Help with 340 compression + cam change

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Honestly, I wouldn’t worry and I would focus on the cam change at hand. Changing the cam should be done carefully because we now know the compression is at a spot where a reasonably size performance camshaft can kill this engines low end.

So think before you leap. ;)

If it were I, I would not increase the intakes duration but only at a max of 6*’s. But it is also the cams timing events. A catalog cam “may” not do what you want it to do. At least installed straight up or as recommended. I’d look for an early intake valve closing to help build cylinder pressure.

If you take a catalog cam and adjust where it is speced to be and move it for an earlier intake closing, you’ll also change the rpm band downwards a few rpm. It will still make more power.

You might want to call a grinder for a custom cam and simply tell them your engine is at 8.5-1 ratio.
 
midnight340 said:
Just found this from "70aarcuda" on this site back in 2011:
"pistons were changed...compression height changed from 70 340 with a 1.84 to about 72 340 with a 1.74....."
Also I've read several times that the compression height change was 0.100". Anyone know for sure?

mn9stheham, this might be more accurate than what I'd stated above. Does this make more sense, or not much difference?
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Thart hangs together exactly with the data sheet I found and linked, the numbers I ran, and matches up with what was going on with all the SBM when the emissions stuff came along in the early 70's. Leads to a cranking compression the 125-135-ish range for such a cam.
 
I’m planning another compression test tomorrow. Picked a new gauge to check accuracy of my existing. (As long as we’re not under blizzard conditions!!)
 
considering the numbers you gave, i come up with 8.4:1 compression ratio. playing with my dyno simulator program, i get two cams that improve both torque and hp, keeping in
mind your converter. and they are comps 275deh and xe274h. they both should increase power by 20-25 hp, and help you shave up to 3 tenths in et. the cam that comp sells that is closest to your lunati cam is the 268h. that one was my baseline cam for cam selection. the 275deh being a little milder than the xe274h, is the one i would choose and they say it could be used with a stock converter, so you know it is not too big.
 
Hummm, I did say, “Just say 8.5-1” now didn’t I?

LMAO!
 
I really appreciate all the input from you guys and I’m taking note of your thoughts on this.

I’ll get a current compression test with my new tester done, hopefully tomorrow.

Another idea I’ve wondered about...
What are your thoughts about trying a good set of 1.6 roller rockers with my existing camshaft?? Keep my duration but adding lift.

I know that would be expensive, but other than that, are there reasons I should not consider this.
 
Wellllll, the expensive low return for the buck is the great issue at hand but.... New rocker cost, new push rod cost is as inexpensive as it gets. You could always add more cost with billet hold downs, ARP studs for the shaft and go through the pains of setting up perfect geometry with a B3 kit.
Heck! You could even have the cryogenics treated.

The accurate & correct ratioed rocker will add a slight bit of intensity to the cam, maybe 2*’s of duration. A more accurate cam timing takes place as intended. And that’s where the power is gained.
Not because it “rolls.”

Is it worth it? YOU tell me.

Hughes Engines
 
Now that was a pretty clear answer! :thumbsup: Gotcha!
I'm gathering what info I can, and considering. I think I've gotten a fair idea of the likely actual compression by now, and as I said will double check cranking pressure. At that point I will call some cam grinders and see what they have to say.
 
I finally got the snow cleared away from my shop doors and did the compression test with a new gauge (which agreed exactly with my old one.) Engine fully warmed up to about 150-160 degrees, carb open, etc.

I had stated I had 160# all around, well I checked and my last reading was THREE YEARS AGO! Time flies when you're having fun! So with at least 40 runs down the strip since, and "daily driver" duty part time, etc. the #s are as follows:

Four cylinders were still between 158# to 160# (...checked with both gauges)
Three cylinders were 150-155# and number 7 was reading at 145-147#s.

So in spite of the 8.5 TRW stock style pistons, and the 70cc head volume, I'm getting decent compression with this Lunati cam. (284adv. 218@.050/.458 lift single pattern)

After all the research and the help here, I should be at 8.4 to 8.5 compression. If that doesn't fit the formulas, it's possible that the decks were not stock before the .015" clean up even though the machine shop said they were???

As Rusty had stated earlier, maybe with this compression, (even though I'll have the timing cover off some time soon), there's not much reason to change the cam right now??? (unless I change everything, including pistons.)
I know there are differing opinions but all are appreciated, and I'm learning some stuff!
 
Well those are very good compression numbers and since you have tried 2 gauges, I'd run with your readings. Says that something is driving up the compression more than is known from the paper numbers: machining, maybe chambers smaller than 70cc or more decking than known, or maybe not those pistons....

But that is all good as it says you can increase the lift a fair degree and/or duration if you like, and still keep the low RPM's stout for the street.

Is your objective to go faster on the strip, or ??? And do want to keep that TC stall speed, or is that an option to change?

If you want higher lift-to duration, then I'd nudge you look at the Howard's cams with the lobes optimized for the larger Mopar lifter size. That gets some really good lift to duration ratio; download their online catalog, and look for the ones with the cam note 15 in the right hand column. Also, they have quite a range of these profiles, many more than are listed in the catalog; you can down load their lobe list, and look for the .903 Mopar profiles. These can be custom ordered through Summit or Jegs and others with a base cam number, profile(s), and LSA.

