Stock 781 BBC Heads - Max rpm? - Team Camaro Tech
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post #1 of 9 (permalink) Old Jan 8th, 20, 03:13 PM Thread Starter
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Harris
 
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Stock 781 BBC Heads - Max rpm?

Hey guys,
do any of you know about the common limit rpm-wise with these heads in stock config?

Im tryring to figure out, till which max rpm i should design my valvetrain to be stable.

No point in an 8500rpm valvetrain, if the stock valves limit to e.g. 7500..

Heads: 336781
454, 73-85, oval-port, open chamber,
113-119cc, 256-258/114-116cc ports, 2.06in 1.72ex

Thanks in advance!

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post #2 of 9 (permalink) Old Jan 9th, 20, 07:11 AM
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Frank
 
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Re: Stock 781 BBC Heads - Max rpm?

Quote:
Originally Posted by Harris Roc Malouda View Post
Hey guys,
do any of you know about the common limit rpm-wise with these heads in stock config?

Im tryring to figure out, till which max rpm i should design my valvetrain to be stable.

No point in an 8500rpm valvetrain, if the stock valves limit to e.g. 7500..

Heads: 336781
454, 73-85, oval-port, open chamber,
113-119cc, 256-258/114-116cc ports, 2.06in 1.72ex

Thanks in advance!
I would check with Chris Straub @ Straub technologies, he would probably know
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'68 Camaro 454, 4sp
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post #3 of 9 (permalink) Old Jan 9th, 20, 10:26 AM
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Dave
 
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Re: Stock 781 BBC Heads - Max rpm?

If your valve guides can keep the valves centered then it will be the valve spring that limits the RPM of the head. Recommend replacing the rocker studs with ARP studs, using PAC springs and a Rocker Arm Stud Girdle to maintain that high an RPM. A CompCam rev kit wouldn't hurt either, but it limits your choice in lifter bodies. The lighter you get things like valves (Ferrea titanium intake valves), with CompCam titanium valve retainers, and either hollow stem 21-4N steel alloy or sodium filled exhaust valves (depending on a power adder raising exhaust temps or normally aspirated) the more stable your valve train will be and the higher the engine can be spun and still make power. If you are not making power why spin it up?

You will need a solid roller cam to hit that high a goal and I recommend a minimum of a 7/16th inch diameter chromoly push rod with an adjustable guide plate to fit. Also the '781 had an exhaust valve rotator that has a deeper seat machined into the head; so you will need a spacer to fill the hole before you set the correct spring height.

Big Dave
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post #4 of 9 (permalink) Old Jan 10th, 20, 02:11 PM Thread Starter
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Harris
 
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Re: Stock 781 BBC Heads - Max rpm?

NEWS!

I talked to Chris Straub, he recommends with fully stock 781s, if:

1.) valvetrain is high-rpm-stable and all those parts are capable of these rpms --> lifters, 7/16s Pushrods, roller rockers, dual springs, light as possible remaining parts

2.) Bottom-End is up to that task:

7000 rpm bullet proof,

7400 absolute limit

so now we know!
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post #5 of 9 (permalink) Old Jan 10th, 20, 06:23 PM
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Dave
 
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Re: Stock 781 BBC Heads - Max rpm?

With what I recommended above I used to twist my tunnel ram two fours with vertex mag to 7,800 RPM all the time. never lost a bottom end. Ate a lot of valves figuring how to make everything live at high RPM, but discovered all it took was money to buy premium parts.

Big Dave
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post #6 of 9 (permalink) Old Jan 24th, 20, 10:57 AM Thread Starter
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Harris
 
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Re: Stock 781 BBC Heads - Max rpm?

Quote:
Originally Posted by Larger Dave View Post
With what I recommended above I used to twist my tunnel ram two fours with vertex mag to 7,800 RPM all the time. never lost a bottom end. Ate a lot of valves figuring how to make everything live at high RPM, but discovered all it took was money to buy premium parts.

Big Dave
Hi Dave,
What is a "two four with vertex mag" ?

And what exactly do you mean by "ate a lot of valve"?

I am impressed by that numbers.. what lifters, rockers, springs, Speck you have?

Btw Chris told me that his rockers are up Tasks in sky high 8500rpm. And the limits travel morel lifters up to even more.. so basically the valve weight + spring Combination determines the max... and in my case - stock valves - that 7400 seem to be a limit...

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post #7 of 9 (permalink) Old Jan 24th, 20, 02:50 PM
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Dave
 
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Re: Stock 781 BBC Heads - Max rpm?

Quote:
Originally Posted by Harris Roc Malouda View Post
Hi Dave,
What is a "two four with vertex mag" ?

And what exactly do you mean by "ate a lot of valve"?

I am impressed by that numbers.. what lifters, rockers, springs, Speck you have?

