Hi there Mike, Roger, and Pete. Nice to see you guys again and thanks for the kind welcome.
I hope you guys are doing well.

Unfortunately, certain people's posts have reminded me why I stopped posting in the first place. It's simply not worth the aggrevation.

Roger, glad you are making use of the info on my site. please continue to utilize it as much as you want, that's what it's there for

 

Well i had the opinion that ported vac was only for emissions also until i recently read Ruggles book on Qjets. For applications with large cams the ported vac can prevent an engine stall and kill as vac drops when loaded by an automatic shifted to drive or energizing the AC compressor. I can recall this from past exprience myself though not everyone will.

The real answer to using ported vac is to try manifold vac then ported vac and compare the results. Larger cammed motors may respond better to a ports vac - but it may not. I can't tell if this is the stock LT1 cam from here in California.

My expectation is that none of the LT1 Qjet's in 1971 used a ported vac - but i could easily be wrong as my '74 Qjet has more vac ports then venturi barrels.

As far as detonation goes it can occur for a number of reasons and it only has to happen in one cylinder to ruin your day. Carbon build up, hot spot on sharp edge in just one chamber, high or higher than normal coolant temps, too much advance (of course). Myself, high compression on a short cammed motor would be istic (>9.5 c.r. with iron heads). The new trend using a large duration camshaft is to calc for a "Dynamic" c.r. and if u (or the owner) have accomplished this then good for you. But on a stock LT1 engine from 1970 who knows exactly what the c.r. and Dynamic C.r. is - unless u open the motor up and measure it. My belive is what Vizard, Lingenfelter, and other pros have published which is too keep the c.r. less than 9.5 in an iron head motor and less than 10.5 with aluminum heads. While that applies to most short street cams it would be useful here.
On the faceless internet u will read dynamic ratios of 8.0 to 8.2 is safe using pump gas with any static c.r.. I admit i don't have the dyno experience to say what won't work - i just use the best information source availible - thats my 2 cents.

From what i read here cooling is the biggest indicator and i would run the coolest T-stat availible (large open disk), check the shroud seals are intact and doing thier job (or replace all of them with new), the centrifical fan clutch is working (or install a fixed blade fan), recore the radiator, add water wetter to the coolant and last just not drive it on hot days.
But the real fix may have to be large chamber heads though it sounds like the engine is marginaly close and possibly just a thick azz head gasket can get it out of detonation - don't gamble on this though unless u think u really want to try it.

Bottom line is, that engine won't last long driving with detonation. And remember that detonation begins long before you can hear it from inside the car. If only 1 or 2 cylinders are detonating u probly can't hear it and it only takes on cylinder in detonation to destroy the engine.

Hope this helps more than it hurts,
cardo0

 

WOW! I'm really getting educated here, can you tell me the correct numbers Qjet ofr a 70 LT-1. Are you by anychance involved with NCRS judging?

PS anyone want to buy a Holley List#4555 for cheap?

 

I have seen at least 100 LT-1's and have never seen one that I thought was real with a Quadrajunk carburetor on it. Must be something real rare

 

Barry it's nice to see you posting here again (as well as the usual fun we have over on VH, nice wave as , they're all watching this). Please keep in mind however that responses like yours above will jeopardize my chance of being Mr. Nasty 2010. I know that Roger thinks I'm a shoe-in but you've beat me to the punch on this one.

 

 

Ah jeez now all the C1/C2 boys have showed up. There goes the neighbourhood.

 

I can't remember how that TCS is set up except for the solenoid. I think there's a temp switch involved too. The fact that it runs fine cold tells me you have something wrong in the TCS side of things or at least something the TCS activation brings to life.

Unhook your vacuum advance. Rev the engine until the mechanical advance stops advancing on the timing mark and note what you have including the initial. Anything up to 36* or so is fine. If you have more than that, start looking for the cause. Once you determine you have no more advance than about 36* total, hook up your vacuum advance straight to manifold vacuum. Record how much total advance you have now at idle. It could be anywhere between 15-25* or so. What is it? It would also be helpful if you post the numbers on the vacuum can.

Take the car out and run it and see what happens. It should run fine on 93 octane. That 11-1 compression isn't a problem with today's gas. Today's 93 octane is roughly equivalent to yesterday's 97 octane and that engine ran well on that.

My own 11-1 engine, tuned to factory specs runs on 89 octane with only a slight rattle on throttle tip in.

Bottom line, I think you're getting too much advance from somewhere.

