[68WarDog]

I have 2 engines here, both 455. This one has a Dynamic compression of 5.37. 455 /4.19 (.040 over).05 below deck/20cc dish pistons / 6.8 eagles rods/ 72cc Eheads. 10.4:1 static compression. Intake valve closes 104 degrees ABDC

Second 455/4.21(.060 over)/ 0 deck. 6cc pistons/ 6.625 eagle rods/ 72cc Eheads. 12.1:1 static compression . Dynamic compression. 6.27 . Intake valve closes 102 ABDC.

Am I doing something wrong? Both of these engines have some very healthy UD solid roller cams. My understanding is that the lower the engines dynamic compression, the lower the octane requirement. Have I got that backwards? And anything less than "8" dynamic can tolerate pump gas. Can someone please explain this process?

[Dragncar]

I would say both, they both matter.
Basing it on static would be the safest way to go. But you might leave some power on the table.
Dynamic, you better know what you are doing if you are going to push the limits.

[Gach]

Dynamic compression of 5.37.

You can run up to 8.00 Dynamic compression. The cam timing is the most critical thing. Hence opening and closing. So at 5.37 shouldn’t have a problem. I’ve ran 12.1 compression with aluminum heads. No issues what so ever, On 93 octane on pump gas. With 8. Dynamic compression.

But since then i’ve turned it down for more street ability. Now it’s 11.1 on 93 octane. Now if your talking on 91 octane I couldn’t really say. But I’d guess at 5.37 you’d be all right. If it’s at all possible and in the budget I would definitely Dyno the motor, you’ll definitely have your answer.

[68WarDog]

Originally Posted by Dragncar

I would say both, they both matter.
Basing it on static would be the safest way to go. But you might leave some power on the table.
Dynamic, you better know what you are doing if you are going to push the limits.

Agree. There was an local high performance engine builder claiming about how I'm leaving 60-80 hp on the table by not building my aluminum head 455 12.5-13:1 static compression for pump gas. Traditionally I have always thought no greater than 11.5 for aluminum and 9.5 iron. Yes, I understood there are some tuners that have gone greater, but unsafe?

[Gach]

When you call any cam company, key words are I want to run pump gas.Here’s the other key for them aluminum head you can run 2. Points more compression vs Iron head. So you give them your compression ratio and say aluminum heads, flow numbers blah blah blah. You know the drill I’m sure. From there they calculate the opening and closing events which brings the Dynamic compression ratio to a safe level the other critical thing is Lube separation.

The question is who’s ( cam company) are they from, And was that information detailed from person ordering cam from them or are they just cams inquired by some individual.

[Stan Weiss]

How you get there matters. Going with a thick head gasket instead of a dished piston and the correct thickness head gasket will want a higher octane.

Stan

[Stan Weiss]

Originally Posted by 68WarDog

I have 2 engines here, both 455. This one has a Dynamic compression of 5.37. 455 /4.19 (.040 over).05 below deck/20cc dish pistons / 6.8 eagles rods/ 72cc Eheads. 10.4:1 static compression. Intake valve closes 104 degrees ABDC

Second 455/4.21(.060 over)/ 0 deck. 6cc pistons/ 6.625 eagle rods/ 72cc Eheads. 12.1:1 static compression . Dynamic compression. 6.27 . Intake valve closes 102 ABDC.

Am I doing something wrong? Both of these engines have some very healthy UD solid roller cams. My understanding is that the lower the engines dynamic compression, the lower the octane requirement. Have I got that backwards? And anything less than "8" dynamic can tolerate pump gas. Can someone please explain this process?

I agree with these calculations. My question is IVC 104 ABDC? That is like a 340 degree cam @ 114 ICL.

Stan

[Gach]

I’m running 16 cc dish with .52 head gasket but the dish mirrors the chamber. Crazy when your chasing every ounce of horsepower. All the different cam companies have their own programs. So are you leaving hp on the table with say 10.1 vs 12.1 yes definitely but question is how much. There’s old rule of thumb every point in compression is worth 20-25 more hp. So at 2 points higher rough idea 50 hp. Maybe more who really knows.

