compresion test of 125lb. Cam question

-
aracer01

aracer01

Well-Known Member
Joined
Jun 5, 2014
Messages
98
Reaction score
4
Location
San Jose, CA
What numbers are best from a compression check? I just checked my 340 and I have 123-126lbs overall. Manifold vac.@ idle is 9+, I use a 3.5 power valve. The cam is D.C. .484Lift, 108 separation, 288Duration. Purple S. I'm thinking this is too much cam, from the cylinder pressure. The .060+ pistons are -.010 down and the 1972 J heads milled/ matched, aprox .050, .020 shim gasket. This was a Chrysler dealer $800 replacement type with heads, we fitted w/2.02 intakes. It ran 12.40 @ the old Fremont strip in a 3300 Ebody Cuda with N50/14" Kellys w/ 4.30 gear @108mph.
Now in the 68, 12.97 @ 104 3.90gear, 275/60-15" in Sonoma. Same old Doug cheepys, 340LD, 780vac. Holley. I added 1.6 steel/bushed roller rockers. for aprox .530 lift. with no improvement in MPH. Now that I have adjustable rockers, IS THERE A BETTER SOLID FLAT TAPPET CAM I CAN USE INSTEAD?
 
i have read that a performance cam will not get as much cyl compression at cranking speed as a mild cam. a history note in the late 60s they found that a high performance cam got better emissions at low RPM that a mild cam. caused by the more over lap on the HP cam. so that went into the stock cams. but that could have changed latter.
 
It is pretty hard to say definitively what cranking cylinder pressures you should have from the cam data...and you did not mention compression ratio which of course is important to the compression numbers...or the craking RPM's, which is also critical to compression numbers. So even if this could be closely 'calculated', important data points are missing. At least your compression numbers are very close.

Also, what is the weight of the old car vs new, with driver? This could explain the whole difference in ET's by itself. You have lot of changes going on that can bear on the ET's old vs new.
 
The Abody is 2900lb w/ driver , old installation in Ebody 3300w/Dr.
No way to tell compression ratio too many unknowns.
Torque conv. is 2800 (flash) stall, custom for this engine/car combo, from local Santa Rosa CA. rebuilder and fellow racer of Abodys, it's allowed the engine to idle in gear without tugging. The old conv. was stock High Stall, Flash/2500.
I use pump gas now, used to use Blue premium Aviation fuel from the Shellville Airport, Sonoma. Probably the difference is fuel, 1.6 rockers and taller gear, but it's lighter now. Same 998 trans.
O.K. What is a good Lunatti solid flat tappet cam for aprox. 9.8-10 to 1 , 355 cu. and stock crank? I can guess, but I need expertise to justify the new work. Thanks people.
 
What kinda pistons are they that they are only .010 in the hole? More like .100 for stock replacements.

The MP .528 solid would be an excellent choice.
 
Stock replacement mopar forged .060 oversize, with 4 valve notches, forged crank .010 off the rods and crank.
Chrysler exchange, factory re-manufactured from a car dealer in 1979. I have to get my notes out, but you are correct, .100 down. One side was , .110 and it had a head with smaller volume so all was equal. You get that Mopar was not to picky? Cylinder pressures are all the same. I added the HP, dampened red Small block Mopar springs with the cam in 1980. Fresh (recently) Cloyes true roller chain and modern cam tensioner. Cam is degreed at 108 as stated on card. I think 110* separation would build more compression with a solid tappet cam. But 112* would build more, if the duration isn't to radical. I see it's overall less @284 duration and faster up at .050 /241*. Is it more reliable than a similar cam from Lunati?
Couldn't find a Lunati solid with 112 separation, so the MP .528 is #1 choice, I always liked how the Purple shafts are polished.
 
It's way overcammed now. With the 528 it will be just a little overcammed......and if you advance it down around 106 it will be about right.
 
Advance the cam you have to 102-104 and see what happens.

Check valve to piston clearance.
 
When I first installed the cam I used the 4* offset key, because the 2* fell in the oil pan and I couldn't fish it out with a magnet. I was so rich running it was like steam from the exhaust. So I ran the stock key @ 110 centerline. That's what ran 108.7 Mph.
When I recently replaced the oil pan gasket and a HV Melling oil pump, I got out the 2* and the centerline is now 108*.
Maybe there was something else wrong I didn't notice w/4* advance @ 106 C.Line. Like the 650 Double pumper was too small and ran rich at idle. Possible? That good working 650dp did the same rich thing when used on the 440. I went to the 780 Vac and it still ran 108mph, back then.
So I am wary to advance it and waste the time. I could replace it for the same amount work.
My limited practical experience is why all the guessing. The Cloyes will allow 4* more advance. Should I try that?
 
