Hello,

I have a few questions of the differnt in types of 390 engines

1) What is the diffenrts in a truck 390 vs a car 390?

2) What is the diffenrts in a reg. car 390 vs a 390 GT?

3) Was there a 390 GT offered in a 1968 T-Bird? How do you tell a 390 GT form a reg. 390?

4) What would I have to change when replacing my 1975 truck 390 with a 1968 T-Bird 390?

 

>Hello,
>
>I have a few questions of
>the differnt in types of
>390 engines
>
>1) What is the diffenrts in
>a truck 390 vs a
>car 390?

Compression Ratio basically (eg. C8AE-H heads were used on trucks and 390 GT's)
>
>2) What is the diffenrts in
>a reg. car 390 vs
>a 390 GT?

Cam, and I believe the S code intake, heads have 14 or 16 bolt exhaust manifolds
>
>3) Was there a 390 GT
>offered in a 1968 T-Bird?
>How do you tell a
>390 GT form a reg.
>390? ]

I dont know if the GT was offered for the T-bird or not. Look for a "S" cast on the #1 intake runner is the best way I know of to tell the 320 horse 390...
>
>4) What would I have to
>change when replacing my 1975
>truck 390 with a 1968
>T-Bird 390?
Nothing unless you need to convert from Manual to Automatic or vice versa, although you might have some exhaust issues if you have 14 bolt heads on the 390 GT

 

I don't think you'll have any problems with the 390 GT heads (if you have 'em) because they have both types of holes in them. They'll accept any type of 390 exhaust manifold. The 390 GT had them because it went into mid-size and smaller cars, so they needed special exhaust manifolds. If that 390 came out of a t-bird, it's just a 390. if you wanted to go fast in a t-bird that year you got a 428 or 429--not sure which.

I have a 390 GT in my '67 LTD, and it has 390 2v exhaust manifolds on it. Bit of a power loss there, but it works.

 

Thanks to you both for responding to JKT. I learned something today. I am a truck guy and don't tend to think in car terms. Your information was concise and accurate. I am fairly new to the 390 experience. I installed a 390 with C6 ( from a 72 F250 )in my 62 F250 about 3 yrs ago. So still learning all the ends and outs of the 390.

Jim

 

I believe the '68 T-bird only came with the 315HP 4V 390 or the optional 360HP 4V 429. The passenger car 390 will bolt in as long as you use the truck's horizontal oil filter adapter to clear the crossmember and exhaust manifolds, the outlets are larger and positioned differently. If the heads on the engine you're swapping in use the 8 vertical exhaust bolts, your fine. If they use the diagonal bolts, there may be a problem. Compare the heights of the top bolt holes, if they're all the same height it will bolt up to the truck manifolds. If the end bolt holes are lower than the center two, your options are headers, swapping heads or making the GT manifolds work in the truck. Header manufacturers usually slot the flange for the top end bolt holes to fit both holes. We had a poster that faced this dilemma and ended up swapping heads.

Barry

 

Truck 390s were of a lower compression ratio than car 390s. This was done by lowering the compression height. If you tear down a truck 390 you'll find it has 410 pistons. This places the compression height about .110 below zero deck. When you add a Fel-Pro .041 gasket you are about at about .151 compression height. The compression ratio is lowered but it is at the expensive of optimum power and economy.

A car 390 came one of three ways, with flat-tops and 9.8-10.5 to 1 CR depending on chamber volume, and with dished pistons and 8.8-9.6 to 1 depending on heads. There are two dished pistons, the early shallow dish 2V piston (not common) and the deeper dished 265 hp 2V regular fuel piston. These all yield a .005 to .010 below zero deck compression height, and with a gasket come in at .046-.051. Great for power and economy as the the closer collision between piston and head purges the curtain area and turbolates the mixture toward the primary flame front.

The common early style D shaped chamber was from 72 to 75 ccs and the kidney shaped chamber was from 69 to 72. This gives an opportunity to mix a little for different applications. You could put a set of D chamber heads on with the deep dished piston and be down at 9.0 to 1, use flat-tops and be at 9.8. You could use the deep dishes and kidney chambers and be at 9.5. to 1 ('68 T-bird) or flat tops and be at about 10.3 or 10.5 to 1 (this is what the GT and premium fuel 390s had). These are approximate as specific chambers vary, you need to cc them to be sure.

You can further enhance power and economy be decking the block to zero deck. In most cases this is .25 of a point of CR, but the better squish motion will actually make the engine less prone to detonate. It would make our 9.0 to 1 dished piston D chamber engine about 9.25, a good truck engine.

Camshaft dictates a great deal of what CR your engine can stand. A camshaft that closes the intake valve early begins to build cylider pressure earlier in the cycle. If too much pressure builds it will need higher octane fuel. You will also by virtue of the early valve closing lose a significant amount of airflow at the point the engine can pull the most in, the end of the intake stroke. A cam with too short of a duration (the stock post-66 cam is awful) overall with be short on overlap, the point where both intake and exhaust are open at the same time compromising the scavenging action of the exhaust pulling in new intake mixture. And overlap can be more forgiving in the low end for keepin cylinder pressure down, but in my mind this isn't a good reason to go with a bigger cam. The closing point is the key. You lose power in the upper rpm range if you close it too early. I have found that a cam with a closing earlier than 55 degrees and a CR over 9.0 to 1 will heve to have premium. The GT cam is a 428CJ hydraulic and works very well with the higher CR.

My 390 has flat-tops, C4AE-G 74 cc D chamber heads (.049-.051 suish, and what I figure is 9.7 to 1 CR ) and a 61 degree closing. I can burn 89 octane, but it likes 93 better.

 

Barry and Bill,

I just learned a whole lot more about the ins and outs.

Thanks,
Jim

 

>My 390 has flat-tops, C4AE-G 74
>cc D chamber heads (.049-.051
>suish, and what I figure
>is 9.7 to 1 CR
>) and a 61 degree
>closing. I can burn 89
>octane, but it likes 93
>better.


Hey bill, do you find any difference in gas milage between the two grades of gas?

 

I haven't really tracked mileage. I can feel the difference in throttle opening needed to maintain speed, not much, but there. I suspect that hurts the mileage.

I have hardly ever noticed any ping, and only when I got too enthusiastic with the base timing. It is a great combination.