The real issue is the tuning, The right heat range plugs, timing and of course jetting. All the things a dyno will tell you. When you’re chasing real horse power. Otherwise you’re making passes ( if thats what your into ) like you know what are the air bleeds.. idle air bleeds mid range high end air bleeds.

So are you leaving hp on table with 14.1 vs 12.1 definitely. But the main thing is what are your goals. Do you want to spend the time chasing your tail without Dyno in it. Or just want a safe combo to reach your ET goal.

[Schurkey]

In-cylinder turbulence, (mixture motion) cylinder head coolant temp, (HOT block, COOL cylinder heads--"Good luck") in addition to the other things mentioned--plug heat range, piston dome/dish shape, combustion chamber shape, fuel curve, the REAL octane ratings of the fuel (not just the "pump octane" or Anti-Knock rating shown on the gas pump) etc. (AND MORE) all influence detonation resistance vs. load; and detonation resistance vs. RPM.

[Stan Weiss]

Originally Posted by Gach

I’m running 16 cc dish with .52 head gasket but the dish mirrors the chamber. Crazy when your chasing every ounce of horsepower. All the different cam companies have their own programs. So are you leaving hp on the table with say 10.1 vs 12.1 yes definitely but question is how much. There’s old rule of thumb every point in compression is worth 20-25 more hp. So at 2 points higher rough idea 50 hp. Maybe more who really knows.

The real issue is the tuning, The right heat range plugs, timing and of course jetting. All the things a dyno will tell you. When you’re chasing real horse power. Otherwise you’re making passes ( if thats what your into ) like you know what are the air bleeds.. idle air bleeds mid range high end air bleeds.

So are you leaving hp on table with 14.1 vs 12.1 definitely. But the main thing is what are your goals. Do you want to spend the time chasing your tail without Dyno in it. Or just want a safe combo to reach your ET goal.

I don't where you found that rule of thumb, but that is not even close to correct.

There is not fixed HP gain for a increase of 1 point in CR, it has more to do with the HP the engine is making.

Also going from 8:1 to 9:1 will see a greater increase in HP that going from 13:1 to 14:1 will.

Stan

[Gach]

Originally Posted by Stan Weiss

I don't where you found that rule of thumb, but that is not even close to correct.

There is not fixed HP gain for a increase of 1 point in CR, it has more to do with the HP the engine is making.

Also going from 8:1 to 9:1 will see a greater increase in HP that going from 13:1 to 14:1 will.

Stan

Your calculations are a little off.

[Half-Inch Stud]

Well my last combo (12.2 ET) overshot on the Pump Gas deal. 87 Octane Street, 89 Strip. No fickleness.

To do over: i'd change 9.0:1 to 9.5:1 or if feeling giddy go 9.8:1. I tink 10:1 is gonna get fickle with iron heads.

Alum heads: seems "add a compr point" is quite accurate. So 10.0:1 ought run on 87 Street, 89 Strip.

[Stan Weiss]

Originally Posted by Half-Inch Stud

Well my last combo (12.2 ET) overshot on the Pump Gas deal. 87 Octane Street, 89 Strip. No fickleness.

To do over: i'd change 9.0:1 to 9.5:1 or if feeling giddy go 9.8:1. I tink 10:1 is gonna get fickle with iron heads.

Alum heads: seems "add a compr point" is quite accurate. So 10.0:1 ought run on 87 Street, 89 Strip.

Mark,
Where did your IVC ABDC?

Stan

[Jay S]

Originally Posted by 68WarDog

I have 2 engines here, both 455. This one has a Dynamic compression of 5.37. 455 /4.19 (.040 over).05 below deck/20cc dish pistons / 6.8 eagles rods/ 72cc Eheads. 10.4:1 static compression. Intake valve closes 104 degrees ABDC

Second 455/4.21(.060 over)/ 0 deck. 6cc pistons/ 6.625 eagle rods/ 72cc Eheads. 12.1:1 static compression . Dynamic compression. 6.27 . Intake valve closes 102 ABDC.

Am I doing something wrong? Both of these engines have some very healthy UD solid roller cams. My understanding is that the lower the engines dynamic compression, the lower the octane requirement. Have I got that backwards? And anything less than "8" dynamic can tolerate pump gas. Can someone please explain this process?

