Is anyone familiar with the measurable difference in torque and HP between the 11/32 and 3/8” valve stem diameters that TA makes for the 455? Any other pros and cons? Thanks much, Matt
Virtually no difference. The primary reason to reduce stem diameter is to control valve train weight. As the valve train weight and mass reduces, higher engine speeds can be achieved without excessive spring pressure (parasitic drag) or self destruction of the valve train. There is a lot more to the subject, but that's a start.
There is a flow gain with 11/32 and more valve spring/retainer options but IMO for a street engine I can see the 3/8 being more durable if only marginal
You mean marginal durability increase or marginal flow increase or both? On these small valve heads I have, I need exhaust seats. Which lead to thinking I could do stage 1 exhaust valves, then maybe a smaller stem diameter. Down the rabbit hole I go!
Down the rabbit hole it is! Big valves won't do you any good unless you have at least a 430 engine, preferably a 455 due to shrouding around the valve head. Hugger is correct that there is a flow increase when using smaller valves, but that increase is negligible unless the builder includes a precision multi angle valve job, port modifications/ balancing, or at least good bowl work. If you are going to go after smaller valve stems, why not go with a valve that has the stem profiled even smaller that 11/32 in the bowl area? Compression ratio, which is drastically affected by cam profile is another enormous factor when designing a performance engine. There are dozens of decisions to make when attempting to improve performance beyond the original design, and by far the most important is "what do you want/ expect from the motor when finished?" Until you can effectively answer that question, and then assign a real budget to the project, you are wasting your time. The dart board approach to performance engine building is profoundly common and rarely if ever successful. No single factor is the golden bullet, IE, you can add a turbo, but if you don't take appropriate measures to accurately supply fuel and control detonation you end up with undesired results. I don't intend to sound harsh, I just want to call it out as I see it. I spent 15 years building ultra high performance engines for quarter midgets and while they aren't nearly the same as Buicks, the basic principles apply. There are a few guys on here that have extensive Buick experience and will be glad to share it with you.
I'm assuming the guides are also needing attention? It's kind of a no brainer to do it all at the same time. At nearly stock there aren't any huge advantages but the inverse is that the larger stem helps nothing. Depending on the exact size of the current guide bore, sometimes you can inexpensively liner them down to the smaller size. Not sure how much you are doing but the bronze liners are several for dollar and the cast guides are about $1.5 or so each.
Basically, my plan is to put ported iron heads on my existing stockish motor. Heads need exhaust seats and guides, so I figured why not put in stage 1 exhaust valves too. A little bigger cam. Running 13.9 with a 1971 455 right now and small comp cams 268h.... not my idea on the cam. That’s just short term stuff for fun in a year or two. Long term in 5+ years when Kids leave is to build a different engin that’s on a stand now.
Don't forget the durability increase in 11/32" stem valves from being able to get away from two piece factory type valves that loose there heads!!
So the smaller 11/32” is stronger than the larger 3/8” stem? I’m still green here and haven’t given the valves a whole lot of thought after having them cleaned and magnafluxed. But I thought since the exhaust needs seats and maybe guides, the money to make improvements isn’t really much more. And these 455 heads will be ported.
No. Strength is not based on stem size. New 11/32 valves are going to be one piece stainless steel.. that is all that is available in that option. Should you stick with your OE valves, or new stock replacements, you might still be dealing with 2 piece exhaust valves. Some makes had real issues with the two piece valves pulling apart under moderate perforamance use, much more than we ever did in Buick-land. Practically, the big Buick valves and 11* locks are much easier to work with, for the home builder who had a set of heads apart once every few years.. Here, I typically use the 11/32 stems in new heads, due to the wider availability of related valvetrain hardware in the aftermarket. The 11* lock is a Buick only thing. JW
I don't think it's being inferred that the stem itself is contributing to any huge flow gains. The valve head profile shape and any undercutting on an aftermarket valve can contribute to that though.
Even a few hp is meaningful if the cost is the same. If I had a dollar for every time I heard “you won’t gain much”, I’d have a few hundred bucks and 30 hp more! (Don’t do the math, I pulled numbers out of the air).
Pocket the cash and call the numbers whatever you want, it sounds like a victory to me. As far as how sensitive a flow bench is, a mason line hung down the port to check how smooth or turbulent things are affects things depending on where you put it. So does hanging a bent velocity probe the size of a small brake line. Agreed that stem size alone and subsequent flow gains would be a huge waste of money to test even if the margin of error on the dyno weren't already larger.
Ziggy, I have a quarter midget I need to get going. I've had it since I was 5(34 now). I know it's outdated as can be but my kids would love it. I don't know anything about the engine that's on it though....?
Even though many 11/32" valves have lower stems necked down to 5/16", it's not so much the stem reduction that makes for better flow, it's the way the small stem allows for a better back angle on the valve!
Please PM me on the new subject. I don't have much left but will be glad to answer whatever question I can.
