1968 c10 396

[mike's1965]

I was wondering if anyone knows what the compression ratio would be a stock 1968 C10 396.

I have one in my 1965 I swapped in years ago, and I got it out of a 1974 C10 truck that was supposed to be a 454, didn't know until a few years later and looked up the numbers.

Was thinking of getting a cam for it, but kind of need to know more.

I recall the heads are 1968-1976 427 oval heads, not sure if these 396 engines came with those heads, or someone put them on.

It may have a cam already, has some lope, about to put on pro flow 4 Edelbrock fuel injection system for oval port big block Chevy.

Are these low compression engines?

[AussieinNC]

Depends on the heads but it is most likely around 9:1....

There were a lot of 396 in Chevelles etc that had 10.25:1 compression ratio but these are getting really hard to find....

To work it out you will need to figure out swept volume against cyl head chamber volume plus head gasket thickness allowance and amount piston is down the cylinder at TDC...

[mike's1965]

Maybe best just to hold off on a cam, since it possibly has one already, see how it does with the fuel injection and get the tune dialed in, maybe it will run pretty good.

Since putting it in truck, I only set timing at idle with vacuum line plugged at 8 degrees btdc.

That's if the chrome timing pointer is even correct, will buy a piston stop to verify.

So most likely the engine has timing not advanced enough, but with the Edelbrock kit I will be able to change it on the tablet, maybe it will have more power.

I was disappointed in the performance after swapping the engine in, hopefully advancing timing and tuning will pick up some power that's left on the table.

Leave tearing it down to investigate pistons and chambers for another time.

[Boog]

What trans and rear end ratio does the truck have? Low gears will make it move. With a 396 in good condition you could run a 3.07 gear and it will still get up and go. Along with initial timing look at the centrifugal advance in the distributor and make sure it is in good condition and moves freely. Lazy advance makes 'em sluggish.
Restrictive exhaust holds them back also.

[mike's1965]

TH400, and 3.73 gears.

Has '74 454 exhaust manifolds with dual exhaust, that dumps out before front tires.

Will be adding long tube headers and new exhaust, but after I get the fuel injection on.

The Edelbrock pro flo 4 also came with a distributor, you set the base or initial timing at 12 degrees btdc, lock distributor clamp, then computer self tunes.

However you can play with the timing advance, and other things with the tablet, and make changes instantly, so that should be nice, and much easier to keep dialing it in.

At the time I put the engine in, I didn't know what total timing was, or how to set it, was always too busy to mess with it and truck got parked.

Anyways it's been parked for years and it's getting back on the road, might even add A/C before it gets hot again, if everything works out, not being driven because it's 120 outside has happened several times.

I'll add pics and try to write up the install of the Pro Flow 4.

[mike's1965]

Originally Posted by Boog

Along with initial timing look at the centrifugal advance in the distributor and make sure it is i Lazy advance makes 'em sluggish.

This right here is what I think was the reason it drove no better than my 327 that was in it before.

Hopefully it will gain some good power with having timing a lot more advanced.

Advance timing to Max performance, new headers and dual exhaust to maximize what it can do.

[NGN]

You say120 outside, holy cow it's 43 here in this part of Jersey as I write. First delivery of just a 150 gallons of fuel heating oil $364.50, send that heat this way,,, well some. lol

[mike's1965]

Originally Posted by NGN

You say120 outside, holy cow it's 43 here in this part of Jersey as I write. First delivery of just a 150 gallons of fuel heating oil $364.50, send that heat this way,,, well some. lol

Come visit next July and August, go for a ride in the 1965, it's so hot it can give you a headache if you are in a vehicle with no A/C, it's hot and the sun is so bright it's almost blinding.

Summers here are like our backwards version of your winter, so hot all you want to do is stay in, kinda like cabin fever.

The only escape is to go tubing the salt river, or drive up to Payson Arizona, went from 114 to 76 in a little over a hour, if you want to be outside and breathe fresh air.

