Serpentine Belt Issues...Gilmer style cog belt for SBF?

Thanks Bruce!

 

Didn't take offense at all. Actually, I was trying not to offend you! Some people get damned sensitive about their hard earned car building decisions! LOL

Now that I know you're not "that guy".. I will come right out and say that 11° base timing is near next to squat for a healthy cammed motor like yours. Even a stocker for that matter and the only reason the old OEM stuff wasn't much higher is due to higher Nox output which mandates even way back then didn't permit. New cars with cat's have hella high low load ignition advance numbers(over 50° at times) not simply because of the computers and EFI allowing it.. but because the cat's can actually mitigate it from coming out the tailpipes. But you have no cats and it's a high performance engine so who cares how high it goes so long as the engine runs better, right?

Only real downside comes into play if you have dumps under the car instead of full exhaust out the back end. Think it may boil down to some guys preferring the sweeter smell of unburned fuel rather than higher Nox levels burning eyeballs out of their head while sitting at the lights. Me.. I like POWER anywhere and everywhere on the go pedal, the more for my money and time the better.. don't care if it's a 100° out and I'll just roll up the damned windows if I have to.

You're giving up at least 3" or more of manifold vacuum on that motor and as you may know.. manifold vacuum is the ideal indicator of engine efficiency and increased cylinder pressure with closed or partially closed throttles. A vac advance pot hooked to full manifold vac source will help idle and cruise a great deal but unfortunately that falls away abruptly when the motor gets leaned on harder so is only a stop gap getting you part of the way towards better low end torque output. Ideally you run upwards of 20 to 20+ base timing which never goes away under heavy throttle angles. Then shorten up the mechanical sweep to hit your total number around 2600-2800 rpm. This will clean up the idle make it more responsive to VERY small throttle openings and the engine will run cooler than ever. 11° will surely generate much extra heat and just heat up the headers with more unburned than is necessary. Leaning it out more with the mixture screws or bleeds will help some but that is just a bandaid for too little timing. Only trying to help you out by sharing hard earned information so add as much salt as you see fit.

 

I should clarify, the 11deg is at idle with vacuum advance hose pulled and plugged off so currently I don’t know what it jumps to when you hook vacuum hose back up to distributor. The only reason I never fooled with it was the guy at terry Walters racing that dialed in distributor said “set it at 11” with vacuum plugged and let it ride. He had all my specs from where my dad and I built the engine. So I assumed he knew I wanted to squeeze out everything I could. Since I have the GM style coil in cap HEI for my 347, hopefully I can get it to idle at 20deg with all in at 2800, I’ll just have to play with it and the vacuum advance to get that right. When it stops raining I’ll do just that and see how it works out. I’ll admit the engine seems sluggish compared to what it put out on dyno but it’s since been adjusted down and several carbs later. Yes the mixture screws have been used as a bandaid for some time. I know the 36deg is all in at 3k. No dumps- I have Doug thorley headers with 3” single exhaust all the way back with original flowmaster 40 series. I’ll let you know

 

You're not alone. I see tons of less than optimized tunes and have fixed or helped fix many. I too remember getting my curves done on machines because that was supposed to be state of the art back then. Unfortunately.. that's nowhere near optimized when compared to what works best in the car running down the road at all sorts of various throttle angles and loads. And dyno's too. Engine or chassis just doesn't truly simulate the real road load situation. Not to mention they generally won't even test much below about 2,500 rpm at WOT much less at part throttle which you'll probably spend even more time at if driving in the city at all.

Here's a quickndirty test to show you what the engine likes and prefers to make max idle/part throttle vacuum. Hook up a vac gauge to full manifold vacuum port. Disconnect the vac advance pot and twist the base timing up till it idles really high. Reset idle speed and adjust mixtute screws to bring it back down and smooth it out. Rinse and repeat till the vacuum gauge starts to get too unsteady and no amount of mixture screw adjustment will correct it. Take a shot at the balancers timing mark, if it even goes high enough, and make note of that setting. THAT is what the engine wants for idle timing. Manifold vacuum will probably go up substantially. Can't drive it or rev it much if any for fear of massively overadvanced when centrifugal sweep kicks in but shows you the extreme difference between WOT and closed throttle combustions dynamic requirements. Easily reversible and DO NOT DRIVE THE CAR LIKE THAT! BOLDED to get the point across and prevent engine damage in case you get power greedy. Lol

 

So for ease and convenience sakes, do I set initial timing at 20deg with vacuum disconnected and plugged, then put a vacuum limiter on it to only bring in the additional 16deg all in at 2800? Or should I hook to manifold vacuum and set to 10 with 10deg vacuum off idle to make it 20 then adjust mechanical to bring in curve to 36 total? I haven’t been inside of my HEI dist before so I don’t know if I need to buy a weights and spring kit or vacuum can with limiter etc.

