Another max compression ratio w/pump gas

[dgibby]

What is safe max ratio for 91 octane (highest we can get here)?? Searched and read as much at I could find but seems to be all over the place on what works.
Building a 410 stroker, aluminum head and engine came in at 10.2:1 static and 9.2:1 dynamic (seems high) builder said it should be fine but pushing it. Have .039 head gaskets wondering if I should go thicker to be safe? Thanks.
Specs:
360 bored to 4.040
4.0 stroke
combustion camber 65 cc
0 deck
- 17 piston volume
Hughes roller cam 238/242 @ .050
Hughes Engines
1.6 ratio rockers

 

[yellow rose]

Run it.

 

[moper]

I wouldn't run it with a dynamic that high. But that seems very high for a cam with those specs. Can you post the cam card for it?

 

[dgibby]

Here it is

 

[RustyRatRod]

Run it. Aluminum heads will make it happen.

 

[moper]

Not knowing what program you're using to calculate...
I use the KB site calculator myself. That spits out 8.55:1 dynamic. I use these upper limits for dynamic based on building for quench with closed chambers, ignoring quench with open chamber iron:
iron head, open chamber - 8.1:1
iron head - closed chamber - 8.25:1
aluminum head - open chamber - 8.5:1
aluminum head closed chamber - 8.75:1
It should run well on pump fuel.

 

[nm9stheham]

The DCR calculation appears to have been done in error, and the .050" lift intake duration value of 238* was used; that produces a DCR of 9.28 but that is incorrect. Duration at a valve lift of around .005" to .010" should be used for computing DCR, and that is not listed on the Hughes cam card.

To get close: Use a 42 degree addition to the .050" duration, and that gives a rough 'advertised' duration of 280* on the intake. That yields a DCR of 8.15, which is around what would be expected. Looks like you are fine...

 

[dgibby]

Thanks for all the input. Based on feedback looks like i'll be good.
This is the calculator that I used but it didnt seem right
Calculators : United Engine & Machine Co. Incorporated, Performance Pistons

 

[nm9stheham]

The reason it did not work out is likely that you forgot to add 15 degrees to the 45* intake closing angle (ABDC at .050" lift) listed for the cam. Putting in 45* gives you the number you listed, but adding 15* to that is 60*, and if that is plugged into the KB (UEMP) calculator, you get 8.52, which is close to what Moper lists.

These calculators will vary depending on when they assume the intake valve is closed 'enough' to start building cylinder pressure; the UEMP computes DCR when the valve is still open roughly in the range of .015-.020". The Pat Kelly calculator computes with a smaller intake valve closure, which occurs several degrees later in the valve timing, and so gives lower DCR numbers.

 

[dgibby]

Yep, that's what I did. Though it was factoring it in...
Thanks again.

 

[Lustle]

I think you guys are forgetting a very important part of "max" compression ratio.

What is your altitude?

You list your location as Utah. Mean elevation in Utah is 6,100 feet.

So your dynamic compression ratio IS 8.52 (with 15 degrees added to cam card, as others mentioned) but that is calculated AT sea level. Add 6,000 feet?

Your DCR is now 7.32.

Do NOT forget all the factors when trying to figure out this stuff. That's how you end up with a dog.

 

[dgibby]

I think you guys are forgetting a very important part of "max" compression ratio.

What is your altitude?

You list your location as Utah. Mean elevation in Utah is 6,100 feet.

So your dynamic compression ratio IS 8.52 (with 15 degrees added to cam card, as others mentioned) but that is calculated AT sea level. Add 6,000 feet?

Your DCR is now 7.32.

Do NOT forget all the factors when trying to figure out this stuff. That's how you end up with a dog.

Good catch. All this is a little over my head that's why I'm asking questions. Im just outside Salt Lake elevation is 4200.
Did find one calculator at asked elevation it came in around 7.50 ran one that came in at 9.2 and one in the high 8's just confused me so I put up the post.

 

[Lustle]

Use: Wallace Racing: Dynamic Compression Ratio Calculator

At 4200 feet. Your DCR is 7.68. You can definitely go more on aluminum heads. Especially if they are closed chamber. I would aim for 8.5-8.75 as someone else mentioned. That's assuming great quench. Strong spark. Good gas. Etc etc. You can probably run in the area of 11.5 to 1 static compression to reach that DCR. Remember, compression ratio isn't what determines pump gas-ability. It's cylinder pressure. You'll be running close to 180 PSI. Which is up there for pump gas 91.

If your builder looks at you sideways. Inform him that the cam specs you gave him are at .050 lift. So he needs to add 15 degrees to the intake valve closing. Well actually it should be approximately 15.5 because you're running 1.6 rockers. But still gives you an idea.