 

The TCS solenoid inhibits any vacuum advance unless

1) the engine is in cold start cycle or almost overheating (that's where the temp switch comes in)

or

2) is in 3rd or 4th gear. 4th gear only on some years.


You might be on to something with a mismatch of the vac can.

 

Mis-match or overstroke of the diaphragm. Stop bushing missing off the mechanical advance, too much initial, vacuum routed wrong, etc. All of this could be dormant until the engine warms up.

It's something simple, whatever it is.

 

welcome to the party...

 

Did someone say party???????????/

Hey, I'm tryin' to learn something, here. I've got one of these LT1 cars!
Chuck

 

I just checked the Corvette Black Book and it confirmed what I thought to be the case: LT1 engines for 1970, 1971, & 1972 were only equipped with Holley carburetors - no Quadrajets are listed.

Furthermore, the 1969 Chassis Service Manual lists a Holley 4150 type carb for the 350/370, AKA LT1. I realize there were no LT1 equipped Corvettes produced for MY 1969, but apparently there were plans to do so until the Camaro "ate" up all the LT1 for use in the Z28.


Pete

 

Well i've never worked on an LT1 carb but from all the good advice u provide i can tell u never have either.

Ok Pete, i have the carb info update now.

No not any kind of judge or want to be but what does that have to do with the OP's problem? Well look who's calling out correct part numbers now?
The OP says the carbs been changed and is considering ported vac. So what i said for ported vacuum still stands whether for Holley or Qjet carbs. It sounds like that TCS soleniod performs basically the same function as ported vacuum. But i don't know since my TCS has always been disconnected. But whichever one works the OP can choose for himself here.

And since we're being totaly critical here, i also think adding water injection is more wishful thinking to solve a problem of over compression. U must be joking? Have u ever installed a water injection? Most are designed with controllers for positive pressure systems like turbos and blowers and would be a handful for the novice to install and tune. And how much $$$ for a water injection kit? Try close to 500 bucks when all done. Why when for <$1000 u can have better flowing aftermarket heads that out perform the stock LT1 heads and will kill the detonation problem. Or even pocket ported large chamber chevy heads for $500 will do the job and still perform well - try Performance Heads: 2995 W Whitton Ave,Phoenix, AZ 85017,(602) 254-9586.
Running 20% race gas all the time? Good luck. Why not just trailer the car to the race track every time to drive it?

cardo0

 

Again, there is nothing wrong with the factory level compression ratio that would cause this problem.

The OP needs to look elswhere for the root cause of the problem.

 

OK lets get the facts down "just the facts" as Joe Friday would say (I know most posters here have no idea who Joe Friday is, hint Google Jack Webb.)

TCS= Transmission Controlled Spark

TCS is a emissions system put on many vehicles starting with the 1970 model year. It blocked the vacuum signal to the advance unit on the distributor unless the motor was in high gear after the motor attained operating temprature. There are a few parts involved, a thermo switch mounted into the passenger side cylinder head that allowed spark advance until the motor warmed up to operation temprature as it was needed for driveability issues while the motor was operating cold. After reaching temprature the thermo switch closed activating the soleniod mounted on the intake manifold to energize and block the vacuum signal. The wiring to the soleniod was switched off by a small micro switch mounted into the transmission side cover of the Muncie 4 speed whan the transmission was in HIGH GEAR (that would be 4th gear ONLY for guys like Mike.)

Vacuum signal for this system is amost always MANIFOLD vacum and not timed vacuum signals, the LT-1 took its signal from the same source as the carb choke pull-off port on the FACTORY HOLLEY 4bbl carb of the LT-1 (sorry LT-1s never came with Qjets or vapor return lines no matter how many people tell me so, I know better.)

This was all done to reduce oxides of nitrogen caused by high combustion tempratures, it was a cheap fix that basically retarded spark timing under most operation conditions (the 11:1 copression of a 70 LT-1 made for some serious combustion tempratures.)

Once again another thread SPAMMED by "experts" thanks for all your contributions.

Not all water injection systems are that expensive and I would say unless you want to do "the cure" a stock 70 LT-1 won't run on todays premium pump fuels without compromising the ignition timing to prevent detonation.

As far as aftermarket heads SCREW THEM, if he has a real 70 LT-1 the 186 heads on the car can be made to perform well and they are the correct heads for the engine. Here is my "cure" for detonation on a 70 LT-1 or 67-9 Z/28 Camaro



Yes I have about the same amount of $$$ into them as a set of aftermarket heads but, they will also outflow most "out of the box" heads I have put on a flow bench and THEY LOOK STOCK! and my car IS a LT-1, the BADA*S of the small block C3 Vettes. This makes it all the more worth while.

