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GMC V6 and V12 Engines Engine repair and rebuilding

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  #1  
Old November 16th, 2013, 07:18 PM
hjennings hjennings is offline
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Default Forced Induction

There has been talk about it, but has anyone actually supercharged or turbocharged one of these GMC V6 engines? Low compression, high torque, and lacking on the top end.... seems like a combo that is begging or a low buck turbo application.
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Old November 18th, 2013, 01:06 AM
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Default Re: Forced Induction

Quote:
Originally Posted by hjennings View Post
There has been talk about it, but has anyone actually supercharged or turbocharged one of these GMC V6 engines? Low compression, high torque, and lacking on the top end.... seems like a combo that is begging or a low buck turbo application.
There was a '64 for sale on CL a couple years ago that had a turbo on a 305....kind of a rough job, but it had 80k on the system and it was a kick to drive.
I'll eventually TC my 351.
No doubt it's a no brainer as far as the engine is concerned.
That 305 went like a scalded cat.
Sorry PETA, but there aint too many things that go like a scalded cat....or so I've been told.
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Old November 18th, 2013, 06:06 AM
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Default Re: Forced Induction

I've written about this 2 or 3 times over the years in the old forum. About 15 years ago now I met a guy in the parking lot at the big annual Portland Swap Meet. He was driving a blue 4WD Suburban (can't remember the year any more) with the 305. He said it had belonged to his Dad who had passed away. His Dad had installed a Paxton supercharger on it. Between the time his Dad died and his Mom decided to hand it down to him, his Mom had sold the supercharger off it. Too bad! At least we know it can be done.
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Old November 22nd, 2013, 05:58 AM
hjennings hjennings is offline
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Default Re: Forced Induction

Rockdriller, I'd love to see that truck! Wonder what kind of numbers it put out?

It doesn't have to be some huge high dollar build. A low buck junkyard turbo would make up for the mediocre heads and make for a cool ride. I wouldn't expect 750hp, but it'd be enough to make it more fun.
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Old December 17th, 2013, 03:30 AM
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Default Re: Forced Induction

It's a sweet deal. Using a tractor set up currently, I'll post some pics and numbers as the experiment moves along.
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Old November 13th, 2014, 07:37 AM
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Post Re: Forced Induction

I now have my own custom shop up and running. I do restorations as well as a lot of resto-rods. While a GMC V6 isn't going to be any big money maker for the shop, it is or sure on the to-do list.

Since the 305 is the most common we'll probably end up using one of those. I'm also thinking that the 305 will make the most power per cubic inch, being the larger engines don't really flow any better, so still bottle-necked at the heads. Plan on machining a custom camshaft, the intake will either be a custom one-off piece of have some serious machining and porting done to it. We'll do what we can with the heads as far as porting, but no matter what they are never going to flow "great."

Since these engines are so freaking heavy, we are hoping they'll be plenty strong for whatever kind of power we'll be able to put out. We'll lighten up the rods and crank some, polish, match weight, and balance it all. Pistons either stick with the 4-ring ones or maybe go with a custom made piston with a standard 3-ring setup (forged if we go that route).

Still in the bench planning stage, of course, but thinking of something around 8:1 compression, electronic fuel injection tuned for E85, a twin-turboo setup, a huge intercooler, and seeing how much boost we can run before something breaks.

We do a lot of work with internal performance coatings (like anti-friction coatings on the piston skirts, thermal barrier coatings on the piston faces, etc) so we'll use various coatings throughout the engine. I think getting the rotating assembly at light as we can without compromising strength will go a long ways. Might even go as far as to extrude hone the heads and intake (stuff is amazing, but still fairly expensive).

These engines are tanks. Way overbuilt on the strength side, so my thought is that it may really surprise us as to how much boost it'll actually be able to take and survive. I also think boost will overcome the lackluster flow of our heads.

Being this is such an unusual build I hate to even try and guess as to what the final horsepower/torque numbers would be. Stock they produced a whopping 120hp and 220ft/lbs of torque. I'm confident we can double those numbers! LOL! While I know these engines are strong and I'm confident they can handle some serious boost, I just don't know how much RPM they'll take before it's just too much. I think that'll be the biggest factor.

I'm going to make a wild *** guess that will probably be way off and say 15psi it'll make 500hp and 550ft/lbs in the 4000rpm range.
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Old November 14th, 2014, 01:56 PM
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Default Re: Forced Induction

this is SO COOL!!!!



Please keep us in the loop and could you mention old specs when you do the new ones for newbs like me that don't know much of the technical stuff?

Kinda like you did with the horses. 120 hp vs 500 hp. Great numbers!

One more thing, I heard somewhere that they used to measure horsepower and torque different in the 60s and that our trucks might be alot higher than 120 hp if measured by today's standards. Is that true?

Thanks for getting your feet wet here. This is an exciting project!
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Old November 14th, 2014, 03:20 PM
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Default Re: Forced Induction

Net vs Gross horsepower. They used to measure the HP without the accessories, now they measure them with (that started that in '71 or '72 I believe).

Some of the specs are hard to find on these old engines. I've found some data on the heads that says the flow about the same as a 350 from that era. That sounds OK, except those flow numbers are total, and the V8 is split between 8 cylinders while our engines are only feeding 6 (so less flow per cylinder).