But if you go higher lift, you're gonna have to look at total lift avialable in places like the retainer-to-guide clearance, check P-V clearance, and also springs if you go with a faster rate cam, like a VooDoo or Howard's.

And 1.6 rocker will do like you think. It won't necessarily be cheap to get reliable ones.
 
I am pretty sure of the pistons and the cc number, so I think The only other factor maybe pushing compression up some would be the deck height.

Thanks for the nudge toward the Howards cams as I've been getting the sense they might be a good choice. As well as the Voodoo cams.

Goals, well, more power under the pedal is always appealing! Two more aspects:
(1) I would like to feel I'm using these heads more, as I am pretty sure they have potential.
(2) and to a lesser extent I run my "baby" brother on the strip each year. (that should explain that part!)

Primarily, I've always felt I wasn't getting at this motor's potential. But there's no end to that rabbit hole is there?!!
 
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More lift ought to work with the idea that the cam is limiting the flow more than the heads, 'specially if it gets the valves open more quickly.

Probably stating the obvious, your 1/4 mile time seem a bit long for what you have.....and the top speed looks to be around 5k RPM for your gears. Might be that things are being choked down.

And there may be other things going on. Others can comment better than me on that....
 
Man, that Howards catalog is pretty extensive!!! And I lived in Wisconsin for a number of years so tend to have a good feeling about midwest companies. They always seemed to me to focus on quality and less hype.
 
I’ve never understood what dynamic compression ratio means or what it would tell me. And prior to this thread wasn’t sure I had the info to figure it out. Consensus here is that my static should be at least 8.5:1.

So using the Wallace calculator with 8.5 static and this cam (34 ABDC) I get:

Static compression ratio of 8.5:1.
Effective stroke is 3.10 inches.
Your dynamic compression ratio is 8.02:1

So what does this tell us???
 
The 351M in my 75 F250 had 160-165 in all eight about 5 years ago when I put the re-man heads on it. They are rated at 8.4:1, but routinely spec out at around 8:1. The cam in mine I am almost positive is the Melling, Elgin, everybody's grind that's like .486/.512 and 204 and 214 ground on a 112. It has a very slight bump when cold but stock when warm.

Your example is a shining one that all the formulas and calculators in the world sometimes come up short. Did you degree that cam? If not, you can add a good chunk to it if you advance the camshaft timing probably 4*. I have threatened to do that with mine, but I have the rebuilt 400 under the work bench.
 
RustyRatRod said:
The 351M in my 75 F250 had 160-165 in all eight about 5 years ago when I put the re-man heads on it. They are rated at 8.4:1, but routinely spec out at around 8:1. The cam in mine I am almost positive is the Melling, Elgin, everybody's grind that's like .486/.512 and 204 and 214 ground on a 112. It has a very slight bump when cold but stock when warm.

Your example is a shining one that all the formulas and calculators in the world sometimes come up short. Did you degree that cam? If not, you can add a good chunk to it if you advance the camshaft timing probably 4*. I have threatened to do that with mine, but I have the rebuilt 400 under the work bench.
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My 351C with all the combustion volumes measured and computations made 45 years ago came up with 160 psi with a 10.3:1 SCR. But the cam was 'long tailed' and knocked down the DCR to the low 8's... which goes right to around 160 psi cranking compression. So it works when you put the right info in it. I don't know what to say about your 351M, but it makes a few assumptions on what is in there, and if the valves are moving as thought....
midnight340 said:
I’ve never understood what dynamic compression ratio means or what it would tell me. And prior to this thread wasn’t sure I had the info to figure it out. Consensus here is that my static should be at least 8.5:1.

So using the Wallace calculator with 8.5 static and this cam (34 ABDC) I get:

Static compression ratio of 8.5:1.
Effective stroke is 3.10 inches.
Your dynamic compression ratio is 8.02:1

So what does this tell us???
Click to expand...
You don't use the intake closing at .050" to compute DCR....the DCR number is computed with intake closing at or near the advertised duration numbers. So 8.02 is not correct for an 8.5 SCR..... Buuut DCR does correspond closely to cranking compression, and if you have 150-160 psi cranking compression, then your DCR is close to 8. (Meaning that the SCR has to be higher than 8.5....or the valves are doing something unexpected when cranking.)

The whole point of running the cranking compression after running the engine is to make sure the lifters are pumped up and closing the intake valve when it is supposed to. If you have Rhodes lifters, which lose duration when oil pressure drops, then they will give a higher DCR and cranking compression.

DCR is a good indicator of the motor's low-mid RPM torque performance. You'll feel a 1/4 point change, a 1/2 point change will make the motor 'wake up' some at lower RPM's, and a full point will make it feel like a different motor. If you are spending your engine operating time up in the higher RPM ranges, then it becomes far less of a factor. So it has meaning for street, cruising uses, and less so for drag racing use.

