline 858 head air flow test results

In addition to the three angle valve work, has any pocket porting been done or is it planned?
Duke

Duke,

I don't believe any pocket porting has been done, but I will find out. It is planned for the next round when I go to the 1.88 " exhaust valves. I don't know the answer to the rest of your questions, but will do my best to figure them out.

Joe

[Joe R.]

Re: Baseline 858 head air flow test results

Duke,

I meant to ask a couple more questions. A while back you posted this:

"These early big block rectangular port heads have a restrictive exhaust. The OE E/I flow ratio is well under 0.75, which means it needs more exhaust than inlet duration on the cam."

When you say that the E/I flow ratio is well under 0.75, is that the average? After looking over the baseline flow numbers, do you still recommend going to the 1.88 " exhaust valve?

Joe

[Duke W.]

Re: Baseline 858 head air flow test results

That comment was based on 1970 test of ''stock" and Bob Joehnck-modified rectangular port, closed chamber, 2.19/1.72" valve heads that is published in "How to Hot Rod Big Block Chevys". Unfortunately (and often typically) the test depression is not listed, so I cannot draw a useful comparison of actual flow numbers (which are much higher) to your data, but the E/I ratio for both production and modified heads were in the range of 0.55 at high lift.

Your data is closer to 0.65 at high lift, and though the linear average is about 0.75 if you flow weighted the average it would likely be less than 0.7, and the larger valve should improve blowdown when flow is limited by the pressure ratio being below the 0.528 critical value, exhaust gas velocity past the seat is sonic, and flow is limited uniquely by curtain area. This does not show up in flow testing because the typical test depression of 28" H20 results in a pressure ratio much higher than critical.

Ideally if we really wanted to understand this you would finish the modifications with the 1.72" valve, then flow test, assemble and dyno test the engine, then remove the heads, install the larger exhaust valves, flow test, assemble the engine and dyno test again, but given time and budget, this is likely not possible, so my recommendation is to go ahead and install the larger exhaust valves as long as sonic testing shows there is enough material behind the valve seat to increase the size without resulting in thin spots. Assuming no signifiant "core shift" when the heads were cast, this should be the case.

The only other data I have on BB heads is from The Chevrolet Power manual for ZL-1 heads, both production and with the modifications they recommend. At 28" H20 the 0.5" lift inlet flow numbers are in the same ball park as yours, but since these ZL-1 heads are of the open chamber variety, I hesitate to draw any conclusions.

On the exhaust side (and these heads should have 1.88" valves) the "production" flow numbers were not much better than a massaged SB head. The modifications produced about a 15 percent improvement, but the numbers were still about the same as what you reported, and I can't explain why yours are currently flowing better than expected. In fact, maybe your heads have already been pocket ported, and a visual inspection will easily reveal if they have.

Duke

[John C.]

Re: Baseline 858 head air flow test results

To compare to the numbers Joe has been posting I've attached the flow numbers I got back from my engine builder on my 1968 3919840 heads I recently had rebuilt. No porting done.

Numbers are after it had been changed to 1.88'" exhaust valve. I don't have any before flow info.

Chamber volume was 110.5cc with a .005" head surface removal.

John

[Clem Z.]

Re: Baseline 858 head air flow test results

Nice data set, but the test depression should be 28" H2O, not Hg.

In addition to the three angle valve work, has any pocket porting been done or is it planned?

Also the chamber volumes are close to the NHRA minimums and in a fairly narrow range. Have the heads been milled, and is there any evidence of chamber grinding? Untouched OE heads have broach marks similar to unmolested OE blocks. If they've been milled in the field there will be circular tool marks. OE chambers are "as cast", so any chamber grinding should be obvious.

Duke

i agree those heads have had work done to them as i never saw a bone stock head chambers that close in CCs and the chamber CCs size are not normal.

[Joe R.]

Re: Baseline 858 head air flow test results

Clem,

What type of work do you think has been done to them?

Joe

[Clem Z.]

Re: Baseline 858 head air flow test results

Clem,

What type of work do you think has been done to them?

Joe

it would seem to me someone has CCed the chambers to equalize them for max performance. this can be done without doing any grinding or polishing on the chamber,just by changing the depth of the valve seats. i have a chart here somewhere that tells me how many CC change for each .001 i "drop"a valve seat because in some racing you are not allowed to do any work to the chamber surface.

[Duke W.]

Re: Baseline 858 head air flow test results

To compare to the numbers Joe has been posting I've attached the flow numbers I got back from my engine builder on my 1968 3919840 heads I recently had rebuilt. No porting done.

Numbers are after it had been changed to 1.88'" exhaust valve. I don't have any before flow info.

Chamber volume was 110.5cc with a .005" head surface removal.

John

Thanks for posting that data. I assume that these heads are basically the same as 858s as far as chamber and port design is concerned.

I don't understand the first row of data with three entries for both inlet and exhaust. Can you explain?

As you can see even with the larger exhaust valves, the E/I ratio is low. If pocket porting produces similar improvement in flow as sixties vintage SB heads - 10-12 percent on the inlet side and 30-40 percent on the exhaust side, the ratio should improve to somewhere in the 0.7-0.75 range. I've seen pocket ported 461s produce similar flow numbers on the exhaust side, so these BB exhaust ports are definitely restrictive.

I don't know why you tested them at lifts above 0.5", which is about the maximum for OE cams. There's no point in testing above 0.5 on either a small or big block unless you have very high lift and this should reduce cost, or you can test more ports to get an idea of cylinder to cylinder variation.

I just reviewed the material in the Chev. Power Manual on ZL-1 heads. The final exhaust port design is very unconventional - from round to D-shaped to C-shaped with the addition of a big hump on the port flow. BB heads appear to require a very different exhaust port geometry compared to SB heads, so it will be interesting to see what a basic pocket port job will do to Joe's heads without the very different as cast shape that Chevrolet developed circa 1960/70.

The first cast iron open chamber head released in 1971 is supposed to incorporate the above exhaust port features plus some changes on the inlet side, including raising the port floor, that were developed at the same time.

Also, this data is further evidence that Joe's heads have already had some "massaging".

Duke

[Clem Z.]

Re: Baseline 858 head air flow test results

with BBC cast iron heads where we were not allowed to do any porting to improve the exhaust flow with open headers we installed "venturi" exhaust valve seat inserts. the later open chamber aluminum heads had these as standard equipment and you would be surprised how many people ground them out !

[John C.]

Re: Baseline 858 head air flow test results

Duke

I don't know what the first row is either.

Kohn

[Duke W.]

Re: Baseline 858 head air flow test results

Call the guy who did the work and ask him to explain. Let us know.

Duke

[Duke W.]

Re: Baseline 858 head air flow test results

with BBC cast iron heads where we were not allowed to do any porting to improve the exhaust flow with open headers we installed "venturi" exhaust valve seat inserts. the later open chamber aluminum heads had these as standard equipment and you would be surprised how many people ground them out !

Since the pressure ratio at exhaust blowdown is less than critical what amounts to a converging-diverging nozzle as used on rocket engines will allow the flow to achieve greater than Mach 1 for a brief length beyond the curtain area at low valve lift. Such will improve blowdown efficiency and reduce exhaust pumping work.

It will also create a very sharp exhaust note.

...but you need to be a "rocket scientist" to understand...

Duke