[gtofreek]

From this post, I by no means am advocating pushing the compression limits with pump gas, but I have some info I thought I would pass along regarding fast ramp cams, and detonation. As most of you know, I am a big fan of the Lunati Voodoo series of cams. We use a lot of them, and have gotten away with running higher compression than what the normal train of thought suggests should be used. Recently we built a pretty much stock 389. It had stock heads, with cast flat top pistons. It came out at a true 10:1 compression. Cam is fairly small at 219/227 @ .050", HFT. Normally I wouldn't recommend a cam this small for this high of compression with iron closed chamber heads. These are 1964 389 high compression heads. I installed this cam 6° advanced, on a 106° ICL. Customer finally got car back together and brought it in to have us fine tune the carb and timing for them. We recommended 91 octane for it, but it inadvertently got filled up with 87 octane. Total timing is set at 36°, with 18° initial. Vacuum advance is ported. At cruise I noticed a real slight pinging from the vacuum advance. If I gave it a little gas, the pinging would go away. No ping under full throttle. Carb felt a little lean at cruise, and when I gave it just a little gas, it didn't really respond, like it was lean. So I changed metering rods to one that was .001" smaller on the tip, and .006" smaller on the cruise step. Now carb responds when a little gas is given, and the slight ping at cruise completely disappeared. When I step on it from a dead stop, and idle, There is no signs of detonation at all. My ear is very sensitive to pinging, and my gas foot also. If I can't hear it[and my hearing is very good] I can feel it in the gas pedal. Spark plugs show no signs of detonation either. Keep in mind, this is a 10:1 engine, with iron heads, running on 87 octane.

This just goes to help support my stand on advancing certain cams. I have always felt that detonation wasn't simply caused by too much cylinder pressure, based off of when the intake valve closed. But rather detonation is more influenced by the valve positioning at TDC, during overlap. The way these cam lobes are designed, advancing them 4°-7° gets the exhaust to intake valve relationship during TDC at overlap, in a position where the pistons suck harder on the intake when they start down the cylinder, thus sucking back in less hot exhaust gasses as it would when the cam is retarded. Advancing a cam gets the intake valve open more, and the exhaust open less at TDC during overlap. Retarding the cam gets the exhaust open more, and the intake open less during TDC at overlap. I bet if I installed the above cam straight up in the engine above, it would have detonated pretty bad.

[STEELCITYFIREBIRD]

Good stuff, interested to know what piston to head clearance you had?
2 or 4 valve relief pistons?

[77 TRASHCAN]

What brand cast Pistons?

[gtofreek]

They were cast Sealed Power, 4 valve relief pistons. They were set at .005" down from deck, with a stock Fel-Pro head gasket, which I believe is .040" crushed.

[Skip Fix]

Paul it would be interesting to make a dyno pull or track times using pump gas then race gas to see if there is inaudible detonation going on. I know my RAIV 400 with an UD had not detonation audible on 93 but picked up 8 HP on race gas so need more octane. 400 0.030 over zero deck FelPro gasket TRWs 6.6cc and a 69 cc chamber.

On the flip side a buddy with a SBC 377 aluminum heads 10:1 Pontiac Acadian ran out of pump gas at the track and had to use race gas and slowed the car down as his motor did not like more octane burning slower.

[gtofreek]

We did dyno this on 91, still no signs of detonation on the plugs.

[STEELCITYFIREBIRD]

Now I'm being grabby, but .....what did it pump compression test pressure, if you happened to check it?

[Skip Fix]

Just wondering if you swapped more octane in on the dyno for a pull. We were surprised mine liked more octane also as plugs were good ears were good for pinging.

[ta man]

A question ,when you change spark plug heat ranges and see it move on the ground strap..a hotter plug will go closer to the base and a colder plug will go closer to the strap tip.Does changing fuel octane show up on the plug?Would a lower octane go towards the base and would a higher octane go towards the tip?

[Cliff R]

There are many other factors that effect or contribute to detonation. For sure the true static compression ratio and cam choice are big players, but there are many other minor players in this deal as well.