If you don’t mind, walk me through the steps to getting my 20deg initial with 36total all in at 2800 if i’m using a vacuum 2nd carb. Although i’m thinking of moving back to a double pumper carb or fitech efi one day so speak to me like i’m a 4yo as I was diagnosed with brain tumor a year ago or so and have a hard time articulating when typing or speaking and I’ve also forgotten more than I ever learned. I was never a genius at timing optimization before anyway. We can build 347 engine in house but that’s why I sent the dist out to be set at race shop to begin with. Thanks for the help, she’s a mean car but is sluggish down low currently.

 

Very sorry to hear that. We'll just take it slow so you can absorb and learn and work at your own pace. Was going to ask for more spec's but the very comprehensive info, although my eyes are getting worse than I thought while trying to read the fine dimmed out print, listed in your sig gave me all that I needed.

Ok, so it's like this. You have 3 different types of advance. Base or initial timing is where you set your distributor. Centrifugal advance sweep is the mechanical portion with the weights and springs. Vacuum advance is used to manipulate idle and/or part throttle timing depending on where you have its vacuum sourced from, ported(also called timed) and full. The overlapping of it all is where most get tripped up when trying to recalibrate the entire curve simply because it's an exercise of robbing Peter just to go pay Paul. That VERY old antiquated system is fatally flawed and we can only do so much without using newer electronics to circumvent their shortcomings but we do what we can. Total timing the engine sees must remain the same but the way it gets divided up is the key part to work towards when setting it all up.

So let's start out with base/initial timing. The reason bigger cams and larger inductions often need more base timing is that the fuel mixes density is quite lean(NOT to be confused with the actual air fuel ratio or AFR) and rather lazy and slow moving. This causes the little available fuel to be in or re-condense back into droplet form as it travels through the slow moving induction pulses(low piston speed/cylinder demand). Compared to a much faster and higher density mass at higher rpms it is quite hard to light off and reduces the burn rate which causes a substantial delay in peak cylinder pressure production. Basically the molecules of mixed and partially gaseous fuel are spread very far apart and takes considerably longer for the flamefront to fully propagate and spread until it is consumed. This is why the manifold vacuum rises as you compensate for that delayed combustion pressure spike when heavily advancing the base timing and/or adding supplemental advance from the vac advance pot when connected to full manifold vacuum sources. That is exactly what you want.. MAX manifold vacuum.

Now, since atmospheric pressure tops out around 14.7 lbs sq/in any help we can get or add to increase an engines initial fuel and air uptake beyond that available static air pressure figure will help power and throttle response. And beyond that the more manifold vacuum created pulls harder on the carbs idle and transfer slot idle feed restriction(IFR) circuit which makes it much more responsive to any tuning changes and helps the venturi based system work much better overall. The more base timing you can get into an engine the better it will initially react to the throttle pedal openings because it is starting from a higher manifold vacuum baseline and is not going to fall away like a supplemental advance coming from the vac advance pot will suddenly disappear under heavy throttle transitions. So you want it as high as possible while still allowing enough left over to hit the total timing number and also allowing some supplemental vacuum assisted ignition advance to be added under very light loads such as steady state cruising speeds. You can crutch a poor base timing number with very fast centrifugal but there is still some slight delay and the motor will still not make as much vacuum as having higher base timing. Fuel control becomes slightly more laggy as the carb tries to compensate with accelerator pump shots and fuel enrichment circuits(power valve) to compensate for the sudden loss in manifold vacuum. Ideally this should be set around 22-25 degrees btdc for a combo like yours.