If you run a "max effort" DCR like that, remember, you are putting more stress on everything. You need to make sure you use good gas. That you have great quench. You must ALWAYS keep it tuned excellently. Timing is EXTREMELY important. Good ignition is a must. Check plugs often. When you run at the top capability of pump gas, you have to really stay on top of your engine to avoid detonation or other problems. Even a 1 degree change in timing can cause problems/detonation. Same with a lean situation. If that carb goes too lean for even a second at full throttle. Problems. Just be aware of that.

 

[rumblefish360]

Good catch there Mr. Lustle.

 

[dgibby]

Use: Wallace Racing: Dynamic Compression Ratio Calculator

At 4200 feet. Your DCR is 7.68. You can definitely go more on aluminum heads. Especially if they are closed chamber. I would aim for 8.5-8.75 as someone else mentioned. That's assuming great quench. Strong spark. Good gas. Etc etc. You can probably run in the area of 11.5 to 1 static compression to reach that DCR. Remember, compression ratio isn't what determines pump gas-ability. It's cylinder pressure. You'll be running close to 180 PSI. Which is up there for pump gas 91.


If your builder looks at you sideways. Inform him that the cam specs you gave him are at .050 lift. So he needs to add 15 degrees to the intake valve closing. Well actually it should be approximately 15.5 because you're running 1.6 rockers. But still gives you an idea.

If you run a "max effort" DCR like that, remember, you are putting more stress on everything. You need to make sure you use good gas. That you have great quench. You must ALWAYS keep it tuned excellently. Timing is EXTREMELY important. Good ignition is a must. Check plugs often. When you run at the top capability of pump gas, you have to really stay on top of your engine to avoid detonation or other problems. Even a 1 degree change in timing can cause problems/detonation. Same with a lean situation. If that carb goes too lean for even a second at full throttle. Problems. Just be aware of that.

Great info, so much to learn, thanks.
Not sure on fuel system yet, thinking FiTech efi and Hei ignition .
Think I'm going to stay were it's at with the current ratio. Job transfer may be in the near future and will end up near sea level if it happens.

 

[Lustle]

Great info, so much to learn, thanks.
Not sure on fuel system yet, thinking FiTech efi and Hei ignition .
Think I'm going to stay were it's at with the current ratio. Job transfer may be in the near future and will end up near sea level if it happens.

It's not "that" hard to change compression ratio if you move to a lower elevation. Could be as simple as head gaskets. Just always keep an eye on quench when you do. Quench is just as or more important than compression ratio.

And who knows. Maybe that lower elevation has better gas. They usually do. Don't need 94/96 octane at 4200 feet because of elevation. At sea level it's a lot more common to find higher octane gas.

EDIT: If you do decide to keep the lower DCR. Just remember that it will be more sluggish off the line. DCR affects more than just overall power. One of the most noticeable effects is off-idle and part throttle power. Without that "crack" that compression gives, engines tend to be softer in those areas.

 

[rumblefish360]

Ummm, doesn't the Fi-Tech also have distributor capability?

Oh! His 10-1+ ratio is more than enough for sea level and the 93 available at every watering hole. With the cam he is running, he'll be OK.




Edit, minor mistake

 

[dgibby]

Guess I need to figure a few more things out. Winter now so engine won't go in until spring, which is when the job transfer will happen, if it does. If I were to push the ratio up another point, what is it worth? 100% street driven car, don't want a full time job keeping it tuned so I can drive it because I'm at the top limit. I understand the importance of a good tune regardless , just don't want to complicateat it any more than needed.

 

[rumblefish360]

Another point rise
To 11.3-1?
To far for pump gas and the small cam IMHO.

 

[nm9stheham]

If this is 100% street driven, and there is a good prospect of moving soon, then my personal decision would be to not push the SCR/DCR up at this time.

100% agreed on the value of higher compressions on the street; it widens the usable torque band and that is what you want to get good drivability and throttle response when you pick up the throttle at any road speed.

In regards to octane on the coast, my impressions from reading numerous posts is that one the west coast, lower maximum octane grades are the norm... like 91. So that conclusion that higher octanes are available near the coast may be in error for the west coast.

 

[moper]

Admittedly I overlooked that and thanks for pointing it out.
But to be honest if I was the builder unless the owner gaurantees the engine will never "come down" I would not build for max power at a higher altitude. Better to be down a little on power (build for more in another way besides compression?) and able to go anywhere, then be relegated to race fuel or damage should the owner want to race it at a show at a lower altitude.