alright I'll play
for those of you that don't already know my background:
R&D Speed Shop is a dedicated Engine Machine Shop located in the Greater New Orleans Kenner area. Since 1958 we have acquired a notorious track record that speaks for itself. From building 6 second drag strip dominators for the all out racer, to factory spec rebuilds for the local Ford, Chevrolet, BMW, Mercedez, Toyota-Lexus, or Cadillac dealerships, we have the knowledge and hands on experience to exceed your expectations. We pride ourselves in the value of being a true SPEC shop, which means we build your engine
the way you want
to meet your specifications
, to ultimately provide you with the most appropriate engine to reach your specific goals
. Our motto is that "EVERY ENGINE IS DIFFERENT" and by applying this motto to every build, we continue to be a true SPEC shop. This mentality coupled with our many satisfied customers, has enabled R&D to be one of the most highly regarded Engine Machine Shops around. This comes to no surprised since the majority of our business is from recommendations.
now since that's out the way, what im about to type has been passed down to me from generation to generation I'll try to explain everything as thorough as i possibly can, if anyone has any questions DO NOT HESITATE TO ASK, If its not evident yet, my sole purpose on this forum is to help others and learn along the way, although it seems I do more of the first part, I can't possibly begin to even repay what I've gathered from this site. I'll try to tackle everything starting from Chris's original post on down.
the amount of seat pressure you need is determined primarily by the cam, valve weight, rate, and profile
usually the seat pressure is going to be determined by the cam grinder
you typically loose about 10% of your spring pressure when you get up to operating temperature just from the thermal expansion
if you setup for 150# on the seat when you get up to operating temp your going to drop down to about 135# on the seat just by the heat, but cam grinders will take this into account when they recommend a seat pressure
ex: if your only going to be spinning up to 6k, it takes a certain amount of energy to operate you motor, nascar goes on a basis of (14.98%) 15% of parasitic losses, but you can even go further than that on an all out drag motor
osses will increase hp, moving from 15% to 10% PL on a 600hp engine is going to net you an increase of about 30hp
this is why they have new technology for rings, pistons, oils, springs, design in crankshafts, everything today is aimed towards less PL and less parasitic drag
Grumpy Jenkins, has seen as much as 8 degrees of torsional twist on a sbc camshaft from high spring rates and loads on the cam shaft, distributor, oil pump, etc.. which effects cam timing and ignition timing
you'll see on newer engines the oil pumps are run off the front of the engine instead of the cam just to help reduce this torsional offset
new valve technology with smaller seats, 7mm or 8mm valve stems, beehive springs with their progressive rates, smaller retainers, and lighter parts, are aimed at less PL because its all basically free hp
now just for example:
the standard performance hydraulic cam for big block chevy engines come with 140-150# o
eat and 350# o
the big valve 572ci 620hp engines with the 'hot' hyd cams come with 180# OTS / 370# OTN, reason being
is that the bigger valve needs more seat pressure to control it at higher revs
if you keep the seat pressure at a lower ratings of ~145# OTS with more valve-train weight, the results aren't going to be to good, since you don't have enough pressure to keep that valve closed at those upper rpms, you'll get valve flutter "float", which will cause the motor to just lay over
when you see an engine that just lays over or runs out of breath that's usually because of valve float
application specific, if your going to run a supercharger, turbo, or nitrous, you will run more seat pressure, the width of the seat is going to determine the amount of pressure, using a wider seat may require more pressure than a thinner seat because your not pressing down as hard
imagine putting your hand on a hot plate, then image putting it down on a hot pin at the same temp, you have a higher load value in that area in a smaller seat so that actually pulls more heat out since you don't have as much area, but that also depends on where you put the seat, cuz if you put the seat all the way out by the margin your actually making a bigger surface area or a larger surface area which then may or may not require more seat pressure depending on the width of the seat, there's a whole bunch of factors, we're just getting into the minutia of the intricacies of knowing whats available
that was kind of technical, so here's another comparison
if you have a hot frying pan on the stove and touch it lightly for 1.2 seconds, it'll probably give you a burn, but if you touch the same spot for the same 1.2 seconds but this time you push hard on it, you will probably blister your finger just because the pressure removed more heat which was transferred to your finger. now try it using your whole palm, the more you can now understand how the surface area is going to come into play
reminds anyone of the turbo thread? lol
now since i know Chris is wondering about that 331 lets put everything into perspective
a 331 making about 400hp
with a mild cam at .500 lift through a hydraulic roller that spins up to about 6k which is where i figured he would want to be, we would put him right at 125-140# seat / 330-350# OTN with the gt40 heads although that won't cut 400hp, for that kind of power you'll probably want a good set of trickflows or even better the RHS heads which come with 140 OTS
now depending on cam selection and valve-train weight that may shift just a little tad but that's typically where we would want to see you at
now if i was building a 331 that i wanted to make 400+hp on a hyd cam, i would be looking to lighten up the valve-train with a cylinder head that can keep the air speed as high as possible, not necessarily the most volume, you want a head thats properly sized which is going to be around the 240-260cfm range
ex: 300 cfm N/A with the right CR, has the potential of making 2hp per cfm, this is just a ball park estimate there's a bit of factoring you need to do for VE but it should have the capacity to make close to 600hp in all honesty
the heads on my friends engine flowed 320cfm and he maked 606hp... and he has a very small cam, he can easily crank out 650+ just by simply swapping the cam
likewise too little seat pressure will cause valve float, to explain better i have the perfect example, well technically I actually have 6 lol
the ford thunderbird supercoupe made from 89-95 with a 3.8L V6 and Eaton m90 supercharger making 210 or 230 hp.
