I have stock Gen IV 454 & 781 Heads, Compression ????
Im going to be working with stock 4 bolt main 454 Gen IV bottom end.. ( in vehicle) I have a set of stock 781 heads.... I'm ready to send the heads off to a machine shop for work ( if needed/recommended) for new parts to match cam and for port&polish job.
Ive been searching and it looks like the cc's of the 781 are 115-118 cc's. not sure on the valves.
The cam im looking at recommends CR to be 9:1+.
Can I and how do I get my CR up without bottom end work?
Should I get the bigger valves 2.19 and 1.88?
What should I tell the machine shop to do with my heads ( mill? cc? P&P )
Should I use 0.020" shims (read that) Port match gasket?
The the cam recommends 2 different valve spring sizes, how Would I determine the answer. so in the future, I can figure it out for myself instead of having to ask. here is what it offers/recommends for the cam
Single Outer Valve Springs: 1.524" O.D., 1.110" I.D. OR
Dual Valve Springs: 1.509" O.D. Outer, .697" I.D. Inner On a guess, Id say the Dual valve springs BC of High RPM and my cam is .51 lift int & exh. and my rocker ratio is 1.72? is that right
Not a daily driver.. Looking to get high tq. rpm range (2200 to 5800)
Any input/advice/direction would be appreciated.... thanks in advance
Building a motor that will make power for a length of time requires doing the same drill every time. It ain't rocket surgery, but it does require that you know what you're doing and that you pay attention. All the parts must be figured out ahead of time to work with each other and generate a product that will do what you want it to do.
I'm constantly amazed at the fellows who come on here and tell us how they don't have a clue what's inside the motor, but they're gonna install a world-beater camshaft in the motor.
Start a log and record all the parts that you use, part name, description, manufacturer, part number and dimensions that you, yourself have taken off the part with your own precision measuring equipment.
Part of the blueprinting process that you must do if you intend to build the motor correctly is "pouring" or "cc-ing" the combustion chambers. Most shops, I think, will charge around 100 bucks to do 8 chambers. You can invest the 100 bucks into equipment and do it yourself. That way, you have the equipment to do another motor down the road or to make a little money pouring heads for friends. I might be thinkin' 60 bucks for a set of V8 heads, 50 bucks for a set of V6 heads, 45 bucks for a straight 6 head and 30 bucks for a 4-banger head. Just doin' a few jobs for friends would pay for the equipment. After that, you'd be makin' pocket money. You would need the heads to be clean and complete with valves and spark plugs. You will not need springs, retainers or locks.
You could also cc dome pistons if you have a cylinder to slide 'em into. If you're interested, I can explain the procedure to you. This cc equipment is very light and could be hauled over to someone's garage to do the measuring (for instance on a block that would be hard to haul around).
Buy this cast iron base....
This 100cc acrylic burette....
This piece of plexiglas....
And these cylinder head work stands....
I don't port match unless it is a max effort motor where every little thing counts. Most times, the grinding is not carried back into the port far enough and it appears in cross section to look like an Anaconda that swallowed a pig. The increased volume at that point will slow down the mixture as it travels the port and may allow fuel to drop out of suspension, giving you tuning headaches that you may not be able to fix.
Dynamic compression ratio with this cam is 8.53:1....
Or, if you want a little more cam....
Dynamic compression ratio with this cam is 8.36:1....
I agree with the above posted reply. Plus you will need more pieces in the heads. Stock 781 heads have VERY short installed heights. After all they are truck heads. If you have larger intake valves installed. Get +.100 long and required retainers and locks/shims.. The exhaust side can use stock length valves with some sort of rotator eliminator (spacer). Your spring spec. did include installed height. Usually 1.875" on a mild big block.. Shorten the guides and cut for decent seals.
Thanks Bob. I just realized that I left out the link to the piston....
Well I guess I kinda am building the engine around the cam but... Not really.. I Know what sound I want and what kind of power I want. I found the heads I want to use. I know for sure in the spring Im going to completely redo the bottom end.. Forged.. Because of my limitations of working on the road and living in a hotel. Im limited to a point.... Going to rent a garage in the spring or thinking about buying an enclosed trailer for a workshop (mobile). Take the engine out and take it apart, down to the bottom end, and off it goes to the machine shop...
both those springs are recommended from the manufacture for that cam. But it depends on the application and its use.