Too low of a DCR will be an engine that bogs at low RPM's..... which is one reason why guys put in higher stall torque converters with a big cam.... the DCR drops and they have to rev the engine past the low RPM bog to get a good launch. This why all the warnings to not over cam a stock mid-late LA 318 which starts with low static CR around 8... over cam it, the DCR drops down into the low 6's and it is bog-city.
 
nm9stheham said:
You don't use the intake closing at .050" to compute DCR....the DCR number is
computed with intake closing at or near the advertised duration numbers. So 8.02 is not correct for an 8.5 SCR..... Buuut DCR does correspond closely to cranking compression, and if you have 150-160 psi cranking compression, then your DCR is close to 8. (Meaning that the SCR has to be higher than 8.5....or the valves are doing something unexpected when cranking.)
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OK, it looks like I'm going to have to assume I have a CR higher than I thought, for whatever reason. So, in order to use the intake closing "advertised" as opposed to what is on the cam card, how is that determined?

One of the reasons I've struggled to figure some things is that this motor never seems to follow what I read about, or others tell me. It likes a fairly cold plug (Autolite 63), it runs fine on 89 but I usually use 91 (it does have the polished chambers and I remember filing the piston edges), it really likes an A/F mix quite a bit richer than what any charts recommend (it wants jetting pretty rich, with sencondaries 3-4 sizes bigger than primaries.) And this is based on both my A/F meter AND many test runs with careful plug readings. Seat of the pants says it really wakes up WOT if I'm seeing A/F of 10.8-11 on the meter. (maybe that's normal, but not the 12.5 some charts list for WOT)

My strip times aren't good, but most from before I figured a lot of this out, before sorting out the combination of timing and curve, A/F, etc. (missed my main events this year due to family stuff, and the test runs I did do I had it jetted way leaner than it should have been.)
 
midnight340 said:
It’s been quite a while, but I believe I advanced it 5* as Lunati recommended.
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I am leaning more and more to advising that you leave the engine alone as far as a cam change. I think it's a good match.
 
RustyRatRod said:
I am leaning more and more to advising that you leave the engine alone as far as a cam change. I think it's a good match.
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When I started this thread I wanted a better sense of where the motor was at compression-wise, and now feel like I'm as close as I'm going to get. And it appears that I could likely get a hp boost with similar duration but with more lift. But you may be right. Maybe not enough to be worth a change at this time.
I'll still call some cam grinders/companies and see what they recommend.

I have the new timing cover sitting on the bench. My old one corroded enough that it started leaking to the outside where it meets the block. Thankfully not into the oil, which I hear they can do? Anyway, some time back I cleaned the coolant passages on each side and applied The Right Stuff inside and so far it has not leaked again. So I can wait until I figure out whether to also do a cam change.

It's not broke, so maybe I shouldn't fix it yet!! :lol:
 
midnight340 said:
When I started this thread I wanted a better sense of where the motor was at compression-wise, and now feel like I'm as close as I'm going to get. And it appears that I could likely get a hp boost with similar duration but with more lift. But you may be right. Maybe not enough to be worth a change at this time.
I'll still call some cam grinders/companies and see what they recommend.

I have the new timing cover sitting on the bench. My old one corroded enough that it started leaking to the outside where it meets the block. Thankfully not into the oil, which I hear they can do? Anyway, some time back I cleaned the coolant passages on each side and applied The Right Stuff inside and so far it has not leaked again. So I can wait until I figure out whether to also do a cam change.

It's not broke, so maybe I shouldn't fix it yet!! :lol:
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The problem with consulting cam companies is that they will certainly tell you that you can add power with one of their grinds. Then you make the change and you might regret it. It's your money and your decision of course, but I think I would spend my money elsewhere.
 
RustyRatRod said:
The problem with consulting cam companies is that they will certainly tell you that you can add power with one of their grinds. Then you make the change and you might regret it. It's your money and your decision of course, but I think I would spend my money elsewhere.
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Yes, I can see that they could likely do that. I think that at this point I'm not messing with a good running motor. I've had the experience of changing something to make it better and then wishing I hadn't, so I hear ya! What "might" work, also "might not" work, and then where are you?

I will put the cam recommendations I get in the "future possible" pile. :)

As far as "my money" goes, I am going to put some of it toward replacing this Edelbrock 750 with one of the new AVS2 carbs. I'm thinking probably the 650 AVS2 as it is more likely to help in normal driving conditions than the 800.
 
midnight340 said:
OK, it looks like I'm going to have to assume I have a CR higher than I thought, for whatever reason. So, in order to use the intake closing "advertised" as opposed to what is on the cam card, how is that determined?
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OR your valve timing events are not what you think, or not the same running versus cranking..... which was the point to suggest timing the valve opening/closing events, and to test compression after running the engine (to make sure the lifters are pumped up).

Not a terrible surprise on the AFR being best below 12.5 for WOT power.
 
Yes you did say to check what is actually going on. ( there is a certain logic to that!!!)

So I assume setting up a dial indicator on the rocker directly over the valve (engine warm) will give me an accurate record of events. And yes, my balancer is marked 360*. I’ll do this as soon as I can get to it and check back. Thanks!

(And i will double check TDC on the balancer...)
 
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