Going from 9.5 to 9.1 Compression 408 Magnum

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thomasfouraker

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Picking up a motor tomorrow that was built for 9.5:1 compression. I am planning on throwing my RT heads on but am hoping I can get the compression low enough to run 87 octane. Going into an older pickup used to tow a boat. I still have to get the motor and run the unknown numbers I don't have available at the moment (piston to deck height, piston dish cc....) in the meantime these are options I am considering to get me closer to to my target CR.

Gaining some cc's unshrouding valves on the heads
Thicker head gasket (cometic goes to .125)
Going to a deeper dish piston (currents are 25cc)
or having the piston relieved if possible.

The easiest might be running premium after all.......

Concerns
Grinding on the combustion chamber of the head
Surface roughness too much on the block for a cometic
Cost/availability of pistons
Weakening the pistons by removing whats needed

Looking forward to hearing responses!

Thanks!
 
IMO, leave it be. Should run ok on 87. If the short block is in good shape and I was really concerned, maybe a thicker gasket. A half point of compression isn’t worth tearing it apart.
 
The fuel you can run is directly related to one thing only,detonation.
Detonation can be avoided by two or three or four things.
The major players, in the building stage are
Combustion chamber temperature, and chamber efficiency.
The former is mostly related to cylinder pressure, which is in some part affected by static compression ratio, but also by the camshaft's Intake closing angle, followed by inlet air temperature, AFR, and coolant temperature.
The latter by head material, combustion chamber size and shape, quench,freedom from hotspots, ignition timing, and loading.
I think I got that right.lol.

As to pressure, for 87 gas you are sorta limited to 150/155 psi with iron open chamber heads. But there are many many ways to achieve that pressure, and Static Compression Ratio is just a tool we use to get what we want.

It is easily possible to run 11/1 on 87E10; I have been doing it since 1999, allbeit in an aluminum-headed 367. Even ran 11.3 with a MP-292/292/108 cam.

Depending on the size and weight of the boat,and the rear gearing, which goes to loading; and the cam used, you may not have to lose a meager 4/10 of a point.

Furthermore, if you have to pay 10% more for the next grade of fuel, but you build your engine to get 10% more miles per gallon; then by the math you break even. But in the real world, the additional cylinder pressure allowed by the better gas, will end up saving you additional fuel during acceleration.
It's a lil tricky, but well worth the effort.

As an example
@9.4Scr, you can get about 155psi@ an Ica of 63* on your cam,which is a nice sized cam for a 408.
@9.1Scr, you can get the same pressure at an Ica of 58*, which is not quite two sizes smaller.

You can push the pressure up a bit by reducing your power-timing in the lower rpms, where, with a stroker, you won't need it anyway. Then you can bring the timing back with Vacuum advance, for cruising with. And you can still have full power-timing after say 3600 for passing.

But, if you have an overdrive, and you intend to tow in it, all bets are off. With a non-computerized ignition timing, it will likely be impossible to get the timing up high enough, and your fuel economy will be in the dumpster because of it.
You see, everything we do in terms of timing controls is designed to do one thing; namely to time the spark in such a way that the maximum amount of energy in the expanding hot gasses,can be delivered to the piston, when it is on the exact right spot to deliver said energy to the crank. If your timing is too early, you lose power and risk detonation. If too late, the fire chases after the piston and you lose power and increase the running temperature. Cruising at 60/65 mph towing a boat, in overdrive, is not the time to be giving up power or overheating. Less power requires more throttle opening to get what you need..... and there goes your fuel-economy. The problem at low rpm/ small throttle opening, is to give the engine the timing it is gonna want. It is not uncommon for a lightly loaded engine to want,at say 2400rpm, 44 to 54 degrees of advance. Say yours likes 50*. You can get that with 22 in the Vcan and 28 in the mechanical. But if 28* at 2400 causes detonation at WOT, well you can't have that. So if by testing you find your engine does not want more than 24* at 2400 at WOT, then your cruise timing can only be 46. Which is likely to be 4 to 8 or more degrees short of where it might need to be for best fuel-economy.
So to marry these, the cylinder pressure would need to rise up, so that it doesn't need the extra cruise-timing.
The take-away here is to not try to cruise at too low an rpm, thinking that you will get better fuel-economy. The potential is definitely there. But if you bias your distributor for cruise-timing, you will very likely get into trouble with your FULL-LOAD WOT PowerTiming. You might be able to overcome this with a smaller than usual carburator. Or a two-step timing curve, which is what I did. For instance;My D has a fast curve to 2800, then it takes another 600 for the rest of it to come in. My engine doesn't get full Power-Timing, until ~3400.
 