At around 5200 rpms they make a complete nose-dive and loose power, now with a supercharger that shouldn't happen, as long as you have the efficiency to back it, a blown engine should keep pulling, 5200 rpms is nowhere near where it should stop making power, much less have a huge decrease in power
the reason being is that Henry only sprung her with ~80# OTS / 230# OTN, with the valve-train weighing in at a massive 230 grams
that's nowhere near the seat pressure that's needed to seat her, you can conclude that at those upper rpm's the valve isn't going to seat, just by swapping the springs over to 120# OTS I can now make power all the way up to 6500 rpms where as before at 5200 the engine wasn't going anywhere fast
VT comparison on the supercoupe
230g = OEM valve, spring, and retainer
170g = new valve, spring, and retainer
that's 60 grams lighter which allows me to spin higher so in return i picked up 40hp just by being able to spin it higher, and then i picked up another 50hp by the bigger valves
also keep in mind more seat pressure will remove more heat or better yet transfer that heat from the valve to the seat then dissipate that to the head and so on...
a more practical example of valve-train weight is the LS7 heads which will give you an idea of how valve-train weight is going to be a large factor in determining the seat pressure needed, the stock titanium intake valve and sodium filled lightweight exhaust valve only require 110# OTS due to being so lightweight, keep in mind we're referring to hydraulic cams. But because the valve-train is so lightweight it doesn't require as much spring tension to control the valve and they spin past 7k
as for the gt40 heads and the stock springs handling .500 lift, no way after about .420 there done.
but any who, the traditional coil spring is old technology compared to beehives, the great properties that beehive springs offer is that they have minimal weight by design, and they have much more controllable spring rates they have dynamic (progressive) spring rates rather than linear rates of older style traditional springs, so its a win-win, this is why you see beehives equipped on all new OEM engines.
moving on to retainers, weight as well as design is going to effect both the seat pressure and the amount of pressure on the seat needed to keep it seated at high rev's
the stock retainer on gt40 head is a POS, its got a cup on it that holds the keepers in place and the rotator cup its just heavy that thing is junk
yes you can shim to get increased seat pressure, you can also go with a different retainer design that has a shorter installed height so more tension is on the seat thus increasing seat pressure.
if you have too much seat pressure, there's a few options you have, although the best and usually most practical would be to get a different spring, but you can go with a deep dish retainer that will give you a higher install height, or you can swap the valves to longer ones, also some companies make drop retainers and drop keepers
keep in mind all of this should be done by a professional not for the dim-witted
on stock gt40 heads the spring isn't as restrictive as the valve, the biggest gains you'll see from a stock gt40 head is doing a valve job and opening up the ports, keep in mind that a very smooth and fine intake port is not ideal and will hurt performance but im not going to get into head porting and what not, the stock ford heads have a crappy valve job to being with its just garbage, it needs one desperately i mean bad!
we have some guys that run 11.90s with gt40 heads and we charge about $600 to do a set of them, you'll then have a decent head that Henry would even approve of, it'll pull to 6200 rpms and includes new seals, valves, retainers, guides, keepers, beehive springs, 5 angle valve job, resurface, bowl work, bronze wall guides, minimal porting, EVERYTHING.
we can actually do you a whole 331 long block using your block and heads fully machined and built for around $3 grand
that includes the cam rockers pushrods everything for the long block if you stayed at 302 you could save about $500
if your guides aren't worn you could put a set of 5/16 bronze wall guides stab it then broach it, then hone it to 8mm stick an ls valve in it, you'll have a lightweight valve-train ls3 1.86 intake valve and 1.55 exhaust valve, thats what i would recommend for gt40s
springs are springs, just like a valve is a valve, they usually list what they were originally designed for but that doesn't mean that's the only thing it will fit, that's just more so for reference.
there is some truth to extra spring pressure needed for boost,
example if you have 20# of boost and you have 2 sq in of surface area on your valve at 20# your going to have 40# pushing on that valve, so now your pushing against the valve when it closes
that extra pressure is going to want to hang that intake valve open, so its going to negate some of that spring pressure if that makes any sense
on the exhaust valve you don't have the same pressure behind it so its really not a problem, but you want to keep the same seat pressure on the exhaust that you want on the intake
visualize the VT in motion, the intake is opening as the piston is coming up, so you need good seat pressure so when the intake valve starts closing, you won't have PTV issues 'during the overlap
while the exhaust valve is closing the piston is chasing the valve, so that's where you need really good over the nose pressure because you don't want the valve floating there because if it comes off the lobe that's when you run into a PTV issue, don't get me wrong their both important but that's where its really important
if you float the intake valve your not really gonna worry about it hitting there unless you run into a lot of valve bounce, your more worried about floating the exhaust, usually you end up hitting the exhaust valve which comes around and hits the intake valve and messes everything up
if your trying to set the national record or if you just want a daily driver that you don't have to wrench on everyday, let the engine builder know, if you tell them you want all out power, be prepared to work on your car, because if your out racing that means your pushing the limit so things will break, if you want it reliable then get what you can and don't try to push the limits of where those parts are expected to live, be honest with yourself and the builder, dreams of going fast cost money, and don't lie to the engine builder it really makes things difficult for us and in the end you probably won't be satisfied with the results.
hope this helps