Cranking compression higher after cam swap

[fourgearsavoy]

I changed from a pretty rowdy flat tappet cam to a milder street roller cam and now my cranking compression is over 200-210 PSI. My cranking compression used to be 185-190.
I might need to step up to E-85 with the new cam.
I never ran a roller cam before so is this a normal condition ?

Gus

 

[Paul_B]

Is the duration and/or LSA different from the cam you had.

 

[70bigblockdodge]

Wow, hope you have aluminum heads and good quench. Will have lots more torque.

 

[fourgearsavoy]

Is the duration and/or LSA different from the cam you had.

Thats an issue because I don't have the card for my new cam

 

[RemCharger]

I changed from a pretty rowdy flat tappet cam to a milder street roller cam and now my cranking compression is over 200-210 PSI. My cranking compression used to be 185-190.
I might need to step up to E-85 with the new cam.
I never ran a roller cam before so is this a normal condition ?

Gus

Makes sense, a roller should trap more compression. Coupled with it being a smaller cam

 

[RemCharger]

What fuel were you using before?

 

[Cranky]

New cam and no card? Can you look up the cam number on the Internet or get it from the maker? If you have a late intake opening and closing and not much overlap yeah you can raise your cranking pressure quite a bit.... that's pretty much what the old RV style cams did. They helped out the old low compression engines build some cylinder pressure to give them a little more spunk.

 

[Geoff 2]

The higher reading is not because it is a roller cam, it is because the cam is milder....
Int valve closes earlier, traps more air....

 

[Cranky]

The higher reading is not because it is a roller cam, it is because the cam is milder....
Int valve closes earlier, traps more air....

Actually, you can have a later intake event and it'll do that too. Think about it....if the intake starts to open say at 10* BTDC and closes a bit after BDC, the vacuum will be high and the air flow will fill the cylinder and continue to move in even when the piston is at BDC....air is elastic.

 

[fourgearsavoy]

Wow, hope you have aluminum heads and good quench. Will have lots more torque.

Yeah I have Indy EZ's with Wiseco flat tops.

What fuel were you using before?

I'm using the pump 93 here in Ohio it's not too bad as I have only had detonation one time when I lugged it down once.

 

[toolmanmike]

Run it

 

[70bigblockdodge]

The reason the engine has more cranking is because the intake is closing earlier because it has faster ramps (roller cam advantage), trapping more intake charge in the cylinder (same as advancing a flat tappet cam)

 

[Geoff 2]

A roller cam has slower ramps, not faster than a FT cam. Up to as much as 40* of duration. After that the roller takes off...
Repeating post #8. The higher reading is because the intake closed earlier. Nothing to do with being roller or FT. When the valve has closed, it has closed, whether a roller or FT cam closed it....

 

[RemCharger]

Actually, you can have a later intake event and it'll do that too. Think about it....if the intake starts to open say at 10* BTDC and closes a bit after BDC, the vacuum will be high and the air flow will fill the cylinder and continue to move in even when the piston is at BDC....air is elastic.

Yes, but that's when you surpass 100% volumetric efficiency. High RPM.

 

[BSB67]

It has nothing to do with being a roller cam. The intake valve is simply closing earlier with this cam. You can knock it down a little by tightening the lash.

 

[fourgearsavoy]

Well I put the fuel and ignition back on today and fired it up and set the timing then drove it for a mile or so and everything feels good. I need to put some fresh fuel in it and do a little tuning on the carb maybe a new power valve to go with the improved vacuum signal. I set the total timing at 34 with the base at 20 and the engine seems to be happy there.
I set the cold lash at .010 and I'll check it tomorrow after I put some more miles on it.
Maybe I was just worried for no reason because it runs really well and I got my low end torque back.

Thanks Men

Gus

 

[lewtot184]

more likely a factor of closing the intake valve earlier.

 

[blue_stocker]

Hey Gus, what were the specs of your old MFT cam?

 

[MoparLeo]

So did someone else install and degree the cam or did you just install it straight up and not know the specs ?
If not degreed, did you check for valve clearance ?

 

[fourgearsavoy]

Hey Gus, what were the specs of your old MFT cam?

It is a Howard's cam 616/620 260/264@ 50 108* LSA , INTAKE C/L 104* W. 1.5 rockers.