Difference between Std. LS9 350 and MD LS8 350 for ‘69?
 

My 69 CST20 came factory equipped with the optional 350 small block but is not the typical LS9. It shows LS8 350 MD. Wondering if that stands for medium duty (?) The block stamped code on the passenger side face is WH Which for whatever reason is not listed in the ID guide I have. Do you any of you have any idea what is different about the LS8. From the standard 350 in 1969?

Steve weim55 Colorado

Re: Difference between Std. LS9 350 and MD LS8 350 for ‘69?
 

Good question. Only other place I've seen (or noticed) an LS8 is on a Canadian-built K20. So not medium duty - but I would have guessed the exact same as you, MD=Medium Duty. Hmmm.

Re: Difference between Std. LS9 350 and MD LS8 350 for ‘69?
 

Suffix code WH shows up here.
http://www.nastyz28.com/gm-chevy-cod...-suffix-10.php

Re: Difference between Std. LS9 350 and MD LS8 350 for ‘69?
 

Found this over at the GM heritage site. Still couldn’t find any information as to the actual differences but this does show the LS8 as a heavy duty version of the standard 350. $27 more for the option over the standard 350 as well. Hmmmm.....

Steve weim55 Colorado

Re: Difference between Std. LS9 350 and MD LS8 350 for ‘69?
 

Maybe it’s for 4 bolt mains.

Re: Difference between Std. LS9 350 and MD LS8 350 for ‘69?
 

My guess would be the 4 bolt 010/020 block with a nodular crank. I've found these several times coming out of "heavy" trucks.

Re: Difference between Std. LS9 350 and MD LS8 350 for ‘69?
 

Heavy duty engines usually have lower compression, smaller valves and a cam designed for lower rpm torque. This engine was probably standard in a C 50 or C 60. More is known on the 366 vs. the 396.

Re: Difference between Std. LS9 350 and MD LS8 350 for ‘69?
 

She has a bad exhaust valve on #1 so I'll be pulling the engine to repair and reseal. I'll report back my findings once I get her apart.......

Steve weim55 Colorado

Re: Difference between Std. LS9 350 and MD LS8 350 for ‘69?
 

It should be the 250hp engine. Let us know about the heads. Your truck is a manual, right?

Re: Difference between Std. LS9 350 and MD LS8 350 for ‘69?
 

Factory turbo 400 automatic truck.

Steve weim55 Colorado

Re: Difference between Std. LS9 350 and MD LS8 350 for ‘69?
 

Found a little literature for '69, LS 8 was the engine used in C 40 C 50's. SPID shows MD ( medium duty) , GM option list shows HD ( heavy duty ). Literature lists Medium Duty. 2 against 1, same engine, so 350 medium duty it is! The only heavy duty Chevy V8's I'm aware of are the tall deck 366 and 427's.

Re: Difference between Std. LS9 350 and MD LS8 350 for ‘69?
 

Another added bit of information, thanks for posting that!

Steve weim55 Colorado

Re: Difference between Std. LS9 350 and MD LS8 350 for ‘69?
 

My interpretation of the words, "light duty", "medium duty", and "heavy duty" are, and always have been...:
Light duty always refers to 1/2, 3/4 and 1 ton pickup trucks. (C-10, C-20, C-30)
Medium duty refers to C-40 and above. I do not know where, what's considered "heavy" duty begins, maybe above C-70???

A medium duty engine in a light duty pickup, doesn't sound proper. Even back then emissions specs were made different for the weight ratings of these trucks.

I'm guessing GM's use of "medium" in that engine description is unique to the light duty engines...

In reference to the original post, I would guess one or more things making the engines different...valve sizes, different cams, possible better valve seats, maybe even a steel crank (I doubt it though...)

The text that factorystock gaave us, I know about...somewhat...the 350 medium duty (as in medium duty truck) engine came with a 2 Bbl Rochester carb. not the ordinary 2 bbl though. It was a special governed carb that would not allow over-revving... WHY??? some of those 350's turned 4000-4500 going down the highway (they had to to make the power needed to pull a heavy truck around). One slip of the throttle pedal and one of those engines would be done, if not governed...