Seems like your doing something wrong in your calculations. Maybe not be taking lash into account on your mechanical cams?

[68WarDog]

Originally Posted by Jay S

Seems like your doing something wrong in your calculations. Maybe not be taking lash into account on your mechanical cams?

That's interesting. I used the dynamic calculator on the Wallaceracing.com site, and lash/lift wasn't one of the variables. I was surprised that net lift wasn't considered.

[Gach]

Originally Posted by 68WarDog

Agree. There was an local high performance engine builder claiming about how I'm leaving 60-80 hp on the table by not building my aluminum head 455 12.5-13:1 static compression for pump gas. Traditionally I have always thought no greater than 11.5 for aluminum and 9.5 iron. Yes, I understood there are some tuners that have gone greater, but unsafe?

My guess is, he’s saying that because he’s had allot of experience doing it and knows the cam company well enough and trust them. It’s really not rocket Science. I’ve definitely pushed the limits with 12.1. Its been done for years now. But it definitely depends on the person and how comfortable you are. As far as determining what octane level for what dramatic compression that’s going to depend on the quality of fuel in your area. On whether or not it has 10% ethanol. I think it’s called ethanol I may be quoting it wrong but. I don’t think I’d be pushing the limits on 91 octane.

[Stan Weiss]

Originally Posted by 68WarDog

That's interesting. I used the dynamic calculator on the Wallaceracing.com site, and lash/lift wasn't one of the variables. I was surprised that net lift wasn't considered.

Which UD lobe are you using for the Intake and what is your installed ICL?

Stan

[Gach]

This is a good read, may explain a little on what Jay mentioned calculating lash. Grab a cup of coffee. Its a long read but will definitely help you understand everything mentioned in all responses. Why cam choice is so critical….along with bunch of other determining factors. Some guys will never get it, but I’m confident that you will. May take a couple of reads.

http://cochise.uia.net/pkelley2/DynamicCR.html

[Stan Weiss]

OK, UltraDyne does have some splid roller lobes which could have IVC 104 ABDC depending on ICL.

IMO I would not be running any of them in these engines.

________________________________UltraDyne_Master_L ist

The_cam_Master_lists_are_arranged_by_family_with_a ll_cams_within_each_family_RPMing_to
the_same_peak_RPM_on_a_spin_fixture._In_an_actual_ operating_engine_the_valve_lift_area
under_the_curve_may_limit_the_peak_RPM.


Master___Adv_____0.050_____0.200_____Lobe_________ _________
Number_Duration_Duration_Duration____Lift_____104_ ____106_____108_____110_____1.5_____1.6_____1.7


ROLLER_Profiles_Adv_=_.020"
---------
These_are_original_designs_with_slower_ramps_and_s tandard_valve_lash_.026-.030
---------

R82______328______290_______207_____0.4578___0.196 ___0.187___0.178___0.169___0.687___0.732___0.778
R00______332______294_______211_____0.4578___0.204 ___0.195___0.187___0.178___0.687___0.732___0.778
R84______336______298_______214_____0.4578___0.212 ___0.203___0.194___0.186___0.687___0.732___0.778
R85______340______302_______217_____0.4578___0.220 ___0.211___0.203___0.194___0.687___0.732___0.778
R86______344______306_______220_____0.4578___0.226 ___0.218___0.210___0.201___0.687___0.732___0.778
R87______348______310_______224_____0.4578___0.233 ___0.225___0.218___0.210___0.687___0.732___0.778

Stan

[68WarDog]

[QUOTE=Stan Weiss;6458494]OK, UltraDyne does have some splid roller lobes which could have IVC 104 ABDC depending on ICL.

IMO I would not be running any of them in these


Stan, the 10.4:1 engine uses the R81/R88 lobes. At the time when building this engine, I was looking for a solid roller comparable to the comp 290 b6 solid flat tappet, almost 20 years ago. There may be better lobes since then, but this cam has a brutal mid range and respectable 1/8 mile time with a 3400 10",Continental converter and 3.70 gear.
What is it about these lobes you don't like?

The 12.1:1 engine has the R79/R80 lobes, this engine wasn't built by me,and I will be replacing the 72cc Eheads with the 87cc version for pump gas safety.