Squish distance is one of them, as is the ability of the engines cooling system to keep temperatures down. Others are vehicle weight, how well the torque converter is coupled, rear gear ratio, etc.

For Pontiac engines with factory iron heads and flat chamber floors more than any others they are very sensitive to squish distance when it comes to combustion efficiency, timing and octane requirements. The most troubled running hot/overheating/detonating Pontiac engines I've had in here to tune all had a lot of squish in them, no exceptions.

We've also had a good many in here that had poor camshaft choices in them that were difficult, if not near impossible to tune for pump gas use even at relatively "low" compression ratios. When you choose the wrong cam, or one that is way too good at cylinder filling at low rpm's, a low compression engine quickly acts like a high compression engine when it comes to octane requirements. I'd also add here that the long stroke engines tend to be more sensitive to this than those with the 3.75" strokes, at least from what I've seen here.......Cliff

[steve25]

Low comp motors like 350s 389s and 400 ( in regards to stroke ) can have a hard time pumping out all the hot Exh Gases and to add fuel to fire so to speak they take longer to burn the air fuel mixture also.

[gtofreek]

Originally Posted by STEELCITYFIREBIRD

Now I'm being grabby, but .....what did it pump compression test pressure, if you happened to check it?

175 lbs. cold.

[ponjohn]

Originally Posted by gtofreek

From this post, I by no means am advocating pushing the compression limits with pump gas, but I have some info I thought I would pass along regarding fast ramp cams, and detonation. As most of you know, I am a big fan of the Lunati Voodoo series of cams. We use a lot of them, and have gotten away with running higher compression than what the normal train of thought suggests should be used. Recently we built a pretty much stock 389. It had stock heads, with cast flat top pistons. It came out at a true 10:1 compression. Cam is fairly small at 219/227 @ .050", HFT. Normally I wouldn't recommend a cam this small for this high of compression with iron closed chamber heads. These are 1964 389 high compression heads. I installed this cam 6° advanced, on a 106° ICL. Customer finally got car back together and brought it in to have us fine tune the carb and timing for them. We recommended 91 octane for it, but it inadvertently got filled up with 87 octane. Total timing is set at 36°, with 18° initial. Vacuum advance is ported. At cruise I noticed a real slight pinging from the vacuum advance. If I gave it a little gas, the pinging would go away. No ping under full throttle. Carb felt a little lean at cruise, and when I gave it just a little gas, it didn't really respond, like it was lean. So I changed metering rods to one that was .001" smaller on the tip, and .006" smaller on the cruise step. Now carb responds when a little gas is given, and the slight ping at cruise completely disappeared. When I step on it from a dead stop, and idle, There is no signs of detonation at all. My ear is very sensitive to pinging, and my gas foot also. If I can't hear it[and my hearing is very good] I can feel it in the gas pedal. Spark plugs show no signs of detonation either. Keep in mind, this is a 10:1 engine, with iron heads, running on 87 octane.

This just goes to help support my stand on advancing certain cams. I have always felt that detonation wasn't simply caused by too much cylinder pressure, based off of when the intake valve closed. But rather detonation is more influenced by the valve positioning at TDC, during overlap. The way these cam lobes are designed, advancing them 4°-7° gets the exhaust to intake valve relationship during TDC at overlap, in a position where the pistons suck harder on the intake when they start down the cylinder, thus sucking back in less hot exhaust gasses as it would when the cam is retarded. Advancing a cam gets the intake valve open more, and the exhaust open less at TDC during overlap. Retarding the cam gets the exhaust open more, and the intake open less during TDC at overlap. I bet if I installed the above cam straight up in the engine above, it would have detonated pretty bad.

What is the LSA of the cam?

[ponchjoe]

Originally Posted by ponjohn

What is the LSA of the cam?

Looks like 112

http://www.lunatipower.com/Product.aspx?id=1775

http://www.lunatipower.com/CamSpecCa...rtNumber=60902

[pastry_chef]

Originally Posted by gtofreek

We did dyno this on 91, still no signs of detonation on the plugs.

HP @ peak RPM?

[gtofreek]

It made a pretty flat 330 HP from 4700-5100 RPM.