Re: Difference between Std. LS9 350 and MD LS8 350 for ‘69?
 

I generally agree. IMO, medium to heavy duty starts about where the CDL kicks in 26,000Lb GVWR and above. Trucks with air brakes and or dual drive ( 10 wheelers) or tractor trailers. In Chevy trucks C 70 and above, and or trucks with 366-427's are in the heavy duty class.

Re: Difference between Std. LS9 350 and MD LS8 350 for ‘69?
 

ok ls8 is a high torque eng made mostly for bigger trucks but put in c10 c20 and c30 trucks as kind of a tow package option most are with manual trand and air injection also 4bbl carbs in the bigger trucks had govenor and low compression smaller trucks the compression was put up to 9 to 1 i have a book with some info will try to put pictures later also my dad worked for chevy in the lat 60and early 70 and gave me some info unfortunately he is gone now
Hope this helps
I have a 69 CST ls8 3 speed 3:73 posi and fully loaded it was purchased to tow a large trailer i know all the owners personally except the first owner who was a working for a company who purchased it for him as a work truck must of liked him alot as it has all the comfort stuff bucket seats AC ps pb and so on very expensive for a work truck in the day

Re: Difference between Std. LS9 350 and MD LS8 350 for ‘69?
 

The medium duty engines generally had steel cranks, forged pistons,& sodium filled exhaust valves with rotators.

Re: Difference between Std. LS9 350 and MD LS8 350 for ‘69?
 

You gotta check out some of the other vehicle history packages. The El Camino one for some reason usually has all the engine stuff.

Re: Difference between Std. LS9 350 and MD LS8 350 for ‘69?
 

HAD an Ls9 in my 69 GMC 30 series motorhome (originally) had 4 bolt mains and a forged crank--don't know about the pistons or valves, I went with a 454

Re: Difference between Std. LS9 350 and MD LS8 350 for ‘69?
 

The only difference I see per the sheets I posted are that LS8 has valve rotators like the Medium Duty C40/50 engines. All the other engines do not have rotators. Looks like LS8 has a cast crank too.

Re: Difference between Std. LS9 350 and MD LS8 350 for ‘69?
 

Looks like the LS-8 has 8.0-1 compression with cast pistons also.
and a governed at 4000 rpm 2 barrel carb.

The 366, and 427 Tall deck Medium engines had a Holley 4 barrel governed at 3800 rpm.
All the Best
Dirk

Re: Difference between Std. LS9 350 and MD LS8 350 for ‘69?
 

Same info in the '69 truck info kit (pages 60 and 61). The footnotes at the bottom seem to show that emissions is likely the biggest difference. The LS-9 has AIR whereas the LS-8 has nothing or CCS. Of course, all have a PCV. My '69 3/4 ton only has PCV and is on the SPID as a LS8.

Also, see page 54 for performance data comparisons.
https://www.gmheritagecenter.com/doc...olet-Truck.pdf

Re: Difference between Std. LS9 350 and MD LS8 350 for ‘69?
 

The footnote shows that one is for series 40 and 50 trucks.

Re: Difference between Std. LS9 350 and MD LS8 350 for ‘69?
 

I see. I usually only root around the '71 stuff and for that year they left stuff out of the truck info pack. The note at the end of page 54 specifics that CCS is used in Automatic, and AIR is used for manual transmission applications. CCS is merely a different carb calibration, distributor curve, and atleast for 1971 a thermostatic air cleaner. Because you have M49, you lucked out and didn't get a smog pump. Page 57 also mentions you should have those valve rotators. Pretty cool I think.

Re: Difference between Std. LS9 350 and MD LS8 350 for ‘69?
 

Not to change the subject, but, the GMC 305 V6 was also available with the valve rotator option for pickups. Low compression and heavy duty valves with rotators were typically standard on all heavy duty gas vehicles, regardless of make.