My intentions is to get everything together and ready to replace the top end. So, I was going to get the heads ready. All NEW components. Then, Cam, Timing Chain, Cover, etc.. so I can get the engine closed back up over the weekend. In the spring, Im now ready for a bottom end rebuild and I also now have a feel for my top-end and if any changes/adjustment need to be made for my preference.. I may not even like that cam/heads.. But I figure experience cost $$$ and adds knowledge. For a few 100 $ its worth it to me..
Techinspector1.... Defiantly buying those items. thanks... Another thing.. All my tools were stolen over the summer while out of town. So, Im starting over with tools.
So, Back to the question and project..Im wanting strong power out the door and good power and tq up to 6000 rpm..
With completely going threw the 781 heads. which way Do i need to go with the fact in mind. Im not doing any bottom end work till the spring. My heart isnt set on anything except these heads. PS.. all this is in a Truck.. 90 SWB std cab.
I need to know what I need to talk/tell the machine shop/head shop. I travel bc of my job. So, I will be finding a reputable machine/head shop online and use them. Im in Dodge City,KS and Im NOT using any machine shop around here.
So, help me out here. lol...
Oh, and what about these valves...
[Oh, and what about these valves...
Im reading and researching.... should I use 2.25" and 1.88" valves... Knowing the bigger means more air. also know bigger is not always better... if the ultimate plan is to bore out to 496... I just wish i knew how to consider sizing everything together? Cylinder volume > head CC's > runners> Intake> Pistons>
How would I go about learning how to do the math myself to see what would be best? is there formulas or trial and error or just plain ol' experience
Also, Ive heard boring out to 496 is bad idea due to the fact of thin walls?
I just know this is my second build. Fist build of bottom end.. and Im gonna do a great deal of talking to everyone on here and researching and researching some more...... I dont really care to much about my ego of trying to look like I know everything.. Id rather chit chat, listen and learn why.
496" is .060" over and a .250" stroke increase.
About the biggest you can bore your way to is 482" (.125" over). That's way too much on the street.
Regarding quench....I'd tighten that up to .038" min. to .045" max.
I would recommend pressure testing over magnafluxing.
stock flat tops on a 454 with 120-123cc heads (781's) makes around 7.7 to 7.9 :1 cr.
you will need a dome piston with 781's to get the compression up or do a 496 stroker kit.
Im going to order the things needed to measure the cc myself as techinspector1 recommended.. How did you do the math to find out the CR. What if or lets say the heads are 118 CC
what would be the CR then?
9cc=head gasket (0.040" thick)
4cc= piston below deck 0.015"
5cc= valve reliefs
(930 + 118 + 9 + 4 + 5) / (118 + 9 + 4 + 5)= (930 + 136) / (136) = 1066/136 = 7.83
Static compression ratio is found by measuring the volumes of all components involved, converting some of them to cc's and doing the math. You will see 2 different constants throughout my math, (.7854) 25% of pi and (16.387) number of cc's in a cubic inch.
You will need 5 values for the math:
1. Volume of one cylinder in cc's. Measure the bore and the stroke of one cylinder. Let's say it is a 350 Chevy block that you have bored +0.030" (cylinder bore measures 4.030" diameter with your dial caliper) and the stroke is stock at 3.480". Begin by multiplying .7854 times 4.03 times 4.03 times 3.48 times 16.387 and find 727.41 cc's in the cylinder.
2. Chamber volume. You have a set of early production heads, #882. You set up your burette and pour them, finding an average of 78 cc's in the combustion chambers. You are so excited about using the burette that you also cc the intake and exhaust ports.
3. Piston Deck Height Volume: This can be found by first measuring the block deck height (the distance from the centerline of the main bearing bore to the flat surface of the block deck where the heads bolt on). You can do this yourself with a 12" dial caliper or you can have the machine shop that will do your machining to do it for you. The nominal block deck height of a 350 Chevy block is ~9.025". Let's say that this block measures 9.015", indicating that it has probably been surfaced by someone else at sometime in its past. Now, we will add up the stack of our reciprocating parts. Half the stroke will be 1.740", found by dividing the stroke by 2 (3.48 / 2 =1.740). A stock 350 will have a rod length of ~5.700" and the piston we will use will have a compression height (distance from the centerline of the wrist pin to the crown of the piston) of 1.560". Now, if we add the 3 values together (1.740, 5.700 and 1.560), we find that our reciprocating (stack) of parts measures to 9.000". Remembering that the block deck height was 9.015", we can subtract the stack from the block deck height (9.015 minus 9.000) and find that the piston will be down in the bore 0.015" from the deck of the block with the piston at top dead center. Now, we can calculate the volume by figuring it the same way we did for finding the displacement of one cylinder. (.7854 times 4.03 times 4.03 times .015 times 16.387) and find 3.13 cc's.
