87 Octane OK?
 

67 LeMans with a 400 engine with a 4.00 stroke, 4.189 bore flat top pistons slightly down in the hole static compression works out to 9.0:1. Cam is a Voodoo hyd roller custom ground on a 114 LSA, 219/227 duration @.050, .515/.530 lift @1.5 but running 1.65 on the intake valve only. Auto trans (TH-400) with a mild converter that stalls around 1800 RPM. I'm using Johnson hyd roller lifters and have some lifter noise that I am unhappy about. Compression test I get 105 to 110 (cold) in all cylinders which I thought was quite low. Vaccuum is 16.5 at idle car idles very nice at 650-700 RPM with a very slight lope. Ported 6X heads and RA/HO exhaust manifolds 2-1/2 pipes going through Walker Dynomax 17749 mufflers, Tomahawk intake with Q-Jet. Engine drives very smooth and very responsive right off of idle and very happy with the power.

My question is do you think I can safely run 87 octane pump gas.

91 octane if it were my engine.

Maybe 87 in the cooler months on low stress flat ground cruising.

You didn’t state what rear end gear ratio you’re using which also is an important factor to consider.

Edit - Also exactly which heads, head gasket thickness and how far down in the hole are your pistons? A tight quench engine would tolerate 87 octane better.

I built a .060 over 400 with 6X-4 heads and the Summit 2801 cam, much milder. Log manifolds. My friend had the engine in his '67 LeMans with turbo 350 trans and 2.78 rear. He found it ran fine on 87. Got good mileage too, like 17-18 on trips. He used a GM open element air cleaner, cast intake, 2 1/4" pipes.

Tim, how high is the cranking psi when hot? Over 150 psi 87 isn’t advisable; 89 would be borderline. I agree with Bart; in hot weather 91 or 93 would be preferable.

6x4s on a .060 400 puts compression in the very low 8s, if not below 8:1 . I'd expect that to run on 87 all day long.

Don't forget it's .068 over and has a 4 inch stroke not 3.75

Heads have been milled quite a bit, 3.08 gears. Didn't do compression hot only cold 105/110

12.2 ET enjoyed Ping-Free WOT on 87 Street, and 89 Strip with the big cam and full Timing ADV.

Could have enjoyed a slightly lower overlap or duration, or more CR.

What is the adv duration (.006") While the 114 LSA reduces overlap for better idle IMO it tells me nothing about octane requirement. What is the installed ICL.

Stan

2051071118 6091118 1800 - 5800 .515"/.530" 219/227 270/278 112/106 Hyd/Hyd
Great mid range torque and HP. Likes 2000 RPM stall converter and works great in towing applications with lower gearing. Largest for inboard/outboard performance marine use.

Using these numbers from the Lunati catalog and the 106 ICL / IVC 61 ABDC with 9.0:1 CR you should get about 180 psi.

Stan

So low, I'd suspect your compression tester is inaccurate.

Did you have the throttle jammed part-way open (or all-the-way open)? I find that "fast idle" is far enough open to give decent compression test results.

If your cranking compression is accurate, probably. Start higher, work your way down. Listen for detonation, and if you hear it, go back up a grade.

I also suspected the tester was in accurate and I borrowed one from Orielley's to back up my readings. The one from Orielley's read 10 PSI lower, Chinese crap mine although old is a Sun Pro made in the USA many years ago when they still made good products. Throttle was wide open during test.

I was responding to android who posted he built a .040 400 with 6x4s that runs well on 87.

2 bad gauges or that thing is way retarded or something. Im thinking 150-165 psi with that little cam. But it seems to run good? Thats strange. To answer. 91 oct. and bring the curve in yesterday

Rhetorical question

How often do you intend to drive the vehicle. If only as a hobby car and infrequently one might consider the end cost of using the highest grade vs a lower grade on a cruise to go get that hamburger.


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