4-screw SU's and the coolant-heated manifold

[SteveInOakland]

The car I bought has 4-screw SU's and the '72-type intake manifold with ports for coolant to enter and exit. See thumbnails.

I'm wondering if, for sure, all 4-screw SU's are earlier than the heated models. In other words, is there such a thing as a 4-screw SU with passages for the coolant.

If my carbs lack those passages, is there any point at all to routing coolant into the manifold? The car might in fact run a little cooler if I simply closed off the ports--larger percentage of the total coolant then sitting in the radiator. ??

Thanks for any thoughts. Looked at ZTherapy photos but couldn't quite determine about the 4-screws.

[Arne]

Here's what I think I know. It might not be 100%. but I think I'm close. (This is for US models, don't know about the rest of the world.)

1.) To the best of my knowledge, no 4 screw carbs have the coolant passages in the base of the carb. Only 3 screw.

2.) All 240Z and 260Z manifolds had coolant passages, but only '72-74 manifolds ran coolant to the carbs. The earlier manifolds just ran it between the two manifold halves.

3.) Any round-top SU (3 or 4 screw) will bolt up to any 240Z or 260Z manifold. If there is a coolant passage mismatch, it will not affect the operation of the carb.

4.) But if you have a manifold that runs coolant to the carbs and you are going to use 4 screw carbs (as I did), you should not connect the coolant passages to the manifold. Depending on the insulator blocks used, either the coolant won't flow at all, or you might have a leak.

In my case, I have 4 screw carbs on an E88 manifold. Unlike the early manifolds, the coolant flow must go in the first manifold half, turn 90 degrees and go into the base of the carb, turn 180 degrees and go back into the manifold, turn 90 again to go to the second manifold half, etc. If the carbs don't have the coolant passages, the path is blocked. So I don't have the coolant connected at all on mine.

The picture below (from ZTherapy) compares the two manifolds and shows the coolant passages. If your manifold is from a '72 or later, you probably don't want to connect the coolant.

[kenz240z]

From the 1972 FSM:

"Manifold heat control thermostat

Manifold thermostat is installed on the 1972 models. This device operates in the following manner; that is, the engine is quickly warmed up and then the coolant in the intake manifold and carburetor is kept in constant temperature at various vehicle speeds. This thermostat shuts down when coolant temperature is above 150 degrees F. Then the flow of coolant is stopped in the intake manifold line."

"Temperature control of SU-type carburetor by engine coolant

On the SU-type carburetor, the flow rate of fuel varies with a change in temperature of gasoline which is largely affected by viscosity variations. The carburetor is heated by engine coolant. This design not only keeps a nearly constant flow rate regardless of ambient temperature variations, but also contributes much to improved atomization into the air intake system. A small-sized thermostat was employed to control the flow of coolant to prevent deterioration under hot weather."

As Arne mentioned, if the carbs don't have the coolant passage, then the coolant won't flow through the intake manifold.

The FSM shows the coolant flow from the thermostat housing into the intake manifold and into the front carb, back into the manifold and into the second carb, back into the manifold and through the thermostat, then back into the coolant pipe & hoses that route to water inlet at the front of the engine on the right side. So, with the 4 screw carbs, the coolant flow would be blocked at the front carb.

[Arne]

...So, with the 4 screw carbs, the coolant flow would be blocked at the front carb.

Good explanation, Ken. But it might be worse than just blocking the coolant flow. Depending on the insulator blocks used, the passage might not be blocked of, and might allow coolant to leak all over the ground!

[SteveInOakland]

Terrific answers from you both, many thanks for your help.

I'm still having trouble determining which manifold I've got. On ZTherapy's photo, one part of the linkage (?) is marked "welded" on the 70-71. But which part? Also the "narrow pad" caption doesn't clear anything up. --Does *only* the '70-71 have the vacuum tap for the brake booster where that's shown? Because my manifold has that. You can see it at the upper right of the first photo, black hose with wire-type hoseclamp, disappearing out the right of the frame.

Thanks again for the help.

Steve

[26th-Z]

From what I can see, you have the earlier manifold.

[montoya_fan01]

The coolant can and will flow through the "manifold" (both halves) whether the carbs have a coolant passage or not. Look at the manifold and you will see that even the manifold at the bottom of the ZTherapy picture is set up to accept engine coolant flowing through it. Note the water fitting at the front and rear of the manifold assembly, and the water pipe connecting the two manifold castings. Also 1971 Z's (I'm not sure about 1970 Z's) had the water pipe that loops around the back of the engine and connects to the back of the manifold by way of a short hose.

However the early manifold doesn't have the holes in the carb mounting surface to allow water into the carbs themselves.

Prior to 72 I don't believe that there was a thermostat to control the water flow through the manifold.

BTW, looking at SteveinOakland's photo's, it appears (from the brake system valve mounted on the firewall) that he has a very late 72, or 73 240Z chassis (70,71, and early 72 Z's didn't have that), but the balance tube shown is a very early one with the vac/brake booster fitting at the rear, where 72/73 (at least) Z's had balance tubes where the vac/brake booster fitting was on top of the balance tube. Also his intake manifold doesn't have the thermostat which would control water flow through the manifold which leads me to believe that his intake/balance tube is from a 70/early 71 engine.

In regard to the "Welded" notation in the ZT picture, It's a pity that the picture is no longer shown with the original explanation that accompanied it when Scott Bruning (original owner of ZTherapy) posted it on the original ZTwebsite. At that time there was text which described in detail the differences in the linkage and what was welded and what wasn't welded.

[a7dz]

I agree with Chris. It is the same set up as my 70. The engine serial number and the data plate Engine number still match on my car. If you would like a picture of this one PM me and I will email it to you.

[SteveInOakland]

BTW, looking at SteveinOakland's photo's, it appears (from the brake system valve mounted on the firewall) that he has a very late 72, or 73 240Z chassis (70,71, and early 72 Z's didn't have that), but the balance tube shown is a very early one with the vac/brake booster fitting at the rear, where 72/73 (at least) Z's had balance tubes where the vac/brake booster fitting was on top of the balance tube. Also his intake manifold doesn't have the thermostat which would control water flow through the manifold which leads me to believe that his intake/balance tube is from a 70/early 71 engine.

Yes, the car is a '73. It has the original engine (block) but the guy I bought it from installed the 4-screw round-top SUs and the manifold shown. From all the good stuff you guys are telling me, it's clear that he probably bought them together and just bolted the whole assembly onto the head of my car.

Anyway I'm sure you guys are right that it's an early-style manifold. Also, I studied the ZTherapy photos a little further and my heat shield is clearly the earliest type.

Montoyafan, this thing you describe -- "Also 1971 Z's (I'm not sure about 1970 Z's) had the water pipe that loops around the back of the engine and connects to the back of the manifold by way of a short hose" -- that's on my car. You can see the manifold connection, the hose, and the start of the water pipe that runs behind the engine, in my second photo.

Thanks to all for all the help! (Looks like I leave things as-is.)

Steve.