Well recently i got the base manifold off my TPI engine. I am planning on replacing all gaskets and several other things while it is apart. Just was wondering why the rear coolant passageway is so restricted by the gasket. Even my shop manual references it. Check out my picture. The small hole in the metal restrictor is like 1/8"!! Why do that, seems like that part of the engine would not get good cooling? I know there is a bypass tube going to the top rear intake.
Also thinking of doing some porting, and gasket matching. I would like to open the casting around the injectors. Seems like i could get better fuel flowing if i open them ports up. What do you think?
Thanks.

 

Sugar, posted this topic in the wrong forum. Can someone move it to the tech/performance forum? I dont know how to do that.....or if i am allowed.
Thanks!

 

This thread will just start a huge debate. Let's just say some use them, and some don't. When you consider Aluminum headed cars were the only TPI to even have this crossover passage in the rear it's an anomaly. I would keep an iron piece (restrictor plate) as far away from my aluminum as possible, and the flow across here is minimal anyway.

 

well i dont want to edit it, just move it! i looked at the edit option, but nothing there to move it. what steps are you talking about? i could copy and paste it into a new thread, then delete i suppose. i just thought maybe an admin could move it for me, but they dont look at all threads i am sure.

 

huge debate....hmmmm. well i do have aluminum heads for sure. my IROC 350 TPI does not, and it doesnt have this restrictor. but after looking at my pictures, i dont see the crossover in the intake. why would you say it is an anomaly? the bypass tube coming over the top rear of the intake, has a fitting right over this passage, and you can see the hole where the coolant would flow. must be there for a reason. GM dont take a poop without a reason.....

I thought of drilling out the restrictor hole to about 1/4" at least. once i get my new gaskets.

 

My cast iron headed L98 has that passage in the intake manifold. It also has the weird EGR passage that connects to the passenger side head AND the EGR pipe to the exhaust manifold. I was wondering if I could eliminate the EGR pipe and use the 1985 EGR valve (the EGR temperature switch mounts to it). I don't know if this would pass California's smog inspection though (assuming they know what to look for -- I don't know if the smog test computer tells them about the EGR pipe).

I think the early '86s were an anomaly. We were told back in 1985 that '86s would have the aluminum heads. THEN we were told that the castings had problems with cracks and they would delay introduction of the aluminum heads. The aluminum heads didn't appear in production cars until around March or April of 1986. So we ended up with this weird hybrid that is half cast iron and half aluminum.

On top of all that, I heard back then that the aluminum heads were 45 pounds lighter than the cast iron heads. The front spring was calibrated based on the aluminum heads and the cars with cast iron heads sit too low in the front because the front spring rate is wrong. There was talk back then of a class action lawsuit to force GM to replace the front springs on all 1986 Corvettes with cast iron heads, but nothing ever came of it.

 

Yes, you do have a weird one. As long as your cast iron heads have the heat riser port you could use the 1005 intake and the egr would function correctly. I don't think even smog check 2 tests for egr operation, and it's a simply visual inspection.
Honestly, I just assumed they used the older intake on yours, but it does make sense that it simplified production to have yours use the aluminum headed parts.

Mike, i guess Cliff here has disproven you GM does everything with research claim?

 

Interesting history lesson. I love learning about our cars. I did know that the early C4s had iron heads, but my C4 book talks about 1987 as being the first official year with aluminum heads. But there were some late 1986 years with the alum heads. Maybe production of the heads vs the intakes was not inline, so some iron heads got the newer intake. I just want to know why they have this nice coolant crossover, then restrict the flow with this tiny 1/8" hole in the gasket.
What about drilling it out a bit, on the new gasket, for more coolant flow?

 

Much debated over the years

http://forums.corvetteforum.com/1565438212-post17.html

http://forums.corvetteforum.com/c4-t...-hose-why.html

Bottom Pg 2
http://www.hotrodlane.cc/PDFFILES/TPIStory.pdf

 

I haven't looked at the smog check computer screen for at least 5 years. I do know that they had a check list of stuff to look at and the technician did the visual inspection and then went back to the computer and checked off everything on the list.

