> Dave, are you spinning on blacktop or gravel???
I can spin them on blacktop. Granted I think it has much to do with how the
GMC is balanced. Unlike a front wheel drive car, the GMC is heavier in the
rear than in the front.
>From your later posts
>your upgrades are HEI ,Injection and headers. How did each effect
>performance, did you do each seperate or altogether?
I did them all separately. My goal was just to get the thing to start
properly. I like an engine that is well tuned so it will start without much
cranking. Just a flick of the key and off it goes. Unfortunately when I
first got my GMC it was far from quick starting. When it was cold it would
start reliably with a bit of cranking but when hot, there where times when
I thought it would never start.
I tuned up the conventional ignition but that didn't help much. So the
first project was the HEI. I didn't know much about the GMC other than the
fact that the drive train was the same as the olds Toronado. So I just got
a rebuilt distributor for the olds and installed it. It didn't help the
starting problem much but it did make it knock with regular gas. After
playing with the vacuum advance (and using premium gas) it works OK. I am
in the process of recurving the centrifugal advance and getting the correct
vacuum advance so maybe I can go back to using regular fuel.
I put headers on it when the manifolds warped. I talked to a few people and
learned that the manifolds are a continuing problem and that headers where
a good solution. So I installed Thorly headers. When I did the headers I
also did the exhaust system. I have a dual muffler system with a crossover
after the mufflers connected to a single exhaust pipe. Pretty much stock
for the GMC. I installed the headers but had a shop do the exhaust system.
I didn't have a welder at the time and the installation required some
welding.
After getting on the GMCnet I have since learned that the manifold problem
could have probably been avoided by getting the carb jetted correctly. But,
at the time, I didn't know about that problem. Although I did suspect that
there where problems with the carb due to the fact that it still didn't
start the way I like it to.
A couple years ago I started having more carb related problems. In addition
to poor starting I was having hesitation and getting a strong fuel smell
after shutting down. After exploring having the carb rebuilt and adjusted
properly I decided that, for a little (OK a lot) more than the cost of
having someone service and adjust the carb, I could install the EFI system.
It was a hard decision but I decided to go for it.
I chose the Howell system because its mostly based on stock GM truck parts.
I did the installation myself. Its relatively easy if you have a little
experience. But then I feel much more confident installing and servicing
the EFI system than I do working on carburetors.
I only had two problems with the EFI system. Both where due to the fact
that I have headers. The headers allow the exhaust to cool before reaching
the oxygen sensor so I had to change my O2 sensor to a heated O2 sensor.
The other problem was related to the mixture at low power settings. Howell
sent me a new PROM and that problem was solved. Since then it works great.
The EFI system solved my starting problems and the engine runs better than
it ever has. Idles smooth and has noticeably more torque.
>about brands and sources.
- - The HEI distributor is a stock GM part. If I had to do it over again I
would get the right one from Cinnabar ($275 + $150 core) or Bob Lamey ($200
+ $? core).
- - The headers are Thorly headers. Cant remember the cost.
- - The EFI system is from Howell engineering ($1200)
>Did you increase exhaust size also?
No. I used a stock exhaust system and had the header flanges welded to the
pipe going into each muffler.
>Did your
>injection require an exhaust crossover?
No, the O2 sensor in the right header. I welded the ring to the header pipe
just past the point where all four tubes converge. It only picks up the
exhaust from the right four cylinders.
>Was it all worth it and how much
>was it worth?
Well, now that I know more maybe I could have saved money by having the
carb rebuilt and jetted properly in the beginning. That probably would have
solved the starting problems and may have solved the warping manifold
problem. I think I would have done the HEI ignition anyway. Just to avoid
time wasted doing tune-ups. But the thing I like most about the EFI system
is the fact that it is closed loop. The mixture is constantly adjusted for
altitude, temperature, fuel speed and load. Its a lot of $ but if you plan
on keeping your motorhome for a long time I would say that these upgrades
are worth it.
I see that you too have a 73 Sequoia. I don't know if you are aware of the
fact that there was a recall on the 73's for upgrades to the rear brakes
and bearings. If this has not been done to your Sequoia then GM will still
do it for you. Its a major safety issue so check to be sure its been done.
Another potential safety problem is the fact that the 73's where shipped
with bias ply tires. The wheels where not strong enough for radial tires.
They will crack and split if you use radial tires on them (as mine did). So
check to see that you have the correct wheels if you are using radial tires.
Dave
73 Sequoia
