Max Dynamic Compression Ratio?

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12-19-2005, 08:01 AM
MainCap

Max Dynamic Compression Ratio?

Anybody now what the recommended maximum allowable DYNAMIC compression ratio is for 87,89,and 91 octane unleaded pump gasoline, for an iron head engine with .040 quench under towing load scenarios?

KB's site suggests no more than an 8:1 dynamic CR under the best (light load?) conditions without getting exotic. They also indicate a static CR of no more than 8.5:1 for an engine used for towing that is running on regular unleaded gasoline. I do not find how much more CR can be run with higher than 87 octane pump gas.

I can drop my dynamic CR by running a cam with a later intake valve closing, or I can choose a 28cc dish top piston that will drop my static CR to 8.8 - which seems low to me. I could also drop a cc or two out of my 64cc heads and stick with an 18cc D-cup piston for a static CR of about 9.5:1 but would 89 or 91 octane keep me out of detonation?
12-19-2005, 09:38 AM
MainCap

I forgot to mention that my truck is also equipped with an electronic spark control/knock sensor that retards the timing when it senses detonation. Do you think 9.5:1 static CR is too high with this system as well?
12-20-2005, 05:44 PM
camaro_fever68

Re: Max Dynamic Compression Ratio?

Quote:

Originally posted by MainCap
Anybody now what the recommended maximum allowable DYNAMIC compression ratio is for 87,89,and 91 octane unleaded pump gasoline, for an iron head engine with .040 quench under towing load scenarios?

KB's site suggests no more than an 8:1 dynamic CR under the best (light load?) conditions without getting exotic. They also indicate a static CR of no more than 8.5:1 for an engine used for towing that is running on regular unleaded gasoline. I do not find how much more CR can be run with higher than 87 octane pump gas.

I can drop my dynamic CR by running a cam with a later intake valve closing, or I can choose a 28cc dish top piston that will drop my static CR to 8.8 - which seems low to me. I could also drop a cc or two out of my 64cc heads and stick with an 18cc D-cup piston for a static CR of about 9.5:1 but would 89 or 91 octane keep me out of detonation?

run a .037-.039 quench area and polish the combustion chambers in the heads to eliminate hot spots
12-21-2005, 12:31 PM
rumrumm

Re: Max Dynamic Compression Ratio?

Quote:

Originally posted by MainCap
Anybody now what the recommended maximum allowable DYNAMIC compression ratio is for 87,89,and 91 octane unleaded pump gasoline, for an iron head engine with .040 quench under towing load scenarios?

KB's site suggests no more than an 8:1 dynamic CR under the best (light load?) conditions without getting exotic. They also indicate a static CR of no more than 8.5:1 for an engine used for towing that is running on regular unleaded gasoline. I do not find how much more CR can be run with higher than 87 octane pump gas.

I can drop my dynamic CR by running a cam with a later intake valve closing, or I can choose a 28cc dish top piston that will drop my static CR to 8.8 - which seems low to me. I could also drop a cc or two out of my 64cc heads and stick with an 18cc D-cup piston for a static CR of about 9.5:1 but would 89 or 91 octane keep me out of detonation?

I think KB is right on about it's suggestions. Yes, you can drop DCR by going with a cam with a later opening or with a lower SCR by using dished pistons. You might talk with CompCams cam help line about an RV cam that would suit your towing/compression requirements. DCR is a better guide to go by than SCR because the choice of cam will delineate how an engine will run on pump gas more accurately. Normally, a performance engine with iron heads should have a DCR under 8.0. Aluminum heads can get away with .5 more. But 8.0 and 8.5 maximum numbers is figured using 92 octane. I would shoot for a DCR of around 7.75 if you want to use 87 or 89 octane.
12-22-2005, 04:54 AM
MainCap

Thanks for all the advice.

I'm trying to get there from here but piston selection seems pretty thin when using 64cc Vortec heads.

As an aside, I was just reading a CHP article posted on CompCams site regarding an equivelent build in both a 355 and 383 intended to run on 87 octane. They chose pistons with a static CR of 9.75:1! Obtained over 500hp as well on 87 Octane. Interesting...

Anyway, with my intended cam the intake closing is just past 60 degrees ABDC. With a 5.7" rod my dynamic CR would be ~7.93 with 18cc D-cup pistons. A 28cc piston yields a dynamic CR of ~7.25:1. These two pistons yield a static CR of ~9.55/8.7:1 respectively, using an .040 quench distance.

