Thread: Need My Head(s) Examined

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Cayne, you worry too much. Take your meds, when the voices go away, you will feel much better.

Originally Posted by 65ny

Cayne, you worry too much. Take your meds, when the voices go away, you will feel much better.

I think, subconciously, that I am worried about your mopar outperforming my bowtie (especially with your shiny new carburator)
Nah...must be something else because we know THAT could never happen!
I am feeling better now. I would feel even better if I could get Pat's .02 although he probably has never assembled a "bad" combo such as mine...right Pat?? Maybe back in his early days... kidding...!

Originally Posted by 65cayne

Here goes...I've got a MkIV 454, with a 770 Holley Avenger sitting on a Weiand Stealth dual plane manifold, 9.5:1 CR, Voodoo 213/219 @.050 (256/262 adv), .515/.530 lift, 112LS, 6272990 heads (2.19/1.88 valves, rect port), ceramic coated headers into 2.25 dual exhaust through a set of FM 40-series mufflers, HEI, ...all this is connected through a TH400 to 3.36 gears in the rear. The cars primary purpose will not be that of a daily driver (gas$$) but it will be street driven with an occasional trip to the track for fun.

I know this is not the best combo in the world and that I have some parts working against each other. Let me just say a few things about that...

1. I got a good deal on the heads
2. the exhaust is brand new, aluminized umm...steel? anyway, it's new and came with the car (for the previously installed SBC)
3. I know alot more now than I did when I started this project.
4. I have a tight budget
5. I have a tight budget
6. I have a set of peanut ports (14092360) that I almost installed in lieu of the RP's

My question is, how well do you think this will behave? Will it be a total dog? Will I be fouling plugs? Street manners? Should I have used the peanuts?Exhaust big enough for decent performance? What changes would you suggest in terms of increasing performance without having to tear the whole motor apart? Any input is appreciated....I am basically wondering if this combo will perform respectable enough to give me some braggin rights and drive for awhile until I can find a decent set of oval ports (plus time and more $$) to put on there. This is my first real build project and turning back is not really an option at this point (I have a kid on the way...need to finish pronto). What do you all think? Has anyone ever thrown together a "bad" combo and been relatively happy with the performance?

It's a real nice looking motor anyway......Does that boost my hp?

-cayne

I.M.O.,the heads are a little large.
I would think it would run better with say a set of 781's with the 2.19/1.88 valves,but don't go selling what you have.
Other than the heads everthing looks good to me.

Are you fairly certain on the c/r?? Sounds pretty good to me. I ran my 291 rect. heads (same size runners, but closed chambers) on my 454 several years ago..my cam was a little bigger, but I can assure you it was not a dog because of the heads...yes, they are a little on the big side, especially with the rest of your combo, but I think they should work out very nicely! You must have a domed piston if you're at 9.5/1, seeing as the 990's are supposed to be 118cc. I think I would just run it and dont worry. Yes, a gear change would compliment the heads, as would a converter and larger exhaust...but one thing at a time!! All that can come over time. Im still running my 291's on my current 505, and they are awesome. Good luck with it. John

Thanks for the comments Erik, John. Just curious, what compression are you running with your 51 extra cubes John?
My pistons were advertized with (approx) 10.32:1cr with 106.9cc head (btw: yes, they are domed). I took those figures to determine the total combustion volume and swept volume. Then it was a simple ratio to calculate the compression for my 118c heads. The approximate value was about 9.47:1 if I remember right.

I get 10.35:1, but close enough.

Originally Posted by Henry Rifle

I get 10.35:1, but close enough.

What do you get 10.35:1 for? My pistons? Or on your own engine?

I had some time to kill at lunch...

for a 454cid motor, you get 56.75cid/cyl = 930cc/cyl = swept volume (SV)
So in this instance, to determine CR, we have:

CR = [SV + (total combustion space)]/(total combustion space)

where: (total combustion space) = volume of the combustion chamber (call it CC) plus other volume (+/- depending on dishes or domes, gasket thickness, etc...call it X)

CR = [SV + (CC + X)]/(CC + X)

tickle the formula a little and you get:
X = [SV/(CR-1)]-CC

So based on a 454cid and the information provided with my pistons
X = (-7.1)

Now using my values... SV=930, CC=118, and X=(-7.1)...I get a CR of 9.39/1 If I have screwed up somewhere...jump on it and let me know. All input is welcome!

Are you some kind of engineer or something?? My head hurts from reading that.

O.K., when you figure c.r. on a cylinder with a domed piston, you have to deduct the volume of the dome from the volume of the cylinder because that dome volume will not fill up with mixture. All other volumes are figured the conventional way, including complete chamber volume, deck height volume and gasket volume. Now, when going back the other way and compressing the volume you have drawn in, you have to deduct the dome volume from the chamber volume. The result of this is as follows:

By back-figuring, I have determined that the dome is worth 22 cc's. If this is a 0.030" over 454, then the bore and stroke are 4.280" X 4.000", resulting in a cylinder volume of 943 cc's. On the intake stroke, we will deduct the dome, so the cylinder will be ingesting 921 cc's. Not knowing whether or not you have decked the block, I assumed not and allowed a deck of 0.030", which computes to 7 cc's. I allowed a gasket diameter of 4.300" and a thickness of 0.040" which computes to 9.5 cc's.

