I am trying to get a rough idea of rear gear i should run. This is what i have. 408 stroker, twin GT45 turbo's (70mm), 850 CSU blow thru, Hughes ported heads, M1 intake, 3800 stall conv, 727 trans,3000lb with driver and 33x21.5x15 Mickey sportsmans, Ford 9inch with four link and coil overs. I left out cam because i forgot specs. But can post those later if needed. Rear has 4.30's in it now.

 

3.50-3.73

 

Thank for the reply.

 

may even want to be in the 3.20 range

 

max rpm and trap speed target would help too

 

This a street truck that I'll take to track couple times a year. Hoping to be on the road in the spring. I want to be in the 9s in the quarter.

 

I agree, trap speed and RPM will help choose the right ratio.
So you are running a stock 360 block with a 4" crank, and two 70mm chargers. How much power are you wanting to make? You would only need to utilize only a part of one of those turbos to get to the point where your block wants to blow up. I have been told that 550hp is about the most you can put through one. Was also told the other day on here that if you fill the block it will hold 600hp reliably. There are guys out there making 500hp running 9's, so you should be able to meet your goal easily with just one of those turbos.

 

This motor was built on the premise of 500hp N/A. Now with that being said I won't be going to a Dyno but Engine builder said with the matched components, head porting and balancing motor should be at or slightly above 500hp NA.

 

I run 3.23's with a 30inch tire, but im thinking of trying 2.94's.

Its been 10.80's on 10psi @128

Remember turbos love load. I would say with a 33 inch youd be happy with some where in 3.23 range but you may have to experiment a lil

 

don't want to hijack this thread, but I have been wondering the same thing BoostedLilRed.
my build is very similar except i'm running single s475. 410 forged internals, no block fill. also 3800 ptc converter.
at the moment i'm running 2.75 gears with 28" tire and getting some converter slip. this is a street stripper. 6000-6500 trap speed, shooting for low 10's.

only doing brake in miles at the mo, but am loving boost. when I give it a foot full and the converter and boost come on, hold on.
cheers

 

I'm running a big tire so thats why i asked the question. i wasn't sure how much effect the taller tire would have. The truck should way in at 2800 to 3000 with driver. I'm shooting for mid to low 9's.

 

Heres a couple pics of my truck. #ad
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Who said the 360 block is only good to 550hp? An awful lot out there that make more than that and that does not include the nitros people. And I don't like NOS. Horsepower is not what breaks motors. RPM and detonation does! Unless its a Ford.

 

I'm not sure which person said that but I am not on here to argue about my set up. I am just trying to get an idea of where to start as everyones set up acts a little different. As you can see above truck is not stock by any means. I now have a pretty good idea what gear i am going to start with. And this truck will see more cruise nights then drag strips for now anyway.

 

exactly.....when I was doing my build I had all sorts telling me it will explode, despite there being 700+ hp boosted 360's out their. people who know not what they are talking about.

don't want to run outa puff mid track so tire and gear size important.

very nice truck by the way BoostedLilRed.

 

Only thing to keep in mind with gearing and load is yes it does load the turbo more which makes more power but that power is then transferred to the Trans which will have a much harder time transmitting that power through the rear gear if that gear is higher. You will need to have a more stout Trans to run a higher gear ratio.

 

always been a fan of the express trucks. Buddy had one back in the day NA . Fun truck. Do you plan to put the stacks back on?

as for gear the tall tire you have makes me think 3.55. As turbostang eluded to the more load you put on it due to gear will push the trans and converter harder. Not an issue but the stall speed and slip need to be considered.

 

I bet the stacks would sound pretty bad ass with the turbos!

 

The less gear it’s got, the harder it is on everything from the pinion forward…. That is true for sure. I will say that I ran my car with a stock 727 and 3.23/2.76 gear for quite a while. The only good part in the trans was the trans brake valve body. Stock raybesto’s tan clutches, stock 4-clutch drum (narrow band), stock sprag, used 2nd gear band, 3.8 lever, etc… nothing special at all. It wasn’t until the car (3700lbs+) hurt any parts until it got down to the 10.60 range? The first failure was the rear sprag (even though it had a low band apply valve body, it still killed the stock sprag). Then the front planetary spun on the output drive, right about the same time it sheared the pinion off the 8 ¾”. This was in the 1.60 60ft range most of the time, although it was in the 1.50’s occasionally.

That being said…I would never make a gear selection based on “turbo’s LOVE load” mentality. The only thing that turbo’s like about load, is it helps spool the turbo up. Load = Heat. Heat = Spool. But once it’s spooled (and you’re waste gating off a good portion of the exhaust volume/heat anyway), it doesn’t make a difference. If it’s got a loose enough converter in it, and a way to manage the timing curve… you’ll be fine with whatever gear you choose. My car has a 3.55 gear in it, and 28” tall tires (so not a ton of load). It will go from 10psi launch pressure to 16psi in 0.3 seconds (from the data logger).

That being said, these things make SOOOO much torque, it doesn’t really need a lot of gear. Most of us with decent power are knocking power out on the launch to make it hook anyway… so more gear doesn’t help anything.

This thing has a REAL big tire on it, so it will be able to take quite a bit of power on the launch. I wouldn’t under gear it on purpose at all because of that…


As for the blocks, many have broken them in the 550~600hp N/A form (not to say that there aren’t exceptions). But to make 550hp NA it requires a LOT more RPM then 550hp with boost. The RPM is what’s hard on stuff…

Again for reference… mine is ½ filled with a girdle and makes 750~800hp with the stock ’91 360 block. It even still has the stock cast crank in it. I have a death pool going which one will fail first… some say crank… some say block… LOL



Ideally this is how you would approach building it… Set a power/MPH goal for yourself. Figure out what fuel you’re going to run, so you have an idea what your boost limit is. Once you know your power, you can figure out roughly how much air the compressor needs to move. You can move more air in two ways… more RPM, or more boost. It’s a balancing act…but you need to find a happy medium between staying below your boost (fuel octane) limit, and keeping the RPM down. Pick an RPM that will move enough air on the compressor map to reach your goal while staying under the boost limit (fuel octane). Then pick a cam to run at that RPM, and pick a gear ratio to match it all.

If you’re running good fuel, what you’ll find on your build is the “sweet spot” between the boost threshold and RPM to move enough air is going to be HUGE. I would suggest keeping the cam fairly small so you don’t have to turn much RPM. That will help the block from coming apart. Hopefully your cam aligns with all of this, and you can pick a gear accordingly.

 

The trans is a race built 727 with new clutches front pump and manual reverse valve body.