4. Piston crown volume. 99% of the time, we will be dealing with flat top or dished pistons on this forum, so I will use a flat top for this exercise. The manufacturer will post the volume of valve eyebrows and/or dishes, so we just have to look at the literature to find it. We'll say that the piston is a flattop that has 7 cc's of eyebrows, so there's our piston value, 7 cc's.
5. Compressed head gasket volume. We measure this the same way as we did the cylinder volume and piston deck height volume, using the gasket bore and thickness. Let's say we're going to use a 10105117 GM gasket that measures 4.000" bore and 0.028" compressed thickness
(.7854 times 4.00 times 4.00 times .028 times 16.387 = 5.76 cc's)
The reason for choosing this thickness of gasket is to set the squish properly (0.035" to 0.045") without having to cut the block decks. Personally, I would want to cut the decks a little just to make sure that each corner of the block is the same distance from the centerline of the main bearing bore. If it took, for instance, 0.015" to get it square, then the block deck height would be 9.000" and the stack would be 9.000", so the piston crown would be dead even with the block deck with the piston at top dead center and you'd want to use a little thicker gasket to set the squish (0.041" gasket would work).
Now, we will add the 5 totals together....
727.41 + 78 + 3.13 + 7 + 5.76 = 821.3 cc's
Now, we will drop out the cylinder volume and add the other 4 values....
78 + 3.13 + 7 + 5.76 = 95.89 cc's
Now, we'll divide the larger value by the smaller value....
821.3 / 95.89
and find a static compression ratio of 8.56:1 with a 0.043" squish (to arrive at this squish, we add the piston deck height of 0.015" and the gasket thickness of 0.028").
Now that you know the SCR of the motor, you are armed with at least one of the determining factors for choosing a camshaft, the relationship of SCR to the closing point of the intake lobe on the cam.
If we look at this chart that I put together, we'll find that the motor (at 8.56) will like a camshaft with around 194 degrees @ 0.050" tappet lift. This is not the only parameter you will use. Others would be engine size, intended use, torque converter stall, transmission used, rear gear used, tire and wheel size, etc. Now, you can move around on that chart by 3/4 of a point either way to tailor the cam to the purpose of the motor. But at least it will give us a starting point to find the proper Dynamic Compression Ratio.
The DCR takes into account the SCR, stroke, rod length and intake closing point on the cam to give you a DCR (an indicator of the relative strength of the motor and it's tolerance for different grades of fuel).
If we use the KB calculator...
We'll plug in our values of 8.560, 3.480, 5.700 and 36.000 and find a dynamic compression ratio of 8.039:1. This would work fine on crummy pump gas.
Here's how I got to the 36.000 figure. I looked at Crane Cams site and chose a camshaft with an intake closing point of 21 degrees after bottom dead center @0.050" tappet lift. You'll see the figures at the bottom of the timing card....I like to use Crane because the intake closing point @0.050 is easy to find....
You'll notice the instructions of the KB calculator that tell you to add 15 degrees to the intake closing @ 0.050 figure, so if we add 21 and 15, we will enter 36.000 on the calculator.
Now, armed with this information, anyone should be able to find the SCR and DCR of any motor combination that uses flat top or dished pistons. To figure SCR with a domed piston, ask me how. I'll expand on this post a little and make a new wiki article from it.
Holy WOW Techinspector1!!! I havent had a moment to let that sink. Work has that affect. lol I had an alert from my phone telling me I had a response to this post.. After looking over it at work. I knew I had to take a moment when I got in, to Thank You for the detailed instructions on SCR and DCR. In which I had no idea on DCR! So I really want you to know how thankful I am for your concern and time to explain in detail the critical knowledge needed when deciding how and which way to go when building an engine properly. And, KNOWING all your components are coordinated together! So, Thank You!
