Freshly rebuilt 2.4L w/2.2s pistons DETONATION.
 

So I'm a big dummy who advised a friend rebuilding his 2.2T motor. We used a 2.4 crank, had the heads ported, used mod-Solex cams, and 2.2S pistons, rebuilt dizzy to E spec's. Started and idled o.k. but under load it knocks. we rotated the dizzy and it still knocks. I drilled the jets out from 140 mains to 155....still knocks. Went to 60 idle jets and continues to detonate under load. it won't if your really soft on the gas pedal. He drained the fuel and used race gas and it doesn't.....is he screwed? can you recurve the dizzy? Is he forced to tear it down and shave the pistons? This sucks.
Nabil

Precisely, how much total timing is he running?

Is the distributor advancing and retarding properly? You can check this with a timing light while watching the timing marks.

Is he using 92 octane or better, fuel?

Thanks for the reply Steve. He had the Dizzy rebuilt to E spec by a reputable local guy that bench tested it. I think it was 30BTDC @6000 rpm. He was using 92 octane (here in Minneapolis) and drained it and went to race gas, which made the nasty sound go away. However it still didn't have the power that you would have expected. Today he did a compression check and had 170psi +- 6psi except for cylinder #6 which had 125psi. He just called me to tell me he hears a "phhtt phht phht" when he cranks it over and its most audiable from the top side of the engine through that throat of the carb. The heads all looked beautiful as they came back from Ollies just before he put them on. He said he loosened the rocker arm but it seemed fine. As an aside, we rotated the dizzy several degrees counterclockwise (retarding I believe) and the other way too, it still detonated under load no matter which way we spun it. He's removing the engine tonight.....sad story, he's trying to remain optimistic. Since I'd hate for him to have to do all this again, I'm wondering if he should have his pistons shaved alittle as I know adding a thicker copper gasket isn't the way to go (at least from what I read). When the engine was idling it had like 5 degrees of timing and like 18-22 at 2500rpm or 3000 I forget, so it seems as though it was advancing correctly. Any advice?
Thanks again,
Nabil

This is only from the peanut gallery since your tech will give you the definitive answer once he has it apart. I suspect the compression ratio might be too much for the fuel octane. As for the "low" cylinder, it might be broken compression ring(s) or a head gasket, although the symptoms you describe sounds like a leaking intake valve. A leak-down test would confirm the exact location - but all sources result in disassembling the same basic pieces.

Hope the damage is relatively minor.

Sherwood

Nabil,

That advance sounds OK to me.

Did he actually measure the CR? Thats VERY critical for any engine build thats not standard!!!

Do a leakdown to see whats going on with #6 and confirm all the rest.

Confirm the jetting in the carbs and make sure its not lean under load.

2.2S pistons are what 9.8:1? and putting them on a 2.4 raises the ratio by .5+, so you are probably running 10.3:1 or better, that's twin plug territory, or race gas as you have found. Think you are going to need to do something to fix the problem besides changing timing.

i thought the deal was this combo produced lower compression in real life than what is shown in calculations?

nabil got any vid of this thing running?

I thought so too? In fact I heard that the compression ratio ends up coming in between 9.5 and 9.8:1 which basically makes for a larger displacement 2.2S motor.
This is the exact same engine I am getting ready to build (except with S MFI) so I am very interested in following this thread.

it does seem clear that compression is a lot higher than you thought it'd be. the change to race gas seems to prove that out.

were the heads milled/decked or whatever? maybe too much was taken off?

are the carbs clean and do you have spacers under the intake runners? or does the detonation occur even when cold (note this is a trick question to find out if you're putting heavy load on the motor before it's up to temp)?

i thought counter-clockwise was advancing on early motors? am i thinking wrong here?

I bet the heads were cut when freshened. Was the case decked as is usually done w a mag case. Was the case line bored for stock sizes bearings? All these can raise compression.

Something you might add to the list, are the cams timed right?

Aggressive cams create a lower effective compression ratio.

I am not an expert, just some thoughts.