DansYellow66

A little ice and sleet last night so I'm stuck at home and mulling over Corvette issues and questions.

Anyone running the LS-7/late L-88 GM aluminum heads (3946074) and vacuum advance? Several years ago I installed a VC-1810 vacuum can on my car and it helped the idle quality and coolant temperature 100%. However, with 36 deg total centrifugal and initial advance, I developed an over-advanced timing miss at highway speeds (which aren't that high with 4.11 gears). Pretty easy to diagnose - plug the vacuum line and the motor runs clean - hook up the vacuum line and it lightly misses/bucks all the way down the highway. So far to limit the amount of vacuum advance I've put a rubber bushiing on the vacuum advance rod. Then I put the biggest piece of aluminum tubing over the rubber that would fit in the vacuum can slot. But, I still had missing. So now I've fabricated a little bracket from sheet aluminum that blocks off part of the slot as shown below.




This helped and the missing is reduced but not gone. So I made up another bracket today that blocks even more of the vacuum advance slot. I've blocked at least half of the vacuum advance by now I would guess. Due to the weather I haven't tried it out again but by the time I've done all of this I will have reduced the total vacuum advance to just about nil. Apparently this thing won't tolerate very much advance at cruise - I'm guessing maybe 42 to 46 degrees. I'll have to check how much vacuum advance I'm getting now when the weather clears up.

Has anyone else who has run these heads had this problem? I'm wondering if this is characteristic of the open chamber head design? I guess the other issue could be the cam. This engine was machined and assembled back in the mid-80s just prior to my buying the car. It's got Ross pistons, (supposedly 12.1) and an old Cam Dynamics #22870 solid cam with .579/.605 lift and 302/312 duration. Any thoughts on this?

Thanks

Dan

L78racer

you can also buy an adjustable can from accel. works great.

DansYellow66

I would like to but my engine only produces 7 to 8 inches of vacuum at idle and the VC-1810 can is the only one I've heard of that will work that low. Have you heard of an adjustable can that will work with low vacuum?

Thanks

Dan

L78racer

nope.

Ironcross

.

Why run a VC anyway. Your close to L88 specs. The distributor probably has 24 degrees built in so run 12-14 initial lead for a total 36-38. But the engine requires 100+ octane. A touch of the gas pedal and your full advance anyway....

PS, LS7`s use cast iron open chamber heads not aluminum

midyearvette

hey dan...you ned to limit your centrifugal advance with a bushing and plug the vacuum and do not use it. try setting your timing at 40 total at first and if too much back it down 2 degrees at a time until it feels right. by doing this you will run high initial timing probably around 14-16 at idle which will let the mill run clean and cool at idle and cruise. with vacuum hooked up, your cruise signal is too high, you can try an adjustable unit but remember at wot it is useless. lots of folks are reluctant to try this but it works great. by limiting the mechanical advance, you can run more constant initial, without being a slave to vacuum signals. this tuning trick is taken right from a chevy power book in the past ,and it still works great today especially with un leaded fuels. the trick is not to run so much initial as to over load the starter with a warm engine. that's why you need to play with a bushing. you won't be disappointed. if indeed you are running that much compression,your total timing may be around only 34-36 btdc. you may have to run a racing fuel anyway. your compression matches that monster cam duration, im assuming duration is around 240-250 at 50 0ff??? .....happy motoring!!!

DansYellow66

Well - my motor is pretty much patterend after an L-88. Has the big bolt rods, L-88 intake, heads, etc. Had an Accell, all mechanical advance distributor in it when I bought it and for all-out performance it was OK. The guy who pieced the motor together probably knew what he was doing. To get a little more original GM parts appearance I switched to a 427/400 HP mag pulse distributor I picked up. I do have a bushing in it to limit centrifugal advance but from memory I couldn't get it below 12 deg x 2 + 24 mech advance. Back then the stock GM vacuum can didn't really activate due to the low motor vacuum and after reading about the advantages of the VC1810 cam, especially with coolant temperatures, I had to give it a try. Seeing that drop on my temperature gage makes it very difficult for me now to go back to all mechanical advance. Plus, the motor will actually run on fast idle when warming up now. Before it was a mother trying to get the thing warmed up and running in cool weather.

I probably have compromised and solved the problem by limiting the amount of vacuum advance - have to try the car out again to make sure. Even an extra 6 or 7 degrees of vacuum advance at idle civilizes the motor a noticeable amount and helps temperatures. I was just wondering if these particular heads had a pattern of tolerating less total advance as I don't hear other BB owners complaining about this very noticeable over-advanced missing at cruise?

Thanks

Dan

DansYellow66

I was wrong again. They are correct for a 1970 LS-6 & 69 L-88. I think I'm dyslexic and reverse the LS-6 and 7 from time to time.

Ironcross

Yep, and you also foiled my attempt at finding a mistake by apologizing for a slip of your 6th and 7th fingers. ...boo. I do it all the time......
.

67pete

L88 didn't use vacuum advance.

muncieman

I think this might work, I sent a email to Accell and ask the operating range of the unit. When they reapond I will post it.

aworks

This is the way I ran my L88. It never ran over 185. The car is in Calif. now and still runs cool. No need for VC. you can just see the Winters mark in this pic. I just left the VC there just like GM did. No hose going to it. And yes you need 100 or better octane. This was a real L88.

Harps

I had a set on my 427 when the motor was rebuilt, but I never could get a good seal with the intake because my set of heads had some bad repair jobs and bad hellicoils in them.

This was how the motor was before it went back in the car:



I had the intake/head to issue going on after breakin and never really got to the point of tuning the motor with my 6074 heads. I also was dealt a wiped cam lobe, so during the new cam install I opted to change out the heads and install a new set of Brodix.



The original 6074's are tucked away for another time.

Even after getting the motor tuned with the new cam and heads, I never really did get it to idle and run right. Then I replaced the stock 435 vacum can and installed a vc-1810. That seemed to help with the idle.

Ironcross

Well now that makes two of us. I guess people are just afraid of them and nearly every one is not real anyway. And some are close as the one on this thread. I dont see any reason to use some L88 parts and not run it as one. I used one for 28,000+ miles and nearly all of them were street and freeway driven. Dependable, 'Hell yes' I put two of them in a 18 foot Hallet drag {ski} boat.

And here's the proof, a pair of real L88`s in that 18 foot Canoe...




.

DansYellow66

That was pretty much my experience too. My motor would start and run OK after warming up, but it wasn't much fun in and around the neighborhood or town. Changing the distributor out again and tuning for 20 deg mechanical advance and 16 to 17 deg initial might make it more liveable. Each to their own, but I tried it both ways and for my driving I do like the effects on the motor of some vacuum advance.

With the Bordrix heads and VC1810 can have you noticed any problems with missing from too much advance while cruising on the highway? I assume the Bordrix heads are an open chamber design too, unless you also changed pistons.

Dan

Harps

I've only put about 400 miles on her, and I have had one problem with some missing at highway speeds. The plugs had fouled on her and I swapped them out. I wanted to tune on it more before it went to the body shop, then I ran out of time. I may be revisiting it all again here when I get the car back from the body shop in Feb.

Yeah, no changing of the pistons, and they are an open chamber design too. The Brodix heads were really nice since they keep your stock exhaust manifold location.