Hello all! First post and hopefully I can get some help on this strange situation. Bought my wife a 92 Camaro with an 87 to 95 vortec 350 4 bolt main. Running Procomp heads, Comp Cams Xtreme Energy (08-432-8) cam, Edelbrock hydraulic roller lifters and 1.5 rockers, 650 Holley double pumper, Accel HEI distributor with "medium" springs, and an Edelbrock RPM intake.

The issue I am having is there is a massive stumble on initial acceleration when seting base timing anywhere close to the 8 - 12° everyone keeps telling me....if the engine will even stay running. It will idle if set around 18°, but runs better if I go closer to 22° and the stumble is not as bad. Adjusting the idle screw up when setting timing lower only helps with keeping it running, but the stumble gets worse and can even die on quick acceleration.

Is this a cam timing issue? I am pretty sure I got the timing marks lined up correctly and set at 0° on the crank. Could it just be a carb issue? Any suggestions are welcome! I will try my best to answer any questions, but I am by no means a master engine builder... Lol

 

That's pretty normal for a cam like that. It's similar to my Comp nitrous cam and mine likes 20-22 degrees base timing.

Number one rule of tuning - give it what it wants. Mine makes 325 RWHP and idles about 850 at 20 degrees. Don't worry about what "everyone" thinks. If it wants 20 then give it 20.

I think when I looked at the Vortec base timing map it was around 14-16 degrees at idle. Bigger cam is going to need to idle faster, and it's going to need more timing. RPM increases timing, and higher cylinder pressure reduces it. With lots of overlap we have low DCR and thus low cylinder pressure at idle. Flame propagation through the cylinder slows down with lower cylinder pressure - fewer molecules for the chain reaction to take advantage of. So you have higher RPM that wants more timing to anticipate the combustion event, and lower cylinder pressure that also wants more timing to get the burn to meet up with the piston. 20-22 degrees is absolutely in ball park.

What you are going to run into with a carb (one of the many reasons fuel injection is just easier) style distributor, is that you have to mechanically manage your maximum timing and if you dial in 22 as the base timing and your distributor dials up another 22 because it's curved for a 10 degree base and 32 all-in, then you now have 44 all in and a basically intolerable situation. At the point that you have a cam such as ours and you need 22 degrees base timing you will have to recurve the distributor to give the proper mechanical advance or possibly use vacuum advance to help get you to 22 at a lower base with vacuum disconnected.

With fuel injection we have the advantage of a timing map and can just put whatever timing we want in for any given operating condition without ever touching the distributor.

GD

 

Probably a timing mark issue. Especially likely if you have a stock crank damper and an aftermarket timing cover and timing tab; or, if your damper is merely old, and the layer of spooge is broke, and the inertia ring has spun on the hub.

Ignore the mark. Set it to where the engine runs best.

 

I have verified timing mark is correct on harmonic balancer by removing spark plug on #1 and inserting a small screw driver to feel for the top of the piston stoke. It lines up with 0° on the timing tab.

 

As mentioned, 20 degrees of base timing for that engine combination isn't unusual. That amount of cam overlap (65 degrees) will have an effect on idle quality and unless the static compression is up there to match the cam, more timing is expected. I've the next step up in Comp catalogue to yours. With 288/292 adv duration and less compression ratio than I'd like, the engine wants close to 30 degrees to idle at 800-850 RPM. I've done this through a combination of base timing (16) plus full manifold vacuum advance for another 14. But tuning with full vacuum advance can be tricky and involves more than OEM parts.
All of that said, to reiterate the previous posts:
1) If you're confident with the accuracy of your timing mark and the engine likes 20 degrees, then that's what is. You don't necessarily need to know what that value is, but I prefer to know. Sort of like using a map AND a GPS for navigation.
2) The other thing to pay attention to is the total timing. With 20 or more degrees available through centrifugal advance, plus the 20 initial, it will be as stated, untenable. And destructive. You'll have to find a means to limit that centrifugal advance. Aftermarket distributors tend to have a limiting bushing that can be changed for this reason. HEI's, do not. And that can be problematic.

 

Handy pic from the Crankshaft Coalition Wiki. Should be helpful.

 

^^^ That would be a modified HEI.
Something I've had to address with the substitute distributor installed when troubleshooting an ignition problem. Welding up the advance slot is another approach but like the mod posted above, it's a cut and try kind of deal. But that's tuning for you. Sometimes never simple.
Nice pic by the way. I may try that approach.
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Tuning is the part that leads to someone having an average engine vs a good or very good engine. The distributor stuff is really important, perhaps the most important part to actually making power. It's also probably one of the more mystical to most as well.

 

I can't say I disagree. A while back while chasing the ultimate is compression ratio and camshaft timing, I soon found that a real and proper timing curve wasn't going to happen without destroying the engine. It was then I learned that timing has a much greater effect on the engine as a whole than does that last half point of compression. I come from a time when I got my tuning chops on an old Sunnen distributor machine. It's times like these (and those to come) when I wish I still had access to it.

 

Are you running any vacuum advance? Ported or manifold?