Mike W, we are still waiting for photos of your Vette on these forums PLEASE! post some. I'm dying to see it

 

Test: Disconnect and plug vacuum advance and I bet your low load ping will go away. What works for me is a vacuum can with about 10* advance rated at 2" vacuum less than your engine's idle vacuum, hooked up to full manifold vacuum. Remember, the smart one sez vacuum advance complements the mechanical advance curve... Just my 2 cents.

 

As usual, you feel the need to lower yourself and come out with personal insults.

Please look up the meaning of 'spam', you consistently mis-use it.

We're waiting for pics of my car? Who is 'we'? I started this thread -as you requested- about three weeks ago. There was 850 or so views last time I looked.

http://forums.corvetteforum.com/c3-g...e-picture.html

Looks like you've (not we've) missed the boat, again.

Almost forgot, a review of GM documents, NCRS technical manuals and personal notes I've taken about known original cars confirms that 1970 Corvettes have a TCS switch for both 3rd and 4th gears. Surely you must be getting tired of being proven wrong?

 

you are right, lets get do just the facts:


fact - as "special and unique" you seem to think the LT1 motor is it's really nothing more than just another SBC motor and works on the same principles as they all do.
This is not saying anything bad at all about the LT1 which is a very nice motor and in so many ways the same as my own '65 L76 - both were the high performance SB motors of their day - but it's still just a SBC motor and the same operating principles apply to it. Talk around the issue all you want but you are simply blowing steam by believing anything else.

fact - your statement "a stock 70 LT-1 won't run on todays premium pump fuels without compromising the ignition timing to prevent detonation" is complete BS.
There is ZERO reason why a stock LT1 should not run perfectly fine on today's pump fuel. I asked you back earlier in the thread originally to give me the reason why you didn't think it would and I'm still waiting on your answer.
I know I'll never get it from you because there is no answer you can supply.
My L76 at 11:1 runs great on pump fuel.
LT1 cars in my local club (100% stock configured including factory timing specs) run great on pump fuel.
Seems like YOUR LT1 is the other one that won't run on pump fuel. Gee, maybe it's your car or your tuning that is the cause of it.

fact - previously you foolishly suggested to the OP that he needs to run race fuel and also recommended a water injection system. Now you seem to also recommend that the OP should gets his stock heads ported??!!

Fact - the "experts" you are slamming really ARE experts although you don't realize it. Maybe you don't like the fact that they disagree with you but that's only because you are wrong. These experts all have DECADES of real life experience with these cars, some of them most likely longer than you have been alive. You would greatly benefit from taking their information and learning from it rather than arguing with them

Really dude, you need to calm down, learn a bit more, and stop foolishly misleading the OP and others that may be reading this . You are WAY misinformed, and seem to love just throwing money and catalog racing parts to a simple problem. You are also extremely rude in this thread and other threads i've seen you post in to anyone that doesn't agree with what you say in your posts. The problem is that you post incorrect and misleading information and others are simply trying to correct you and you than you attack them and tell them they don't know what they are talking about. Classic defensive moves from someone not wanting to accept his own mistakes.

unless the engine in question was rebuilt at MUCH higher CR and there are other modification that have not been mentioned there is NO reason that the motor should not run great on pump fuel at factory specs and timing settings.
As MikeM already posted this should end up being a SIMPLE solution. Most likely just too much advance coming in from somewhere that needs to be found - could be a messed up curve on the distributor, incorrect timing belt cover so the timing indexing is wrong, or something else causing too much timing.
Also, as I mentioned in my earlier post, since the car is not being used for NCRS judging, the OP will also most likely gain better throttle response, performance, and lower operating temps if he bypasses the TCS system and connects the VA up to full manifold vacuum.

I see from your profile you are a mechanic. If you give your customers at the shop you work at the same advice you give here on the forums I feel sorry for them.

BTW, I entered this thread to help the OP with his problem, not deal with your problems so I'm done with you and will not waste more of my time responding to you, only to the OP.

 

Obviously you work on these cars and I'm guessing you know what causes the micro switch to open and close.
Why don't you think back to when you assembled your tranny and recall how many divots were in the 3-4 shift shaft. If you recall 2 divots then I think you owe someone an apology. If there is only 1 and your car is a 70 I'm fairly certain its the wrong shaft for the car.