Been torn on swapping to 3.7s or 3.5s this winter in my 9”. I know boost loves load and all that but what is the case on a Procharged car that builds boost the faster you can spin it up?
currently unknown power. But car made 430s NA
car is 62cc prc225 heads with a 235/243 | .630/.610 | 111LSA with a t56 and is now running 8-10PSI depending on air..

currently it’s running 4.11s for the NA setup…car gears out before 1/4 at the 6500spark cut(lowered from 7,000 once boosted) and is so fast through the gears i feel it would pickup just by holding a gear longer through the higher boost.

so should I just jump straight to the 3.5s? This would put me at 131 at 6,000 and 144 at 6500. Do you think it would possibly be faster than the 3.7s and 4.11s due to being able to hold a gear longer while making higher boost. Thanks a bunch guys. I’m leaning more towards the 3.5s due to the 6,000rpm shift points being nicer for everyday hooning around yet still have the legs to stretch out once in 4th. The only crappy part about 3.5s is this donkey d*ck NA cam will need some help lugging around at 60mph.

 

If your on the limiter, it'll pick up for sure. I always geared to be at peak rpm at the stripe, NA or FI. With the T56 you can always gear down if it's lugging at cruise.

 

Yea I think either the 3.7s or the 3.5s will give me enough gear, but what about holding each lower gear for longer 3.7s vs 3.5s?..NA obviously that’s not faster but with boost that builds with rpm Im a bit torn on what I think I know…, I’d think it would keep me on the peak hp and higher boost longer per gear and make the car a bit faster down the strip, right or wrong? And it’s not so much lugging just that this big cam bucks a bit below 14-1500rpm and/or below 25mph speeds..I’m a tooner and not a tuner so I can’t get rid of it lmao.

 

I was also 4:11 gears with a NA big cam motor and on my turbo build I'm jumping to 3.70 in my 9" on a 28" tall tire.

Look at what ET you want to run in the quarter and how many rpms you want to turn your motor.

In my case I'm hoping for a 165 trap speed in the 1/4 which will be around 7400rpms and I plan on having my rev limiter at 8000rpm.

 

Yea I plan on running stock height 25.7? I think is what they are. 275/40/17s

if I recall my little bit of calculating, With a 6500limiter 3.7s would gear out at 133. 3.5s would be 144(if I recall correctly). Not saying I think my car will trap even close to mid 130s or 140s but I’m just waiting on this motor to give it up to go with something that will handle more so I think just in that regards 3.5s would be a one and done move unless you think I’m leaving a lot on the table going with the 3.5s vs the 3.7s while I wait for this motor to check out... My only gripe with the 3.5s is it will push this cam down low for most of my highway driving at 60mph in 6th gear.

so I know everyone talks about boost liking load of lower gears and all that stuff, idk how true that really is vs gearing the car to cross perfectly but where do centrifugal blowers come in on that? Gear them to just barely make the pass or let it load up a bit more on the lower gears?
I guess in your opinion would the longer gears be quicker or just as quick on a Procharged car that builds boost by strictly rpm vs a turbo? Id think by holding each lower gear for longer and higher rpm/boost through the lower gears it may actually pickup some even in the lower speeds vs my 4.11s currently. Seems in my car it really really likes 3rd and 4th, 1st isn’t usable and 2nd is so damn fast across the rpms it feels like it just starts to make power and it’s time to shift. If I recall on an hptuners log 2nd gear is over in sometimes less than 1.5seconds. I used to love the 4.11s, now they are so bad I gotta change to something and I’d like some opinions. Thanks guys.

 

Not everyone and the ones that do cannot explain what they are talking about. Once you toss that idea out of your head what everyone in this thread is telling you will make more sense.

 

Most of the discussions I’ve found on the big wide web turn out to be a big argument lol.
Usually it’s always to do with turbos and whatnots. But hell I don’t know either so I figured I’d ask on the one site I figure someone would know based off experience etc.


the more I think about it though the more of a dumbass I become for even questioning gearing ratios, initially I was worried about the lower gear out on the 3.7s.…I took the time once I got home today after seeing what you all are shifting around and played around on my gearing calculator..the 3.7s start to become really appealing the more I look at it, room to grow in terms of trap speed would come from a more rpm capable motor rather than a different ratio rear.