Btw Chris told me that his rockers are up Tasks in sky high 8500rpm. And the limits travel morel lifters up to even more.. so basically the valve weight + spring Combination determines the max... and in my case - stock valves - that 7400 seem to be a limit...
"two four with vertex mag" I raced back in the good old days. I had a 1968 Camaro RS/SS clone of Grumpy Jenkin's Pro Stock from 1970 (Grumpy's Toy IV). It was powered by an over the counter 427 L88 with an Edelbrock tunnel ram and two 660 cfm Holley carbs. Because point bounce and voltage was an issue with points style distributors I ran a Vertex Magneto that had strong springs on the points (that burnt up quickly) and out put ever increasing voltage to the plugs the faster I spun up the engine.

The BBC Chevy engine was designed to be a race engine. But to pay for the tooling it was produced mostly as a truck engine. This means the bottom end was designed to run 300,000 miles at wide open throttle. The bottom end was bullet proof. However because of the canted valves there were strong harmonics that developed in the springs (valves were tilted on two different axis). If you didn't run a damper on a dual spring the spring would break from metal fatigue as the engine reved. You also had to run over 450 pounds of open pressure on the springs to control the valves at high RPM. Because of this you were required to run a solid ROLLER lifter (the spring pressure would wipe the lobe off of a flat tappet cam). This didn't prevent spring failure (were the valve dropped down into a running engine destroying the piston, head and valve by the engine "eating" the valve), it just prolonged the interval between failures.

If you checked the spring pressure on the heads with this tool:

https://www.summitracing.com/parts/mor-62391?rrec=true

You could find a weak spring about to break before it created carnage. I also ran triple wound springs instead of double wound springs to get one more spring to keep the valve from falling into the engine. Parts are cheap compared to the price of a complete engine being destroyed.

If you are going to wind a BBC up (Pro Stocks are now limited to only 10,500 RPM on shifts instead of winding them even higher) then you have to consider valve springs to be a consumable and replace them regularly.

Back in the early seventies I couldn't afford Titanium valves so I was limited to only 10.13 time slips while "da Grump" was capable of turning in 9.70 ETs.

Stock valves are made of a 5100 steel alloy that is friction welded (valve head to valve stem welded by heat of friction). I prefer tuliped stainless steel swirl polished valves that offer more flow at low valve lifts and are lighter than the stock steel valve. Titanium valves and retainers are a race only part due to the titanium metal quickly metal fatiguing and breaking apart. You also have to run lash caps with a titanium valves to keep the ends from mushrooming under the beating of a solid cam and 400 plus pounds of spring pressure.

One other thing is at high RPM the the actual rocker arm studs flex under the pressure so you have to run a stud girdle (in addition to ARP studs) or roller rockers on a pedestal (Jessel or T&D) which drives up the price. Since I keep my big blocks at a more reasonable 7,800 RPM instead of over 10 grand I can get by with a rev kit under the head and a stud girdle (old school). I run an HEI style distributor triggering a seven series ignition box now instead of a Mag, which has no points to bounce at high RPM.

Big Dave
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post #8 of 9 (permalink) Old Feb 2nd, 20, 06:44 AM Thread Starter
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Harris
 
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Re: Stock 781 BBC Heads - Max rpm?

Quote:
Originally Posted by Larger Dave View Post
"two four with vertex mag" I raced back in the good old days. I had a 1968 Camaro RS/SS clone of Grumpy Jenkin's Pro Stock from 1970 (Grumpy's Toy IV). It was powered by an over the counter 427 L88 with an Edelbrock tunnel ram and two 660 cfm Holley carbs. Because point bounce and voltage was an issue with points style distributors I ran a Vertex Magneto that had strong springs on the points (that burnt up quickly) and out put ever increasing voltage to the plugs the faster I spun up the engine.

The BBC Chevy engine was designed to be a race engine. But to pay for the tooling it was produced mostly as a truck engine. This means the bottom end was designed to run 300,000 miles at wide open throttle. The bottom end was bullet proof. However because of the canted valves there were strong harmonics that developed in the springs (valves were tilted on two different axis). If you didn't run a damper on a dual spring the spring would break from metal fatigue as the engine reved. You also had to run over 450 pounds of open pressure on the springs to control the valves at high RPM. Because of this you were required to run a solid ROLLER lifter (the spring pressure would wipe the lobe off of a flat tappet cam). This didn't prevent spring failure (were the valve dropped down into a running engine destroying the piston, head and valve by the engine "eating" the valve), it just prolonged the interval between failures.

If you checked the spring pressure on the heads with this tool:

https://www.summitracing.com/parts/mor-62391?rrec=true

You could find a weak spring about to break before it created carnage. I also ran triple wound springs instead of double wound springs to get one more spring to keep the valve from falling into the engine. Parts are cheap compared to the price of a complete engine being destroyed.

If you are going to wind a BBC up (Pro Stocks are now limited to only 10,500 RPM on shifts instead of winding them even higher) then you have to consider valve springs to be a consumable and replace them regularly.

Back in the early seventies I couldn't afford Titanium valves so I was limited to only 10.13 time slips while "da Grump" was capable of turning in 9.70 ETs.