The fuel you can run is directly related to one thing only,detonation.
Detonation can be avoided by two or three or four things.
The major players, in the building stage are
Combustion chamber temperature, and chamber efficiency.
The former is mostly related to cylinder pressure, which is in some part affected by static compression ratio, but also by the camshaft's Intake closing angle, followed by inlet air temperature, AFR, and coolant temperature.
The latter by head material, combustion chamber size and shape, quench,freedom from hotspots, ignition timing, and loading.
I think I got that right.lol.

As to pressure, for 87 gas you are sorta limited to 150/155 psi with iron open chamber heads. But there are many many ways to achieve that pressure, and Static Compression Ratio is just a tool we use to get what we want.

It is easily possible to run 11/1 on 87E10; I have been doing it since 1999, allbeit in an aluminum-headed 367. Even ran 11.3 with a MP-292/292/108 cam.

Depending on the size and weight of the boat,and the rear gearing, which goes to loading; and the cam used, you may not have to lose a meager 4/10 of a point.

Furthermore, if you have to pay 10% more for the next grade of fuel, but you build your engine to get 10% more miles per gallon; then by the math you break even. But in the real world, the additional cylinder pressure allowed by the better gas, will end up saving you additional fuel during acceleration.
It's a lil tricky, but well worth the effort.

As an example
@9.4Scr, you can get about 155psi@ an Ica of 63* on your cam,which is a nice sized cam for a 408.
@9.1Scr, you can get the same pressure at an Ica of 58*, which is not quite two sizes smaller.

You can push the pressure up a bit by reducing your power-timing in the lower rpms, where, with a stroker, you won't need it anyway. Then you can bring the timing back with Vacuum advance, for cruising with. And you can still have full power-timing after say 3600 for passing.

But, if you have an overdrive, and you intend to tow in it, all bets are off. With a non-computerized ignition timing, it will likely be impossible to get the timing up high enough, and your fuel economy will be in the dumpster because of it.
You see, everything we do in terms of timing controls is designed to do one thing; namely to time the spark in such a way that the maximum amount of energy in the expanding hot gasses,can be delivered to the piston, when it is on the exact right spot to deliver said energy to the crank. If your timing is too early, you lose power and risk detonation. If too late, the fire chases after the piston and you lose power and increase the running temperature. Cruising at 60/65 mph towing a boat, in overdrive, is not the time to be giving up power or overheating. Less power requires more throttle opening to get what you need..... and there goes your fuel-economy. The problem at low rpm/ small throttle opening, is to give the engine the timing it is gonna want. It is not uncommon for a lightly loaded engine to want,at say 2400rpm, 44 to 54 degrees of advance. Say yours likes 50*. You can get that with 22 in the Vcan and 28 in the mechanical. But if 28* at 2400 causes detonation at WOT, well you can't have that. So if by testing you find your engine does not want more than 24* at 2400 at WOT, then your cruise timing can only be 46. Which is likely to be 4 to 8 or more degrees short of where it might need to be for best fuel-economy.
So to marry these, the cylinder pressure would need to rise up, so that it doesn't need the extra cruise-timing.
The take-away here is to not try to cruise at too low an rpm, thinking that you will get better fuel-economy. The potential is definitely there. But if you bias your distributor for cruise-timing, you will very likely get into trouble with your FULL-LOAD WOT PowerTiming. You might be able to overcome this with a smaller than usual carburator. Or a two-step timing curve, which is what I did. For instance;My D has a fast curve to 2800, then it takes another 600 for the rest of it to come in. My engine doesn't get full Power-Timing, until ~3400.

First, thank you so much for taking the time to provide such a comprehensive answer. I obviously over simplified the concept using only static Compression Ratio.
I should have provided more detail as well in my original post. I plan on retaining factory efi and dialing a custom tune in on a dyno here locally. Will be running iron heads with 63cc chambers.

Also, I never tow with OD on. In a 2000 Ram that’s transmission homicide.
 