Wish I could find a D-Cup stroker piston in the 22-24cc range as I'd like to hit a static CR of ~9.0:1...
12-22-2005, 05:31 AM
erik erikson

Quote:

Originally posted by MainCap
Thanks for all the advice.

I'm trying to get there from here but piston selection seems pretty thin when using 64cc Vortec heads.

As an aside, I was just reading a CHP article posted on CompCams site regarding an equivelent build in both a 355 and 383 intended to run on 87 octane. They chose pistons with a static CR of 9.75:1! Obtained over 500hp as well on 87 Octane. Interesting...

Anyway, with my intended cam the intake closing is just past 60 degrees ABDC. With a 5.7" rod my dynamic CR would be ~7.93 with 18cc D-cup pistons. A 28cc piston yields a dynamic CR of ~7.25:1. These two pistons yield a static CR of ~9.55/8.7:1 respectively, using an .040 quench distance.

Wish I could find a D-Cup stroker piston in the 22-24cc range as I'd like to hit a static CR of ~9.0:1...

You never said how much money you wanted to spend.These pistons might be over -kill for you but I think they are what you are looking for.JE part #131636.Yes they are exspensive.Check out there web site www.jepistons.com.
12-22-2005, 05:57 AM
MainCap

Thanks Eric, but those JE 131636 pistons are for a 355ci (3.48 stroke). I need a 22-24cc dish for a 383 (3.75 stroke with a 5.7 rod).

Do you happen to know if it is possible to machine 4cc's out of an 18cc D-Cup piston, or would this throw the CG all out of whack.

Wonder if KB custom builds/modifies pistons?

I guess I'd be willing to spend a few hundred extra to get it right as it would not grossly effect the end cost - pushing over $4k when all is said and done anyway.
12-22-2005, 08:59 AM
tcodi

Does adding lead have any adverse affects on the engine (non emission vehicle), or just the environment.
12-22-2005, 09:18 AM
erik erikson

Quote:

Originally posted by MainCap
Thanks Eric, but those JE 131636 pistons are for a 355ci (3.48 stroke). I need a 22-24cc dish for a 383 (3.75 stroke with a 5.7 rod).

Do you happen to know if it is possible to machine 4cc's out of an 18cc D-Cup piston, or would this throw the CG all out of whack.

Wonder if KB custom builds/modifies pistons?

I guess I'd be willing to spend a few hundred extra to get it right as it would not grossly effect the end cost - pushing over $4k when all is said and done anyway.

I mis-understood.I thought I read about both a 355 and a 383.I was un-sure about what you where building.Why not get a little thicker head gasket?
12-22-2005, 09:19 AM
techinspector1

On a non-emission vehicle, tetraethyl lead will slow down the burn by interspersing molecules of a non-combustable material in between the molecules of the fuel/air mixture and thus helping to prevent pre-ignition and detonation, allow the use of more ignition lead and coat the valve seat and valve faces which in effect "dirties them up" so they cannot weld to each other resulting in valve recession.
12-22-2005, 09:26 AM
erik erikson

Another great thing about tetraethyllead is it's lubricating properties in the valve seat area.I would where gloves if you handle it.
12-22-2005, 11:06 AM
MainCap

Eric,

I'm working up a 383.

I'm trying not to exceed an .040" quench distance and so don't want to run a thicker head gasket. Most data suggests that running a higher quench than this also increases potential for detonation.
12-22-2005, 01:15 PM
tcodi

so if I need to use lead it won't hurt my engine?

Where do you pick that stuff up nowadays anyhow?
What is a good amount of lead/gallon to start with to see
if it solves the problem?
12-22-2005, 02:33 PM
pat mccarthy

if you need more cc .work out the heads grind and polish them i would do this any ways scribe them to the block or head gaskets do not go right up to the edge keep back from it by0.030 or so and get all the junk out off the head and cc one and see were you are at
12-22-2005, 04:30 PM
erik erikson

Re: Max Dynamic Compression Ratio?

Quote:

Originally posted by MainCap
Anybody now what the recommended maximum allowable DYNAMIC compression ratio is for 87,89,and 91 octane unleaded pump gasoline, for an iron head engine with .040 quench under towing load scenarios?

KB's site suggests no more than an 8:1 dynamic CR under the best (light load?) conditions without getting exotic. They also indicate a static CR of no more than 8.5:1 for an engine used for towing that is running on regular unleaded gasoline. I do not find how much more CR can be run with higher than 87 octane pump gas.