 

mstansbury - I would spend some time crunching numbers on the this calculator http://www.wallaceracing.com/calc-gear-tire-rpm-mph.php

 

Bear in mind that the calculators are not an exact science either, if you're looking for a given trap speed then I would add some cushion to account for variables.
There are also different series of carriers for some manufacturers like the GM 12 bolt uses a 3 and 4 series carrier for different ratios.
If the 3.73 gets you right to the limit of your speed target I'd drop down one more to 3.50, the difference will negligible plus you'll have the option to spin the engine higher or add a little more power later and not have to open the diff up again.
For the highway you'll just end up cruising down the highway in 5th instead of 6th, or just go 75 mph lol.

 

Keep the gear thats in it and raise the limiter. The load thing doesn't apply to blower cars and and only roughly does to turbo cars. Blower cars are RPM dependent, the more the better. Is there a specific reason for the limiter being so low? Gear the car to be a few rpm from the limiter at the end of the 1/4 in 4th.

 

Interesting there appears to be back and forth on load for blower cars. I’ve been trying to determine a good ratio for a forced induction build I’m doing and had some of the same questions presented here. Thinking I’m going with a 3.70 or 3.45 and just see how it performs.

 

I'd experiment shifting at 6500 but also 7000.

I went low 9s @ 145 years ago with a 5.7 + YSI combo, I was TH400 and 3.50 gear on 28s.

Never run a stick combo, but I think one of those options would make sense, more so if you decide 6500 max rpm.

 

Can you elaborate on the load for turbo cars?

I ran 4.10s in an 02 RCSB with a 4.8/SS2/7875 and it took forever to spool up. It was partially a function of very conservative timing but never really seemed to get going. Going to be running 2.88s in the Jaguar I'm wrapping up (6.0/SS2/7875) so I'm curious to see how the other end of the spectrum feels.

 

A lot more than gear comes into play in most cases. I’d guess not enough converter for your combo if you had spooling issues. Ask 10 different people, you’ll get 10 different answers. I’m less concerned with my “spool” and more concerned with my ET. In my experience, gear the car so that you are at or darn close to your limiter as you cross the finish line. That gives you the best mechanical advantage. Then build the combo around that. Easy to say… hard to do in practice. There is a “window” that performs "ok" and a smaller window that performs well. You run out of gear if you have too much gear, and you lug the motor if you don’t have enough.

I ran a 2.73 in my daily with a TH400. I used it like a glide with OD. Would do 140 in 2nd gear and had similar ratios as a glide with 3.73. AS far as using a 2.88 gear “normally”… that’s a horrible idea. When u shift into third, it will lug the motor WAY down and that’s horrible for the bottom end. Causes cylinder pressure spikes, detonation etc....

 

I guess I should have elaborated - this will be purely a street/highway fun car, mostly cruiser. I was hoping the reduced mechanical advantage from the tall gears would help get the converter up faster. Since I'm not spending time at the dragstrip the gearing for hitting the limiter at specific speeds is pretty irrelevant. More focused on response so that if I put my foot in it at 40mph I don't have to wait 4 seconds for it to wake up.

 

The answer is pretty much the same. If you are racing A to B you want the car to perform its quickest. Most “Street races” don’t last longer than a ¼ of a mile. (More like 1/8th IMO) Similarly, highway pull roll racers don’t want crazy low gear in them either. If you want a car that does 40-120 or 60-140… same deal. Gear it to hit 140 near its peak RPM if you want the most out of the setup. You need a goal to pick a gear and have it performed well. If you have your weight and HP you can guess gear that will be pretty close.

Running a 2.88 to try and improve turbo response is just a bad idea all around. RPM and horsepower are your friends in race cars. Lugging the motor with a huge gear and a ton of boost is a recipe for disaster. It’s not a diesel. And if you pair a big stall with a ton of gear you just ride the converter and never couples properly. I’ve done it and learned the hard way. Just sharing the information. The car will stil run of course, just not as well as it could. Also I get you may not have a huge selection of gears for that rearend.

 

Appreciate the insight, I'm only going with a 2.88 because it's what's already in the car haha. Open diff means it'll be a one tire fire anyways so once it comes out I will probably end up changing ratios to something in the 3.xx range. The pumpkin is actually based on the Dana 44 architecture so there are some decent options for ring and pinions.