Casey Butt

Something of interest: These are the flow numbers for Trick Flow's 23* heads on a 310 (305 0.030" over) and a 350.

3.766" Bore
Lift Value: Intake Flow (CFM): Exhaust Flow (CFM):
0.100 61 53
0.200 127 105
0.300 180 142
0.400 216 160
0.500 242 172
0.600 242 180

4.000" Bore
Lift Value: Intake Flow (CFM): Exhaust Flow (CFM):
0.100 62 57
0.200 127 109
0.300 180 142
0.400 219 167
0.500 242 181
0.600 245 192

The intake flow is practically identical and the 310 flows about 5% less on the exhaust. Therefore, valve shrouding has no effect on flow on the intake and only causes a 5% decrease on the exhaust. The combustion chamber on these heads overhangs the 3.766" bore, the result would be an expected disruption of flow along the cylinder wall side. However, the 310 is an 11% smaller engine than the 350, so with a 5% decrease in exhaust flow, these heads, in proportion to engine displacement, flow better on a 310 over the lift range tested. The effect of valve shrouding is, therefore, not significant.

BMmonteSS

Way to bring the tech where did you find that at? I'm kinda surprised that flow wasn't effected on the intake side until .600 lift, and the exhaust was effected. Kinda backwards from the way I would have assumed. You also have to consider that the combustion chamber overhang isn't a good thing, it's a perfect place to trap unburned gasses. But with a head that has a combustion chamber designed for a 305 it wouldn't be an issue.

If some one wanted to argue, they could bring up that the velocities that are tested on flow benches aren't any where near what the heads see in the real world, and some times flow disturbances don't rear their ugly head untill velocities start to pick up.....if some one wanted to argue they could bring that up..... Which I don't.

Casey Butt

That's directly from Trick Flow's website.

I'd be very interested to see what would happen if that overhang didn't exist ...I'd bet exhaust flow would improve at least a few cfm.

I think there's no difference on the intake side because the majority of the airflow travels out towards the spark plug boss and very little comes out along the cylinder wall side anyway (at least on heads with good swirl). On the exhaust side most of the flow is directed from the center of the cylinder and not near the wall. So the shrouding effect doesn't appear to be much of a factor.

"velocities that are tested on flow benches aren't anywhere near what the heads see in the real world, and some times flow disturbances don't rear their ugly head untill velocities start to pick up"

I know, I know ...but we'd only be guessing. I've seen some very shrouded 350 heads (23*) - with 2.125" intake and 1.625" exhaust valves - but they flowed over 300 and 200 cfm, respectively. So I really doubt that shrouding along the cylinder wall has much of a detrimental effect at all on SBCs - that area simply isn't a high flow area to begin with. The exhaust seems to suffer a little more because swirl isn't a factor and so the flow isn't as "directed" away from the wall.

I think the moral of the story is: If you port heads for a 305 concentrate on making good swirl on the intake and try to bias both ports so most of the flow is directed to and from the center of the cylinder. Of course, those "rules" also apply to any street driven small block, but are probably especially important on a 305. In any case, with good heads, valve shrouding doesn't appear to be that big of an issue (at least on "street style" heads).

I can't argue with you ...I'll need your advice when I try to tune in my new cam.

BronYrAur

iTrader: (1)

Good comparison data, what size were the valves in the heads though?

mw66nova

iTrader: (13)

those are 1.94/1.5 valves, just like what's in my 305 (310)

BMmonteSS

LOL, argue away.....a good hearted "discussion" that's backed up with facts is the best way to further everyone's knowledge on the subject being discussed.

It is a very good test and it shows that the 305 bores aren't as big of a hinderence as one might think. You only have to look as far as the LS1 to see that a slightly smaller bore isn't the end of the world. It would also be interesting to see what would have happend had the combustion chamber been designed for the smaller bore, and also what would have happened with bigger valves, even though they can contact the cylinder bores at higher lifts.

Casey Butt

I'm putting 1.94/1.6 on a set of 601s before the end of summer. I'll unshroud them out to the 3.766" bore ...I might even go to the 3.87" gasket and do the same at the top of the cylinders by the valves (down to the top ring "line" anyway).

If possible, I'd love to get them flow benched with a 3.766" bore.

contact

3.766 bore
1.94 valve

how close is the valve to the cylinder wall?

Casey Butt

about 2 mm at the closest point (just under 5 mm on a 350).

BMmonteSS

Wouldn't that vary depending on valve lift? That's the problem with 23 degree heads, the valves open towards the cylinder walls. This is why the LS1 heads flow so well, they get the valve mounted at a much shallower angle (12 deg?) as well as raising the intake ports for a straiter shot at the valve.

Damon

You might be interested to know that there is another popular Chevy engine that often has the chambers overhanging the engine's bores...... the Big Block Chevy! Look at any aftermarket BBC head and they'll probably tell you it's just peachy using it on anything from 396, 427, 454 and the "bigger cube" versions of the big block (502, 572, etc.).

Then look at how the chambers ACTUALLY overhang the bore on anything 454 or smaller. Sometimes they overhang by a frightening amount. My AFR 305cc rectangular port heads overhang the bore on my 30-over 454 block by almost 1/8" in some places (most notably near the exhaust valve)!

A dead give-away that the chamber overhangs the bore is to look at the manufacturer's head gasket recommendation. It will often be for a gasket with a hole much larger than the bore of your motor. For instance... my 454 has a bore of 4.280" with the bore job (30-over stock). But AFR recommends the use of head gaskets with a much larger 4.540" bore on my motor. And I can tell you exactly why.... you NEVER want the edge of the head gasket hanging out in the combustion area! I had a spare 4.370" bore BBC head gasket laying around so I lined it up over the chambers..... no way, not even close, way too small for the chambers.

