Thread: Max Dynamic Compression Ratio?

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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 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?

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

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.

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...

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.

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.

Does adding lead have any adverse affects on the engine (non emission vehicle), or just the environment.

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?

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.

Another great thing about tetraethyllead is it's lubricating properties in the valve seat area.I would where gloves if you handle it.

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.

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?

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

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.