compression and timing

[doug428]

looking for a little help with 440 engine in the super bee I bought
I dont have a lot of info on the cam used
this is what I do know

440 bored .060 over
the heads are from a 73-73 motor home no porting visible on intake or exhaust ports
heads were re-furbished and have the hardened seats.
the pistons are raised dome Keith Black

the cam is supposed to be close to a stock 6 pack cam??
I show about .455 lift with 1.5 rockers and about .485 gold crane rockers installed
I think owner said cam overlap is 108*
in does sound lopey

my issue is I can only seam to get about 27 degrees total advance before I get spark knock with 92 octane

when I did a compression check on cold engine I got about 155 psi

this doesn't seem too be very high of psi for having raised dome pistons

i read a lot of post about these motors like to run mid 30's for total timing

even though I only have 155 psi, is the compression ratio too high for pump gas?
should I be running race fuel?

has anyone ran the raised dome pistons on a pump gas street engine?

 

[66Satellite47]

Off the top of my head, without the specific dimensions (piston deck height down the hole, chamber volume, net dome volume) it's hard to know what the static CR is. However with any dome & an iron head it's not too surprising you may get detonation with total timing below 30*. You're probably above 10.0 CR which is a problem with today's gas. The 155# seems in the ball park with non-stock cam & some dome. Longer overlap bleeds down cranking pressure pretty quick.

 

[PurpleBeeper]

Off the top of my head, without the specific dimensions (piston deck height down the hole, chamber volume, net dome volume) it's hard to know what the static CR is. However with any dome & an iron head it's not too surprising you may get detonation with total timing below 30*. You're probably above 10.0 CR which is a problem with today's gas. The 155# seems in the ball park with non-stock cam & some dome. Longer overlap beeds down cranking pressure pretty quick.

I bought a used set of TRW domed 440 pistons many years ago & the tops look like they'd been shot point-blank with a shotgun (tons of little divots everywhere) and that motor was run on 93-octane. You might want to consider some of the super-thick head gaskets to cut a 1/2-point off your compression ratio (or run race gas like you mentioned). As 66Satellite47 mentioned, you could also try a cam with more overlap (more lope) to bleed off some of that compression.

 

[66Satellite47]

I should clarify, I meant that 155# cranking compression seems about right for a longer duration cam with domed pistons. I'd try blending race gas with 92 before going to all race gas, or stick with the 27*.

 

[bandit]

I have domed TRW's on my 383 and it likes around 100 octane but it all comes down to your CR. I time purely by the ping and don't pay much attention to the timing marks. If you can get more advance out of higher octane with no ping then I say keep pushing it up until no more benefit. You will hit a point where it cannot advance anymore (car will start to labor to turn over and start)and you will learn what best octane hits that sweet spot right before that.


Also check for air leaks and make sure your jets are big enough.

Good luck

 

[Kern Dog]

What is the testing method you used to determine the cranking compression numbers?
You will get a low number if you do it wrong.
Remove all 8 spark plugs.
Disable the spark to the distributor.
Connect a battery charger to the car to maintain a consistent battery voltage.
Hold throttle open, crank for 5 seconds on every cylinder.

You have to do it this way to get reliable, accurate results. The engine will not crank as fast if the other 7 cylinders have the plugs in...It will be building compression instead. The throttle needs to be wide open to reduce restriction. The battery needs to have a steady reading to be able to spin the engine the same speed for each cylinder.
I had almost 11.0 to one and had to limit my timing to 30 degrees to keep from knocking. Once I added thicker head gaskets and got my CR to 10 to 1, I was able to step it up to 34-35 degrees. The car runs fantastic now with no knocking.

 

[miller]

Exactly how Kern Dog says...

Though, going by what I read years ago, allow each cylinder to 'hit' three times on compression stroke. Use the third hit reading.

 

[doug428]

thanks for replies
its been a while since I did the compression test, but I;m pretty sure I did it with throttle open and plugs out.
I will try the race gas to see how far I can advance it and if I get more power out of it.
I've heard you can run a colder plug also.

another guy I talked to ran E85 because of the higher octane and lower cost, I dont think I want to go that route.

I may check out the thicker gaskets, anyone have part numbers for these.

 

[lewtot184]

the cam is probably the mopar 272-.455. i've used this cam and with the crane adjustable rockers it's about .490" at the valve. the "domed" KB pistons would be quench domes and 155psi blow tells me that maybe they're KB184's. so if i'm right somebody has tried a quench dome unleaded build. if done right the compression will be around 9.3:1 and will work with 91-93 octane gas. having detonation problems at only 27 degrees total tells me that either the timing mark on the balancer may be off, you may not be setting the timing correctly, or your getting water or oil contamination in the combustion chambers. the 184's don't need anymore than 34 degrees total timing.

 

[doug428]

I found additional info on engine
kb-236-.060 keith black
455 lift with 1.5 rockers 108int-114ex duration. I added 1.6 rockers so about 485 lift now
head casting number 3751213

 

[lewtot184]

I found additional info on engine
kb-236-.060 keith black
455 lift with 1.5 rockers 108int-114ex duration. I added 1.6 rockers so about 485 lift now
head casting number 3751213

if that piston was set up correctly then you probably have 10.5:1 compression. it should blow well over 200psi. the "108int-114ex duration" don't make sense; maybe installed centerline. when i ran the 272-.455" cam with crane 1.5 rockers (they checked at a true 1.6) i had a around .490" lift at the valve.

 

[Kern Dog]

The 108 is the lobe center. The 114 is the LSA. This looks a lot like the Crane copy of the factory 440 Magnum cam.

 

[doug428]

The 108 is the lobe center. The 114 is the LSA. This looks a lot like the Crane copy of the factory 440 Magnum cam.

I think that is correct, the guy wasnt too sure, he remembered those numbers but didnt know what they applied to.

I did find the receipt for the kb 236 pistons though

should I assume he already put thicker gaskets on if I'm only gettig 155 psi on cylinders

I guess I back off timing to run pump gas and on special days I bump it up and mix race gas

 

[Cranky]

27 total isn't much timing for an engine with 155 psi cranking pressure. Have you verified true TDC and that the balancer is marked correctly? What is your initial timing showing and at what engine temps do you notice it pinging? There's a lot of engines out there with much more than 155 psi running on pump gas without pinging issues.....