Flow Testing on Different Bore Sizes
 

Yes I know that the are Flow Testing a SBM Edelbrock Head. But with all of the 350 Pontiac talk here it results might be of some interesting

BORE SHROUDING TEST: 4.03" vs 3.91" Edelbrock RPM Heads

https://www.youtube.com/watch?v=JNXpCxxVFT0

Stan

The SBM as used in this video has a 18 degree valve inclination angle and in terms of our Pontiac heads with a 14 degree would see even a greater loss if the same test as in this video where done.

I think the evaluation would have to be done with a chamfered 350 block and a non-chamfered one for accuracy. I'd be interested though since I'm running 6x-4 heads on 350 block bored .060 over.

In the video they flow tested on a 4.03 bore a 3.91 bore and then modified the 3.91 bore to simulate a valve relief at the top of the bore.

Stan

His notches in the tape aren't nearly smooth as machined chamfers though.

Those SBM heads come with 2.020 intake and 1.60 exhaust valves making them slightly smaller than what we would get in an over the counter aftermarket head.

The one thing I don’t like that was done in that video is that the bore length of his flow bench adapter was real short .

The reason I posted the video was so people could see trends not absolutes. That bore was 3.91" a 350 Pontiac is 3.875" That intake valve was 2.02" and most Pontiac people are looking at 2.11". Cutting the tape did show that a valve relief does help / work.

Stan

I agree. I watched the video, it was interesting. I don’t think that narrow tape for a smaller bore translates to very good data. The knotch combined with the short tape is pretty strange data to. There is something to be learned from it, but the lesson might have more to do with turbulence than bore size.

I build some LA mopars, and have a 71 340 duster. A few years ago Nick’s garage did a 390 CID based 318 stroker that made 406Hp thru A body exhaust manifolds like my 340 Duster. It had ported iron heads with 2.02s and a HFT, similar to my 340. But my 340 is closer to 380HP thru manifolds, might be able to split the difference with a cam like the 318 stroker used, it definitely had a better cam. But overall it appeared the extra stroke and cubes were more important than the bore.

I think thats probably going to hold true in all but extreme examples. As long as you have a reasonable valve size, adding more CI is going to have a larger impact on power, than adding more valve size without adding CI. Of course if you can it would always be better to add both.

When you are CI limited, bigger valve will make more power. Assuming supporting modifications of course. The DZ and Boss 302 from the Trans AM Series come to mind. The Ford motor flat out made more power because its heads had a 2.23 intake valve compared to the Chevy 2.02. They should have won more of those races, but the Ford's couldnt stay on the track. Jokes write themselves. I also think Ive read that a wedge head motor is a more efficient design than a hemispherical chamber, but the HEMI simply allows you to shove larger valves in it, which on the dyno or at the top end of the race track is going to make bigger numbers. Engine masters did something like that with a Hemi and a wedge mopar, where the HEMI would have won a drag race but lost everywhere else.

Im sure if you graphed a static CI engine with different valve sizes and a static valve size with varying CI there would likely be an equilibrium somewhere.

Always helpful to see videos like this, thanks. I did some grinding almost to the extent of that block srm showed above. Kinda pumped up to see the results when its back in.

John

In the Boss 302 engines off the show room floor they did not even start to get into there peak torque band until 5000 rpm due to there overkill minimum intake port area in those heads, then let’s add a bigger cam for road race usage and its easy to see why they had problems winning races, and especially on short tracks.

Too bad Ford did not combine the 2 bbl Cleveland heads exh ports with a modified 35% smaller intake port from the 4 bbl boss head , then they would have been on the right track for a 302 CID motor spinning to 7200 and the motor would have pulled far better out of the corners.

When a street engine is not a max effort build I don’t think the bore shrouding becomes as much of a bad deal as it is often made out to be. On a street engine I think port volume has similar issues, and gets over blown. Large port volume usually needs less cam than a smaller port volume. To keep the engine responsive at part throttle a bigger port wants more compression than a smaller port, which can be done by cam and compression. The small bore that is shrouded is effected similarly with compression in terms of it may need more compression to make good power (less efficient do to the bore shrouding), but should still have crisp throttle response. Both seem to like the cam events spread out versus something more conventional.

Here are the dyno graphs of a BOSS 302 (Blue line) and a BOSS 351 (Red Line), both are very close to stock except for headers. The other graph is a Boss 302 (red) against a SBC DZ302 (blue). I think the biggest issue with the BOSS 302 was it did not have enough compression for the RPM’s it could turn, it only had 10.5, and that was over rated versus what they are actually. The cams had very wide LSA’s, the big ports on the small engines did not need a lot of cam overlap. Ford learned from that and increased the compression on the BOSS 351, and it also gained from the extra cubes making the cam act smaller. Both those Fords have fairly small cams compare to a SBC, and flat power bands that start about 3600 RPM. The Boss 302 had no issues beating the DZ302 when Richard Holdener tested them. The DZ 302 had slightly stronger midrange, but it had a more compression. BOSS 351 was a very well designed street combo…For a Ford (LOL)

What he did on the video reminds some of some testing I did last winter on some heads messing with the intake port fixture going into the head. I flow tested a bunch of heads with and without the transition that funnels to air into the intake port, and modified the transition to see what it did to the flow numbers. I did ported 990 BBC head rectangle port head that flowed over 360CFM, and the transition barely affected the flow. On Pontiac ported d port heads there was a slight difference, more than the BBC. I thought that was interesting, so I tried a mopar big block ported Stage 6 head that was sitting there, and it made more difference than I had seen yet. Then I took a big block mopar CNC ported Trick flow 240 head and flowed it, and I was shocked it barely out flowed a stock head without that fixture. It gained about 100 cfm with that fixture on. I checked it a bunch of times thinking I was doing something wrong. What it said to me was the bigger difference the fixture made, the more efficient the port was. After doing that I understand why some shops flow the heads with the intake on.

It would be interesting to see a full length bore fixture tested on that video, and test with that short tape in the bore, and try that small knotch on both. Maybe it made a difference, maybe it didn’t. I think it made a difference more than what he realized.

I agree with you Stan and thank you for the posting. I think it helps justify my head choice on a 3.935 bore with the machined chamfers.

Maybe Steve25 remembers for sure. As I recall the block chamfers on an early 350 really do not even line up with the combustion chamber on a 6X head. I have my doubts the early 350 block with larger chamfers are doing much with a 6X head when they may not even line up together. It has been a bit, but as I recall a 6X has a narrower combustion chamber than even an early 70s small valve 350 head. The 6X combustion chamber design appeared to be narrow enough that on the late 350s with factory 6x heads Pontiac eliminated the bore chamfer all together. The 1974 350s kept an exhaust chamfer, mid way through 75 the exhaust chamfer was eliminated altogether. To get the most out of the early 350 cylinder scallops the 6X might need to have the valves unshrouded and the chamber widened.

Jay,
Do you have a swirl meter on your bench? It would have been interesting to see if it changed, and if so how during those different tests.

Stan

I don’t have a swirl meter. Stan, that is a good point, something to consider might also be how the swirl is effected when the bore is smaller like the in the video, or with cylinder scallops.

They redid the tests with 3D printed bore adapters.

https://www.youtube.com/watch?v=6v-fO3GRLiU

Stan

The info was a lot better on this last video than the first one. At .450” lift his tape lost 10 cfm against the longer bore adapter, but got it back at .50”, then it was back and forth thru the lift range, there was some turbulence going on with the tape.
Offsetting the cylinder bore seemed like a good solution for unshrouding the intake valve on his project. On a 350 Pontiac the exhaust lift would be more limited, still could work though.