I've been trying to flush out detonation issues on my rebuilt L46 the past couple days, and could use some advice. Background:

Motor is mostly stock, bored 0.030". Stock L46 camshaft advanced 4°. Ignition is transistorized ignition with the 1111491 distributor, using a B28 vacuum advance. Carburetor is service replace 7029207 rebuilt by Lars. Fueled w/ 93 octane.

I used Lars timing paper to set the timing, and then to set idle speed and idle mixture.

Total timing is 30° at 3000 rpm, initial timing is 12°. Engine is pulling 16 in. Hg of vacuum (measured at manifold port for vac system).

Part of my problem, or so I thought, is that the service replacement 7029207 only has one manifold vacuum port, which the PVC is hooked up to, with the vac. advance hooked up to ported vacuum. At this point I had total timing at 33° and initial at 16° and I was getting some slight "marbles in a can" sound while accelerating up a hill. Idle is excellent and very responsive set at 750 rpm.

I backed off the timing, and then tried to tee the vacuum advance with the manifold vacuum port for the vacuum system. It seems that I have even more detonation with the reduced timing, and my wiper door is now raised.

Is 30° really too much advance? Does the TI ignition system have any affect? Any ideas for manifold vacuum source?

 

The first thing I saw was the 4 degrees of cam advance. That would hurt you in the detonation department.

Another concern is the static compression ratio. What did you use for heads, pistons, and head gasket? The L46 was an 11:1 factory rating. Depends on the parts you used here.

 

Stock 186 cylinder heads shaved and measured at 64cc.
Pistons are federal mogul L2304F, which are 2.4cc.
Head gasket is Fel-Pro 1044 head gaskets (4.200" bore, 0.051" compressed thickness)
I calculate compression ratio of 10.4:1.

 

Did you degree the camshaft to see what your actual advance is? By advancing your timing your building more cylinder pressure obviously. I’d look there to see if you have a tolerance stack up issue and have closer to 6-7 degrees advance. Also, if your LSA on the camshaft is wider than OE (114 I believe) for whatever reason you’ll build more cylinder pressure.

It shouldn’t have an issue if your compression ratio is as you declare but you don’t have a lot of room for error if something isn’t manufactured/ground exactly to spec.

Another consideration here. Is it possibly terribly lean? You have validated that it’s holding fuel pressure as the RPMs climb?

 

No I didn't degree the cam - took it as advertised.

Didn't consider the fuel ratio. What's the best method for measuring fuel pressure on a stock set up?

 

If YOU advanced the cam it would increase the dynamic compression. If you installed it "straight up" with the standard timing gear set then the cam does not have any additional advance. I believe that cam has 4 degrees of advance "ground in" already.

I run an L46 with the cam installed "straight up" using some modern design aftermarket iron heads. My static compression ratio is 10.4 and it runs fine on pump gas without detonation.

 

The head gasket may have killed your quench because it is so thick. Your pistons probably have a compression height of 1.56" The thinner .015" steel shim gasket would have been a better match assuming you did not deck cut the block. Stock original block deck 9.025" or so. Those piston decks would be about .025" down in the hole. Add the .015" steel shim gasket and the quench would right on target for .040". Based on some assumption you are at .076"? That is not going to allow quench to do its job to limit detonation.

Your pistons look a lot like my flat tops with single trough but yours have a small dome added.

Are you setting the ignition timing at the top of the curve with the vacuum advance disabled at 30 degrees?

 

The vacuum advance can is pulling a lot and not backing off fast enough when you accelerate.
Do when you say "4 degrees advanced" on the cam......do you mean you used the 4 degree option on the crank sprocket? If so.....put this back to the "dot" (There is a dot, circle, and square).

Do a compression check and see if you are over 180 psi......which is the threshold for for pump gas.

Jebby

 

Do you have the piston to deck clearance. I just looked at the cam card for Speed Pro CS-1095R (L46) LSA 114 and intake centerline 114 so there’s no advance built in.

 

When I run the numbers using .015 head gasket it equals 11.35 SCR. An L46 originally had a .026 head gasket.

 

I get the same numbers. Not suggesting he just use that gasket and try to run it "as-is". His current C/R is not really that far off but the quench really is. No dome and tight quench works better. Once you bore it and get new pistons it all has to work together.

Based on what is here I would cut the dome off and run the cam straight up. Might cut the valve relief a bit just for good measure. Needs the thin gasket for the quench to work. Get C/R down a little closer to 10:1 and run it.

It's got to be either quench or curve because I know mine works fine.

 

Do you know your piston to deck clearance? Gotta know that to accurately calculate your compression ratio.

 

Hey Stingr,
Deck height is 9.023" and the piston is 0.22" down in the hole.
Yep, setting ignition so all in at 3000 rpm w/ advance disabled at 30°.

 

Compression test is agood idea. At this point, I'm not going to adjust the cam advance, it's a little late at this point.

 

Yes I do, here are the complete engine specs:

 

The 4 degree cam advance is too much. That needs to be brought back to straight up "as ground". This is not that much work IMO.

If you retard the cam timing it might help but I would not do it. Not enough evidence that it would work and too much work involved to experiment IMO. Hurts the bottom end torque more than I would accept.

Head gasket is too thick. You really need the quench effect to work right.

Dome should be cut off.

 

Gotcha. So approximately 10.4:1 Now that we have all the data...... UGH...... Might be a fuel pressure issue like I suggested earlier but I wouldn't get my hopes up alot on that. While it might be there is some advance ground in the cam or tolerance stack up, I don't think that is the root cause at this point having the data.

Your quench is definitely off what what it should be optimally (.040 is optimal). You've made your compression in a manner that quite frankly is old school (dome) and less than optimal with unleaded fuels. As another poster suggested a .020 head gasket with flat top pistons would be ideal and they are correct. If you work out what your compression would be with a set of flat top pistons and a .020 head gasket with a 4.060 gasket bore it puts you right in the same ballpark your in compression wise right now. Quite frankly your quench is double what it should be and it is likely causing some of the detonation issues in conjunction with a 4 degree advanced camshaft.

https://help.summitracing.com/app/an.../engine-quench
https://www.dragzine.com/tech-storie...ston-and-head/

How to fix it...... Well, check your fuel pressure and when that checks ok decide how you want to go about it. Being you just got the motor together I would take the advance out and see how it runs. It's very likely that will solve the problem. That's obviously not what you intended but I think given how the rest of the motor is built you're kinda backed into a corner with this one as it is.

 

Thanks guys. At this point the motor is what it is - I don't have the time or space to tear back into it. Took me long enough to get to this point. Lesson learned - I did not consider quench when building.
I'll start with some av gas and see if that helps.

 

A 1/2 can of Torco in the tank is a LOT easier than a tear down.

 

Exactly! At this point anything is easier than a tear down.

One thought - if I put stiffer springs on my centrifugal advance weights and shift that advance to higher RPMs, would this reduce the detonation under load in say 4th gear at 2000 RPM?