Stock valves are made of a 5100 steel alloy that is friction welded (valve head to valve stem welded by heat of friction). I prefer tuliped stainless steel swirl polished valves that offer more flow at low valve lifts and are lighter than the stock steel valve. Titanium valves and retainers are a race only part due to the titanium metal quickly metal fatiguing and breaking apart. You also have to run lash caps with a titanium valves to keep the ends from mushrooming under the beating of a solid cam and 400 plus pounds of spring pressure.

One other thing is at high RPM the the actual rocker arm studs flex under the pressure so you have to run a stud girdle (in addition to ARP studs) or roller rockers on a pedestal (Jessel or T&D) which drives up the price. Since I keep my big blocks at a more reasonable 7,800 RPM instead of over 10 grand I can get by with a rev kit under the head and a stud girdle (old school). I run an HEI style distributor triggering a seven series ignition box now instead of a Mag, which has no points to bounce at high RPM.

Big Dave
Hi Dave,
thanks for the info!

I didnt really hear a lot about rev kits up to now. The only one for BBC i can find is the -> COMP Cams Rev Kits 4003.

Is there any downside to using them?

Stud-girdle is also something i have been thinking about.

I know they are recommended for heavy-duty use (tracks etc.).

But what about a car that never sees a track, but only few occasional visit up to 7000rpm? (Of course with rpm-capable valvetrain)

Is this a must-have in that case,too? or rather an nice-to-have?

Thanks for your input!

1967 Camaro SS/RS 396 Convertible 4-Speed L78-M20. I am happy
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post #9 of 9 (permalink) Old Feb 2nd, 20, 10:02 AM
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Dave
 
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Re: Stock 781 BBC Heads - Max rpm?

Rev kit does two things. It subtracts the mass of the weight of the lifter (something that is considerable with a hydraulic roller) by having a separate spring pushing it down allowing the valve spring to control just the valve. And in the case of a spring failure they keep the lifter in the bore preventing a massive internal oil leak if you loose a valve.

With rollers it is not really an issue as tie bar holds it in place anchored by the other lifter. But if you are run a flat tappet or rely upon a key way to index a roller (due to your valve lift being above the ability of a tie bar to reach) then it is useful in this regard. Only down side is the spring rides on the top of the lifter body and modern rollers have a sculptured top in an effort to minimize the lifters total weight so they are not as common today.

Today as in "those thrilling days of yesteryear" (referencing the intro to the Lone Ranger) we worship our idols and place on our cars whatever they had on theirs. We don't think that our car will never be driven as a race car but after all it can't hurt and it's only money. If you do not run strong springs, a solid roller cam and use a stud girdle you won't have to worry about it on the street because your motor will never see high RPM operation. Once you loose control of your valves the dyno graph will show you the loss of horsepower and the fact that your RPM has stopped climbing. You need those items to exceed 5,800 RPM (the usual upper limit on a hydraulic cam). The stock rocker studs can handle the stress of a hydraulic cam (though the weight is much more massive the RPM that magnifies the stress is not).

Why and to what tolerance do you balance a motor? It depends upon it's intended operation. A motor that is off by a quarter of an ounce at a point on the end of a 454 counter weight (four inch stroke crank) will have a ounce imbalance when static; but spin it to 2,000 RPM and the measured imbalance is now 7 pounds, add 2,000 more RPM to 4,000 RPM and the observed weight is now 28 pounds, at 6,000 RPM it is 63 pounds, at 8,000 RPM and that weight is now 110 pounds. Same crank, same imbalance, only difference is in how high you spin it. If you don't run a high RPM you don't need anything other than stock parts (the factory engineers worked on the Big Block Chevy design for three years before it appeared at it's first race; and it was a modification of an existing engine, the 409!).

This is my problem with a street strip car. There is no such thing! If you bring a street car to a race that allows race cars at the track expect to lose every time (what fun is that). By the same token the only time Bill Jenkins ever drove his TOY on the street was seven miles out side a race track in Arizona where his car hauler died and he drove his race car to the track for help (in the days before cell phones).

Do you "need" a stud girdle, rev kit and stiff springs? No not for the street, they are a race only item (I do it, but I had no kids which will cost you $647,380 each over their time with you, and I was well paid as a managing engineer with a big company; so I could afford it). High RPM is desired for one purpose only. The more power strokes per minute (RPM) then the more power you can make. That is why Formula One cars which are limited by the rules to 2.1 liters spin to 15,000 RPM. It is why a Jap bike will beat a Harley every time unless the rules are changed so that it isn't a true heads up race. The computer designed by a large team of Jap engineers Suzuki or Kawasaki bike will rev to the moon while the Harley can't (it was designed by William S. Harley fresh out of engineering school back in 1911).

The BBC days are numbered because the newer LS series is a better more modern engine. Not bigger, just more efficient so it gets more power out of a smaller engine the same way a F1 engine does. In the old days of muscle cars bigger engines where the solution to every problem. Like me, someone who got an engineering degree with a slide rule and a book of logs, my days are also numbered.

Big Dave
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