cam selection/cyl. pressure

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71violet

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Right now I have a 360, MP cam, .474 lift and 280 advertised, 110 lsa. I'm looking to go bigger on the cam, for starters I was looking at another MP, it's got .509 lift and 292 advertised, 108 lsa. The engine runs on 91 octane, timing at 34 and it'll ping with any lower octane. WIll the cylinder pressure go up too high for 91 octane? Will the new cam have enough vacuum for power brakes and will there be any spring binding or valve to piston clearance issues? All responses are appreciated.
750 Demon
KB107 (unsure of deck height)
adjustable rockers (MP I believe)
MP valve springs (p4120249)
Hedman headers
MSD ign.
2,500 stall (willing to go higher if need be)
3.91 gears
street driven
 
71violet said:
Right now I have a 360, MP cam, .474 lift and 280 advertised, 110 lsa. I'm looking to go bigger on the cam, for starters I was looking at another MP, it's got .509 lift and 292 advertised, 108 lsa. The engine runs on 91 octane, timing at 34 and it'll ping with any lower octane. WIll the cylinder pressure go up too high for 91 octane? Will the new cam have enough vacuum for power brakes and will there be any spring binding or valve to piston clearance issues? All responses are appreciated.
750 Demon
KB107 (unsure of deck height)
adjustable rockers (MP I believe)
MP valve springs (p4120249)
Hedman headers
MSD ign.
2,500 stall (willing to go higher if need be)
3.91 gears
street driven
Click to expand...

Hey man, call Clay Smith Cams right now and tell them what you just wrote. You will not be sorry.

http://claysmithcams.com/ 714-523-0530
 
You are adding more duration so the cylinder pressure will go down.
 
does't longer duration mean more air fuel in the cylinder , hence increase in cyl. pressure, giving the power increase?
 
Yes and no. Depends on the given combo and rpm.

More duration usually means more intake/exhaust overlap (both open at the same time). This CAN bleed down cylinder pressure, especially at lower RPM.

Increasing cam duration can increase power but it is usually due to increasing the volumetric efficiency. Or simply the ability to fill the cylinder with more air/fuel mixture. Ideally for every combustion event, the cylinder would be fully filled with the air fill mixture equal to volume of the cylinder when the piston is at bottom dead center. For many normally aspirated non-race engines engines the volumetric efficiency is between 80 to 90%.

Free flowing intake, heads, exhausts will help with volumetric efficiency too.
Especially when pressure waves in the intake or exhaust are in sync.

The increased intake/exhaust overlap allows the intake charge to start escaping out the hot exhaust (pressure bleed) but it also jump starts the charge's velocity. Now that the intake charge has some velocity, a reflective wave from the exhaust will then slow the intake charge's escape out the still open exhaust valve and instead direct it to the rapidly increasing volume of the cylinder (as the piston goes down - vacuum). Once the piston reaches bottom dead center and starts on the compression stroke, the intake will start to close but due to the velocity it will continue to "pack" the cylinder even though the piston is starting to move up. This will effectively increase cylinder pressure.

When the combo is right for a given rpm, a normally aspirated engine can exceed 100% volumetric efficiency at certain RPMs - a ram air effect.

This phenomenon is why going longer duration often makes the engine softer down low but make more power up top.

As always, it always depends on the engine's particular combo.
 
nope. It actually has nothing to do with the mixture coming in.. Try to only consider the mechanics of the parts. Pressure in the cylinder results from a sealed cylinder meaning the intake valve closed and the piston coming up. A longer duration cam must close the valves later therefore less area swept by the piston after the valve is closed. Now, in terms of what you have... I dont think you should be having issues with pinging. The .509 is way too large for your engine and will be a turd nor to mention you wont have vacuum for brakes. The one you have should be fine. So let me ask... with that Demon... did you tune it? On the 34° of timing... what is initial timing? What rpm is it all in? Do you have the vacuum advance hooked up?
 
what I meant was at the sweet spot for the cam, not during a compression test, when the overlap and duration is helping to fill the cylinder more efficiently, it would increase cylinder pressure at that point compared to a cam that doesn't fill the cylinder as efficiently. As socal' said packing the cyl. with more air/fuel thus more cyl. pressure.
Was a long time ago since I set the timing, initial I believe was 14 (that is more of a guess than my next two figures lol) I believe the timing came all in at 2,200, for a total of 34. I set it at 36 and it would ping a little under load so went back to 34 total where it never pings on 91. I'll do a check to confirm this. I had some opinions that my motor was a little under cammed, that's why I was looking for something bigger. Being used to small Chevy's, anything over .500 lift sound like alot to me.
The Demon i set up from the jetting info on Demon's web site, then left it alone. It makes WAY more power than the vac. sec. Holley I had on there that was prepped by a shop, but maybe a lean spot at part throttle? (over half) Oh ya, I have a MSD billet distributor with no vacuum advance and a 6A box.
 
Slow down the advance and tune the carb. Every Demon regardless of size I've ever tuned had issues with lean spots in the fuel curve OOTB.
 
dgc333 said:
You are adding more duration so the cylinder pressure will go down.
Click to expand...

And that was the biggest mistake I made years ago installing a cam into a relatively stock magnum engine. All I did was bleed off cylinder pressure.

It lost all of it's snap down low that I loved so much.
I think it probably made less torque than it did stock hahahaha.
I was so pissed!!!!
 
The parts have to work together. You can and need to run a higher static compression ratio with a higher duration cam.
 
OK, my compression test showed 140 psi, (I'm at 3000 ft.) So, question is, is there hp to be gained going to say a xe274 or xe284, or something more modern than my MP cam? without losing much in lower revs?
 
The cylinder pressure tells me the pinging is either tune up, or oil getting into the chambers. More than likely tuning will remove it. The Demon 750 is a little large for that engine and that may have something to do with it. Proper carb sizing is critical with Demons.
 
thanks for the help guys, I guess I'll just leave the motor alone for now, do some tuning and hopefully it'll be a 408 next winter. I guess a couple seasons of that holley boiling over everytime I parked and washin down the cylinders with gas was wearing it out quicker. The Demon doesn't do that at all.
 
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