Hmm, both extreme heat and cold suck. Lol

[HO455]

You can ball park the compression ratio by doing a compression test of the cylinders. My 10.75:1 GTO is 185 to 190 psi with a Ram 4 cam. Lots of test results on the interweb to compare yours to. You would at least have some idea and also know if all the cylinders are relatively in the same condition.

[Steeveedee]

I had an RA III 400 in a '66 Le Mans (obviously a transplant, as it had a 326 originally). The compression ratio for that engine was advertised as 11:1. Don't know if that was true for this particular engine, but it had 210-230 PSI for cranking compression. Even with the good grades of gas you could buy back then (mid-'70s) it wasn't really happy unless I put in an octane booster. I had a compression tester that you would put against the plug hole, and it split the rubber fitting. I had to go and buy a gauge that screwed into the plug hole, and had an o-ring for a seal. You can get a reasonable number for the compression ratio by looking at the numbers, but if the engine is really worn, it might not help.

[mike's1965]

Originally Posted by HO455

You can ball park the compression ratio by doing a compression test of the cylinders. My 10.75:1 GTO is 185 to 190 psi with a Ram 4 cam. Lots of test results on the interweb to compare yours to. You would at least have some idea and also know if all the cylinders are relatively in the same condition.

Recently did this, 155 ish psi.

[HO455]

With lots of reservations about cams and lifters and other things 155 would generally be in the 9:1 range.

[mike's1965]

Originally Posted by HO455

With lots of reservations about cams and lifters and other things 155 would generally be in the 9:1 range.

I guess that's not too bad?

Should I run regular or premium once I start getting timing advanced?

[HO455]

With iron heads and today's fuel 9:1 is right where you want to be.
Personally I would run it on regular with as much timing as it will safely take. Then if it just won't run without pinging with a reasonable amount of timing then step up the octane rating.

[HO455]

Originally Posted by Steeveedee

I had an RA III 400 in a '66 Le Mans (obviously a transplant, as it had a 326 originally). The compression ratio for that engine was advertised as 11:1. Don't know if that was true for this particular engine, but it had 210-230 PSI for cranking compression. Even with the good grades of gas you could buy back then (mid-'70s) it wasn't really happy unless I put in an octane booster. I had a compression tester that you would put against the plug hole, and it split the rubber fitting. I had to go and buy a gauge that screwed into the plug hole, and had an o-ring for a seal. You can get a reasonable number for the compression ratio by looking at the numbers, but if the engine is really worn, it might not help.

It sounds like someone may have done some head swapping and you ended up with a higher than advertised compression ratio.

[mike's1965]

So, are these engines low performance?

I know it's not a 427, or 454 but it can't be that horrible?

It should at least out muscle a basic stock 350 small block?

Today at pull n save I saw a 1984 crew cab truck with a 454, according to the tag on air breather.

Odometer said 25,000 ish miles, but probably 125 or 225, it was a landscaping company truck, thinking of maybe getting the engine, was missing carb, distributor, and some other stuff, but had heads exhuast manifolds, etc, but I will probably skip it as my truck isn't running, and a big block can't be hauled in a PT Cruiser (I don't think so anyways) or a C5 Corvette. Lol

Passenger side valve cover was popped off, I think you can get a complete engine for about $400.00.

If any local people want it, it's Pull N Save in Gilbert of off Cooper Road.

Didn't crawl under but probably a TH400 if anyone wants those anymore.

[Aus69c20]

I’ve got the original 396 in my c20 and gets up and goes alright.
The engines virtually stock with headers, a performance dual plane intake, a sniper efi unit and electronic ignition.
Compression tested at around 175psi give or take a few psi and I run 36deg of timing advance. With that set up it runs better on a higher octane fuel. It still runs fine on Australia’s standard unleaded but can diesel a little on shut down.
The truck spins both tyres off the line but the party’s over by 4000rpm. I drove a few c10s with stock 350’s and there was definitely a decent difference in power, even with the extra weight of the c20.
The 396 just does it easier. Hopefully that’s a useful point of reference.

