[cabhop]

350p built with leftover parts accumulated over the years. I did the +55 over mopar 318 cast pistons that were .030 down in the bore. I used the 1969 small valve 46 head that had been rebuilt with low miles. I used nos felpro 350 head gaskets from ebay that had the scallops in them along with a Summit 2800 cam. When I assembled this motor a factory intake would not bolt up to the heads (heavily milled?) so I ended up putting on a craigslist performer manifold that did.

My issue that I need help with is-
with moderate acceleration this motor detonates on 92 octane plus a can of booster. I've tried all the tricks with different carbs and distributors, with and without vacuum advance connected, and with timing pulled way back. The warm compression test is 190-195 across the board. Any thoughts on this?
Thanks

[PunchT37]

Cam too far advanced?

[steve25]

What do you have for piston to valve clearance with those Mopar pistons?

What do you have for total mechanical timing and when does it detonate?
Are you detonating at full throttle, part throttle or both?

If a intake would not bolt up then the heads had to be milled at least .040”.

If your heads are dated pre 1970 then with the mill work I will guess that you have a compression of between 9.5 and 10 to 1.

Since the heads where milled and on a small bore you may have a sharp edge on the chamber lip that is getting hot enough to light off the fuel charge and is giving you the detonation, but in general with your hot cranking compression you have I would check to confirm that your cam is in no more then straight up and not mistakenly advanced.

I would also suggest that even without looking at your plugs you start off your troubleshooting by dropping down one heat range and see how that pans out.

[PunchT37]

Yeah, it`s hard enough to get decent compression out of these 350`s, much less 30 in the hole. That kills quench BUT, he`s got 190 to 195 psi. Cam timing comes to mind.

[steve25]

The normal factory deck clearance on a 400 is .023”, so he’s only .007” deeper which I would no say is a issue in and of itself!

[cabhop]

Originally Posted by steve25

What do you have for piston to valve clearance with those Mopar pistons?

What do you have for total mechanical timing and when does it detonate?
Are you detonating at full throttle, part throttle or both?

If a intake would not bolt up then the heads had to be milled at least .040”.

If your heads are dated pre 1970 then with the mill work I will guess that you have a compression of between 9.5 and 10 to 1.

Since the heads where milled and on a small bore you may have a sharp edge on the chamber lip that is getting hot enough to light off the fuel charge and is giving you the detonation, but in general with your hot cranking compression you have I would check to confirm that your cam is in no more then straight up and not mistakenly advanced.

I would also suggest that even without looking at your plugs you start off your troubleshooting by dropping down one heat range and see how that pans out.

I didn't check valve clearance since another member who used these mopar pistons said they had 1/4" clearance (I know, rolling the dice). The mechanical timing is about 21 degrees and I have backed off the initial to 2 degrees with disconnected vacuum advance and still it detonates as soon as the secondary start opening into full throttle. I did install autolite 303s after pulling out the Motorcraft AS42C with no change. I'm running through a quiet single exhaust so hope that doesn't' add to the problem.

I'll pull the timing cover and verify cam timing and report back; I do appreciate the help with this.

[cabhop]

Originally Posted by PunchT37

Cam too far advanced?

Looks like I'll be pulling the timing cover and checking my handy work...

[Formulabruce]

195 psi is more like 11:1 . You can run E85 in it now.. but not premium pump gas..

[Stan Weiss]

Did you degree the cam? If so what is the ICL?

Stan

[Jay S]

According to my figures, assuming your not too far above sea level, with that cam and cranking compression reported, the static compression is around 9.5 to 9.7. Too get that much compression with 46 heads on a 350, which generally have cylinder scallops that lower the compression some, the head would need to be milled a ton, and the pistons would have too be .030” above the deck instead of below the deck. Most of the cast pistons for a 318 are taller than Pontiac pistons. Usually if installed in a Pontiac 350, a 1.74” compression height, will stick out of the deck some.. There are some industrial 318 pistons that are .080” shorter, but the would be below the deck by a lot in a Pontiac. I don’t know what pistons are being used to end up with .030” down in from the deck??? Do you have a piston part number?

190 to 195 cold cranking is pretty high on cranking compression for that particular combo. The compression really needs to be dropped about 30 lbs for pump gas. Which is pretty much 4X what you could do by adjusting the cam timing, the only exception might be if it was really advance on the install, off a full tooth. If you taking time to look at the cam timing, and it turns out to be pretty close, it would be time well spent to go ahead a change cams to something that will drop the cranking compression down more. As is, if the pistons are down in the deck as much as mentioned(.030” down) create a pretty poor quench, the intake didn’t fit is an indicator a decent amount of metal was machined off the head surface, those heads often start out at 78 to 80cc, yours appear to be closer to 70cc In addition, the stock felpro head gasket used (which is .045” thick, and makes the poor quench from the reported .030” down pistons worse yet), combine with the small 2800 cam), and 190 to 195 cranking compression), plus a restrictive exhaust system, all put together, unless something in the original description was not accurate for how it is set up, that engine is going to be an extraordinary tuning challenge on pump gas.