Here's a link to an article on AFR's website where they KIND OF tell you this is happening, although they never state it directly......

Air Flow Research

Here's the relevant part of the article to this discussion:

But they never really tell you outright that the chamber overhangs the bore on a 454 or smaller bore motor, do they? No, they like to keep that kind of stuff quietly hidden away. At least until I buy a set and start checking verything for myself. On a larger bore 502 motor (4.47" bore) these heads would probably not overhang at all, but I don't own a 502 so I live with the overhang.

Is it the end of world that the chamber would overhang the bore? Obviously not, if you comapre AFRs own flow figures for that same head on a 4.25" bore vs. a 4.50" bore fixture. It's just one of those many little details that costs you a few horsepower here and there. Add a few of those "little" things up, though, and suddenly you're down a ton from where you thought you should be.

Casey Butt

BMmonteSS, I don't think the shallower angle would significantly relieve shrouding until valve lift gets high enough for the long-side tip of the valve to leave the combustion chamber. The reason I say that is because you can port the chamber to have enough clearance around the valve (i.e. clearance = current valve lift) ...until the tip of the valve reaches past the chamber.

I don't think 23* heads are a significant limitation until high lifts are used. I don't know much about the LS1s but I think they use pretty high lifts don't they?

Damon, doesn't it irritate you when companies don't give specs like the width of the combustion chamber? You'd think something fairly significant like that would be part of the spec sheet on the heads. The reason I knew that the Trick Flow heads overhang the 3.766" bore is because they specify that a 4.00" bore gasket must be used.

kdrolt

Excellent -- another TGO myth busted.

I will add however that the heads used in the test flow very well and therefore do not much rely on airflow past the valve/chamber side. That may not be true for all heads (factory e.g.) so that the bore shrouding might have more of an effect than shown here.

Good post.

Fast355

iTrader: (2)

Just goes to show what I have been saying for a while too. Shrouding in a 305 is overated and minimal. If a 305 was as bad a people claim, I would never have been able to make 301 RWHP or the current 423 FWHP.

Casey Butt

I think we can make a generalization from this data anyway:

305 heads with good swirl won't be significantly affected by shrouding along the chamber wall up to 0.600" valve lift. Heads with low swirl numbers will likely be affected more (as evidenced by the decrease in exhaust flow), and that is probably on the order of a few percent decrease (estimating from the ~5% decrease on the exhaust side).

Above 0.600" lift I'd expect the shrouding to become more of an issue because the 23* valve angle starts appearing to put the valve a little close to the cylinder wall in a portion of the high flow region at least. How dramatic any shrouding effect would be is only conjecture at this point ...but lift that high is wayyy too much for a 305 anyway, thus it becomes irrelevant.

Fast355, in your case, "the proof is in the pudding".

Damon

Be careful comparing intake flow to exhaust flow. They're going in opposite directions past their respective valves. Different things going on inbound vs. outbound. Not saying how this affects shrouding issues (becuase I honestly don't know enough to make an intelligent observation) but don't assume that becuase it affects the intake flow by X that similar conditions on the exhaust side would also affect it by X.

BMmonteSS

I think you are correct Damon, the intake side of things deffinately has a flow path. I noticed that the combustion chamber of the lastest AFR heads have the chamber shaped to guide air towards the center of the chamber in a swirling pattern. On the exhaust side air is just being shoved towards the valve from every angle, so the exhaust ports just needs to form a funnel, thats where I think the cylinder wall gets in the way of things.

11 Flat

I posted these numbers in a thread where a guy was wondering what size valve to run. and what type of flow was possible out of Pro-topline iron lighting heads

221cc after porting

2.08 Valve on a 4.00 bore flow fixture

Lift---Int
.200-134.4
.300-195.1
.400-240.9
.450-259.8
.500-275.0
.550-286.2
.600-293.3
.650-297.6
.700-302.8
.750-308.3
.800-311.0
.850-314.2
.900-317.0
.950-317.7

same head but on a 4.125 bore flow fixture
Lift---Int
.200-138.0
.300-199.1
.400-245.6
.450-263.1
.500-278.7
.550-289.7
.600-296.1
.650-302.0
.700-307.5
.750-311.4
.800-315.8
.850-319.3
.900-322.5
.950-324.0

Casey Butt

That's exactly why I guessed that if you don't have good swirl on the intake then the flow will start to drop and the intake port will act more like an exhaust port flowing backwards. The exhaust ports don't have as much of a directed flow from the center (though the majority of gases enter that way), so I roughed that the ~5% decrease will apply to an intake with low swirl also. Of course, that's a real rough guesstimate, but that's where I was coming from.

I think this shows the importance of swirl on 305 heads ...and also on grinding the chamber walls around the exhaust valves (especially) on an angle (like a partial cone) to not only relieve shrouding but to funnel exhaust gases into the exhaust runners as smoothly as possible. 305 head porting seems to have a set of rules much more critical than when porting heads with larger combustion chambers for bigger bores. Of course, a flow bench would be nice to test these modifications, but this is what the data (and results from other heads) seems to suggest...

If anyone's doing this (as I am currently doing myself) I think it's important to set the quench in the 0.040" to 0.045" range to compensate for the small amount of material being removed from the quench side of the combustion chamber deck. That's being picky, but if you're doing something you might as well do it right and get the most out of your setup.