[mike's1965]

That's a pretty good description.

[weq92f]

.
Running one from 1970 here with stock 60 over bottom end and stock iron heads ( #063 ). Best I could figure with all the measurements and calculators out there was my particular combination is at about 9 or 9.5 to 1. I mic'd everything I could and cc'd the chambers but ended up having to estimate the piston dome still.

It's a very strong running motor with 12* initial and 36 total, no vacuum advance. Definitely beats the crap out of any stock 350 but it's done below 5000 rpm ( as per the cam selection ). Speaking of...Comp XR270HR.

Hth,

-klb

[mike's1965]

Hi everyone, last night I was looking up 1968 396 310 HP, saw a sample of a book, may be a book on Chevy truck history.

Anyways it mentions the debut of the 396 in 1968 trucks and says it had 9.0-1 compression ratio, with 4 barrel carb, cool.

I wonder if it could handle mild boost from a turbo, or set of turbos?

The pro flow 4 supports boost with upgraded sensors.

No plans right now, but down the road it's an idea to step up to 454 or 496 power out of the 396.

[mike's1965]

Hey guys I got truck on road in June and have been driving it once or twice a week, usually at midnight because it's just too hot in the Phoenix AZ area in the day, it's usually 100 or more at that time of night.

Anyways still running Q-Jet, had to replace wiring harnesses for lights, plan on pro flo 4 swap and Summit coated headers BBC headers when it cools off, maybe after Halloween at nights.

New spark plugs too as others had carbon buildup after years of starting and idling, it runs pretty good, and a few times I've put the TH400 in 2nd and got on freeway onramp and stomped it and let it wind out, hear/feel the Q-Jet secondaries open then shift when I think rpm sounds like it's time to shift.

In June it felt more powerful, but since it's gotten hotter outside its down on power, another plus to looking forward to fall/winter.

The 396 has pretty good power.

[68Gold/white]

I drove my 68 with the original 396 for about 6 years. Had high compression. I had to be careful with timing, it was finicky. Anyhoo it's been apart in the garage for quite a while.
i DO remember mine has domed pistons (not a big dome, though) and small closed chamber heads. No wonder it was finicky about timing. I think mine is/was 10:1 or over a tad.

IF you ever re-cam your 396, try to get a good handle on what the chamber volume of your heads are, and if your piston are flat top or not. A proper cam COULD help your engine some. Just don't dive off and buy a XE cam, that will be worse...

[mike's1965]

I think the 396 has stock cam, but could be wrong.

Will have to get the head casting #s, have them highlighted in a book about how to build max power BBC in a box somewhere, I remember on the page with the casting numbers and description it said 1968-1976 427 heads with oval port.

Not sure if that's what these 1968 truck 396 engines came with, or someone put those on.

Anyways was thinking of putting in a cam to go with the Edelbrock fuel injection swap and the new summit headers.

Even if putting in a better cam, it's still going to run out of rpm at 4000-5000 rpm as others have said?

Over the years people have told me don't waste time on the the 396, go 454. However it runs pretty good, and I already have it.

[mike's1965]

Originally Posted by 68Gold/white

I drove my 68 with the original 396 for about 6 years. Had high compression. I had to be careful with timing, it was finicky. Anyhoo it's been apart in the garage for quite a while.
i DO remember mine has domed pistons (not a big dome, though) and small closed chamber heads. No wonder it was finicky about timing. I think mine is/was 10:1 or over a tad.

IF you ever re-cam your 396, try to get a good handle on what the chamber volume of your heads are, and if your piston are flat top or not. A proper cam COULD help your engine some. Just don't dive off and buy a XE cam, that will be worse...

Might have to pull a head to see.

[geezer#99]

Don’t pull the head.
Have a look see with one of these.
Lots of mechanics or plumbers have them.

https://www.amazon.com/dp/B084H8G2V1/ref=dp_prsubs_1