GMC V6 engine compression ratio

[LSGarage]

I'm a couple of years down the road from this but might as well get some feedback on it. I have a 67 GMC 4WD with a 305E V6, manual trans. Specs say it has 7.75:1 compression.

Question: How much compression can you add to this engine before you run into overheating, head gasket sealing, valve clash and other problems?

I'm thinking of taking it up to 8 or 8.5:1 when I do the engine. But I would like to know how high you can go before these issues begin to surface. I'm looking increase the torque. Maybe a low and flat torque curve. No revs over about 4,500. Maybe going to the Holley 2bbl on the stock manifold, headers, duel exhaust and all that stuff. Its got 3.73 gears in it now but I think I want to go back to the original 3.54 or there about ratio. And I running 31x10.50 tires.

[Longhorn Man]

I would imagine 3.07 would be even better in all honesty.
Being it has iron heads, and old school chaimbers, I wouldn't go over 9 to 1... if you get a custom cam with a little more overlap that stock, one that'll bleed off some pressure, you may be able to get away with 9.5 or so, but I don't know how you would even get those numbers. That is way too much to mill the heads.

[Fred T]

Getting the revs over 3,500 is going to take some major work on the crank, which has some big weights. One of the problems with the GMC V6 was with balance and it's inability to handle high revs. The engine doesn't like a diet of sustained rpm of over 2,500 rpm, and will spit out the rods like there's no tomorrow. Been there, done that when the engine was in production.

If you haven't already been there, go to www.6066gmcguy.org

[chevy_fatman]

..............................and the big dogs have spoken.....................

[LSGarage]

I haven't got a tach on it yet and haven't done the math on everything but it feels like it doesn't want alot of RPM. In high gear the motor feels and sounds like it doesn't want to rev any higher at around 50 mph. Which is fine with me, I'm just exploring ways to improve on the torque in and around the current RPM range.

I'm afraid that if I got too low on the rear ratio I'll loose the power that it has now. When it all done I'm only going to use the truck in the winter to get around.

[rwgregory]

You are the only person I have ever heard of that wants to get more bottom
end grunt out of a GMC V-6. These engines are ALL GRUNT!! As others have
said, they don't like to spin, but you can lug them all day.

[Erl]

I am rebuilding my 305 V6 and doing several performance improvements. First I bored it .030 over. I had a the cam reground for more performance, since you can not buy a new one. I have GMC 351 V6 magnum heads, intake and exhaust which have much better flow than stock. The heads were ported and polished, by me. The heads have been milled .070 to give 8.8:1 compression ratio. I am using a Holley 500 CFM carb and a custom Buick V6 HEI. I hope to pick-up about 40 horsepower. When it is finished I will take it to a local dyno, then post the results. I am sure someone will tell me to buy a Chevy 350 to get better performance and save money. I want the look of the original engine with some more power.

[hoser]

Originally Posted by rwgregory

You are the only person I have ever heard of that wants to get more bottom
end grunt out of a GMC V-6. These engines are ALL GRUNT!! As others have
said, they don't like to spin, but you can lug them all day.

I am with you on that one I remember back in the late 70's a guy with an old GMC cleaning up at the local truck pulls with his stock V6 he won the stock and modified for his class against modified v8's he was slow and quiet but made full pulls every time!

[LSGarage]

Thanks Erl, I will be very interested in what your results will be. I guess I'm just a rookie with this engine. I'm fast learning that you can lug this engine more than the norm. I'm also finding out that my old truck is kinda rare so I think changing the engine would be a disgrace to its history. Keep the progress posted.

[JAKES 68GMC]

Hey Erl, how'd your V6 turn out after you got it all done??

[WyoRoadDog]

I'm interested that this surfaced as well, I had not come across this thread in my back reading on the board. I'd very much like to know what all has been/can be done to these engines, beyond the tips on Jolly's site. Any rebuild tips and tricks could help as well. I'm already planning on the Magnum heads/HEI mod, but wondered about balancing and porting/polishing. Any real world input would be appreciated!

[JAKES 68GMC]

The Holley 500 2bbl. carb seems to work wonders on them, and my truck doesn't run quite 100%. They're still slow, but have better pick-up than with the stromjunk. Another thing you could do is get a set of heavy-duty truck exhaust manifolds, with the 3in. outlets. (like cast iron headers IMO) I've heard that back in the 60's, Hot Rod Mag had a 305 V6 on the cover with a Offanhauser 3x2 intake, and Hooker headers. But thats just a rumor.

