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LS_5

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Discussion starter · #1 ·
Is it possible to generalize about pump gas vs. mixed gas when it comes to compression?

I've got a BB (454)with fully ported heads. We're having a "debate" here about what the max compression is to safely run pump gas in connection with a rebuild that includes new forged pistons (yet to be spec'ed).

One school of thought is that if you go more than 10.25 - 10.5 you risk detonation problems that will eventually burn holes in the tops of the pistons.

The other camp thinks that's "old school" and that you can safely go to as high as 12:1 on pump gas with today's fuel and pistons and other components and compression shouldn't be an issue - the more the merrier!

Personally, I'm no engine builder - but want to run pump gas and absolutely do NOT want to risk damage to the components! This stuff is expensive!!

This may be like asking who's the prettiest girl in town, but what's the max compression you'd feel comfortable with to maximize performance, yet be safe you arten't damaging anything on pump gas?

Andy
 
#2 ·
I run 92-93 octane with 10.59 to 1 compression 454
*38 total timing
My cam also creates alot of cylinder pressure too.
I would say thats the limit IMHO
Theres plenty of 500+ HP 454's that run well with less compression than 10.5 too
12 to 1 in a street/strip big block motor is just ridiculous (not in a bad way,just not needed),unless its some crazy small block with real steep gears in a light car.
 
#4 ·
It also depends on the intake closing point of the cam. Generally the longer duration a cam has, the later the intake closing point and more compression it bleeds off. The more compression is bled off, the higher a compression ratio can be utilized on a given octane. The advance/retard and LSA of a cam also has an effect on the intake closing point. It's called Dynamic Compression Ratio. It's thought a DCR of around 8.25 to 8.5 is the max for pump gas with iron heads. Do a search on Dynamic Compression Ratio to find info on this and a calculator.
 
Discussion starter · #6 ·
Pat/Von

Thanks for the link. I really mean that!! But
I've never built an engine and have only a very basic idea about what some of the things in the article are referring to.

I kinda need a layman's version of this paper, or something to read that a novice can understand as background before jumping into something this technical.

What's BDC and TDC stand for? I assume DC is Dynamic Compression - but what do the B and T stand for?

How about the A and B in ABDC?

Thanks!

Andy
 
#7 ·
Andy, BDC Is Bottom Dead Center, or when the piston is at its lowest point in the cylinder. TDC is Top Dead Center, at its highest point. ABDC is After Bottom Dead Center. To simplify things, with the cam you intend to run (somewhere in the 230-240 deg duration at .050 lift if I remember right) and iron heads, you'll want to not go much over 10.0-1 static(the commonly known rating)compression ratio to run pump gas. I'd say 10.5-1 is really pushing it. The school of thought that says you shouldn't go above that number without risking detonation damage is correct. You may be able to go a little higher with a really long duration (read wild) cam but that's pretty much race only.
 
#8 ·
Here is an explination that may be a little simpler. First, think of the four strokes of the engine. Intake(as the piston drops with the intake valve open) compression or the power stroke(as the piston rises and compresses fresh fuel and air with both valves closed) combustion(Spark plug goes off...boom, gas expands and forces the piston donw on the power stroke) and the exhaust(piston rises and exhast valve opens to let out the burnt gasses).

Now look specifically at the intake event. As I was sayin, the intake valve is open as the piston is dropping. Because the piston is dropping, pressure drops, so fresh fuel and air enter through the open intake valve to equalize the pressure in that cylinder. But, the valve does not close as soon as the piston gets to the bottom of the stroke(bottom dead center). The piston will reach the bottom of the stroke, change direction and start to rise(the compression stroke), but the intake valve is still open as the piston rises. Not the whole time, but depending on the cam choice, the valve could be open for a fairly long time, or just a short time. At this time, you cant actually really compress much cause the valve is still open with the piston rising. You cant start compressing anything to any great degree until the valve closes.

So say you build two motors, and both are 10:1 compression. Lets say one motor has an intake valve closing angle of 60 degrees(after bottom dead center). That means that the intake valve wont close and you cant compress anything for 60 degrees worth of rotation after bottom dead center. Once it closes, the piston will be partially up in the bore, so the mixture actually gets compressed at a rate less than 10:1. In order for the mixture to actually be compressed 10 times to one, the valve would have to close right at bottom dead center, but it doesnt. Lets say your other 10:1 engine has a different cam with an intake valve closing angle of 74 degrees. Thats a bigger number than 60, and it means that it will take even longer for the valve to shut, and once it does, you be able to compress even less than with a 60 degree closing angle, even though the static compression is the same in both motors. That means that even though both motors, have 10:1 static compression, because of the different camshafts, each engine will actually produce different cylinder pressures while running. The one with the bigger camshaft with the 74 degree angle will produce less cylinder pressure. That make sense so far? All in all, there really isnt an exact number for static compression ratio. You could build a 9:1 engine and have tons of detonation problems on premium fuel cause of the cam choice, or you could build an 11:1 motor and be able to run happily on 91 octane.