[tom s]

With our Calif piss gas I have found 180 to be a sweet spot for 91 gas.Tom

[cabhop]

Originally Posted by Jay S

According to my figures, assuming your not too far above sea level, with that cam and cranking compression reported, the static compression is around 9.5 to 9.7. Too get that much compression with 46 heads on a 350, which generally have cylinder scallops that lower the compression some, the head would need to be milled a ton, and the pistons would have too be .030” above the deck instead of below the deck. Most of the cast pistons for a 318 are taller than Pontiac pistons. Usually if installed in a Pontiac 350, a 1.74” compression height, will stick out of the deck some.. There are some industrial 318 pistons that are .080” shorter, but the would be below the deck by a lot in a Pontiac. I don’t know what pistons are being used to end up with .030” down in from the deck??? Do you have a piston part number?

190 to 195 cold cranking is pretty high on cranking compression for that particular combo. The compression really needs to be dropped about 30 lbs for pump gas. Which is pretty much 4X what you could do by adjusting the cam timing, the only exception might be if it was really advance on the install, off a full tooth. If you taking time to look at the cam timing, and it turns out to be pretty close, it would be time well spent to go ahead a change cams to something that will drop the cranking compression down more. As is, if the pistons are down in the deck as much as mentioned(.030” down) create a pretty poor quench, the intake didn’t fit is an indicator a decent amount of metal was machined off the head surface, those heads often start out at 78 to 80cc, yours appear to be closer to 70cc In addition, the stock felpro head gasket used (which is .045” thick, and makes the poor quench from the reported .030” down pistons worse yet), combine with the small 2800 cam), and 190 to 195 cranking compression), plus a restrictive exhaust system, all put together, unless something in the original description was not accurate for how it is set up, that engine is going to be an extraordinary tuning challenge on pump gas.

Sealed Power Cast Pistons 285AP20. They were the Summit $99 dollar specials at the time.

[cabhop]

Originally Posted by Stan Weiss

Did you degree the cam? If so what is the ICL?

Stan

It was just dot to dot. The sad part is, I have an unused degree wheel.

[PunchT37]

Originally Posted by cabhop

It was just dot to dot. The sad part is, I have an unused degree wheel.

Is that a multi slot crank sprocket?


If so, I`ve seen them put on wrong, as in, not understanding the instructions.


I was called to help on a Mopar 440 that wouldn`t start. It would backfire loudly out the exhaust though. I checked it out. Came to the conclusion that the valve timing was off. They said, bull$h!t, that`s a brand new Cloyes chain. I said, why did y`all call me? " Did y`all put the chain on right?" Yeah,they said. I said, show me the instructions. They threw em out. They DID have some old fouled NGK plugs saved in the new plug boxes though. I bitched them for that.



New, pretty engine they just put into a `69 Dodge Coronet. I said, pull the cover and figure it out, and left.


They got it running. Chain on wrong. True story.

[77 TRASHCAN]

Originally Posted by Jay S

According to my figures, assuming your not too far above sea level, with that cam and cranking compression reported, the static compression is around 9.5 to 9.7. Too get that much compression with 46 heads on a 350, which generally have cylinder scallops that lower the compression some, the head would need to be milled a ton, and the pistons would have too be .030” above the deck instead of below the deck. Most of the cast pistons for a 318 are taller than Pontiac pistons. Usually if installed in a Pontiac 350, a 1.74” compression height, will stick out of the deck some.. There are some industrial 318 pistons that are .080” shorter, but the would be below the deck by a lot in a Pontiac. I don’t know what pistons are being used to end up with .030” down in from the deck??? Do you have a piston part number?

190 to 195 cold cranking is pretty high on cranking compression for that particular combo. The compression really needs to be dropped about 30 lbs for pump gas. Which is pretty much 4X what you could do by adjusting the cam timing, the only exception might be if it was really advance on the install, off a full tooth. If you taking time to look at the cam timing, and it turns out to be pretty close, it would be time well spent to go ahead a change cams to something that will drop the cranking compression down more. As is, if the pistons are down in the deck as much as mentioned(.030” down) create a pretty poor quench, the intake didn’t fit is an indicator a decent amount of metal was machined off the head surface, those heads often start out at 78 to 80cc, yours appear to be closer to 70cc In addition, the stock felpro head gasket used (which is .045” thick, and makes the poor quench from the reported .030” down pistons worse yet), combine with the small 2800 cam), and 190 to 195 cranking compression), plus a restrictive exhaust system, all put together, unless something in the original description was not accurate for how it is set up, that engine is going to be an extraordinary tuning challenge on pump gas.

