Jim Martley2

I am working on a friend's 70 LT-1 and have a question about the vacuum advance supply. Is it supposed to be "ported", as in no vacuum supplied at idle and progressive supply as the throttle is opened, or is it supposed to be "manifold" vacuum, as in 100% vacuum at idle and drop off as manifold vacuum decreases at wide open throttle?

There IS an electrical solenoid inline with the hose coming from the carb to the Vacuum Advance and I believe this to be factory. The carb has been changed from original and does NOT have a vacuum port that I remember being there for "ported" vacuum in the primary metering body as most Holleys have.

Thanks,
Jim

...Roger...

Are you wanting it to be factory ? Manifold vac will help the engine run cooler.
The solenoid is part of the TCS wiring, advance only in 4th poss 3rd depending on the year.

Jim Martley2

I would like it to be factory but he is having issues with severe detonation at part throttle with todays so called Premium gas. The engine is completely original internally so it has the stock 11 to 1compression and I am believing that THIS is the main cause of the detonation. I have told him to try putting a half tank of "Cam2" racing gas in it mixed with 93 octane Premium and see how it runs then. He says the car runs GREAT until it gets warmed up, 180-190 degrees, and then it "pings " badly. He just had the Transistor Ignition distributor professionaly rebuilt/restored so I am VERY confident that the advance rate and all that is what it should be for a 70 LT-1.

Jim

Solid LT1

Hook the line to the factory TCS soleniod on the intake manifold so it will NOT have vacuum advance, 11:1 compression and pump gas won't work in today's world. You need to retard ignition timing about 4-6 degrees and then your fighting heating problems due to retarding the timing and throwing extra heat into the exhaust ports. TCS spark control soleniod should only allow vacuum advance when motor is cold or in high gear (little switch on Muncie side cover is for TCS activation) when it is warm, otherwise it blocks vacuum signal to advance on distributor.

The best solution is to run 20% race gas and leave it at the factory settings, you'll knock out 20-40HP out of that motor retarding the ignition timing to run on normal pump fuel. There are also water injection systems from places like AEM or Snow Performance that will allow it to run on normal pump gas.

Jim Martley2

THANKS!!

7T1vette

If it doesn't have a 'stock' carb on it, why does he care about the accuracy of the hose routing? That line going to a 'timed' manifold port [or, ported, as you state] is only for emission purposes. The retarded timing at idle burns off more emission when at idle.

Other than that, nearly all setups would benefit from moving the vacuum advance to 'manifold' vacuum so that your advance will be engaged at closed throttle. The engine will run cooler, smoother and burn less fuel [but produce some increased emissions at idle].

Jim Martley2

It's not so much wanting a "stock appearance" as it is having a good starting point in knowing wheather there is SUPPOSED to be vacuum to the advance at idle or not. I always remember having ported vacuum on my older muscle cars and they didn't have a problem with timing induced detonation. I am trying to find a happy medium with this situation of todays gas vs. yesterdays compression.

Thanks,
Jim

BarryK

wow, so much information and ALL of it completely wrong.

please tell me why 11:1 CR won't work on todays fuel.
please tell me why connecting the vacuum advance so that it doesn't operate helps anything.
please tell me why you are recommending to the OP that he needs to spend money foolishly on racing fuel.
Please tell me why the OP needs to spend more foolish money on catalog racing parts like water injection to make his car run on pump fuel.

For the record, my '65 L76 motor, which is completely stock specs including 11.0:1 CR runs GREAT on pump fuel. Timing is NOT retarded by any means to lose performance. It's set at 12º initial, 24º mechanical advance = 36º total timing all in by 2800rpm PLUS vacuum advance (connected to full manifold vacuum) of 16º so a total timing plus VA of 52º.
I run 93 octane pump fuel with zero detonation issues and at times I haven't been able to get 93 i've used 91 octane also with zero detonation issues.

BarryK

Jim
keep this in mind before deciding how you want to configure the car vacuum advance set-up.

1. Ported vacuum was used ONLY for emission purposes, period. It is NOT the best set-up for performance
2. the TCS was designed for emission purposes ONLY, period. Again, it's not good for performance.
GM was attempting to keep the cars within the new federal emission requirements so were doing all sorts of screwy things that killed performance but helps emission outputs (such as ported vacuum source, retarded timing settings, TCS system, A.I.R. system, etc.

If your friend is not worried about NCRS judging where everything, including having the operating TCS system in place is important than I'd personally set the timing so the total timing of 36º is all in by between 2500-3000rpm, hook the vacuum advance up to a full manifold vacuum source, and enjoy driving the car.
SOME motors, not all but some, don't seem to like the full 15-16º of vacuum advance on todays fuel so if you get jerking or chugging you may need to limit the amount of advance the VA cannister gives you but other than that you should be good to go and you don't need racing fuels, or any catalog parts

Jim Martley2

BarryK,

Thanks for the info. The owner is NOT concerned too much about NCRS type originality as he is about being able to DRIVE the car ECONOMICALY. The cost running even a SMALL amout of Cam2 in the car would take a big bite out of his "fun money" that he operates the car with. I will try your suggestions and see how the engine reacts.

Thanks again!

Jim

7T1vette

Economy will NOT be improved by hooking vacuum advance to a 'timed' port. The only difference between a 'timed' port and manifold vacuum is that at CLOSED THROTTLE only, the 'timed' port has no [or very low] vacuum on it. But, as soon as you touch the throttle, it becomes the same as manifold vacuum.

Solid LT1

OK Smart Feller you tell us how the TCS functions on a car? Why did GM spend the money on it?

Tell me how a 3.25 stroke motor would have same fuel requirments as a 3.48 stroke motor running the same compression, I guess any SBC is a Chevy in your book.

Only way you can answer this is to look it up in some book, if you can read at all. You don't have the slightest clue what your talking about. Are you sure your 327 is still 11:1 compression? does that great intake and 600CFM Holley on it give the same amount of fuel mixture as a LT-1 Hi-rise and 780CFM Holley? Would piston dome shape have any effect on octane requirements? (the dome on a 327 11:1 motor was designed in the early 60's and isn't a very good shape but, will be less prone to detonation than a 302/LT-1 dome.)

Please post some pictures of the LT-1 Vettes that you have owned or go back to the General forum of the C1/2 crowd, this is a "Tech/Performance" forum maybe you need to return to the C1/2 side of things. We tend to really drive/use our C3 Vettes compared to most C1/2 owners.

noonie

That's too simple for most to accept.

BarryK

so in other words you CAN'T answer the questions I posted above.
I thought so.

BTW, besides my '65 I did also own a C3 and I do drive my C2, it's never seen a trailer since I've had it.

Before recommending foolish ways for people to needlessly spend money on their cars maybe you need to learn and educate yourself first.

7T1vette

There's not much on a '70s vintage vehicle that requires 'exotic thinking'; they are pretty simple devices. But, the lack of understanding of them is often surprising. This Forum is a good place to learn about some of the C3's operating systems (i.e., like the wiper door and headlights vacuum systems). But many just want the answer to their immediate problem...not enough knowledge to understand it themselves. Oh, well...we're all different. And it's all good!

69427

Barry, welcome back! Haven't seen you lately. Drop me a PM sometime and let me know what you've been up to.

69427

FWIW, any Small Block Chevy is a Chevy in my book too.

69427

...Roger...

Nice to see you post here Barry , I still reference your info on your site on a regular basis , thanks.

PeteZO6

I second that Barry. I wasn't a member of the forum very long before you sold your C3 but I did enjoy reading your posts, especially about tuning.


Pete