What's the highest CR you can safely run with pump gas?

[EL5DEMON340]

That's what I'm running old school j's cheap hyper pistons, stock crank, rods, stock type cooling system with a 6 pack. If you know what your doing you can run hi compression on pump, if you don't know how to tune you better keep it 9 to 1 or lower but then if you can't tune you might mess up that low compression engine anyway LOL...

 

[RustyRatRod]

That's what I'm running old school j's cheap hyper pistons, stock crank, rods, stock type cooling system with a 6 pack. If you know what your doing you can run hi compression on pump, if you don't know how to tune you better keep it 9 to 1 or lower but then if you can't tune you might mess up that low compression engine anyway LOL...

I understand that completely. I've done it too. A lot. But the fact remains that compression as high as yours with no quench and iron heads requires a pretty late intake closing event.

That means your engine is nowhere near as efficient as it could be with an earlier closing event and race gas.

My responses were not "geared toward people who cannot tune" but more around how I perceived the question was asked.

This is the reason these type threads always go sideways, because people always try to throw technicalities in. When I saw "old school" I answered in that fashion. Your engine is actually the reverse old school.

Plenty of us built stock compression engines back in the day with cams that were way too big, and they ran ok. That was before DCR and camshaft valve events were widely understood. Now that we have the benefit of that knowledge, it's not "old school" anymore.

That was my point. To answer the question the way I saw it asked.

 

[PocketAces]

This thread is making me nervous. I built my 273 at 8.5:1 dynamic but have not fired it yet. Pump premium is only 91 octane here in California. Luckily the head gasket is only .029 so I have an easy fix if I went too far.

 

[mderoy340]

This article by Pat Kelly is a good read on compression and pump gas.
http://cochise.uia.net/pkelley2/DynamicCR.html
I built my 340 to 8.3 dynamic compression and it runs good. 93 is available and what I use, 91 also works fine since recurving the distributor and dialing in the TQ with a wideband.

 

[rumblefish360]

Aluminum of coarse..what other heads are there

He he he he, but you didn't mention it. But to answer yours, LMAO, W2. The only iron head that would be considered, if at all.

 

[rklein383]

No one has mentioned this, but you can run higher CR at higher elevations.
Rod

 

[toolmanmike]

This thread is making me nervous. I built my 273 at 8.5:1 dynamic but have not fired it yet. Pump premium is only 91 octane here in California. Luckily the head gasket is only .029 so I have an easy fix if I went too far.

OK guys, remind us all (including me) , what's the procedure for measuring static compression and the formula for measuring dynamic compression and why do we need to know both. I need to go back to basics to understand this. As always, thanks for the education. tmm

 

[PocketAces]

OK guys, remind us all (including me) , what's the procedure for measuring static compression and the formula for measuring dynamic compression and why do we need to know both. I need to go back to basics to understand this. As always, thanks for the education. tmm

Static CR is the ratio between the total volume and the chamber volume. It would be accurate if the intake valve closed at bottom dead center. But in reality, the intake valve closes sometime after bottom dead center. Since the cylinder doesn't actually begin building pressure until the valves are closed, this effectively lowers the compression. The bigger the cam, the later the intake valve closes and the more the compression is reduced.

Dynamic compression ratio take this into consideration.

I use a compression ratio calculator like this one.
https://www.uempistons.com/index.php?main_page=calculators&type=comp

To get the DCR you have to enter some additional info, specifically the rod length and the intake closing event based on your cam.

You and I both used the Isky E4 cam. I called Isky and they told me that the Intake closes at 33 degrees ABDC measured with .050" lift, so I entered 48 into the calculator (33+15). Not sure why they want you to add 15 but that's how I understood it.

 

[Louie70Dart]

Any one try that new stuff, Race Gas Concentrate? What about toluene solvent, or xylene? Toluene was once used in F-1 racing. NHRA banned it though.

 

[AJ/FormS]

Better to find an on-line calculator. Theres grade 11 trig and algebra involved.For me thats nearly 45 years ago.

Static cr is (SweptA + cv) / cv

Dynamic cr is (SweptB + cv ) / cv.

Where cv is the total chamber volume above the piston at TDC,
and sweptA is the volume that the piston sweeps from BDC to TDC(bore X stroke)
and SweptB is the volume swept by the piston from its position in the bore at the point that the intake valve closes, to TDC

The engine only sees Dcr
We just quote static c/r for bragging brights.
You can build an engine to a static number. Get it all put together and calculate the Dcr. Then you can change, the cam, or cam timing, or valve lash, or the rocker-arm ratio,and all of these change the Dcr, but have no effect on the static.
Thats why its so important if you are close to the detonation limit for your combo, to consider how every contemplated change will affect the Dcr. If after the engine is built to a safe Dcr, you go and upsize the cam a couple of sizes, the Dcr falls. Maybe enough to turn the bottom end soft. On the track its no big deal. you just compensate with gears and stall. But on a streeter, a soft bottom end is lousy deal.And on a DD the fun factor is history.
And if you built to a High Dcr, and later decide the cams too big. Then the new smaller cam gets into detonation right away. Now what? Well the Dcr will need to be reduced back to where it was safe.And many times a thicker headgasket will just make it worse. When that happens, its time for new pistons. Again.
Or a different fuel. Or additives. Or water injection.
Designing a sharpe engine for todays fuel, is a lot different than it used to be.