Jay do you think a 068 cam could be the solution?

[Stan Weiss]

Originally Posted by Jay S

According to my figures, assuming your not too far above sea level, with that cam and cranking compression reported, the static compression is around 9.5 to 9.7. Too get that much compression with 46 heads on a 350, which generally have cylinder scallops that lower the compression some, the head would need to be milled a ton, and the pistons would have too be .030” above the deck instead of below the deck. Most of the cast pistons for a 318 are taller than Pontiac pistons. Usually if installed in a Pontiac 350, a 1.74” compression height, will stick out of the deck some.. There are some industrial 318 pistons that are .080” shorter, but the would be below the deck by a lot in a Pontiac. I don’t know what pistons are being used to end up with .030” down in from the deck??? Do you have a piston part number?

190 to 195 cold cranking is pretty high on cranking compression for that particular combo. The compression really needs to be dropped about 30 lbs for pump gas. Which is pretty much 4X what you could do by adjusting the cam timing, the only exception might be if it was really advance on the install, off a full tooth. If you taking time to look at the cam timing, and it turns out to be pretty close, it would be time well spent to go ahead a change cams to something that will drop the cranking compression down more. As is, if the pistons are down in the deck as much as mentioned(.030” down) create a pretty poor quench, the intake didn’t fit is an indicator a decent amount of metal was machined off the head surface, those heads often start out at 78 to 80cc, yours appear to be closer to 70cc In addition, the stock felpro head gasket used (which is .045” thick, and makes the poor quench from the reported .030” down pistons worse yet), combine with the small 2800 cam), and 190 to 195 cranking compression), plus a restrictive exhaust system, all put together, unless something in the original description was not accurate for how it is set up, that engine is going to be an extraordinary tuning challenge on pump gas.

Jay,
Not being sure where the cam was installed, I was getting between 9.8:1 and 9.9:1

Stan

[Stan Weiss]

Originally Posted by 77 TRASHCAN

Jay do you think a 068 cam could be the solution?

Be interested to see what Jay thinks. I think it will.

Stan

[Jay S]

Originally Posted by 77 TRASHCAN

Jay do you think a 068 cam could be the solution?

Jeff I think the 068 is a good suggestion, and agree with Stan’s opinion it would work. It should drop the cranking compression about 15 lbs down to that 180 psi area Tom mentioned.. Which can potentially lower the octane it require around 3 points from the 2800 summit. It sounds like that would be in a 91 octane range from were it runs out now.

The most common dot to dot setting on the 2800 grinds is 107 ICL. I think that is what Summit’s are set at. At 195 pumping compression it would be as high as 9.9 depending on what ICL it is set at, or if it is a bit higher elevation than the number I used.

That 318 piston has a compression height of 1.72. The piston should be higher in the deck than .030” down. The cheap pistons are not always what they are advertised as though.

[Schurkey]

Originally Posted by cabhop

mopar 318 cast pistons that were .030 down in the bore...

...I used nos felpro 350 head gaskets...

...this motor detonates on 92 octane plus a can of booster...

...The warm compression test is 190-195 across the board.

Crappy quench/squish, coupled to high compression pressure and iron heads is a recipe for disaster. As best I can tell, quench distance between .050 and .100 or so is the "danger zone". Greater than .100--.120 results in an "open" chamber that doesn't detonate so much, and tighter than .050 (and preferably tighter than that--.030--.045 depending on engine and piston design) produces few end-gasses and good turbulence that also reduce detonation.

Are you sure the compression gauge is accurate? Seems like nobody tests them against other gauges.

I REFUSE to install a cam without a degree wheel and dial indicator. Saves frustration, and diagnosis time later.

Aftermarket "replacement" pistons are often at least .010 short, and sometimes .020 short on compression height. Bastages.

[Jay S]

FWIW..A friend recently went from a summit HEI to a DUI and his engine went from running on 91 octane too 87. Made a huge difference. I don’t know what you have though.

I am not that familar with auto lite plugs, but I think the 303 plugs should be cold enough. Not sure what else you can do tuning from what has been tried. I have done quite a few of those cam grinds, at least 10 in various engine platforms. It gets pretty edge on 91 at 9.5 on most engines, 9:1 is about perfect.