[MisterMan]

compression ratio has nothing to do with valve slap. compression ratio is simply the compression of one, or all of the combusion cylinders. im runing 9:5:1 with 87 octain.

[TP from Cntl PA]

Originally Posted by LSGarage

I'm afraid that if I got too low on the rear ratio I'll loose the power that it has now. When it all done I'm only going to use the truck in the winter to get around.

ROTFLMAO................Sorry, but IMO, if your having a torque issue you probably aren't running it right. These engines aren't like a V-8, you pull these down and they absolutely LOVE it. I think most people can't get used to that fact. Neither my dad or brother who both run mine alot don't care much for it, because on a hill they won't let it get down low enough in RPM's to where it preforms the best on the torque curve. When it drops acouple hundred, they want to drop gears................Of-course, the engine is screaming and not do much but sucking up gas. I on the other hand, leave it and let the old torque monster lug its way through. They are pure Torque monsters. I got a 305 in a '69 1-ton, and I wouldn't trade the V-6 for 10 of the same trucks with 350. Heck, the 454 in my K-20 didn't run any better loaded and used 2x the fuel doing it. If most people knew what I was grossing when I rolled over the scales with that old 1-ton in the past, they'd have a DOT van at the end of the lane to our house waiting for me.

The only bad thing for a V-6 is RPM's...........

[Bob B.]

As far as increasing the compression ratio on the GMC V-6, you are really limited. About all you can do is mill the head. Stock pistons are hard enough to find, and to my knowedge high compression pistons were only availble for a short time in the 60's, when they were made for industrial V-6's running on propane or natural gas. Good luck finding a set of those! No doubt the 'Magnum' heads and manifolds are better than the stock 305's, and the 500 cfm Holley is a big improvement, too. But I wouldn't try making a hot rod out of a GMC V-6. It just wasn't designed for it.

[JAKES 68GMC]

Originally Posted by TP from Cntl PA

The only bad thing for a V-6 is RPM's...........

I had my V6 going today, and I was revving it up. The tach was going well over 4000rpm, close to 5K. I thought "It's a dream come true, my V6 is revving higher!" I finally snapped out of stupid-mode realizing this isn't udderly possible, and shut the motor off. Restarted it again and the tach was working normal again.

[Erl]

I am still working on the rebuild. I have switched gears since that last post. I have a set of 351 magnium heads, intake and exhaust manifolds ready to go on it. This did cause a problem. The combustion chamber volume of the 305 heads is 100cc and the volume of the 351 heads is 120cc. This will bring the compression ratio down to 6.8:1. To make this work I had to purchase custom pistons from Arias Piston Co. That set me back $700. They could not guarantee any more than 8:1, because of the size of the thier stock piston forging. The big V6 has a large compression height and wrist pin dia, which limited the dome volume. I will know in a couple of weeks when they come in. If you think this is a waste of money, keep it to yourself, this is a hobby not a business. Desk Top Dyno predicts about 275 HP and about 325lb-ft of torque. I will take some pictures and post them later.

[TP from Cntl PA]

Originally Posted by Erl

If you think this is a waste of money, keep it to yourself, this is a hobby not a business. Desk Top Dyno predicts about 275 HP and about 325lb-ft of torque. I will take some pictures and post them later.

Can't wait to see what it makes................I bet it will beat the DTD estimate on the torque end.

[T Herder]

Howdy,
I'm new to the board but have been running these trucks for over thirty years.
Check the exhaust valves, some of these motors have sodium filled valves with hardened valve seats, you want those for unleaded fuel. The top end oiling is fed from the rear main camshaft bearing, it has a partial groove around its circumference, have the groove machined all the way around. Have close tolerance valve guides installed, if nothing else do this, the valve guides wear and the performance drops. If you can get them use the pistons with four rings and follow the factory original spacing and clearances. Install a five pass radiator, you'll think your temp gauge is broken, don't care how hot it is or what you do, it won't overheat.
Best regards,
Butch
----
65 3/4T 4x4, fuel injected 305E V6, SM 420, Gear Vendors, NP 205, BFG 35's on Weld SS, and some other stuff.