Anyways, this whole idea that you cant compress anything til the valve closes might sound bad, but its not all bad. At high engine rpm's the air/fuel is traveling so fast and has so much momentum, that even after the piston starts to rise(pressure isnt dropping any more) the air can continue to force itself into the cylinder with its own momentum. This allows for better breathing at high rpms. If you shut the valve too early, you may not allow as much fuel and air in as you could, so you wont produce as much high rpm power. This is just one of the reason that big cams are better for higher rpm, and small cams are better for low rpm. Small cams close the valve quicker and build better cylinder pressure and torque down at low rpm, but the dont allow the motor to breath as well at higher rpm. Bigger cams do just the opposite. Hope that helped some.
 
Discussion starter · #10 ·
Greg/Von

Thanks for the explanation! That helps alot!!
Image


The cam specs I have are:

Lunati 50249

Power Range: 2000 - 6000
Adv Dur: .290"/.300"
.232/.242 @.050
.578"/.595" lift
112 Deg SEP

It's ordered but hasn't come in yet, so I don't have the card yet to pull other details.

Andy

ps Greg - what part of the 'Burgh are you from?
 
#11 ·
Just kidding :D

The Engine Masters competition has pretty much narrowed down what you can get away with. However they tolerate some detonation to win the competition. To me that's not quite kosher, but anyways.
With a big cam and aluminum heads, 12:1 looks to be the max. At that level it will sometimes detonate, sometimes not, depending on the environment.
So you can on theory build a 12:1 motor, and 'cruise' around with it on pump gas, but you play roulette every time you hammer it.
 
#12 ·
Andy,

I had comsidered that same cam (50249) for my 408 motor, but wound up going a little smaller. JMHO, I think the 50249 should be a real good hyd roller in a 454 motor. I don't quite get Lunati's position on only using a single spring for .578/.595 lift though. Also, when you get the cam, please let us know for sure if it is made on a billet or a cast core. Also, if you get the actual seat timing numbers that would be nice to know. Lunati(Holley) only lists the .050 timing numbers, the seat durations appear to be a little on the long side. Makes me think their advertised numbers are based on .004 rather than .006 lift.
 
Discussion starter · #13 ·
Will do.

It's scheduled to be delivered on 2/3 directly to the builder but I can get the card info and will ask him about the core.

Andy
 
#14 ·
Im running 11.1:1 compression in my 350. I successfully run 89 octane ethanol at the drag strip. I could probably run 87 but 89 is cheaper around here. The reason being im grossly over cammed with 268/278@.050... wich i might add is going to change this spring. I also dont go over 170 degrees with the water. I also pretty much have aluminum everywhere on my motor.
 
#15 ·
LS_5, based on those cam specs you gave, Von is right that 10:1 compression is a pretty good place to be. That should do you just fine with that cam and engine. Im actually about 30 miles north of pittsburgh in beaver county(yes, there is such a place). Thats just above Allegheney county, and I live pretty close to the ohio border.

Hey TJC, nope, Im not a lawyer(maybe I should re-consider). I usually just get carried away and use more examples than are probably needed and I when I re-read my posts, it seems I often repeat important stuff more than it probably needs to be. As long as it gets its point across, I dont mind typing all that. But I guess many times I do write something about the size of a magazine article just to make one or two points, where a good editor can cram 30 concepts into an article of that size and make pretty good sense of all of it. Eh well, I guess it works fur me.
 
#17 ·
I think it was Bruce Crower that came up with an Ecomomy engine/cam. The compression was like 15.0.1 ultrahigh compression and cam had so much duration it would push the charge out. Wild design that never caught on. Hot Rod even built a 350 with this set up.
 
#19 ·
More pressure = more power. For a datapoint Andy, at 10.51:1 with a relatively short ( 276) duration cam, it's defintely at the max. I was shooting for 10.3 exactly and over shot, losing a margin of error for the occasional bad tank of gas.

Not much on mixing stuff in, although I'm doing that now. Might end up dialing the cam back 2 degrees. Would prefer there to be a margin of error.

I agree with school of thought A. 10.3-10.5 max.

School of thought B works with cams I would consider more 'race' and it's more like 11:1. ( or the older LT-1 L88 type cams) Something as small as the old Ultradyne 288/296 might fall in that camp; your Lunati, on the cusp. When you get above 310, then , sure 12:1 would be fine.

10.75:1 would be a good margin of error target for a cam like that. 10.5:1 would be conservative. I ran 304 ( 296 in hyd terms) seat duration on a 114 lsa at 10.75:1 without a hint of trouble. Chris runs a smaller cam on a 112 lsa at 10.59:1 with no issue.

old school, to me, is more like building a pure L88 or LS7 -type 7200 rpm screamer. And yeah, 11.5-12:1 might actually tolerate 93 with those cams. I figured on 8.3 dynamic compression with the L88 grind and 11.5:1. 12:1 and that cam puts it real close to right where Old Red's 427 is now. 8.5-8.6 dynamic or effective compression. Sensitive old bastid ; to poor fuel that is.
 
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