 

It is running vacuum advance and it is connected to the side of the carb for vacuum only when throttle is open. Tried tuning with and without it connected. Only seems to help when quickly releasing the throttle to go back to idle, but always seems to want to idle high with it connected. Adjusting the idle screw to lower the rpm causes a HUGE drop in RPM almost instantly to the point the motor will stumble and die.

 

While the advanced timing isn't anything unusual with your combination of parts (and without knowing what the actual compression ratio is), the other symptoms your describing seem carb related.
Do you know the history of this carb? It may not be a bad idea to pull it and check the relationship between the primary butterflies and the transfer slot. Then revisit your idle mixture screws and baseline them to 1 1/2 turns out from fully seated.
A picture of the port your vacuum advance is connected to might clear up the collective understanding of how it's hooked up. (The "side of the carb" is a little vague.
Once the carb is baselined, set it up with the timing it runs best at and work through the "stickies" tuning procedure.

 

You absolutely need to hook it to FULL manifold vacuum for the time being. Without doing that you are going to have WAY too much advance at WOT.

GD

 

The pistons are Sealed Power 423NPS .040 which I believe are 8.4:1 based on some googling, but that is what came with the motor. I did have the vacuum advance connected to full vacuum and I did not run into the strange high idle after revving up the motor and letting it come back down.

 

Bingo! That's an issue.
I don't wish to be condescending and I don't know what your experience is relating the general SBC platform but this is certainly a problem if the data pans out.
Take that compression ration of 8.6:1 (and in all reality we really don't know what it is) combined with that camshaft (installed straight up) you end up with a running compression of about 6.8:1. That's fine if you want to go forced induction but for a normally aspirated engine, I'm surprised that it runs at all! Not really, but this is where the problem lies and also why it takes 20 degrees of timing to idle.
Are you able to do a compression test? Even a couple of cylinders (or as many as is practical) and post back? My best guess is less than 145 PSI.

 

I have a cheap compression tester, but cold motor here are a couple of cylinders :
Cylinder 1 = 160PSI
Cylinder 2 = 155PSI
Cylinder 3 = 160PSI

 

Well, not the end of the world but it definitely explains why you need the timing you do, barring anything fundamentally wrong (other than weak compression pressure).
So let's work with that.
The carb problems that you've described, and the low compression and consequent weak signal to the carb booster are part of this, points to an issue with the carb in general. While this posting and re-posting thing is difficult in and of itself, I'm thinking that with all of the experimenting you may have lost the thread in the carb setup in general.
Perhaps you might want to re-visit the baseline and establish the settings in their entirety. Get the throttle balde opening to spec regarding the transfer slot. Same with the idle mixture screws. The double pumper (an ill-advised choice) should have secondary metering as well as an obscure secondary throttle idle stop screw (only visible from underneath). It too has be set appropriately.
Go here.
https://www.thirdgen.org/forums/carb...ey-tuning.html
It's fairly comprehensive.
One thing I can say, that with the weak(ish) compression, carb tuning becomes more difficult. Just keep the timing advanced to where the engine likes to be and once you've established that the carb is set up reasonably, you may find that you can dial back the timing somewhat. That said, with compression pressures as you've posted, 20 degrees is nothing, but the overall timing (base+centrifugal+ vacuum) needs to be addressed. That can come later before you compete in the next National event!

 

I concur, you probably need an adjustable vacuum can, or one that takes less vacuum to pull it in. I've seen your issue, put it in gear and the rpm drop, with a corresponding drop in vacuum, causes the timing to back out of the advance can. Timing drops, rpm drops, engine stalls. Adjust your vacuum can to take less vacuum to keep it pulled full, and it'll stay steady at idle. Here's a good link to an article.
https://www.streetmusclemag.com/tech...g-in-a-vacuum/

 

So... Had a local, more knowledgeable friend than me helping with it and we installed an Edelbrock 600 on it instead of the Holley 650 and all my problems went away instantly. Revs perfect, no more shutter off idle, and base timing is at 15° with vacuum advance connect to full vacuum.

 

Two thumbs up!

 

That's low and commensurate with the relatively low static compression. Very likely the compression is around 8.2 to 8.4 given those numbers. Having done compression tests on a lot of turbocharged engines - that's very close to what you see with an 8.2:1 Subaru Turbo engine. For comparison my stock LB9 305 was 190-195 on every cylinder.

With a nice big cam you actually want MORE compression for NA purposes. This is because what actually matters is DCR or Dynamic Compression Ratio which takes into account the loss of pumping efficiency inherent with cam overlap.

Unfortunately that engine is going to be a poor performer - or at least a lot poorer than it could be.

GD

 

Excellent. That should be close to what you need. Still need to make sure that you're not at too high of a timing value at WOT but with that super low compression it will probably need a ton of timing anyway due to the low cylinder pressure and it's relatively inefficient VE.

GD

 

I could put a small super charger or procharger on it if needed :P Just have to find the right one. Not scared to run a bit more power from it if needed so I don't have to tear down the motor again. I could change the jets on the Holley and tune it to the car a bit better.

 

You need about 20 psi from a decent sized turbo in that engine.

GD

 

Ouch... Yeah, I may just run this motor as is. Not much room to fit anything in the sides and that would require a bunch of modifications. It easily breaks the tires loose now and my wife is happy with it so far Maybe I can build a separate motor that I can put in later.....