Rebuilt 350 - detonation --> Cam swap?
 

Okay I rebuilt a 350
87 TBI block, .060 over, flat top slugs with 4 releifs
65cc TBI "193" 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.

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

..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.

Re: Rebuilt 350 - detonation --> Cam swap?
 

Where do you get the detonation?
Vortec’s don’t like more than 32 degrees.

I’d cut the mechanical back to 18 and then see how much initial it wants.
You’re getting up there in elevation. Your motor needs more initial.

Re: Rebuilt 350 - detonation --> Cam swap?
 

Running Non-Vortec TBI heads.
32* total would be fine, I can't safely run 30* total with this setup.
Elevation is 2200' here
Getting detonation around 3800RPMs, no problems on the bottom end.
It will run up to 15* initial with no problems as long as the RPM's are low enough.
Stiffer advance springs help a little but not enough.

Re: Rebuilt 350 - detonation --> Cam swap?
 

You need to limit your mechanical.
Like in here on an hei.
http://www.crankshaftcoalition.com/w...EI_distributor

Re: Rebuilt 350 - detonation --> Cam swap?
 

How will that help me on top end?

Re: Rebuilt 350 - detonation --> Cam swap?
 

It’ll reduce your total.
With 15 initial aim for 15 mechanical.
Aim for 30 degrees all in by 3000 rpm.
Right now you’ve got 36 total.

Re: Rebuilt 350 - detonation --> Cam swap?
 

Right now I have 28* total, and slight detonation.

I see where that was confusing.
I should have said
8* initial, and 28 * Total
There is 12* VA on top of that, but I get detonation whether the VA is hooked up or not.

Re: Rebuilt 350 - detonation --> Cam swap?
 

What’s the compressed thickness of your head gaskets?
How far down in the hole are your pistons?

At what rpm is your timing all in?

Re: Rebuilt 350 - detonation --> Cam swap?
 

Decked .020
felpro rebuilder gaskets .041
Tried all different combos of advance springs, no help. Right now it's all in around 2800.

Re: Rebuilt 350 - detonation --> Cam swap?
 

I got nothing else!

Re: Rebuilt 350 - detonation --> Cam swap?
 

---->cam swap
But which cam?

Re: Rebuilt 350 - detonation --> Cam swap?
 

I did some searches.
Are you sure about your compression ratio?
I’ve read that 193 heads with 64 cc’s Had 9.4 to 1 using dished pistons.
Your 4 valve relief pistons could put you up there past 11 to 1.
How did you check your ratio?

Re: Rebuilt 350 - detonation --> Cam swap?
 

Online compression calculators.
4 valve relief flat top slugs should be (about) -7cc ...I think


Compression test was 185psi

Re: Rebuilt 350 - detonation --> Cam swap?
 

In maintaining decent low end torque, you probably don't want to go much bigger than this cam.

https://www.summitracing.com/parts/sum-1103

You might try running the numbers in one of the online dynamic compression calculator and see where your dynamic compression comes in at with this cam.

Re: Rebuilt 350 - detonation --> Cam swap?
 

that's pretty close to what I was thinking. I was looking at this one:
https://www.summitracing.com/parts/m...make/chevrolet

Re: Rebuilt 350 - detonation --> Cam swap?
 

It sounds like you have a lot of compression. I have seen variances in compression gauges so I like to know more about the gauge when evaluating the numbers. What does your compression gauge read on a stock 350? I have only seen 180 on my gauge twice. On most stock engines mine reads 120-140.

Those TBI heads are probably one of the worst heads for any kind of performance but they should work on a low rpm truck engine. Most sane people would say to change them but I would think they could work on a low rpm truck engine. I have a set of rebuilt tbi heads with an aluminum intake sitting on a shelf just waiting for that low performance build someday in the future.

I thought detonation happened more at lower rpms, like part throttle hill climbing and such. Having detonation at 3800 rpm sounds odd to me so I agree with geezer about limiting mechanical advance. I think 4000 rpm is when those heads start to run out of breath. Maybe something strange is going on in that range.

Re: Rebuilt 350 - detonation --> Cam swap?
 

Any cam change is just throwing good money after bad.
You’ve got an iron headed high compression detonating slug.
The right cam to make that combo work will defeat your intended purpose.
I think you need to get better heads.

Re: Rebuilt 350 - detonation --> Cam swap?
 

Fuel grade(s) being used?

I see TBI heads mentioned, are you running a carb or some version of FI?

Are you running rich or lean on air fuel mix? (rich burns faster - needs less advance, lean burns slower - need more advance).

How does the motor run? If it runs just fine with less timing, quit arguing with it and enjoy. You adjust timing to match the motor and usage, not the motor to meet some arbitrary timing specification.

Re: Rebuilt 350 - detonation --> Cam swap?
 