Picking up a motor tomorrow that was built for 9.5:1 compression. I am planning on throwing my RT heads on but am hoping I can get the compression low enough to run 87 octane. Going into an older pickup used to tow a boat. I still have to get the motor and run the unknown numbers I don't have available at the moment (piston to deck height, piston dish cc....) in the meantime these are options I am considering to get me closer to to my target CR.

Gaining some cc's unshrouding valves on the heads
Thicker head gasket (cometic goes to .125)
Going to a deeper dish piston (currents are 25cc)
or having the piston relieved if possible.

The easiest might be running premium after all.......

Concerns
Grinding on the combustion chamber of the head
Surface roughness too much on the block for a cometic
Cost/availability of pistons
Weakening the pistons by removing whats needed

Looking forward to hearing responses!

Thanks!

Been down this road and there are two things you can do.
One cheap, one free.

Cheap, thicker head gasket. A standard Fel-Pro at .043 or the thicker one at .05x which is about double the thickness of a factory head gasket. Cometic will make you one of darn nearly any thickness for you. There pricey for head gaskets though!

Free, retard the timing a few degrees. The draw back is power loss but 87 octane is assured.
 
Thank you everyone for taking the time to respond. Picked up the motor yesterday and my concerns were premature. All the pistons are below deck enough where I should be able to dial everything in through gasket thickness alone.

I will have some questions forthcoming regarding external/internal balancing but thats a few weeks away I assume.

Been involved with quite a few internet communities for trucks, hobbies and interest and this one seems far above the others in getting quick, thorough and no condescending responses. That along with patience for someone trying to learn is truly appreciated!
 
Your welcome and I missed you when you joined so a belated “WELCOME ABOARD” to you.

Balance? It can be tricky. What the factory did and what you have could be two different things. Engine history is the first leg of homework.
 
Your welcome and I missed you when you joined so a belated “WELCOME ABOARD” to you.

Balance? It can be tricky. What the factory did and what you have could be two different things. Engine history is the first leg of homework.

The history will be the hardest part of this puzzle. I am the third owner as of yesterday. The original owner had it built for a late 90s Dakota. He ended up parting the Dakota out due to a rusted out roof after putting "15K miles on it". Purchased for a Duster project that ended up changing direction to a 340. The guy I purchased it from said he heard the motor run and helped pull it. I felt good about the purchase, the seller was very forthcoming and accommodating allowing me to pull and inspect main and rod bearings.

The kit is a Eagle Forged crank and rods. It appears there had been some light grinding done to the crank and rods. Doesn't look like any mallory was added to the crank but there are places metal was removed. The damper is a pro/street unit but can't tell if it has the weight bolted to the back of the damper as its still on the motor. Also the flex plate didn't have any weight on it.

This will be a learning process for me. I plan on pulling everything down, giving it a good cleaning and checking clearances then reassembly. Might take the block to the machine shop to get it honed for Moly Rings......gotta spend more time on here reading before decisions are made.

Excited to get it going though!
 
Oh! I’d be totally stoked myself.
I’m pretty sure it is internally balanced but do so check the damper anyway. That’s easy enough.

Jacksonville in what state?
 
Oh! I’d be totally stoked myself.
I’m pretty sure it is internally balanced but do so check the damper anyway. That’s easy enough.

Jacksonville in what state?

Florida. So I will have about two weeks of comfortable weather this year to get this motor done without suffering!
 
what heads?
what's the dish look like D or circle
what is the deck clerance
unshrouding the valves is always good and clean up the chambers
 
what heads?
what's the dish look like D or circle
what is the deck clerance
unshrouding the valves is always good and clean up the chambers

This pistons are Keith Black 356. They have already been machined down where the elevated dome part above the valve reliefs are. It is machined flush with the rest of the piston. Going to try and get one cleaned up and post a photo.

I haven't measured deck clearance. Don't have an deck bridge for that but will run a straight edge across and put some feeler gauges in there to get me close. Its definitely below deck a good bit.

Will be running Magnum R/T heads. 2.02 Intake Valves, Bowl blended and light porting. I haven't confirmed myself but I believe they are 63cc chambers according to everything I have read.

Will try and get some actual numbers to go by today but I think I am going to be fine to dial in my CR through head gasket alone.
 
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