I said above I'd probably go with a 305E engine. My thinking there is it probably has the most flow per cubic inch of any of the GMC V6 engines. If somebody can confirm or deny this please let me know. I'm sure it'd be between a 305E and a 351E (Magnum heads). The 351 may be the one to use. Not sure until I find the flow numbers.

500hp is a complete wild *** guess. I hate evening saying that, as we have built several engines and can usually tell a customer what a certain combo will make and the dyno is always really close. With a buildup of a turbo GMC V6, it's never been done. So who knows. A big part of it is how much boost and how many RPMs the block/crank/rods can take. I'm guessing that these engines are very strong (as far as how much HP they can take.... they've never been tested at any sort of limits). So who knows, might be able to handle some insane boost levels.
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Old November 14th, 2014, 08:59 PM
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Default Re: Forced Induction

Quote:
Originally Posted by hjennings View Post
Net vs Gross horsepower. They used to measure the HP without the accessories, now they measure them with (that started that in '71 or '72 I believe).
The difference between measurement at the engine output, and output to the wheels is what it was explained as to me. Sounds pretty similar though, previous was output of just the motor, current is output of motor and everything else. '71-'72 sounds about right as well. *Somewhere* in my stuff I actually have a report and charts that I did back in college (in early 80s) on just this subject, the change in HP rating.

Quote:
Some of the specs are hard to find on these old engines. I've found some data on the heads that says the flow about the same as a 350 from that era. That sounds OK, except those flow numbers are total, and the V8 is split between 8 cylinders while our engines are only feeding 6 (so less flow per cylinder).
Okay, I guess I don't understand flow...
Flow of "X" divided by 8 cylinders will be less than the same flow divided by 6 cylinders. Flow of 48 divided by 8 would be 6 per cylinder while the same 48 divided by 6 would be 8 per cylinder.
The other way to look at it is that if the flow is the same as a v8, they might mean the flow per cylinder is comparable.

Quote:
I said above I'd probably go with a 305E engine. My thinking there is it probably has the most flow per cubic inch of any of the GMC V6 engines. If somebody can confirm or deny this please let me know. I'm sure it'd be between a 305E and a 351E (Magnum heads). The 351 may be the one to use. Not sure until I find the flow numbers.
305 E isn't magnum. a 305 is a 305 is a 305, essentially.
351E or M is the magnum vs. 351C.

Quote:
500hp is a complete wild *** guess. I hate evening saying that, as we have built several engines and can usually tell a customer what a certain combo will make and the dyno is always really close. With a buildup of a turbo GMC V6, it's never been done. So who knows. A big part of it is how much boost and how many RPMs the block/crank/rods can take. I'm guessing that these engines are very strong (as far as how much HP they can take.... they've never been tested at any sort of limits). So who knows, might be able to handle some insane boost levels.
Since you have in a previous posting mentioned changing the weight of the internal components, previous experiences may not apply... but stock these engines do not take high RPM well. Being so solid and heavy they seem to come apart if the revolutions are taken too high. Got mine stuck, tried to power out, and spun/destroyed bearings (ultimately threw a rod, but that was my fault for using it before fixing it, but indicates damage was done due to the high revolutions).
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Old November 14th, 2014, 09:22 PM
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Default Re: Forced Induction

Quote:
Originally Posted by David R Leifheit View Post
Okay, I guess I don't understand flow...
Flow of "X" divided by 8 cylinders will be less than the same flow divided by 6 cylinders. Flow of 48 divided by 8 would be 6 per cylinder while the same 48 divided by 6 would be 8 per cylinder.
The other way to look at it is that if the flow is the same as a v8, they might mean the flow per cylinder is comparable.



305 E isn't magnum. a 305 is a 305 is a 305, essentially.
351E or M is the magnum vs. 351C.



Since you have in a previous posting mentioned changing the weight of the internal components, previous experiences may not apply... but stock these engines do not take high RPM well. Being so solid and heavy they seem to come apart if the revolutions are taken too high. Got mine stuck, tried to power out, and spun/destroyed bearings (ultimately threw a rod, but that was my fault for using it before fixing it, but indicates damage was done due to the high revolutions).

Yeah you are looking at the flow backwards. The same flow of the V8 heads has to fill 8 smaller cylinders as opposed to 6 larger cylinders. Same flow rate trying to fill a much larger cylinder equals out to a lesser flow rate (effectively).

I'm aware the 305E is not a magnum, however there are different 305s. There is an A, B, C, D etc. Difference in intakes and heads is my understanding. Still not a "magnum" though. But back to my point.... a 305E head flow rate vs a 351E..... I think obviously the 351 Magnum (E) heads would outflow 305 heads, but you have to do the math (per above) and see if it makes up for the extra 46 cubic inches or not. The 305, while having less flow, may have more flow per cubic inches than the 351E. Maybe. I don't know without knowing the numbers and doing the math. Either way, I want to start with the GMC V6 that has the most flow per cubic inches.

As far as removing weight.... we will definitely be doing a LOT of machine work to the crank and probably the rods as well. Custom made pistons. Stronger stud kit will replace all the bolts. Everything will be matched weight and fully balanced.

Of course we still have no clue as to how many RPMs we can squeeze out of one of these engines because nobody has done this before. We'll have to see how much weight we can remove, judge the strength of everything, do some calculations, and guestimate a max RPM.

Fun stuff, right?
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