Minimum Octane for High Compression?

id say set it as close to the mark as possible,a couple degrees doesnt hurt just dont go nuts.
as far as the cam timing id say they figure it for peak power,430ish cams werent really designed for making lots of power at 3500 rpms! :laugh:

79MKII wrote:

Maybe you guys can help with this question too...

I set my cam timing according to what web cams recommended. What do they base this setting on? Peak hp at upper RPM's? Good mid-range for the street? Anyone know? I never thought of it before but the cam timing would impact the dynamic compression, and therefore the combustion right? Maybe I should re-degree my cams...



Lobe Center: 106° int 108° ex

I think the lobe center recommended in the cams spec sheet, is what the cam manufacturer feels the cams they designed, will safely run well at, with stock valves.
U have oversized valves so u have to check every thing that u do to avoid a collision. When u set up cam timing, it is essential that u check ALL clearances before u turn over the motor. With larger valves and high domes, u must check the V to V, and V to piston clearances to find your min and max lobe center settings.
If u just change a LC because someone says it will run better, u run a big risk of a collision.
U are in control of your motor , don’t EVER take anyone’s word for it, that a mod will work, without thorough research AND measurement.

If u reduce the exht LC to a smaller # , u can gain some bottom end at the expense of some top. But before u change anything, u need to find the minimums.
I would just set the ignition timing to stock (40) until u find a need to change it.
As Larry mentioned, check the plugs for specks of alum. If there ok then I wouldn’t worry about it .if I get to a Dyno or gain more knowledge of what symptoms to look for, after a change , I will be leaving mine as is.

Very good points, Mark. That's pretty much why I went with Web's settings. I figured that's what they designed them for. The shop that rebuilt my head had everything basically set up and ready to install when I got it. They checked the v to v clearance and assured me that the p to v clearance would be OK too. They never told me what the min and max lobe centers are - that would be good to know before you make any adjustments.

Here's what Web says about my cams:

Excellent mid and upper end performance for hot street. Requires High Compression Piston, Shim Under Followers, and Performance Valve Spring Kit. Price Per Set (2).

Sounds like they have engineered them for mid AND top end performance - not just top end. Probably best to leave them as is.

As a side question - why would a cam dictate that a high compression piston is necessary? I never did understand that. My guess would be that since the cams overlap, higher compression is needed??

79MKII wrote:

Very good points, Mark. That's pretty much why I went with Web's settings. I figured that's what they designed them for. The shop that rebuilt my head had everything basically set up and ready to install when I got it. They checked the v to v clearance and assured me that the p to v clearance would be OK too. They never told me what the min and max lobe centers are - that would be good to know before you make any adjustments.

Here's what Web says about my cams:

Excellent mid and upper end performance for hot street. Requires High Compression Piston, Shim Under Followers, and Performance Valve Spring Kit. Price Per Set (2).

Sounds like they have engineered them for mid AND top end performance - not just top end. Probably best to leave them as is.

As a side question - why would a cam dictate that a high compression piston is necessary? I never did understand that. My guess would be that since the cams overlap, higher compression is needed??

you are right in your thinking. cams with higher lift usually come with more overlap. so booth valves are open at the same time for a longer period of time. this is why u loose bottom end, less compression. so by using smaller LC's u can decrease the overlap, but only slightly(like fine tuning). if u want a lot more bottom end , u would buy a cam designed for that.
the extra duration helps at high rpm's due to a super charging affect. as the piston nears BDC the intake charge is moving at a very fast rate. so if u keep the intake open a little longer u can get a big gain but only at higher rpms. at low rpms this works against you.

79MKII wrote:

Maybe you guys can help with this question too...

I set my cam timing according to what web cams recommended. What do they base this setting on? Peak hp at upper RPM's? Good mid-range for the street? Anyone know? I never thought of it before but the cam timing would impact the dynamic compression, and therefore the combustion right? Maybe I should re-degree my cams...

Lobe Center: 106° int 108° ex

79MKII wrote:

Here's what Web says about my cams:

Excellent mid and upper end performance for hot street. Requires High Compression Piston, Shim Under Followers, and Performance Valve Spring Kit.

Are these cams over .430" lift? I might of missed that while reading the thread.



As a side question - why would a cam dictate that a high compression piston is necessary? I never did understand that. My guess would be that since the cams overlap, higher compression is needed??

You are dumping a lot more fuel into the motor with increased duration & overlap. Higher compression is needed to compress and burn, that extra fuel.

79MKII wrote:

...why would a cam dictate that a high compression piston is necessary?....

To achieve the correct dynamic compression ratio.
What the engine actually sees with the given grind and degree of rotation.

Good Fortune!

That "changes" information is very useful. I always forget what does what - now it's in writing.

I wonder if a degree or 2 would make a noticeable difference? I don't plan to adjust anything - just curious about minor changes - and if "a degree or 2" is even a "minor" change.

Also makes me very curious to know what my stock, non adjustable cam sprockets were actually set to. I never degreed the cams with my last build - they might have been quite a bit off the recommended lobe center.

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I had a buddy years ago that had a Camaro with 12-1 compression. He ran on pump gas and that thing would deisel for about a minute after shutting it off. we were young and dumb and didnt know what the cause was so he would shut it off while still in drive. :laugh:

Patton wrote:

79MKII wrote:

...why would a cam dictate that a high compression piston is necessary?....

To achieve the correct dynamic compression ratio.
What the engine actually sees with the given grind and degree of rotation.

Good Fortune!

Give that man a cigar

maybe you could give him one of yours! :woohoo: