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350 having detonation problems.

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Greasy Harley:

--- Quote from: tayto on February 04, 2019, 06:04:12 PM ---I just got my set of 191 TBI heads cc'd last week and they were 65 cc. 193 castings will be the same FYI. You may want to check your static compression again unless you've worked the combustion chamber then ignore me.

--- End quote ---
I thought for sure they were 72cc combustion chambers,

...So yeah, I think I miscalculated my compression ratio :o
Okay I rebuilt a 350
87 TBI block, .060 over, flat top slugs with 4 releifs
65cc TBI "191" heads, These heads are susceptible to detonation (I later learned)
10:1 compression
Compression test reads 185psi (rings aren't seated yet, about 200mi on engine)

I put a "torque cam" in it trying to make a good truck motor. Problem is, I have my timing retarded to 8* initial, and 28* mechanical and still have slight detonation around 3500-3800 RPM's -I have to estimate because I don't have a tach.

Cam specs are:
Elgine MTC-1
Intake Duration at 050 inch Lift: 204
Exhaust Duration at 050 inch Lift: 214
Duration at 050 inch Lift: 204 int./214 exh.
Advertised Intake Duration: 278
Advertised Exhaust Duration: 288
Advertised Duration: 278 int./288 exh.
Intake Valve Lift with Factory Rocker Arm Ratio: 0.420 in.
Exhaust Valve Lift with Factory Rocker Arm Ratio: 0.433 in.
Valve Lift with Factory Rocker Arm Ratio: 0.420 int./0.438 exh.
Lobe Separation (degrees): 112


This motor doesn't like this cam, so my options seem to be,
1)decrease compression----> different heads with 72cc chambers I DON'T REALLY WANT TO SWAP HEADS
-Maybe thicker head gaskets?
..or

2)swap cams

I'm thinking I need a cam with around
Intake Valve Duration (degrees @ .050" lift) in the 220ish range to compensate for the high CR

I don't want to swap springs if I don't have to, and I would like to keep the RPM Power Range as low as possible.
anyone have any cam suggestions ?
Can I fix this problem with a simple cam swap?
Should I just buy different heads? ---Really don't want to.

Greasy Harley:

--- Quote from: Greasy Harley on April 20, 2019, 04:07:24 PM ---.... Problem is, I have my timing retarded to 8* initial, and 28* mechanical and still have slight detonation around 3500-3800 RPM's -I have to estimate because I don't have a tach.
...

--- End quote ---
I should clarify, My timing is 8* initial, and 28* Total (not additional mechanical) with VA unhooked.;
There is an additional 12* VA on top of that, but I get the same detonation whether the VA is hooked up or not.

Greasy Harley:
Been doing some research.
I currently have this cam:
https://www.summitracing.com/parts/mel-mtc-1
was thinking this one might do the trick:
https://www.summitracing.com/parts/mel-22215
Maybe this one?
https://www.summitracing.com/parts/sum-1103

Cliff Ruggles:
Move up to the 214/224/112 cam and it will lower dynamic compression close to 1 point.

Another option is to simply retard the timing till it doesn't ping.

Decades ago I built a 400 SBC with flat top pistons and 76cc heads and installed a small cam in it.

Had to run the initial timing down around 4 degrees and total at 26, but it ran over 100,000 miles and made TONS of power everyplace.  I also added 8 degrees from the vacuum advance.

The engine was in a 1979 K-5 Blazer, TH350, 3.07 rear gears and 32" tall tires.  It was one of the best running engines I've ever owned, and got excellent fuel economy as well.  I knocked down 13-14 for normal driving and 15-17 on the highway.  I could tow anything with it, the torque production was excellent.......Cliff

tayto:
The TBI heads like 28-30* TOTAL timing. It sounds like you are running the "old school" 36* total timing if I am reading it right...
The TBI heads are very efficient cylinder head it just sounds like you are throwing to much timing at them? I am building an engine similar to yours 191 heads, 10:1 static compression, running a TPI cam 202/207 @. 050 " 114.5* LSA with 1.7 roller rockers . Sounds like i should goto a bigger cam as well....

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