Whats the highest compression I can have and still be able to run pump gas?

 

In my honest opinion!! for what its worth? I wouldnt go over 10 to 1......

 

I'd consider 9.5 or 10:1 for Iron heads and 10-10.5 for ALuminum heads. Your cam selection makes a big difference. If you've got some possible selections in mind, maybe you can get SCouder to get some dynamic compression ratios calculated.

 

As stated above there are many variables that dictate maximum compression, such as:

Cam selection
Head material (iron or aluminum)
Quench distance
Piston to deck clearance
Cylinder cleanliness
Altitude and temperature
The grade of gas you want to run

Assuming sea level, iron heads, stock cam, miles on the engine, and low grade fuel, I would say 9.0:1 to be safe.

You can however increase compression if you are making other changes that allow it. My engine, for example, has aluminum heads, 295 degrees advertised duration on the cam, 0 deck height, .041 quench distance, and 12.5:1 compression. I expect it to run fine on 92 octane pump gas here at 7000 feet. The tradeoff is that I cannot reduce altitude or go with a smaller cam without running racing gas or changing pistons.

-Scouder

 

Scouder, If my 360 is +.040 over with Edelbrock Rpm Heads, 93 Octane and Comp Cam with 280 Advertised Duration and .530 lift. Im in Fort Worth, Tx i dont know the altitude here.

 

I'll run some numbers at home tonite and post em.

-Scouder

 

OK, ran some numbers. I made the following assumptions:

Sea level
0 Deck Height
Cam degreed straight up/no advance or retard
gasket thickness .041
Chamber vol 72cc

Typically an engine will like between 7.5 and 8.5 dynamic compression ratio (DCR) on premium pump gas.

The common theory is that you can add 1 point static for aluminum heads. I myself am a little skeptical, and would probably err on the side of caution. So, the numbers work out as follows on your combo:

7.5 DCR = 9.88 Static
8.0 DCR = 10.50 Static
8.5 DCR = 11.25 Static

You can add compression for the aluminum heads if you wish. Good luck.

-Scouder

 

Scouder, I thought that the effects of bleed off from the cam were eliminated when the cam reached the max of it's power band. I've seen where different cams on DD2K will have a volumetric efficiency peaking at a different RPM, but it still does peak at or above the stock cam, unless the cam is way too wild for the flow through the heads. Would this mean that the cylinder is getting filled as good as the stock cam at that rpm, and if so would that negate the effects of the DCR? Please explain it to me, as I thought that once the cam reaches it's power band the effects of bleed off were eliminated by intake port velocity and exhaust scavanging.

 

I wish I knew the details that well. Here's what I do know. The primary theory of DCR is that no matter how much mixture you put in the cylinder, it cannot make compression until the intake valve closes. I do not know all of the physics behind the intertial "ramming" of the cylinder, or how all the variables come together, but I did do a little bit of informal research. I knew that the typical top of the DCR range for pump gas was supposed to be 8.5, so I simply gathered data on existing FE's with various cams, heads, bores, strokes, etc... The results were that they pretty universally started detonating at around 8.6-8.7 DCR. So the limited evidence I have supports the theory.

Also, I have been told that the bleed off is only a low rpm phenomenon, but the fact that the cam manufacturers recommend higher compression ratios with higher duration also tends to support the theory. If not, wouldn't we see detonation at higher RPMs if we followed their advice?

I myself am putting it to the test. If it is wrong I'll let everybody know about it. With my combo an 8.0 DCR, corrected for altitude, comes out to 12.5:1 static. I didn't make any compensation for aluminum heads. So I have my fingers crossed.

As a side note, I was talking to a local hi performance guy about my compression, and he said that he used to run a pump gas 350 Camaro at 13.0:1 at this altitude with iron heads without problems.

-Scouder

 

Maybe the cam manufacturers are counting on racing gas? You may very well be right, but it still just doesn't make sense to me. I'd like to see the formulas you're using. I would like to apply them to my cam to see what octane I can use. (it's a 343941, and the engine is going to have about 9.8:1 static compression)

 

The only problem with that is that the numbers they suggest on some of the cams are obviously not race gas territory. For example, they will say something like 10.0:1 compression and headers recommended. If race gas were intended it would say much more than that. And as the duration increases, so do their recommendations.

I'll see if I can find the site where I downloaded my DCR calculator and post it.

-Scouder

 

Hi Scouder, you may be the guru with the knowledge to help me. I am building a 390 for my F250 highboy and I live at 9000' in the Rockies. I am looking for some cam guidelines for my engine. .060 over 74 block with L2291 pistons and the block is decked .010. It will be fresh honed during a refresh. I'm looking at a Lunati cam 10330205 with 210/210 duration, 508/508 lift on 110 LSA. I think it will be good for shorter duration and a good steep lobe on a tighter LSA. It seems 110 LSA helps build the compression lost due to altitude. Your advice and or correction of misunderstanding would be great. Thanks, Kent

 

Scouder's last post was in 2012. Feel free to start a new thread with your info, I think you might have better results.

 

Hi 85, I honestly don't know how to start a thread. Since you are a moderator you can help me. Thanks.

 

You would do better with a wider LSA, like 112-114 in that case. What's the pin height on those pistons ? And dish volume ?