2x Yes, for at least this experiment try some high octane to see if it gets rid of that combustion knock. In the lab I'd try aviation gasoline, 100LL (one hundred, low-lead, the blue stuff), but you're on road so maybe some unleaded racing gas.

Re: Rebuilt 350 - detonation --> Cam swap?
 

got this at autozone the other day . if i was to try a bottle of booster i figure VP is good stuff so i try it . 1 bottle does 10 gal 8 octane jump .

and i was told av gas is not intended for ground level its octane is rated for altitude . so i have never used it . guy that told me this has been building engines 2 and 4 stroke for over 20+ years with lots of dyno testing .

Re: Rebuilt 350 - detonation --> Cam swap?
 

It would be interesting to see how much difference premium fuel would make. It's hard to believe some 65cc heads on flat top pistons would make the compression too high. I hate math so I'm not the guy to calculate it.

Re: Rebuilt 350 - detonation --> Cam swap?
 

Actually, AvGas works great in these Chevy 350s, but it's technically not legal for road use because of the lead content. I used it in the dyno lab because I was looking at knock and power loss and needed a fuel that had no knock even with crazy spark advance. As it turns out, light knock increased power (according to VW, Lee and Schaeffer), but it was a tricky balance. A little more knock and you'd lose power and risk losing the engine.
(https://saemobilus.sae.org/content/890161/#abstract)

And, I'm dating myself here, but back in the mid 80s AvGas was around $3/gallon compared to $10/gallon for pure isooctane reference fuel. Good memories.

Re: Rebuilt 350 - detonation --> Cam swap?
 

Fine, I give up.
I bought a set of heads. smaller 1.72" valves, bigger 70cc chambers.
I know I said I didn't want to swap heads, but that was mostly a money thing.
A buddy of mine had these setting on a shelf, freshly rebuilt and sold them to me for $200.
IF my calculations are correct, it should drop my compression down around 10:1 and still make good torque.
A lot of good information here. I appreciate all the help. I'll update this post after I swap them out.

-R

Re: Rebuilt 350 - detonation --> Cam swap?
 

Those TBI heads get a bad rap that's usually undeserved. They use less timing than vortecs do, and the reason those TBI engines run out of air is because of the puny 187* @ .050 intake duration cam they were teamed with.
They have around a 185cc intake port.
They should be perfect for a truck build, and I am sorry but I don't have a link to the timing thread that would tell you where to start tuning the timing correctly for them. They are very fast burn heads.
(If you'd like the info I'd be happy to find it again. The gentleman that had posted the info has used them successfully on more than a few builds)

I specifically sought-out a set of 305 TBI heads for a 307 build that I will likely angle mill to achieve an approx 10-1 compression or slightly more (small cam - approx 207* to 192* intake duration @ .050), and I'd rather use them for my goal than vortecs or 305 HO heads.

If I were you I'd do more adjusting first, you very likely have too much advance at 2,800+ rpm !

EDIT:

"ThirdGen.org" is where you'll find great swirlport head info, and "FAST355" is the gentleman with the info & alot experience with them, and it's his thread with great timing info also!

Re: Rebuilt 350 - detonation --> Cam swap?
 

Here's some info :

Re: 193 swirl ports or Vortecs for 350 truck?
Unread post by Tuner » Thu Feb 27, 2014 6:58 pm

Do not think of the swirl-port heads and “sensitive to timing” as if were a bad thing.

A Hudson Hornet combustion chamber is so efficient it only needs 17° total, some Flathead Ford cylinder heads only need 15° total.

The key word is “need”, as in, “that is all they need for best power and any more than that is bad because it makes less power, whether it knocks or not.”

The swirl-port head combination of intake port and combustion chamber is so efficient that ‘only’ 24° total is required to complete combustion at the desired crank angle for best power.

This is a good thing because less pressure before TDC is less negative work. Less combustion force is pushing the crankshaft backwards and more of the combustion force is applied to pushing the crank forward. The result is dramatically improved efficiency.

I recently installed an Innovate MTX-L to improve the mileage on a ’90 TBI 350. The MTX-L was set it up to feedback the ECU O2 input with normal NB O2 voltage oscillation, which is .1V ~ 1V, but at 16.5 AFR. So far, it is getting 17 MPG in mixed city street and freeway driving, and that is up from the 12 MPG it was getting with the stock O2 sensor. The plan is to try it a little richer and leaner than 16.5/1 to see if it can improve from there.

This ’90 TBI 350 has the swirl-port heads and stock hyd-roller cam. It is a used stock engine and TBI electronics package and the 700R4 and transfer case out of a ’90 Suburban in a 4WD Toyota Tacoma with 34” tires and (I think) 4.11 gears.

I like the swirl-port heads.

End quote.