Engine Run On/detonation - PART II

[cdog]

I should have been more specific on this from the get go...

The 514 BB car is running on 93 octane. It has 10.25 : 1 compression.

It does have pretty severe run on / detonation when I shut it off for 5-15 seconds..

What causes this? How is it harmful to the motor?

Also, under full load in 5th gear it has some detonation like at 2,500 rpms or so...what causes this?

One other issue, under full throttle for 3-4 sec it emits a large, large cloud of brownish smoke/dust... what causes this?

The motor is newly rebuilt. But the last motor had the same setup same problems and ending up self destructing by chewing up the cam, clogging the oil pump... Could this have been caused by detonation issues?


As to carb size, Demon and other makers agree that 975 cfm is about right for a built 514. Some guys even run 1200 cfm+ dominators with no ill effects. My 975 RS Demon should be right on the money.

I'm going to the dyno/carb guy on friday to work on the problem.
Any thoughts on how to approach it?

[Randy Klein]

cdog

RE: "The 514 BB car is running on 93 octane. It has 10.25 : 1 compression"

I have some problems in 5th at ~2200 - 2500 RPM, especially up steep hills, and I only have 9.75:1 compression. I believe that an octane additive, to kick you up closer to 95-100 octane would help the part of your problem that sounds like predetonation.

Good luck on your dyno work this weekend

[ERA535]

Regarding the run on after shut down, be sure that the base idle adjustment is ok - in other words the butterflies should not expose too much of the idle transfer slots. Read what the Demon literature says about this and follow it.

Are you sure about how much compression it has?

[dalola]

cdog,
Go back to your original post! All that information there still applies. I would also strongly suggest you limit operations until you resolve this issue! Engines do not last long under your described conditions.
Simply put, the combustion in your engine is happening at the wrong time, and this puts tremendous loads on internal engine parts.
Dave

[Cobranut]

You're not going to get from 93 oct pump gas to 95-100 octane with an "additive". Run good race fuel, either 100 oct unleaded, or blend the pump gas 50/50 with CAM-2 110 oct. That, along with the other items mentioned, should cure your problem.

Later,

[HighPlainsDrifter]

Besides the usual things to check you didn't mention what cam specs you're running. A big cubic inch motor with compession needs lots of duration to bleed off some cylinder pressure at part throttle or you will detonate also.You will need a minimum of 240 degrees duration at .050 lift ground on a 108 or 110 degree lobe centerline.For exaple a comp cams 292 magnum cam has 244 degrees of duration and a 110 lobe center,would be a good street/strip cam for you.As for the brown puff on acceleration -your intake probably isn't sealed properly and you are sucking oil into the intake ports from the lifter gallery.Use a fel-pro gasket # 1230 with sealer already on it.
Perry.