350 having detonation problems.

[Greasy Harley]

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.

I thought for sure they were 72cc combustion chambers,

...So yeah, I think I miscalculated my compression ratio
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]

.... 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.
...

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....

[Greasy Harley]

I actually loaned my distributor to a friend that needed to get his truck running.
As soon as I get it back, I will experiment with different timing.
I have been doing a lot of reading about tbi heads, swirl port heads, compression ratios, quench, cams, etc.
I will try to tune it before I throw more money at it. I'll update this post when I know something.
Thanks for all the help

-Russ

[tayto]

keep us posted. my engine is currently a short block with no oil pan or timing cover on and i won't be getting around to reassembling it for another month. if a cam change is in order then would be easier to do it now before i button everything up. i have a 2 piece timing cover and LSx style lifter trays, so changing a cam in situation will be easy. remove rad, waterpump, rockers, crank pulley, rotate engine twice and pull cam. dont need to remove intake or oil pan (in theory)

[tayto]

BTW what octane are you running? 87?

[Cliff Ruggles]

I have been doing a lot of reading about tbi heads, swirl port heads, compression ratios, quench, cams, etc.

The more things you do to make the engine efficient the LESS timing and fuel it will need to make optimum power.  Less octane as well.

There are quite a few "players" in the deal, static compression ratio, shape of the piston tops, combustion chamber shape, cam size and timing, intake/exhaust efficiency, quench distance (the big player), engine temps, etc.

These days we shoot for .035" quench distance and never over .040" for any reason.  We also run high compression ratios than most folks do.  Like many other things associated with this hobby folks continue to regurgitate old/outdated information.

The proverbial "brick-wall" of 9.5 to 1 for pump gas is at the top of the list.

Inaccurate information in other areas that make the list are "upgrading" to a double roller timing chain (the stock Morse type is at least twice as strong), high volume/high pressure oil pumps (they just cost power and reduce timing chain/distributor gear life and can shear the drive shaft).

Another big myth is upgrading to a bug-zapping ignition system.  Stock type points will make the same power as HEI's if not a tad more (natural high rpm timing retard) and last 25-30,000 miles without issue.

In recent years I've yanked out and round-filed more Petronix units than I can count, and not a big fan of MSD stuff either.  It works OK, but NEVER takes a chit when you are in front of Jegs or Summit stores.

Anyhow, I'd tune the combo so it doesn't ping even if you are ending up with at or slightly less than 30 degrees total timing.  Avoid light springs and goofy weights in the distributor, a timing curve starting right off idle and all in around 2800-3200rpm's is adequate.  Use vacuum advance and be conservative until you find the ideal amount to add at light engine load.

Most well thought out engine combo's I build or tune here will like about 10-14 degrees initial timing, 18-20 mechanical, and 10-15 from the vacuum unit.  I seldom use manifold vacuum to the advance and never try to "crutch" a lumpy idle with a ton of initial timing.

A very well though out engine build isn't going to like, want or respond well to tons of timing at idle speed, I don't care what the "guru's" tell you on the Forums........Cliff

[Greasy Harley]

So, I finally got back around to working on my truck.
Here's what I know;
I reinstalled the MSD street fire dizzy. (might swap that out later, we'll see)
I set the initial timing to 4* without vac-adv
mechanical came in at 25*
set the vacuum advance for an extra 10*
and my timing at idle jumped to 15* with vac-adv attached

Seems happy, idles smooth, decent torque, starts easy.
Probably not 100% dialed in, but it is a solid starting point.My truck runs great with the original cam and heads, No parts swapping required
There is no pinging and it has decent power. I'm gonna leave it here, at least until the engine is broke in.

This has been a learning experience, and I have had to admit that I might not understand timing and combustion quite as well as I thought I did. (I know a little more now though   )
I'm glad I listened before I started swapping parts.

Thanks for all the help,
-Russ

[tayto]

good news! so if i am understanding right it sounds like you have 29* TOTAL timing. You will probably be want initial timing around 8-12* and then keep your TOTAL timing around 28-30. I don't know if you read my post earlier but grade of fuel are you running?

[Cliff Ruggles]

With high compression and small cam there is nothing wrong with a low initial timing number. 

I ran my high compression 400 SBC with tiny cam at 4 BTDC and put nearly 200,000 miles on it w/o issues anyplace.

Years later I built a 1967 327 with high compression and small cam and it didn't want much timing either, but had a smooth idle and great power, plus managed pump gas just fine.

In any and all cases with these things give the engine the timing it wants even if you end up a little below the "norm".  We've learned over the years that things done to improve combustion efficiency have the engine wanting LESS timing at every RPM, not more......Cliff