Now, adding all the incoming mixture, 921, 7, 9.5 and 118, we get 1,055.5 cc's. Now we'll deduct the dome volume from the chamber and compute the chamber at 96 cc's. So the squeezed mixture will be 7, 9.5 and 96 which totals 112.5 cc's. Now, dividing 1,055.5 by 112.5 reveals a static compression ratio of 9.382:1

Now cayne, if you will bless us with the intake closing point of the camshaft, we'll be able to figure the dynamic compression ratio of the motor and maybe get a handle on the type of fuel you'll be able to use.

Originally Posted by techinspector1

O.K., when you figure c.r. on a cylinder with a domed piston, you have to deduct the volume of the dome from the volume of the cylinder because that dome volume will not fill up with mixture.

I compensated for the dome when I figured the total combustion volume (piston @ TDC).
I compared your numbers with mine (my answer was actually 9.389, very close to yours, which I rounded to 9.4), and it seems that you did the same thing that I did, only in a more "technical" way (hence, your name I guess.. ). Sorry, I can be a lame joker somtimes...no offense intended.
Anyhow...my cam specs are as follow:

213/219@.050
256/262 adv dur
.515/.530 lift
112 LS

I have a feeling that dymanic compression is much more complicated. Is there some type of "approximation" or do you have to wear out a pencil and 20 sheets of paper?

I just re-read what you requested....intake CLOSING pont and realized that I did not provide that for you (I dont think)... my cam card says the following:

IO @ -1.5 btdc
IC @ 34.5 abdc
EO @ 45.5 bbdc
EC @ -6.5 atdc

The intake centerline (??) is 108 degs. (says "the above centerline will make this cam 4 degs advanced") I have no idea what all this means. It looks like my exhaust valve is closed before my intake even starts to open. I must be misinterpreting it because I always thought there was some overlap there.

The numbers look screwy because the cam grinder is giving you numbers based on a tappet lift of 0.050". They would look very much different if the figures were based off the tappet just beginning to lift off the base circle of the lobe, or even at advertised lift figures which are usually based off tappet lift of 0.006".

Anyway, this will be the easiest calculations you will have to do, just add 15 degrees to the IC and plug them into the electronic calculator provided by KB........
http://kb-silvolite.com/calc.php?action=comp2

Compression ratio-9.382
Stroke-4.000
Rod Length- 6.135
Cam timing 49.5

The result is 8.314:1

Most of us shoot for a figure between 8.00 and 8.25 for a street motor on pump gas, but I wouldn't think 8.314 would be out of line too much. You may have to run a little better grade of fuel than 87 regular, but it should be ok. If it has a little too much cylinder pressure, you can bleed some of it off by retarding the camshaft in relation to the crank.

I don't know if you understand, so I'll explain. On the intake stroke, the piston reaches bottom dead center and starts back up in the bore to compress the mixture so the plug can fire it. The intake valve is still open as the piston reaches bottom and does not close until the piston is headed back up in the bore. The reason for this is that the slug of mixture that is being drawn in has inertia and will continue to jam itself into the cylinder even though the piston is coming back up. Now, this is a juggling act and if the valve remains open too long, the piston will push some of the fresh mixture back out past the valve seat and up into the intake port. When this happens with a really radical cam, you can see the cloud of foggy mixture hovering above the open carburetor top. This plays hell with the metering in the carb because it isn't getting a clean vacuum signal from the cylinder, so the motor tends to idle erratically, load up and generally just be a pain in the butt. That's where you get the rumpety-rump idle from. It's just the sound of the motor being inefficient at low rpm's.

Anyway, this closing point can be altered to make the motor do what you want it to to a certain degree. For instance, if your motor tends to rattle on pump gas, that means that the dynamic compression ratio is too high and the motor is making too much cylinder pressure. To lower the pressure, you could leave the intake valve open a little longer and trap less of the mixture. For instance, retarding the cam by 4 degrees from its present position would put the intake closing point at 53.5 degrees after bottom dead center and would change the DCR to 8.133

I'm going to continue this conversation by asking you if you know the on-seat and full-lift pressure of your valve springs. I've seen a bunch of brand new cams wiped out by starting up a new motor with stiff springs. Let's get this sorted out before you do your initial startup.

We need to address the proper procedure for startup too, if you don't know.

Have you adjusted your valves and you POSITIVELY know they are correct?

I dont know about the springs. Heads are basically a clean swap from another motor (the guy bought some aluminum heads and had no use for these). So I just bolted them on there after cleaning them real good. I understand the terms but I dont know what the values are. I hope this wont be a problem

I adjusted the valves by first positioning the motor (i.e. watch for exhaust valve open, adjust intake...dont recall precisely how to explain it but that is one thing that I want to check again before intial fire up) and then tightening rocker until I feel the resistance and then putting another 1/2 to 3/4 of a turn on it.
Suffice it to say that I am not POSITIVE that they are correct and I would much rather adjust them again to be sure.
My plan, when I am done with all of the nit-noid stuff I need to do to finish this installation, is to prime the oil system, prime the carb, start it on the first try, set the timing (not sure to what degree), set the carb (if necessary??...brand new out of box), and rev it up to 2500 or so for about 30 minutes to break in the cam. That is my plan and I am not necessarily "sticking to it" because I could be wrong. That's why I am here.

btw: does dynamic CR change with RPM? I am guessing yes?