Help with 340 compression + cam change

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midnight340

midnight340

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Hi all, I am considering a cam change to make better use of my combo. I'm hoping that I can include enough info that someone with more experience can help me zero in on my compression.
I built this motor myself quite a few years back (with the help of excellent machine work!) and cc'd the heads, but not the cylinders, so:

Question 1)
Can someone get me close to likely compression with the following info?
.040 over, decks cleaned up .015 from stock, stock stroke, #2385 TRW pistons, .040” Felpro gaskets, heads cc at 70. Static reads an even 160# all around, carb open, no plugs.


Question 2)
What should I consider for a different cam choice for mostly street with maybe 15 runs down the strip per year? And mostly to make better use of my very good heads? (head details below in build notes.)

Current camshaft :
Lunati Bracket Master 284 w/ 218”@ .050/.458” lift, 110* installed 5* advanced. (64* overlap, ICA 34* abdc)

Have 750 Eddy on now, but will change to AVS2 soon.
Running 4:10 gears , with 518 Cope built trans. Wt. 3200+/-



For any further questions, or if anyone is interested here are my build notes:

340 Block (1972) .030 over with plates (4.070”)
Align honed, decks squared, milled .015 from stock, with
“Cross-over” oiling system, AN line across valley engraved with
“A.W. Auto Full Comp Oiling. 1002”

Note: Inside of block de-burred/smoothed with white epoxy finish.

Crank: Forged 340, full groove bearings rods/mains, (installed with windage tray)
Pistons: TRW #2385’s w/ Speed Pro chromoly rings
Balanced, forged 340 rods, reconditioned, side-clearanced

Heads: “X” 2.02/1.60
(ported by Bob Mullen in ’70s, ports shaped and fully polished, haven’t had them flowed). They cc @ 70.
Resurfaced, O-ringed, 3-angle valve job, hardened seats, 273 adjust. rockers, K-Line guides, Erson chromoly pushrods.

Springs: Lunati 110# / 300#, Comp retainers, Lunati valve locks 7*,

Head gaskets: Felpro #P4120093 (.040” compressed)

Camshaft: Lunati Bracket Master 284 w/ 218”@ .050/.458” lift, 110* installed 5* advanced. (64* overlap, ICA 34* abdc)

Intake: LD340 port-matched
Exhaust: 1-5/8” port-matched mid-length with 16” tubes and 10” collector, 2-1/2” exhaust with MagnaFlows

Misc. 360* fully degreed balancer, Mellings Hi-Volume oil pump, Cloyes double roller timing chain.
 
IMG_2187.jpg

Here is the cam card from the current (old style) Lunati
 
Good complete info! But something is not hanging together......

With those pistons with their loooow CH of 1.740", I come up with 8.2 static CR. No way with that cam that you would come up with 160 psi cranking compression, unless perhaps the lifters were all drained of oil and the intake closing was reeeeally early... (Or there is a turbo running on it sitting still LOL)
 
OK, I wondered about that, having realized my piston choice from way back was probably not a very good one. Didn't know that much and was following advice from "someone".

(I'll start by getting a new gauge and see what happens.)

If indeed the compression really is that low, I don't think I'm ready to completely tear down a nice running motor, but a different cam might make this work better?

I have long recognized I likely have a mis-match of low compression and high flowing heads.
(This was very high compression motor when I got it, but couldn't keep those "mountain" pistons for street.)
 
If those are listed with a Deck Clearance .082" and my decks were cleaned up .015" does that mean they are maybe about .067" below the deck???
 
Yes, that would be about right and agrees with computing the numbers from 1/2 stroke, rod length, and CH; that's the number used in my computations. Again, something is not matching up with the compression numbers.... has the engine been running / can it run? The take numbers after running if possible so the lifters are all pumped up properly and closing the intake valves at the correct angle.

BTW, if you run DCR computations with the cam numbers, the intake vavle closing angle listed is for .050" lobe lift, which is not what is to be used when doing DCR computations. You use intake closing angle based on 'advertised' info for most DCR calculators. IIRC, the one that Comp has/had online is the only one I know of that may use .050" duration numbers....

BTW, can you explain a bit more what you want achieve? More 'snap' on the street at lower RPM's or ????? For sure, there are much better cams for that, with better lift to duration ratios.

BTW, did you mean to say '360' balancer?
 
Motor runs great. Has run a 1/4 mile best of 13.95 @ 98 but more commonly mid to low 14’s @ 92-93.

So you are saying to make sure the static compr. reading is with motor warmed up, right?

I don’t understand much about figuring DCR.

Yes, I’m thinking I could get more low/ mid performance without giving up 1/4 mile times. (I do have a PTC converter with “2400-2600” stall, as well as the 4:10’s.)

(I meant to say the balancer has degree marks all around, 360* markings.)
 
I should say I’m not unhappy with the street performance necessarily, but feel like more lift would wake it up some. Say, the 702/703 Voodoo, or the like.
 
midnight340 said:
I'm hoping that I can include enough info that someone with more experience can help me zero in on my compression.
I built this motor myself quite a few years back

Question 1)
Can someone get me close to likely compression with the following info?
.040 over, decks cleaned up .015 from stock, stock stroke, #2385 TRW pistons, .040” Felpro gaskets, heads cc at 70. Static reads an even 160# all around, carb open, no plugs.
Click to expand...
No, it can’t be done without assuming. I don’t like to assume on things like this at all.
Since you built the engine, where do the pistons sit in the cylinder?
Beingbthat you get a 160# reading, I’d say this is good. It is also due to the cams timing events. A larger cam would drop the number and a smaller cam would raise the number.