Im going to have to play with those equations and use different engines and pistons and gaskets in order to really grasp and KNOW the math! It appears Im going to have to use dome pistons for where Im going. If not, I may in the future, so..... Please do. Explain Dome Piston SCR and DCR.... MY guess would be to read the same literature as the piston eyebrows and find the negative side of the dome piston and just subtract that? then, Also having to check clearance from deck height to be sure valves are in the clear and the quench is where it needs to be.. Of coarse I have no clue how that process would need to go. But, Using what Ive just learned and what your about to teach me on DOME, I know Im about to learn!!!
On another Note..... I have another engine.. exact same as Im wanting to work on and talking about in this forum.. SO, My plan is to rent a climate controlled storage unit. use that as my temporary shop.. and Build that engine Correctly. Instead of building half now and half in the spring.. Why not just tear it down, send it to the machine shop and start. ground up!
i am no expert, and there are guys here that know way more than me on this subject, but........
i would check the math on the costs of doing those heads, parts and labor.
unless you are able to do some of the work yourself (porting/bowl blending for new larger valves)
also the cost of the springs and other pieces for the cam etc.
you might find it with be worth the couple extra bucks and just buy new heads
it all adds up quick
i'm building a set of 781's and if i was to do it over, i'd just sell them
you may be able to find a set that will work better with your flat top pistons
Yep wild_man, it's just that simple. We'll start with this piston....
Subtract the dome volume from the combustion chamber volume before you begin your calculations. ex: 118-25.5=92.5, so that's our new combustion chamber volume to calculate from. We have whacked off the piston dome and stuffed it up into the chamber, reducing the volume of the combustion chamber. Now, visualize the piston being perfectly flat on top with a slight valve relief divot which is worth 3 cc's.....see here.....
We'll call the notch "piston crown volume" and figure it in when we calculate. Or, we can borrow 3 cc's of material from the combustion chamber and stick it into the valve relief of the piston. We'll magically fill the valve relief with 3 cc's of material, leaving a perfectly smooth piston crown and an 95.5 cc combustion chamber volume (92.5-3=95.5). Doing your own calcs allows you to be whimsical if you want to.
What to do with the pop-up on the intake stroke though? It's sitting there on the piston crown and taking up space (volume) as the piston descends in the bore with the intake valve open. That 25.5 cc's of piston dome could have been 25.5 cc's of fuel/air mixture if a flat-top piston had been used, so I think we have to consider it and deduct 25.5 cc's from the cylinder volume.
So, BBC bored whatever, let's use 4.310" as a bore size with a 4.000" crank.
(.7854 X 4.310 X 4.310 X 4 X 16.387 = 956.32 cc's. Now, we deduct the 25.5 cc's from the 956.32 cc's and find an actual cylinder volume of 930.82 cc's.
Next, combustion chamber volume. You have become proficient at pouring voids and doing calculations to arrive at an exact answer. You're makin' a little money on the side, pourin' for the local speed shop. You pour these heads at 121.0 cc's across both heads. Someone has spent some time on them, equalizing the volumes chamber to chamber. If they aren't all the same, then you have different static compression ratios cylinder to cylinder.
We've stuffed 'em with the domes off the pistons and now we'll take back 3 cc's in just a minute, leaving a chamber volume of 95.5 cc's.
Now, we know that the block deck height is ~9.800" stock, but you measured this block at 9.785" with your 12" dial caliper...someone has taken a light cut on it sometime in the past. You hate electronic calipers because everytime you pick them up for use, the battery is dead. Anyway, we need that figure to know how much space there will be for the reciprocating assembly (crank, rods, pistons). Half the crank throw is 2.000", rod length is 6.135", piston compression height is 1.645"
so the length of the "stack" is these 3 added together and arriving at 9.780". This will leave a piston deck height volume of (.7854 x 4.31 x 4.31 x .005 x 16.387 = 1.19 cc's).
The value of the smooth piston crown will be zero.
A commonly used head gasket would have a bore of 4.370" and a thickness of 0.039", yielding 9.58 cc's.
1. cylinder volume 930.82 cc's
2. chamber volume 95.5 cc's
3. piston deck height volume 1.19
4. piston crown volume 0
5. head gasket volume 9.58
Thanks again Techinspector1! I havent forgotten about this thread or post. Really been putting in the hours to be able to get off for Christmas. SOOO, I will get back to this post this afternoon and see what Ive learned..
|All times are GMT -6. The time now is 12:40 PM.|
Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2015, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0 PL2
Copyright Hotrodders.com 1999 - 2012. All Rights Reserved.