I put a new front Y-pipe on last year. It's a Magnaflow (the only one approved for California). The tech spent a LONG time looking at the driver's side cat (from above). The smog approval numbers are on the bottom, so I guess he finally decided that he wasn't going to work that hard and went on.

I did a smog check about 15 years ago and the tech DID check the EGR valve by lifting it with a distributor wrench handle. He told me that most techs don't check it and I have never seen any other tech do this.

 

All '86 convertibles have the aluminum heads. When the convertibles went into production they started putting aluminum heads on the coupes also.

When I had a head gasket blowout several years ago the intake manifold gaskets came with metal plugs to put in the rear holes. I left them out.

When I had the second head gasket blowout about a month ago, I bought a 1985 Corvette head gasket set because you sometimes have to fool the web site and I wanted to make sure I got the correct gaskets for a cast iron head. The rear passage was NOT blocked.

It turns out that because of the rear water passage and the steam tube that the rear passage needs the restriction. The FSM specifically mentions that the gasket must have the restriction. But on the other hand, I think I mentioned earlier that I believe the shop manual was written as if all '86s had the aluminum heads.

It got a little weird on the Fel-Pro web site, as they showed BOTH the cast iron and aluminum head intake manifold gaskets with the small holes. The strange part for me was that if you looked at the full head gasket set the cast iron set did NOT have the restrictions in the intake manifold gaskets but the aluminum head set DID have the restrictions.

So -- small holes or not? I don't think anybody knows for sure. I haven't read the links yet. I put in the gaskets with the small holes this time. I think they affect the way the coolant flows through the heads.

 

Good info, and interesting reading. I just like to understand some of these things that are different with our C4s. I am now thinking i will just leave things stock, the way it was. I need to order new gaskets soon. We'll see if they come with the restictors.

 

I dont think it really matters which heads you have, iron or AL. It matters what intake you have, whether or not it has the rear crossover. If no crossover, then no coolant flows thru, so it doesnt matter if your gaskets have the restrictor or not. My '88 iron headed IROC, has the ports in the heads for the coolant, but the intake does not have the crossover. But i did read above that this restricted rear passage system was unique to only Corvettes.

 

That's what I was thinking. I bought an extra intake manifold gasket set to get the ones with the small holes.

The last blown head gasket I had was only 30K miles ago and I don't want to go through that again. On the other hand, I'm retired now and even though I drive this car every day the mileage is very low because I live close to all the places I need to go. It could take a LONG time to put on another 30K miles.

 

So whaasssup with these blown head gaskets? A known problem? Iron heads and Alum intake? I bought my '88 IROC in '92, put on it like 48000 miles, and no head gasket issues...

 

It is a known issue. For some reason, it's always #7 that goes out. My theory is that the coolant is relatively stagnant back there and it gets hot enough to cause problems. I plugged the hole where the problem occurs. See my thread on this:

http://forums.corvetteforum.com/c4-t...umber-7-a.html

 

Interesting story......so, after all that are you recommending to stick with the stock restrictor intake gaskets, if you have alum heads and the crossover in the rear of the intake? Do you think the first time you had the problem was because you did not use these gaskets? Or was it possible the coolant breaking down?

 

Fel-Pro said it was caused by electrolysis, which was caused by not changing the coolant often enough. They said the coolant turns acidic after a while and then the electrolysis starts. I can't say if that was true or not. I will be using CoolTrak pH strips in the future to monitor the pH of my coolant.

I do know that putting Dexcool in my system without thoroughly flushing out the green stuff caused a rusty looking muddy mess. That happened back in the '90s and I didn't completely recover from the mess until about 6 months ago when I did a really thorough flush. I drained and replaced the coolant about 10 times by draining, filling, driving, draining, etc. until the coolant was completely clear.

 

I believe it was a " what if ,just in case measure " .
Alum heads where relatively exotic back in '86 and the GM engineers were taking no chances
with potential hot spots

Few aftermarket intakes have a rear crossover and many have fitted them to C4 alum heads without a rash of cooling problems being reported

 

Well that could be the cause of your failure. Changing your coolant out often can only be a good thing. and my mechanic said not a good idea to change coolants, green to the Dexcool or vice versa. Just too hard to clean out all coolant from one or the other. Just stick to what it came with and game over. The green coolant will still be available for long time.