 

[Captainkirk]

I didn't think his question was open ended at all. I read "'old-school A-body' guys running LA engines" to mean no modern quench technology and iron heads on the cheap.

Bingo.

Maybe I was mistaken.

You were not mistaken. Two things to remember about Rusty, folks:
1) He's always right;
2) He's never wrong. O

Allow me to clarify, if I may:
1) Iron 'J' ( 1.88 ) or 'X' ( 2.02 ) heads
2) Chrysler electronic ignition with fast advance curve kit and no vacuum set to 36 total
3) Pump gas with or without additive...possibly blended with 100 octane avgas if available (but we all know that's not always possible)

I will probably be using a long duration/overlap cam but haven't chosen the grind yet as CR comes first.

 

[toolmanmike]

Any one try that new stuff, Race Gas Concentrate? What about toluene solvent, or xylene? Toluene was once used in F-1 racing. NHRA banned it though.

And I had 2 friends that experimented with Xylene, benzenes and toluenes. They both died of brain cancer in the last few years. There might be a reason why NHRA banned it.

 

[AJ/FormS]

If you want a sharp engine; Cam and cr are engaged, Intake closing point and Dcr are married.And although divorce is possible, its some times expensive, and sometimes sucks the life out of it,and sometimes just not worth the hassle.

 

[Captainkirk]

If you want a sharp engine; Cam and cr are engaged, Intake closing point and Dcr are married.And although divorce is possible, its some times expensive, and sometimes sucks the life out of it,and sometimes just not worth the hassle.

Right. Which is why I have to determine the static CR first.......

 

[toolmanmike]

Mine is 9.6 static. I need to figure out the dynamic just for fun.

 

[RustyRatRod]

1) He's always right;
2) He's never wrong. O

Oh how I wish that was true right now. I will be rich by the end of the week. lol

 

[Captainkirk]

Oh how I wish that was true right now. I will be rich by the end of the week. lol

You are already rich in Mopar knowledge, my friend. rayer:
See...told ya.

 

[krazykuda]

OK guys, remind us all (including me) , what's the procedure for measuring static compression and the formula for measuring dynamic compression and why do we need to know both. I need to go back to basics to understand this. As always, thanks for the education. tmm

Here's how to check static compression on any engine:

http://www.forabodiesonly.com/mopar/showthread.php?t=278215

 

[AJ/FormS]

Heres the easy way to determine the approximate Dcr of an assembled engine, on the engine stand.If youre careful, its pretty accurate.
Pick a cylinder.(I use no 1) Roll the engine over till the sparkplug hole is up, and is the highest point of the chamber.This is very important. All the air in that chamber has to come out of that hole, or your work will be for nothing.Put the piston at TDC.Measure out exactly one liter of heavy oil. Fill the entire chamber to the bottom of the plug hole.Write down the amount of fluid that went into there.If the oil level doesnt stabilize, its running past the rings, and your done.Either the oil is too thin or your ring gaps are too big, or the cylinder-to-ring interface is ndg.Move to another hole and start over, with thicker oil.
Next,back the piston down the bore to put the piston near the bottom of the compression stroke. Put your thumb on the hole, rotate the crank to bring the piston back up. As soon as you feel the slightest compression. stop.This represents the moment and point in the compression stroke that the intake valve is closed.( I like to use the short hose that comes with a Quality compression tester, and stick it in my mouth.This is very sensitive)So, next,carefully continue to pour the oil into the cylinder until its full.Determine the total volume of oil that went into the hole. I just look at how much is left in the beaker, and subtract that amount from the start amount. Write it down.
Using the formula Dcr = (SweptB + cv) / cv Plug in the numbers.
In this case, the first, smaller number is cv, and the second, larger number is actually SweptB + cv .
Example; lets say the first was 78cc, and the second was 702. Dcr = 702/78 = 9.0
This is a typical 366 with closed chamber heads, and a hot street cam(mine).
Have fun

 

[stroked340]

He he he he, but you didn't mention it. But to answer yours, LMAO, W2. The only iron head that would be considered, if at all.

Ya' got that right

 

[Captainkirk]

OK, so from what the majority of you have posted, it seems that for an old-school (read: "iron head") 340 smallblock street/strip motor, 10:1 would be the upper limit for pump gas with 9:1 being preferential with today's 91-93 octane gasoline/ethanol...
Sorta what I figured.

 

[NC Engine Builder]

Those old dome top pistons you are wanting to use aint gonna help either. Throw those things on ebay and buy some pistons with lower compression, tighter tolerance, and lighter.

 

[Captainkirk]

Those old dome top pistons you are wanting to use aint gonna help either. Throw those things on ebay and buy some pistons with lower compression, tighter tolerance, and lighter.

This is why we are having this discussion, my friend!:coffee2:

 

[AJ/FormS]

KB107s made my 360 rev like a 260/289 ford.Thats not to say the stock 360 slugs were that heavy. But the 340 slugs; now those are heavy.
I seem to remember the factory pistons were over 700 grams, maybe 740 is what I remember. Thats 40 yrs ago. So that could be a little fat, much like my wife..s sister. lol

 

[Captainkirk]

KB107s made my 360 rev like a 260/289 ford.Thats not to say the stock 360 slugs were that heavy. But the 340 slugs; now those are heavy.

My TRWs are ranging between 657-662 grams apiece......