Will large valve heads fit 428 bore?

[Marx427]

Will Medium Riser or High Riser Heads fit the 428 bore size? 

I'm contemplating upgrading from iron Low Riser heads  to Medium Risers on my 427 stroker  (468in).

I was wondering if those same heads could also be used on  the two 428 based projects I'm working on?

Thanks.    M

[e philpott]

is your 428 standard or bored over ??

[Marx427]

Ive got two blocks......one is a 391 truck block bored to standard 4.13  and a 428 CJ/ service block that cleaned up at .040 over.

I have an old late 70s publication that says they will fit, but that authors work has since been proven to be full of holes.....

M

[hotrodfeguy]

They do but I think the results are counter productive in the sense that the shrouding of the bore does not help the overall flow.

[Marx427]

hotrodfeguy,  I see your point..... 
 Does anyone out there concur?   Is anybody using medium risers on a 428?

[jayb]

I guess I would tend to disagree, although I don't have any numbers to back that feeling up.  But think about the geometry involved.  If your intake valve is .100" from the bore at the closest point with a 390 block, and you go from a 2.09" valve to a 2.25" valve, you are now within .020" of the bore.  But how much flow are you losing at that one point?  You've only got .100" to start with, so I would say probably not that much.  Compare this with, let's say, 80% of the valve which is completely unshrouded.  By going to the bigger valves you have increased the valve circumference by 7.6%, so around 80% of the valve you have a 7.6% increase in valve curtain area.  Then the remaining 20% may be shrouded, but only the closest point is really shrouded badly, and the rest of that 20% probably still flows pretty well.  Then of course consider that the further the valve is open, the further away from the cylinder the valve head is, and so at higher lifts there is less shrouding of the big valve yet.  When shrouding has been a big concern, I've gone as far as notching the bores in some engines to fit the largest valves possible into the heads, which also alleviates this problem.

What is really needed is a back to back flow bench test on a reasonably sized bore, where two different valve sizes could be tested.  That would answer the question fairly conclusively, I think.  But until I see some data disproving my own thoughts on this, I'm going with the biggest valves that make sense in the head, given the port configuration.

[Joe-jdc]

I just went through this with a forum member and 428CJ heads.  He wanted me to rework his heads and port them some.  When I receive a set of heads, I always flow them to see where they are as received.  Then whatever modifications I make, whether valve job, oversize valves, porting bowl, chambers, or competition port and polish is completed, I re-flow the heads to see what, if any improvement was achieved with the modification.  The heads above were flowed out of the box and only flowed in the 235cfm range @ .500" and did not pick up through .750".  When I pulled the valves out and measured them they were 2.190" valves in a 2.090 seat, and 1.750" in a 1.665seat.  Just by sticking a stock CJ valve in and reflowing, it picked up nearly 15cfm, no work involved.  I flow on a 4.200 fixture, or a 4.350 fixture for FE heads.  Unless the walls of the chamber are laid back to the bore size, the flow will diminish and not change above a certain value.  A case in point is a set of heads I did for this same customer a few years ago, and they flowed 313cfm without a valve job and using a 2.150 intake valve.  He wanted a local machine shop to do the valve job and install a 2.190 intake valve.  The car did not run any faster, and when the heads were sent to Blair P, they only flowed 260cfm with the oversize valve.  I have had other heads flowed before Blair, and after Blair's bench was used, and our flow benches are very close in cfm figures, so I know the larger valve job killed over 50cfm.  Just my own experiences with FEs, I have seen very similiar results on SBFs and brand X.   Putting a MR head on the 4.130 bore is doable with the intake, but the exhaust needs to be chamfered similiar to the 427 block if you are going to use anything larger than the 1.665 valve.  You might squeak by with a 1.710, but the block needs attention before I would feel comfortable with it.  The flow difference is negligible between the 1.710 and that bore, and a good 1.665 valve.   Joe-JDC.

[jayb]

Joe, that's great info!  But I have one question:  Was the diameter of the seat under the valve opened up to match the valve size on the heads you tested?  I've been told that the inside diameter of the valve seat opening should be about 90% of the valve diameter.  If larger valves were installed, but that seat ID wasn't opened up, I'd think the gains would be negligible, or maybe flow would decrease as you have seen.  I assume that whoever installs larger valves in the heads would re-work the seat to match.

[Marx427]

Okay..........thanks guys.........somewhere in there  Ive got the idea that yes ....medium risers will fit...at least my 4.170  bore block with maybe some chamfering   and valve shrouding will be negligible........?

[jayb]

I wouldn't hesitate to do it, but I'd also make sure to mock up the heads on the block and check for clearance between the valves and the bore.  I'd also chamfer the top of  the bore if you don't have a minimum of about .060" clearance there.  I've put 2.19/1.71 valves in a .040" over 390 block (4.09" bore), using Edelbrock heads, and didn't have to do any clearancing.  But medium riser heads have the valves spaced farther apart than the Edelbrock heads if I recall correctly, so you may have to chamfer the bores in your case.

[GJCAT427]

Every thing I`ve read says that 390-428s must have the bores notched to prevent the valves from hitting the clyinder walls. Factor in your valve lift and I think its right on advice. Just becarefull when notching and don`t overdo it. 

[Joe-jdc]

Jay, on the CJ head where the 2.190/1.750 valves were installed, the seats and throats were stock size, the shop simply installed oversized valves.  Bad move.   On the C6-R style heads that flowed 313 cfm here after I ported them, the valve job was done by a shop somewhere there in Illinois that was supposedly more into Chevrolet work, and they cut the throat out, did a poor valve job, and simple killed the port with their work and the shrouding from the larger valve.  As for throat percentages, I still follow the rule of 88%on a rectangular port, 89% on more oval, 90% on round ports for throat sizing.  If the target flow is not achieved with these percentages, sometimes I will open them up to 91-92% and retest.  If that fixed the problem, then great, but most times the velocity is better with the smaller diameter throat values.  Also, the smaller the throat % to valve size will help the transition around the face of the valve in most instances.  The problem with the Fe combustion chamber is the 90* wall the incoming air must negotiate between the valve edge and the combustion chamber, which kills flow.  Hopefully the new head Barry R. is working on will have a better combustion wall transition, helping low lift flow as well as being a quick burn feature.  Joe-JDC.