improving performance through rear end ratio

I am new to the forum and not for sure how to post a question. Here is my dilemma.. I have a 56 ford customline show car with an almost stock 390 fe. the only thing not stock is dual carbs and high performance coil and dist. setup.. I am going to get carbs adjusted to run right. They are way to rich at this time but can be calibrated down. I can not add anymore hp because there is no room to add headers (have had it looked at by pro custom people). Did 250 hp on dyno (17 inch wheels on rear) and a 325 rear end ratio positrac with 4 speed shifter. Not for sure how much power loss due to the dual carb set up but am looking at how to increase performance on zero to 60 and eight mile runs. At 65 mph in 4th I am tacking 2400 rpm and it will only do 4400 rpm. ( that's borderline to much) If I go to a 6 speed manual shifter and change rear end gears to a 370, will the 6 speed (.63 in 6th compared to 1 to 1 presently in 4th) get me down the road quicker with out hurting my top end or cruising speed?? there are no power adds possible at all because of the header situation.
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What 6 speed transmission? What 1st and 2nd gear ratios? Lets assume a 1st gear of 2.96. With a rear end ratio of 3.70 * 2.96= 10.952. A ratio like that means you are going to run out of 1st gear fairly quickly...that isn't a big deal for a SBF that revs to 6000RPM, but its a much bigger deal if you only have 4400 RPM to work with. You will end up with something that is worthless after about 30 feet. A 6 speed is just going to make you shift more and slow you down...especially with a 390 paired with a 3.70 rear end ratio. A T4 or a C6 would be more appropriate with that rear end gearing.

I don't believe for a second that there isn't room to add headers, as an example, I built headers for my engine and only had this much room to work with:



Most times shops just don't want to take on tight clearances when building headers, too many issues that come with it.

That being said, there are other ways to add power...such as turbocharging...or keep it simple and run a reasonably sized shot of nitrous. 250HP(Call it 285 to the wheels) isn't a whole lot of power to work with...a small block could easily make that and have a lot more room to work with for things like headers as well as being able to rev a lot higher and take advantage of lower gearing better.
Internal engine mods will be your best bet for a performance upgrade
Make it a 428 and tell them it's a 352 or stock 390
Dual carbs is a bit too much for a stock 390 IMHO (yours 10,5:1 and 315 HP)
I agree that headers can be made for the vehicle
I have a 56 too with a 429 in it and Headers are next.
I agree there is not much room. I am going with custom made fenderwell exit headers
I think we're missing something here, the OP knows it's running like crap but is going to ignore that and swap trans and Diff. I think you need to fix the engine issue before you start throwing cash at the rest of the car.
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I think we're missing something here, the OP knows it's running like crap but is going to ignore that and swap trans and Diff. I think you need to fix the engine issue before you start throwing cash at the rest of the car.
Agreed. Fixing the engine is #1. A pretty common mistake is too much carb for the engine. Assuming the carbs are reasonably sized and you just have them for the cool factor(no judgement here, I do after all have individual throttle bodies and while potential is good, setup on multiple throttle plate and linkage systems is always a pain), they still need to be correctly synched up or you are just going to be chasing your tail
I just am finishing putting a Tremec 5 speed in (287 - .68) this gets me a lower 1,2, and 3rd gear while solving my cruising problem (previously 2450 rpm @ 70 mph). ordered an Edelbrock "Multiport injection" system. This should free up a lot of horsepower! I took it down to performance shop to jet carbs down and discovered it was running non 1 degree of timing... they jetted it down which fixed most of the "too rich issue" and found that it ran a whole lot better at 10 degrees (they said at least another 50 hp and 700 rpm gain) but wont start once warm without having it to sit for 5 minutes. The best right now is timing set at 5 degrees where it will start warm. The Edelbrock comes with a handheld tuner and will allow the timing to be set at 12 degrees on full throttle but at startup and idle 5 degrees. So I am hoping to see a gain of 50 to 65 more hp thru the timing and good fuel system. when done, I will probably spent $8000 for good gearing and fuel delivery (OUCH). On the header issue, I have added the pics. notice the poer brake setup is a half inch above the valve covers so raisiing up motor to put hearers on dont work. Car was built to show, not for hp...


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