Cranking comp/dynamic comp ratio vs pump gas

[slowmobile]

Is there a rule of thumb as to what the max is for say 92 octane?

Right now I am at 165 psi cranking comp and 8.22 dynamic with 11.10 static and aluminum heads. I've been running 20% 110 with 92 pump gas. Wondering if it's even necessary to add the 110.

thx.

[bigjoe1]

I had a guy come in for dyno testing with almost the exact same numbers. When we went to straight 91 octane, it picked 25 to 26 HP on the top end. What kind of engine are we talking about here ??

JOE SHERMAN RACING ENGINES

[plovett]

Is there a rule of thumb as to what the max is for say 92 octane?

Right now I am at 165 psi cranking comp and 8.22 dynamic with 11.10 static and aluminum heads. I've been running 20% 110 with 92 pump gas. Wondering if it's even necessary to add the 110.

thx.

A rule of thumb is 8.5 max DCR for 91-92 octane pump gas. Also, something like 185 psi cranking compression for 92 octane gas would be another rule of thumb.

Those are just rules of thumb, though. There a lot of other factors. How big is the motor? How heavy is the car? How high is the stall speed? How high (numerically) is the gearing? What altitude are you at? How big is the bore on the motor? What's the timing curve? yada yada yada.......

For what it's worth, my motor has about 8.4 DCR and runs fine on 91 octane. Cranking compression is around 170 psi. Car is approximately 3500 lb., has a 432 cid motor with 517 hp, 3000 rpm stall, and 3.75 gears. I'm at about 1100 feet above sea level. I run 18 degrees initial timing and have 36 total in before 3000 rpm.

Sounds like yours should probably run fine on 91 octane.

JMO,

paulie

[slowmobile]

Guys thanks.

It's a 482 ci Olds. Heavy car (66 Cutlass) with 3.90's soon to be 4.33's. 3500 stall.

The compression figures were done at 4500 ft elevation.

I'd like this to be a true pump gas motor since that's what it was built to run on, just wanted a warm fuzzy feeling before I did it.

Thanks again.

[curl'd]

I'm interested in this topic as well.

What's the highest DCR anyone has seen on say 4v Heads (Import and local) with 92/93 Fuel ?

[1989TransAm]

There may very well be a difference on the rule of thumb as to whether we are talking about an aluminum headed motor versus and iron headed motor. With an iron headed motor you might want to keep the dcr lower versus an aluminum headed motor.

I also believe the rule of thumb posted above is regarding street motors that typically run at lower rpms. From what I have read most of the engine builders on this forum don't worry about DCR for the high end motors they are building.

Just trying to differentiate between the motors as a lot of people will be looking at these threads. Google searches will send them here. Being a rookie myself if my statements are wrong please correct.

Edit: LT1 motors with the reverse cooling seem to tolerate a little more dcr.

[Stan Weiss]

Iron Heads
PSI Octane
185 93
180 92
175 91
Aluminum Heads add another 10 PSI to each above
My calculated DCR's are lower than those posted

[slowmobile]

Iron Heads
PSI Octane
185 93
180 92
175 91
Aluminum Heads add another 10 PSI to each above
My calculated DCR's are lower than those posted

That's good info.

Thank you.

[gmc406]

interesting stuff guys

[Stan Weiss]

interesting stuff guys

255psi and runs fine on 91 octane

How about a little more information on your combination

[gmc406]

The statement I made was towards the dynamic and static comparisons. I've learned something again. I also forgot about my signiture.
Here it goes.... my engine builder deserves all the credit for this. I want to tell you as much as I can, but out of respect for Lyndon at Westers's Garage, I must hide the fine details. He put the rotating assembly and tuning together and I did the rest. Let just say I was scared to put it together because even I thought it was impossible. I regret telling him "Lyndon if I have to burn rocket fuel in this, your going to get it back." he proved me wrong.
The engine is and TBI 406 in a 91 pickup. It has 12.2:1 and 255psi cranking pressure(engine was cold and still had 7 plugs in it, cause I was too lazy to take them out). It also has a small cam, actually small heads that only flow about 230-240cfm, and small headers(1.5 inch). The engine is not a race engine by all means. Lyndon is an EFI genius and knows what he's doing.
The truck runs 13.1@102.3mph(oh ya, this is at 2400 feet and the truck weighs 4400lbs with me in it and a 3/4 tank of fuel. It runs out of fuel out of the hole.) and still get 21+mpg. It's better on fuel than when it was stock. I have to go see him in the spring, cause he wants to throw it on the dyno and maybe get me in the 12's. I'm only shifting at a little better than 5000(can't tell cause the tach stops there). It's a torque monster. I could run faster by putting in a bigger cam but, we wanted drivability. I've taken it on many 400 mile round trips without fail in 90 degree heat and the A/C on.
Remember... What ever you guys have said, trumps what I just said. Cheers guys!

[stroker]

i have small block 383 with aluminium head 64cc flat top deck at 5 gasket 1003 felpro041 small roller 242 /250 crower lift 610 compression220 pounds. run petro canada 94 streetand strip for 3000 km 210 runs strip. never had problem. engine 535 hp and 506 tq my experience RICHARD

[bigjoe1]

None of you guys have told us what you can run for total advance. Everybody in the Engine Master contest was pumping 245 or so, and some you could hear it ping real BAD, but that is where is still make best HP and torque. On a real street engine, I still want to know where the timing was on these real high compression numbers.

JOE SHERMAN RACING ENGINES

[stroker]

Big Joe,
I run on street 36 degree lock
Richard

[camaro_fever68]

My prostreet 383 w/FT's in my Luv with .034" quench

Iron Eagle heads
11.1:1 Static
8.83 Dynamic
215# Cranking
18* initial 36* total @3200
93 Pump Gas

NO Rattle or Ping


My Chevelle 385 w/-12.5cc bowls with .039" quench

Brodix aluminum heads
10.3:1 Static
8.36 Dynamic
200# Cranking
16* Initial 32* total @ 3200
93 Pump Gas

Will Rattle and Ping above 34* total timing


I think it has a lot to do with piston design and quench area. My iron headed 383 runs excellent on pump gas with flat tops and tight quench/squish area. My aluminum headed 385 with -12.5cc dished pistons don't like timing and will rattle at cruise and ping under acceleration at anything over 34" total timing.