Two 1969 Exhaust Questions

[Bill stephens]

I have a 1969 with a 350 that had headers on it when I bought it. I bought exhaust manifolds on e-bay that were supposed to have come off of a 1968 350 corvette engine. The passenger side manifold had the heat riser valve on it, but I have two questions if anyone knows the answer.

First, the two manifolds are the same height. I thought the passenger side one was supposed to be shorter to accommodate the heat riser. Are the two exhaust pipes leading from the manifold different sizes?

Second, what does the heat riser do anyway?

Thanks in advance for your help.

[Joe L.]

Re: Two 1969 Exhaust Questions

Bill------

Corvette big blocks use different "length" manifolds to "adjust" for the heat riser length, but small blocks do not. The manifolds are the same "length" on either side. The heat riser "length" is accomodated in the forward exhaust pipe configuration.

If by "different sizes" for the exhaust pipe length you mean different configurations ("lengths") to accomodate the heat riser "length", then the answer is yes. They are also different configurations otherwise and are side specific.

ALL 1968 Corvette exhaust systems from 300 hp 327 to L-88 were 2-1/2". ALL 1969 exhaust systems, from 300 hp 350 to ZL-1 were 2". However, all 1966+ small block manifolds had a 2" outlet. So, for 1968, there was originally a swaged reduction section on each forward exhaust pipe to effect the change from 2" outlet to 2-1/2" exhaust pipe.

For 1969 big blocks, there was a reduction section to effect the change from 2-1/2" manifold outlet to the 2" exhaust pipe size. The 1969 big block heat riser was also unique to that year and exhaust pipe configuration.

As far as your question about the heat riser, the purpose of that device is to aid in engine warm-up and choke operation. Under cold, low RPM conditions, the heat riser remains closed, forcing exhaust heat from the right side cylinder bank through the intake manifold crossover passage and out through the left side exhaust pipe. When the engine warms up, a thermostatic spring on the heat riser is supposed to open the valve and allow exhaust gasses to flow out through the right side pipe. Also, under higher rpm (and exhaust pressure) conditions, regardless of engine temperature, the valve is "forced" open and exhaust gasses exit through the right side.

[Bill Stephens]

Thanks

Thanks Joe. That's a big help.

[Richard D.]

Re: Two 1969 Exhaust Questions

Joe:

What benefit, if any, would be derived by wiring the heat riser in the open position as others have advised in the forum?

My 68 has an electric choke (SB327/350).

Thanks,

Rich

[Joe L.]

Re: Two 1969 Exhaust Questions

Richard------

The benefit is "full time" dual exhaust. The other benefit is MUCH longer life for the right side muffler.

On the negative side, you will get slower engine warm-up, especially noticable if you operate in cold ambient conditions, and the choke may not operate as well. In your case, with an electric choke this aspect is irrelevent.