Perhaps someone that knows there static compression ratio and the cam timing events and tested there cranking psi could chime in.

Question 2)
What should I consider for a different cam choice for mostly street with maybe 15 runs down the strip per year? And mostly to make better use of my very good heads? (head details below in build notes.)

Current camshaft :
Lunati Bracket Master 284 w/ 218”@ .050/.458” lift, 110* installed 5* advanced. (64* overlap, ICA 34* abdc)

Have 750 Eddy on now, but will change to AVS2 soon.
Running 4:10 gears , with 518 Cope built trans. Wt. 3200+/-
Click to expand...
Well that sounds like an excellent street combo ready to drive anywhere!
The rest of the build looks really good and I’d bet those heads are done really well. Just how far they will go is another issue.
 
Thanks for the responses!!

Unfortunately the one thing I don’t know is where the pistons sit in the cylinder.

I may check static compression with a second gauge, but I’ve always gotten a reading close to the 160#s. If a another gauge gives me a different number I’ll report back.

Motor has about 65,000 miles on it. (Includes cross country trips, lots of runs down 1/8 and 1/4 mile, a couple of loops around a nascar oval, and some SCCA autocross) so I’m happy the readings are still very even.

I have to change the timing cover due to corrosion and an external coolant leak, so thought it would be good time to consider whether I’d want to do a cam change.
 
I see and not a bad idea considering how deep your getting into it.

Well, if I knew where the piston sat in the hole or above or at zero deck, of the hole, that would basically be the missing calculation number. (To a lesser degree, how wide the cylinder head gasket is.)
Do you know the height of the piston? Or exactly what piston it is. That would help some and from there , In theory, working with the manufacturers listed deck height, we can do the math from there.
It would not be exact, but it would be an educated guess.

A known piston height really only leaves 1 thing to guess at. The factory said there deck height is 9.6 - 9.58? Then minus the .040....
 
Pistons are the old TRW L2385 stock pistons for the 72-73 340s. (Lower compression motors) Four valve reliefs.

From an old Moparts forum post:
“Compression height 1.591” (vs 1.74” for the hi compr. 2-valve reliefs) ...and allow 8 or so cc’s for the 4 reliefs.”
 
If you're gettin 160 PSI, that's a decent combo as is. I say leave it alone. What's going to happen is, you're going to go "bigger" and lower cylinder pressure and lose power. Or you're going to go "smaller" and risk detonation on pump gas.

I use quotation marks because bigger and smaller are not good words to use regarding camshafts. What you are looking for regarding cylinder pressure are several things. Perhaps the most important is when that intake valve closes. If one cam closes the intake valve earlier than another, it will probably build more cylinder pressure. A cam that closes the intake valve later, may make less cylinder pressure.

I use "probably" and "may" because the reverse could also be true. It's all dependent on the rest of the valve events and how the camshaft is ground. Duration @ .050", lobe separation and so on.

Plus, all the formulas and online calculators in the world are really useless as tits on a boarhog, because they know nothing of real world performance. You need experience and know how with camshafts to choose wisely......and to me......it sounds like someone did that.

I agree that engine probably has a static compression ratio of "around" 8.5. So, if that thing really has 160 PSI, it's pretty snappy like it is. Ultimately, IMO, to improve upon it you will have to change the whole combo. In other words, more compression, more cam and so on.

I think you are fine right where you are with THAT particular combo.

Again, just my opinion.
 
Just found this from FOABO post:
"72-73 piston sits 0.054 below the block deck" so with my decks surfaced .015", 70cc heads, .040 gasket, and allowing 8cc for valve reliefs, running the Wallace calculator I come up with 8.44 compression. So that's probably close???
 
midnight340 said:
Just found this from FOABO post:
"72-73 piston sits 0.054 below the block deck" so with my decks surfaced .015", 70cc heads, .040 gasket, and allowing 8cc for valve reliefs, running the Wallace calculator I come up with 8.44 compression. So that's probably close???
Click to expand...

Probably so, yes. But don't put a lot of worry in that. Low compression motors can be made to RUN. With 160 PSI, you have a LOT to work with. Have you recurved the distributor? That can make a REAL difference.
 
Yes, using a MSD box and distributor, running 22* intial and 34* total.

And using an A/F meter have gotten my 750 Edelbrock dialed in as good as it's going to go. Trying to decide if the the Eddy AVS2 650 would be a better choice than the AVS2 800. (probably?)
 
Static and dynamic CR's numbers always hang consistently together with the cranking compression numbers. That 160 psi indicates a DCR of around 8-8.1 and that is not consistent with a low-mid 8's static CR.... not even close with the cam and piston combination posted or even with a cam that has 20 degree shorter duration.

And, the 1/4 miles times don't seem to be up to the potential of the combination....So IMHO something is up with the cam/lifters and the intakes are not open as much as thought....
  • the cam listed might not be the actual cam or it is considerably advanced.
  • Or deck was milled more......
  • and compression gauges can be off.
I'd encourage the OP to do some lift checking/profiling on the cam and try a different compression gauge.
 
nm9stagehands,
Understood. I’ll start with a different gauge as mine might not be accurate.

As for how much the decks might have been milled, I only have what I was told.

When I can get to it I should be able to check and get an idea of the actual intake opening.
 
OK..... just wanting to see if things will 'hang together' better as that gives confidence in the data. And realize that if the lifters leak down, you lose considerable duration... which will artificially elevate the cranking compression numbers. That is why to test after it has been run...the lifters are pumped up.

If you really have 360 degree markings on the damper, that would make it easy to see where the valves were opening/closing.
 
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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