Intake Gasket//Exhaust Crossover Passage

[Joe C.]

I have decided to replace the intake manifold gaskets on my 1965 L76, because of a pesky leak near the draft tube. The rear transverse gasket probably slipped during installation. Any suggestions on adhesives that I should use on the front and rear transverse rubber seals, to prevent them from creeping in the future. This is a "correct" gasket set, with tabs. How about laying a very thin bead of RTV black between the seals and the block, and then letting it cure completely, to actually cement the rubbers in place before installing the manifold.

While the manifold is off, does anyone think it is a good idea to block off the exhaust crossover. The car will be driven occasionally, in the summer. The motor now has an OCCASIONAL stumble/pop thru carb on sharp acceleration. The accelerator pump works fine, and is adjusted correctly (.010" @ WOT, plunger fully depressed), so I am thinking a slightly lean condition. Will blocking the crossover keep the manifold cooler, and enrich the fuel mixture? What is the downside to doing this. The heat riser is new, works fine, and I am not ready to tie it open just yet.

As always, thanks in advance.

Joe

[Clem Z.]

Re: Intake Gasket//Exhaust Crossover Passage

i would use contact cement if you use the rubber end seals. blocking the heat riser cross over will make the fuel mixture denser but you will lose your choke heat so you must disconnect the choke. check to make sure that your carb has the correct power valve because one that opens too later will cause your problem. post the list # from the choke housing and i will post the specs for your carb.

[Joe L.]

Re: Intake Gasket//Exhaust Crossover Passage

Joe-----

I would use the contact cement that clem suggests if you want to retain the manifold end seals in place. It will work far better than RTV since the contact cement is "sticky" and the RTV decidedly not so. 3M Super Weatherstrip Cement, which has MANY other automotive uses, is the perfect cement for holding the seals in place. You DO NEED to use a daub of RTV on the ends of the seals, where the side gasket tabs interface with the seals. Actually, as I've stated many times before, replacing the end seals with RTV is the best solution if oil leak prevention is your first priority. Oil leaks cost judging points just like end seals without the "tabs" or "GM" markings. Oil leaks are also very gauche whereas the lack of "tabs" or "GM" logos is not so. Pick your poison, though.

I do not recommend blocking the exhaust heat crossover in the manifold. As clem says, you will lose your choke. Since you still need a choke, even in temperate weather conditions, you will have to convert to a manual or electric choke. I recommend neither. Also, if you block the exhaust crossover, then you MUST wire open the heat riser valve or remove it and replace with a spacer. Otherwise, when the heat riser valve is closed, the right side exhaust "will have no place to go". That creates problems.

[John H.]

Re: Intake Gasket//Exhaust Crossover Passage

Joe -

Your '65 L-76 doesn't use a remote choke thermostat (it uses the hot air tube from the exhaust manifold to the choke housing), so blocking the crossover won't affect your choke operation. But, it will affect the first 10-15 minutes of cold weather operation due to lack of pre-heat under the intake plenum until the manifold heats up, and (as Joe L. points out), if your heat riser sticks closed with the crossover blocked, you have real problems. I'd leave the crossover open and just make sure your heat riser is operating freely, or wire it open. Also, as a personal preference, I use RTV instead of the intake end seals; I can't stand oil leaks, and would rather lose a point or two than have to contend with oil dribbles from inadequate seals.

[Joe L.]

Re: Intake Gasket//Exhaust Crossover Passage

John-----

Yes, you're absolutely correct. I was mistakenly responding thinking the question was about a 67. There seems to be so many 67 questions, that whenever I'm responding to a mid-year question, I guess that becomes my "point of reference".

[george romano]

Yellowing with age Joe? *NM*

[Joe L.]

Re: Yellowing with age Joe?

George-----

Without a doubt!

[Joe C.]

Thanks Clem, Joe, John......

.....I will take your good advice, and leave the crossover open, and use contact cement to hold the end seals fast, with RTV in the corners only. When the carb is off, I'll go inside and recheck the PV and PV orifice as per Clem's suggestion of last month. It's great to have unanimity---makes the choice that much easier.

Joe

[Michael Jones]

Re: Intake Gasket//Exhaust Crossover Passage

Another reason you don't want to block the exhaust crossover is:

I did it on my '57. The car ran OK but it completely changed the exhaust tone which I really always enjoyed. It became flat and sounded like a Renault 4 cylinder!

I changed back to the stock type gaskets and the good tone returned!

Mike

[Clem Z.]

Re: Intake Gasket//Exhaust Crossover Passage

the change in tone was why we did it in the early 50s to flat head fords,gave the dual pipes a rapping sound. it took a copper penney to block the cross over. i do not think i have owned a performance car with a aluminum intake that i have not blocked the crossover. my 68 Z-28 had the carb air intake plumbed into the air intake area in front of the windshield and when i would start the car up in cold weather with snow on that area and it would backfire thru the carb the snow would jump up on the windshield and scare who ever was setting in the passenger.

[Duke W.]

Re: Intake Gasket//Exhaust Crossover Passage

The crossover means that all eight cylinder exhaust pulses appear in each
pipe. That's why C2 FI engines have a more gutteral tone. No heat riser crossover and no exhaust system crossover means an uneven four cylinder beat - 270-180-90-180 - in each pipe, 90 degrees out of phase.

Exhaust crossovers generally make the sound mellower because it raises the fundamental frequency, which is more pleasant to the human ear.

Duke

[Clem Z.]

Re: Intake Gasket//Exhaust Crossover Passage

that is why the C-5s have the H pipe and they have a washer in the cross pipe which on some years has a different size hole and you get a different loudness to the exhaust. my 97 was quiet my 99 was louder and my 02 is quiet like my 97. some guys have taken out the H pipe and install a X pipe which gives a different tone. did not some of the old FI cars have a crossover pipe under the car because they had no heat riser crossover?

[Duke W.]

Re: Intake Gasket//Exhaust Crossover Passage

Solid axle cars have a crossover that routes in front of the engine as I recall, but I don't know what years or engine options it applies to. I think it may have been on both FI and some carbureted engines.

Maybe one of the solid axle experts can fill us in.

I believe virtually all modern V8s have exhaust crossovers because they don't need heat risers with modern EFI induction systems, and it would be tougher to meet noise standards without the crossover. Crossovers are also supposed to be worth some power either with or without mufflers.

Duke