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Max Dynamic CR for 87 octane?

 
doublesun
V-6 Camaros rule!
Posts: 40
Joined: 07/10
Posted: 07/24/10 08:31 AM

I playing with a program that calculates Dynamic Compression Ratio. My question is what should my Dynamic Compression Ratio upper limit be for regular 87 octane?

A link to an article provided in an earlier post suggests between 7.5 to 8.5 for 91 octane. The author goes on to say he's not quite sure what the range would be for 87 octane.

Anyone have an upper range number?

Thanks,

Mike  

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frotosride
My first time was in a Chevy
Posts: 162
Joined: 02/10
Posted: 07/24/10 06:10 PM

Thats a good question and it is really a game of chance.example: 2 sbc 350 same castings heads and everything all stock. leave one completely oem and take the other and do a simple porting and polishing job and adjust spark plug heat range. The second engine will tolerate a little higher compression for a few reasons One: porting and polishing the heads, specifically in the combustion chamber will eliminate  hot spots from say casting flash or any thin metal in the combustion chamber Second: pulg heat range obviouse.. less heat retained in the plug. now this engine will have less chance premature detonation becasue of the lack of hot spots where it counts.
On a stock block and heads there's no real way to tell but what i have learned fro a lot of the really talented guys here 7.9 to maybe 8.5 will hold on a well prepped engine. So in all reality it depends on the engine and set up. The more you can control hot spots and latent heat the better chances you will have with raising compression.  
"Beat it like a red-headed FORD"

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gettnlarge01
I mow my lawn and find Chevys
Posts: 3330
Joined: 09/09
Posted: 07/24/10 06:23 PM

7.0 DCR is acceptable for 87octane. You can run a higher DCR with a properly built engine and still use the same 87octane fuel. Things start to get more in depth to accomplish and require more info about your engine.
The best way to determine which fuel to use is by going of the cranking compression. Is your engine already together or are we just talking theories?  
luck is when skill and opportunity meet

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frotosride
My first time was in a Chevy
Posts: 162
Joined: 02/10
Posted: 07/24/10 06:35 PM

gettlarge, don't take this as though i'm bashing you or anything and i only say that because some people get really temperamental on this forum when you are simply asking a question. that said. With my set up I.m running 9.7:1 static and 7.9:1 dynamic. So are you saying if you don't take into account all the dynamics of the engine and more specifically the cam he will be better off winning this battle with his motor/future motor.
If so wouldn't it be misleading to him if say his intake valve close degree is is smaller and bleeds off more compression before it shuts which obviously won't change static compression but will effect the octane level for the engine...? Just wondering your thought process?  
"Beat it like a red-headed FORD"

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chevelle83
I mow my lawn and find Chevys
Posts: 2280
Joined: 11/09
Posted: 07/24/10 06:43 PM

dont forget it also depends on what the engine is doing,,, if you are pulling a long hill with heavy car and high gears it will be less tolerant to low octane,  opposite for the other way around.  

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gettnlarge01
I mow my lawn and find Chevys
Posts: 3330
Joined: 09/09
Posted: 07/24/10 06:44 PM

IMO static compression is worthless. Why worry about something that never happens in an running engine?  Proper quench,reshaped combustion chambers,valve timing,etc all play a role in an engines ability to stay out of detonation  
luck is when skill and opportunity meet

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doublesun
V-6 Camaros rule!
Posts: 40
Joined: 07/10
Posted: 07/24/10 08:36 PM

Engine has yet to be built, block at machinist right now. Trying to get component selection right in my head. The DCR program relies on cam data and I can play with the cam to move my DCR.

Looking at Vortec heads with a +22 piston for a SCR of about 9:1. I can also select a +18 for a SCR of about 9.25:1.

Dropping different cam data into the DCR program definitely changes things as the intake closing angle changes. I'm looking for a low end torquey 383 and thought "mild" cams with their smaller intake close angles. (I use the term mild to describe what cam manufacturers call daily driver, smooth idle, etc.). When I put the "mild" cam data in the DCR program the DCR is higher than a so called "hotter" cam. Which makes sense as the intake close angle tends to get larger with the hot cams.

Given the Vortecs with either +22 or +18 pistons does anyone see any problems with running a cam with a range of 52 to 62 or so degrees intake closing angle and still running this baby on 87? My gut says go for it but looking for input.

I'm also under the impression that a higher DCR gives me better low end torque. Is that a fairly true statement?  

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gettnlarge01
I mow my lawn and find Chevys
Posts: 3330
Joined: 09/09
Posted: 07/24/10 08:46 PM

to a certain extent a higher DCR will give better low rpm tq.
with .000 deck clearance,.039 compressed thickness headgasket,at least duburred combustion chambers I would see no problem with an good ignition system.  
Keep in mind that DCR calculators are never 100% accurate and thats putting it mildly.  
luck is when skill and opportunity meet

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doublesun
V-6 Camaros rule!
Posts: 40
Joined: 07/10
Posted: 07/25/10 05:45 AM

Ok, any cam selections for a 383 daily driver, smooth idle, low end torque over speed, with Vortec 64cc heads and +22cc pistons? Looking at both Crane and Comp. Keeping mind I would like to use 87 octane.

Feel free to explain your reasoning for choosing a certain cam. Not only building an engine, trying to learn the why's and wherefore's.

Thanks,

Mike  

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gettnlarge01
I mow my lawn and find Chevys
Posts: 3330
Joined: 09/09
Posted: 07/25/10 08:55 AM

ok next set of questions
truck or car?
trans type?
gear ratio?
will it be used for any towing?
2wd or 4wd?
tire size?  
luck is when skill and opportunity meet

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doublesun
V-6 Camaros rule!
Posts: 40
Joined: 07/10
Posted: 07/25/10 02:03 PM

1976 Camaro

4 Speed Manual - factory

Gear Ratio Unknown - Assume it's stock however

No towing

Tire size - 295/50R15 on rear, more than likely

Performance requirement is: strong, reliable, not a hot rod, lower rpm driver, no race, more low end torque if I can get it, probably 4bbl 650 (?), regular 87 gas  

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tyler659
I mow my lawn and find Chevys
Posts: 1710
Joined: 05/09
Posted: 07/25/10 02:25 PM

what kind of cam flat tappet, roller, solid...and are you wanting to replace the springs on the heads?  
tyler659

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doublesun
V-6 Camaros rule!
Posts: 40
Joined: 07/10
Posted: 07/25/10 02:35 PM

Hydraulic flat tappet.

Didn't consider swapping valve springs. Planned on "stock" Vortec setup. Where are you going with that question? Lift?  

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tyler659
I mow my lawn and find Chevys
Posts: 1710
Joined: 05/09
Posted: 07/25/10 03:00 PM

stock vortec v-spring are good up to .500 lift i ran a comp xtreme energy XE262 in my 350 with world heads and headers and it had a nice idle and with 1.5 rockers it only has .462 lift  
tyler659

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tuffnuff
Moderator
Posts: 7841
Joined: 12/09
Posted: 07/25/10 05:03 PM

Vortec heads will only take .480" lift, by design. The valve stem seal, sits right on top of the valve guide boss, in a machined recess and any more lift, could cause retainer/seal interference.
Guide bosses can be machined and then more lift is possible.
Smile  
When The Flag Drops.,.

tuffnuff

The Bull ***t Stops.,.
tuffnuff

P. Engineer, Engine Builder

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