Ok onto mechanical advance which is just as it sounds, mechanically controlled by means of weights and springs. Very easy to understand and see how it works with the cap and rotor out of the way. The biggest issue here is that the OEM based designs are somewhat limited on what you can do to fully tailor the curves. Mixing and matching springs are even stretching them slightly can help with fine tuning. Especially in the case when much higher base timing is desired and the need for shorter mechanical sweeps are needed to hit a specific total timing advance number at higher rpms. Depending on the specific design being used there are many ways to go about this but all boil down to limiting the travel of the mechanical advance mechanism in some shape or form. In your case, since it a GM style HEI setup you could reply on some of the kits on the market but even they have been modeled far to closely to the original stock factory timing requirements so still fall short on limiting maximum advance sweep lengths. Stiff springs can work but are bandaids and don't allow faster but still shortened sweep curves. Can also use solder, shrink wrap, rubber or metal tubing, etc, etc. About the only way I have found that works without any longer term durability issues is to actually drill a hole in the plate below the weighted/sprung advance mechanism and tap it with machine thread to install a bottonhead type set screw to limit the plates movement length. From there you can file one edge of the bottonhead screw flat where it actually contacts the advance plate to more finely adjust and limit its travel distance. Be sure to use some loctite to keep in from vibrating out over time. Can be tricky because you want a little more limiter than actually required to then sneak up on the exact setting by filing some of it away to increase total advance plate travel to where you want it. Limit too much and you end up filing all the way towards the phillips or flathead slotting and makes it impossible to remove without damaging it later on. Been a long time since I fooled about with stock style HEI stuff so specific sizes and measurements would probably best be googled to verify. For your engine/gear/weight combo I'd be leaning towards about 10-12 degrees total sweep capability and you want it to be rather quick with light'ish spring rates to be all in at around 2600-2800 rpm. Maybe 2 lighter springs or 1 light and 1 medium tension spring should get you there.

Last is vacuum advance. Rather simple concept but there are more pitfalls to overcome here as well. First of all, ideally you'll need an adjustable unit to end up where you need to be. Non-adjustable's are a definite no-go with WAYYY too much advance built in to compensate for weak base timing and lazy mechanical sweeps. You can also do something similar to the above mentioned mechanical advance limiter with a set screw in similar fashion but they are cheap enough to just buy an adjustable version rather than add more modifications to the list of things to do here. Your's may already have an adjustable one attached. Some have adjustable stroke lengths too. These are typically adjusted inside the nipple of the vac pot with an allen head screw(5/32?.. that sounds too big so maybe it's 5/64"?). You essentially want the unit to only offer a minimal amount since the base timing and advance curve have become so aggressive compared to their original baseline settings. This should be hooked to a FULL manifold source so it may add its available supplemental advance under no load(idle)/light load(steady state cruise) situations. Many use ported sources but these do not offer any supplemental idle assist which the motor will enjoy for the reasons mentioned above. So limit this adjustable vac advance pot to about the minimum setting it will offer.. usually around 10-12 degrees assist/travel lengths.

So let's recap for your desired advance setting numbers. Set distributor housing to achieve 25° base timing. Limit and adjust mechanical advance sweep to 12°. Adjust vac advance pot and hook it to full manifold vacuum source to achieve additional supplemental advance of minimum capability.. usually around 10-12°.

This gives you.. 25° base + 12° mechanical to = 37° at anything over about 2600-2800 rpm @WOT. At idle it will amount to.. 25° base + 10-12° vac assist to = 35-37°. At lighter throttle cruise speeds it will go as high as.. 25 base° + 12° mechanical + 10-12° vac assist to = 47-49°. Sounds extremely high but remember that the vac advance supplementation instantly falls away under heavier throttle openings so it's only intended to be that high under extremely low loads and idle conditions.

With the vac assist hooked up and cruising at higher speeds it will likely cause the carb's IFR and main jet to lean out the mixture a bit which could cause surging, light popping and even possible backfiring. At this level carb tuning will likely be required but some carb's, especially Holleys, are already very fatly calibrated and may actually work with limited modifications to low speed air bleeds to richen it back up for happy high speed cruising. Accelerator pump shot will also likely now need to be made fatter(bigger nozzle # with high flow nozzle screw.. #38's work well for most) and come in quicker(brown cams work well here).. especially VS setups since they only have one accelerator pump up front to rely on. Basically want a fatter shot earlier on in the initial throttle movement.