I can drop my dynamic CR by running a cam with a later intake valve closing, or I can choose a 28cc dish top piston that will drop my static CR to 8.8 - which seems low to me. I could also drop a cc or two out of my 64cc heads and stick with an 18cc D-cup piston for a static CR of about 9.5:1 but would 89 or 91 octane keep me out of detonation?

I would just try and keep the comp. ratio 9.25 to 1 or less.This is a truck not a race car.It sounds to me like you are trying to pick fly poop out of the pepper.:3dSMILE:
12-22-2005, 05:25 PM
techinspector1

tcodi, I don't think you want to start fooling around with lead. I suspect there are some pretty hefty fines if the authorities find out you're using it on the street.
12-22-2005, 07:26 PM
MainCap

I don't know about you Eric, But I don't like "fly poop in my pepper". Yes, it is just a truck - but I figure while I'm at it I might as well try to make it as efficient as I can versus just tossing a bunch of stuff together, but that's just me. Coulda just bought a crate motor for that matter.

I intend to follow that plan Pat. Picked up my CC'ng stuff and die grinder's today. I was going to ask how many CC's is typically lost to polishing but I figured I'd find out soon enough. There is some flash to be removed around the perimeter and the chamber surface is like sandpaper. I intend to leave the ridge (but not the flash) around the intake valve to help dispurse any wet flow, will smooth out around the exhaust valve, and polish the chamber surface and exhaust port. I'll then CC and see where I'm at, balance them up, and go from there. Will also port match the intake.

I'll let you know what I end up with.
12-22-2005, 11:27 PM
camaro_fever68

Quote:

Originally posted by pat mccarthy
if you need more cc .work out the heads grind and polish them i would do this any ways scribe them to the block or head gaskets do not go right up to the edge keep back from it by0.030 or so and get all the junk out off the head and cc one and see were you are at

This is some good advice. Watch what you do to those L-31's though. They're pretty efficient with high swirl as cast, but as long as you only polish like Pat says you should be just fine. The valves are very unshrouded so only enough metal to polish it should be taken from there. If you enlarge the chamber to fit the bore a little bit better, make sure you don't change the shape of the chamber as it will hurt efficiency. If worse comes to worse and you can't get the CR where you need it with KB pistons, you can get custom pistons fairly easy. I got a set of custom Ross pistons exactly like I wanted them. I went through www.campbellenterprises.com They also have a nice CR calculator on their site.
12-23-2005, 05:14 AM
MainCap

Thanks for the help c_f68,

I don't intend to change the chamber shape for the reasons you mentioned - just going to remove the casting flash (thin jagged flanges that can't be contributing anything but hot spots) around the valves and chamber perimeter, and polish out the rough chamber surface.

I've read several articles regarding porting/polishing vortec heads and it seems there is little to be gained by messing with the pockets & runners, especially for low/mid range torque applications.

As an aside, I don't subscribe to the rough chamber surface (dimple) theory as being beneficial so long as there is an edge around the intake valve to re-launch potential wet flow. In this application the rough chamber surface just gives carbon a place to stick which smooths the surface back out and/or accumulates irregularities, so I figure polishing is a worthy endeavor to reduce carbon buildup, hotspots, and potential detonation. Rough intake runner surfaces is a different story and probably does aid in fluid suspension so long as there is sufficient velocity and the surface remains clean.

My background is aerodynamics. In aviation dimples and "trip strips" (hard edges) are used to re-attach/re-energize seperated laminar airflow boundary layers on airfoils to decrease wake drag. Works well, as long as the surface does not accumulate dirt or ice - which is more likely on rough surfaces. I've spent considerable time in wind tunnels messing with this kind of stuff.

For what it's worth?
12-23-2005, 06:21 AM
erik erikson

Quote:

Originally posted by MainCap
I don't know about you Eric, But I don't like "fly poop in my pepper". Yes, it is just a truck - but I figure while I'm at it I might as well try to make it as efficient as I can versus just tossing a bunch of stuff together, but that's just me. Coulda just bought a crate motor for that matter.

I intend to follow that plan Pat. Picked up my CC'ng stuff and die grinder's today. I was going to ask how many CC's is typically lost to polishing but I figured I'd find out soon enough. There is some flash to be removed around the perimeter and the chamber surface is like sandpaper. I intend to leave the ridge (but not the flash) around the intake valve to help dispurse any wet flow, will smooth out around the exhaust valve, and polish the chamber surface and exhaust port. I'll then CC and see where I'm at, balance them up, and go from there. Will also port match the intake.

I'll let you know what I end up with.

As long as you have the time.Do you own a magnetic deck checker and a dial indicator?