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Discussion Starter #1
Hi, guys. Just thought I'd run this past you before I went ahead and built it.


A fella I know is building a 1956 Mercury 4 door sedan. Someone has already swapped a 390FE into it, but it needs a rebuild. The owner is a body man, but has zero engine building experience and has asked me to assemble his engine. I have done stock builds on several FE's, but have not tried to go much past it other than a mild cam swap. So, what do you think of this combo, in a '56 monster heavy car?


Engine

390FE
10.00:1 compression
194*/204* @.050", .458/.484 lift, on 110* seperation, with a 105* intake c/l
780cfm carb on dual plane intake
Large tube full length headers through Flowmaster 50's, with H pipe
Pocket port, with intake port match

DD2000 says 440 ft/lbs @2000rpm, peaking at 462 ft/lbs @ 3500rpm, peak horsepower 400 @ 5000rpm. Looking at the graph, I estimate 360 ft/lbs off idle, and over 400 from 1500rpm through 5000rpm.

Is 10:1 too much for 91 octane on this engine? 9.5:1 only drops peak tq to 453 ft/lbs @ 3500rpm, and peak hp to 391 @ 5000rpm. If 9.5 works w/o detonating, I'll be happ, as I am not looking for huge #'s here, just something to give this old beast some grunt.
 

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That is very high compression for a very small cam. You should calculate the dynamic compression ratio to see what you are getting.

I'm running 9.0:1 static compression ratio in a 396 with a 206/212 cam in a 4500 pound truck with a 3.08 gear, 3 speed auto, and it will detonate if I don't use 93 octane. I can turn the timing down a little and run 91 octane but it reduces power. It will run on 87 as long as I don't make any hard accelerations, but what is the point of that. Be aware that a engine will detonate more with higher loads on it (heavy car with highway gear).

I don't have my notes in front of me, but I remember my 396 dynamic compression ratio was about 7.9:1, which I thought was a little too high but I ran it any ways. I should had set it up with 7.5:1 DCR due the the high load on the engine and BBC's appetite for ignition timing. I set my 454 up with 8.75:1 cr with that same cam (206/212) and it runs fine on 91 octane.

I think you need to use a bigger cam, like a compcam 252H (206/206 on 110 LC) and turn the compression down to 9:1. FYI, a 252H compcam can use stock springs and rockers. Or maybe invest in some aluminum heads and run 9.25:1 cr with your little cam (194/204). I would use 8.25:1-8.5:1 cr with the cam you picked with iron heads.
 

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Agreed on the compression, especially with old-school FE heads' chambers.

I strongly suggest not using DD's pre-calculated flow figures. They are optimistic for old-school heads and not enough for modern heads. Find actual flow numbers for your heads, then estimate how much you gain from pocket porting. The reason I say that is 400 hp is really optimistic from just pocket porting FE heads.

With outdated chamber designs, compression is not your friend with today's gas. I'm actually not a big fan of high compression for large cars. Big compression is matched to big cams which (as you obviously know) don't go well with big cars. I personally chose 8.7:1 for my 454 in my station wagon with a 218/224 cam. Its very well matched, excellent throttle response, no detonation on 87 octane, and mountains of torque. It makes about 400 hp with 049 heads. Sure I might have picked up 1 more mpg and 10 more hp with 9.5:1, but I didn't see the need to run premium in Los Angeles at $3 a gallon just for 10 more hp in a 5200-lb car :)
 

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Discussion Starter #4
Thanks for the replys, I will likely do as advised. I have not calculated dynamic compression yet, but will do and post what I find. Thanks again. :thumbup:

Garth
 

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yep, drop that compression down. I also think you would be over-carbing it with that big of a carb with that cam. A 650-700cfm carb would be fine. Holley makes a good Street Avenger carb in 670cfm for motors between 300-400hp and the pretty much come tuned in out of the box.

To give you an idea (and yes I know fords and Gms are apples and oranges) but the '69 Olds 455 w/ C heads made 375hp stock and only rolled with a 600cfm quadra-jet. It did the job back then with that size carb and you aren't going for much more. I had a 455 installed in my '57 olds 98 4 dr sedan connected up to TH-400 auto tranny (now that's a beast of a car), and I was smokin' my buddy in his 55 chevy coupe with small block chevy 355ci w/ aluminum heads with manual 5 speed tranny. The low end torque was amazing also.

I think if you do any big work, pay attention to the heads. Make sure it breaths good.

another tip... if you are running an auto tranny with that big heavy car, make sure to use a good tranny cooler.
 

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Discussion Starter #6
So I downloaded a DCR calculator, and came up with 8.1:1 DCR. From what has been posted, and what I have been able to find out, this is too high. If I drop my static to 9.0:1, this gives me a DCR of 7.67:1 with the cam indicated. If this is still too high, I will try other cams, with a little more duration and overlap.


Premium fuel is not too much of an issue, as the car will not be a daily driver.


EDIT: I tried the DCR with a Comp Cams 268H. I used this cam, as the 252H is a chevy grind, and not availiable for the 390. Ther DCR came out at 7.37:1. The Static compression had been reduced to 9.0:1, with only a slight decrease in performance figures. I also reduced the carb CFM, and dropped it to 700cfm. This gave me peak hp of 409 @ 5500rpm, and peak tq of 439 ft/lbs @ 4000rpm. I know these #'s are optimistic with factory heads, but they knid of give me a general curve to work with. As for making 400+ hp, I could care less. On this build, I want lots of torque, availiable right off idle.
 

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If your willing to try something new then do a search for singh grooves. It should give you the ability to run 10:1 on pump gas, make more power and get better fuel economy.

Jordon
 

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... IF the heads and pistons provide adequate quench. Grooves will help, but not very well if its an open chamber or lacks quench. Still helpful, but you can't say absolutes without knowing more.
 

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Agreed, The words "should" and "do a search" are key points. If its being rebuilt then then you may be able to use the singh grooves.

Jordon
 
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