Sorry for the longer post but hopefully it helps you wrap your thoughts around the mechanisms and combustion requirements needed to get the most out of a high performance engine. Although these are not highly refined for your specific engine, gear, weight combo.. they are damned close and a far better curve than what you currently run right now. Just use some caution as you sneak up on the higher timing numbers and listen hard for detonation when the motor is lugged in a taller gear while going up a long hill. Push the throttle as hard as possible without actually causing a downshift so it loads and lugs the motor harder. I myself usually purposely add extra weight to help load the car down as it would be with additional passengers and also roll the windows up to better hear the engine over the road noise and exhaust. Even the slightest audible detonation should be squashed right away by twisting the base timing down till the mechanical or vac advance sweeps can be readjusted to allow base timing to be put back towards its optimum. I have purposely left the distributor adjustment bolt just loose enough to wrestle some adjustment with two hands but not enough that the adjustments can change during runtime. Just be sure not to grab the cap while doing that, don't ask how I know this. lol. T

After all this hubbub the car will certainly not be sluggish anymore, that I can promise you. Most of the stuff I do/did that has/had lazy OEM style ignition advance curves and weak fuel delivery suddenly starts smoking the tires from standstill.. with no power-braking required to start them spinning. Also very common for the extra low rpm torque to start sqwauking the tires during shifts too. Most people do not expect such results and are simply amazed at what they have been missing out on. And the icing on the cake?.. MILEAGE IMPROVES AS WELL. Just a tiny crack of the throttle and you're on your way towards highway speeds without much extra throttle effort required to get there.

If you do want to pursue this in more detail you can PM me and I'll talk you through it over the phone.

Take care,

Greg

 

See above comments and questions. Thanks Greg for helping me thru this can’t wait for car to run cooler at idle and lower cruising speeds. Also attached is an image of a similar carb to mine and my dist vacuum hose is connected to far left port on front of carb. It says that’s timed so I need to move to far right manifold port on carb? Or directly into performer rpm air gap manifold? Thanks again Greg. Any other thoughts and theories? I’d like to get this dialed in this weekend atleast most of it

 

whew.. tough for me to answer all that online here. Phone support would be much better and faster.

I wasn't really trying to push you towards the perfect ideal so much as explain what that ideal may be. Start more conservative at around 20 base timing, 16 mechanical, and add FULL ported vacuum of around 10-12°. Your cars weight, rear gearing and camshaft will easily allow that without detonation. Still need to keep an ear open for it.. but the potential for detonation or preignition is very slim unless something else isn't right with the tuneup.

Base timing should always be set with the vacuum pot disabled.

Yes.. you always want FULL manifold vacuum so that the assist occurs during idle AND lighter throttle cruise speeds. Ported vacuum source that you now run will only come into play once the throttle has opened more during slightly higher loads and faster speeds. Swapping that over to a full manifold source, by either connecting to the carbs FULL manifold port or directly to the manifold itself, whichever is easier and looks cleaner for your install, will immediately increase your idle speeds and manifold vacuum will rise. You will then need to reset the idle speed back down to what you want, should be somewhere around 850'ish in neutral and fall downwards of 650-700 in gear, and then reset the mixture screws to achieve smoothest and most consistent needle readings on the vacuum gauge.

As for what a motor tolerates for ignition timing advance?.. at very low loads with nearly closed or light throttle positions the engine becomes MUCH more tolerant of heavy ignition spark lead. It goes back to what I said above about fuel density and slower piston speed mixture movement. Plus once you step into the throttle more that manifold vacuum drops and takes the vacuum advance assisted portion of the timing curve away with it. So, in essence vacuum advance a load based configuration and no.. 40° or more is not too much. Most EFI cars these days run upwards of 50° or more during deceleration and light loads.

I can look around to see if there's an online tutorial or pics for an HEI advance limiter mod for you to study. As I said prior, it's just been too long for me to remember the specific geometry requirements of HEI style stuff since everything I generally do is aftermarket and has external coils. Here's a link with pic's to get you started on what's involved.

https://gbodyforum.com/threads/easy-way-to-limit-the-hei-mechanical-advance.39352/page-2

Just try to feel around with the 3/32" allen wrench but be careful not to push it too deep or too hard or you could puncture the vacuum diaphragm. Here's an instruction sheet listed at Summit racing, should be similar enough to the one included with your distributor. Disregard and don't get confused with their other "cookie cutter" timing instructions as I'm trying to retrain you away from standard practices which will not produce the same results for which I'm steering you towards here.

https://static.summitracing.com/global/images/instructions/sum-850030.pdf

About all I have time for right now. Good luck with it!

 

Thanks for the insight and help