I found a Lunati cam 270/270 duration, 245@ .050, a fast ramp, 110* lobe sep, solid lifter .545 lift, cL @ 104* Part#404A1LUN
This has less over lap, less duration, more lift and it's solid for race constancy.
QUESTION? Will this build more cylinder pressure than the old 288l288- 108* lobe separation, .484 lift Purple C.?
 
Already recommended one. Pick one yourself. Good luck.
 
I don't doubt your selection, but I keep finding similar cams and that's what I was getting at. Lunati # 60440 [SIZE=-1][FONT=Arial, Helvetica, sans-serif]SOLID: Great for street machines and mild bracket racing 9.5:1 compression with 2800 stall, lopey idle.
[FONT=Arial, Helvetica, sans-serif].014[/FONT]​
[FONT=Arial, Helvetica, sans-serif].014[/FONT]​
[FONT=Arial, Helvetica, sans-serif]2400-6600[/FONT]​
[/FONT][/SIZE]
[SIZE=-1][SIZE=-1]
[FONT=Arial, Helvetica, sans-serif]264​
​
[FONT=Arial, Helvetica, sans-serif]272[/FONT]​
[FONT=Arial, Helvetica, sans-serif]235[/FONT]​
[FONT=Arial, Helvetica, sans-serif]243[/FONT]​
[FONT=Arial, Helvetica, sans-serif].526[/FONT]​
[FONT=Arial, Helvetica, sans-serif].546[/FONT]​
[FONT=Arial, Helvetica, sans-serif]110[/FONT]​
[/FONT][/SIZE][/SIZE]
 
IMHO, you need to prove and or improve the cylinder pressure. I run a proven 185psi on 87 octane with al. heads, with a Hughes 230*/.050 cam. I have run other cams from 223*/.050 to the MP292/509( around 239*/.050). As I changed duration or retimed them the pressure always moved around. Pressures below 140psi were always soft on the bottom.Over 200psi I had to run premium fuel.160 to 185 worked for me. I know this doesnt answer your question but I thought it was worth mentioning. Hope it helps. AJ
 
AJ/FormS said:
IMHO, you need to prove and or improve the cylinder pressure. I run a proven 185psi on 87 octane with al. heads, with a Hughes 230*/.050 cam. I have run other cams from 223*/.050 to the MP292/509( around 239*/.050). As I changed duration or retimed them the pressure always moved around. Pressures below 140psi were always soft on the bottom.Over 200psi I had to run premium fuel.160 to 185 worked for me. I know this doesnt answer your question but I thought it was worth mentioning. Hope it helps. AJ
Click to expand...
 
Please excuse me for jumping in the thread, but AJ?FormS are you running that set up with the late model over drive 4 speed?
 
RustyRatRod said:
Back from the dEaD.
Click to expand...
Today the engine runs fine. I had to put back the stock rockers, since advancing the cam 4* using the Mopar offset key, still put the valves in jeopardy. I bent a few hollow push rods, scared myself, changed one lifter on the worst of bent rods, twice on that valve. The cam advance on the Cloyes timing set was crazy, like 10* or more. Add to the mix, milled heads and .020 gaskets. I was stubborn and did not check valves since this a 20,000 mile old block, and heads, I had ported and replaced guides and 2.02 intakes, I feel bigger valves was a wasted effort, since it only put the valves closer to pistons. I have the new MP replacement double roller chain, it fit the Cloyes gears, a new tension-er. And it ran in August @ 106 MPH @ 13 et. also driving to and from track 100 miles away. I bought a new harmonic SFI Summit balancer, and a shinny new pulley for this season.
Just wrestled in a new head liner, so I'm ready. Auto 998, reverse Direct Connection era valve plate and casting for a manual only shift, no kick down rod needed, a local shops 2800 stall, 3.91 axle, sure grip, and 28"x 8.5" Cooper 70 R Cobras. New 750 Eddy performer is very satisfying after seat increased to .110 and float raised 1/4", secondary 4% jet increase and also choke works perfect. New heater core too. I'll post pictures tomorrow.
I'll have to build another engine before I try a different cam,, so I'll drive this until it uses oil. I have an original 80K miles, 1974, 318 forged crank block, to bore and try aluminum heads and stroker crank. By then, gasoline will be the cost problem.
The head liner looks like a puckered coffin, it's the non perforated $70 Ebay type, and not flexable. In the heat of summer I'll tighten it.
 
Last edited:
Photos to prove ownership?

P1010179.JPG


P1010180.JPG


P1010181.JPG


P1010182.JPG
 
You must log in or register to reply here.
-

Back
Top