Rebuilt 350 - detonation --> Cam swap?

[cadillac_al]

Interesting read. I guess the OP has some timing to back off.

[Greasey Harley]

Originally Posted by Dead Parrot

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.

Running a Q-Jet, The sparkplugs read like my AF is pretty spot on.
It sounds like some motors like less advance, I guess.

Originally Posted by Greasey Harley

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

Okay, I may have jumped the gun a little on that one.

Originally Posted by 68post

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!

I'm getting an edumication here.

Originally Posted by cadillac_al

Interesting read. I guess the OP has some timing to back off.

...Yep looks like it

I was ALWAYS taught that Small block Chevys like [Need] between 32 and 36 degrees total timing or you're giving up power.
I will play with the timing and see what happens.

I'll keep this post updated.

[Sawlog]

Isn't your .061 quench in the detonation danger zone? Need .035-.045.

[Greasey Harley]

So, I finally got back around to working on my truck.
Here's what I learned;
I reinstalled a brand new MSD street fire dizzy.
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

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 )
Seriously, thanks for all the help and suggestions.

[geezer#99]

Some timing info.

http://www.badasscars.com/index.cfm/...d=76/prd76.htm

[MySons68C20]

geezer is right on the link.....I run 16 deg initial with 20 deg mechanical advance in the distributor for a total of 36 deg mechanical timing all inby 2500 rpm. At idle with 10 degrees of vacuum advance it idles at 26 degrees. Fires up at a touch of the key and shuts down without dieseling using the full manifold vacuum port on the carb.
This method works great just takes a bit of time and patience.

[Sawlog]

If your running that much compression with iron heads you should of decked your block to zero deck and went with a .039 head gasket.
Your cam is also too small for that much compression. You need to bleed some off.
A ran I to all of this last winter with my build.
I built a 350 bored .040 over. 9.9 compression with iron heads. Dart iron eagles.
7cc 2 valve relief pistons, zero deck, my quench is .039.
My cam is .221-.221 duration @.50 lift is .470-. 470
4 barrel holly with performer intake, headers.
My compression test was 190psi - 192psi per cylinder.
16 degrees of Initial timing with 20 degrees built in. 36 total.
No detination on 89 octane.
The key is to have your quench on the money if your going to try to have that much compression.
Needs to be around .035-.045.
I didn't loose any botton end tourqe with my cam. I have 30in tall rear tires and can break them loose at a 25mph roll. I do have a 2000 rpm stall with a th400 tranny.

[Sparky dave]

Are we talking ally heads here? Just checking. Cos If steel heads should be looking for 9-1 surely

[Sparky dave]

185 cranking pressure is perfect for a performance aluminium headed motor on pump gas. If you have iron heads that’s surely your problem. Backing the timing off is a way round it but the consequences are a big loss in performance and economy. Total timing should be 34-36 as previously stated. If you haven’t milled the block. 76 cc heads and flat tops puts you around 9-1 compression which would be perfect with a small ish torque cam. Good luck hope you get it dialled

[Greasey Harley]

THREAD RESURECTION:
I got tired of the mediocre power of this motor with 28* total timing. I decided to tear it down and do an exploratory surgery.
This is what I found.
SPOILER ALERT, some of my previous calculations were off.
Actual numbers:
Pistons---------------8cc, Flat top with 4 valve reliefs, Measured with a syringe
Head-----------------65cc, Advertised, also measured with a syringe
Piston to deck------.020, Measured with a Mitotoyo caliper
Gasket thickness---.041, advertised
Bore:----------------4.06", bored.060 over
Stroke:--------------3.48, stock

My new calculations:
This yields a compression ratio of 9.68 with a quench of .061 and intake duration @ .050 is 204
I think I have solved my detonation mystery.

-Russ



Summit racing compression calculator
https://www.summitracing.com/newsand...ion-calculator

[Greasey Harley]

New plan;
Lunati Voodoo cam 10120702
Intake Duration at 050 inch Lift:219
Exhaust Duration at 050 inch Lift:227
Duration at 050 inch Lift:219 int./227 exh.
Advertised Intake Duration:262
Advertised Exhaust Duration:268
Advertised Duration:262 int./268 exh.
Intake Valve Lift with Factory Rocker Arm Ratio:0.468 in.
Exhaust Valve Lift with Factory Rocker Arm Ratio:0.489 in.
Valve Lift with Factory Rocker Arm Ratio:0.468 int./0.489 exh.
Lobe Separation (degrees):112

Fel-Pro 1142-026 PermaTorque Multi-Layer Steel Head Gasket
.026 installed thickness

Should yield .046 quench
10.02:1 compression
219 Intake Duration at 050 inch Lift

Hopefully this will wake the old truck up. It will certainly sound cooler.

I did limit the mechanical advance in the distributor to +16°, I will likely need to adjust that to around +20°-22° with the new configuration. We'll see

[Bigdav160]

If it were me, I'd be saving for cylinder heads. I've had detonation issues with TBI heads in anything but stock builds.

[Greasey Harley]

Originally Posted by Bigdav160

If it were me, I'd be saving for cylinder heads. I've had detonation issues with TBI heads in anything but stock builds.

That's probably great advice. I'm not going to take it
Will I regret that decision? possibly, but I didn't hate the way it ran with the smaller cam, It was just kind of anemic. This should be an improvement. It's just an old truck.

[68post]

Have you been using 87 octane ? If so tune it a little rich and run E15/88 in if it still detonates - it's cheaper than 87. Probably have to play with timing again but your getting a good hang of it

(edit; I like thread updates and here's "how it's doing now" )

[Greasey Harley]

I've been running 91 octane, 10% ethanol.

[68post]

Originally Posted by Greasey Harley

I've been running 91 octane, 10% ethanol.

Is that locally your highest available ? Notice a diff between different brands ?

[Greasey Harley]

Originally Posted by 68post

Is that locally your highest available ? Notice a diff between different brands ?

You can find 92 octane here, but it's not really convenient. No E85 around here. I can get 91 and 92 non ethanol, but it doesn't seem to help and it's considerably more expensive.

I think my quench was a big part of the problem. A tighter quench coupled with a longer intake duration should make it less susceptible to detonation.
I hope

[68post]

It will help. But also you definitely cannot always hear detonation. I consider .046 on the very edge of wide.

I'd love to have a spark-knock retard type ignition such as a J&S, not cheap but good I believe.
I'd really prefer electronic fuel injection and timing control together and run high compression much easier with better octane tolerance, but that's thousands of dollars to just replace a cheap carb & hei setup (and more time and trouble than I want to spend).

[Greasey Harley]

Originally Posted by 68post

It will help. But also you definitely cannot always hear detonation. I consider .046 on the very edge of wide...

100% agreed. however my piston heights aren't all exactly the same, they vary by .006".
.020 is the deepest hole.
My quench will actually range from .038 to .046.
The shallowest piston is .014, otherwise I would have used a .015" steel gasket, I just wasn't comfortable with .029 clearance on a couple cyls, so I compromised with a .026 head gasket.
Is .006 a lot? I don't know and it really doesn't matter at this point, The motor is going back in. I need my truck.
No matter what, too much clearance is better than not enough.
Piston----><----Head