The Stovebolt Site Forums The Shop Area Engine Shop optimal RPM range for 235

Hi guys i did a search and read a few threads but wanted to ask my specific question anyways...... i currently have a 3.45 rear and a s10 4 speed with a 4th gear of 1.00 and 26.9 diameter tires. which puts me at about 58mph at 2,500 rpms

i used this caculator: http://www.angelfire.com/fl/procrastination/rear.html

i'm shopping for a s10 5 speed and have a set of 3.08 rear end gears at home ready to go in. but i wanted to find out what the optimal operating rpms for a healthy 235 is?!

i just drove about 700 miles roundtrip to a show and keped it at 2500rpms the whole time, the truck ran great other than eating a little oil and wasting a lil bit of gas! but i feel this should be lower, maybe in the 1900-2100 range tops?!?!?! i dont care much about power on this truck, i care more about long distance trips, gas millage and most importantly engine life. i'm trying to figure out what year s10 t5 to get, since they all have diff. 5th gear ratios.

i've found a 89 s10 t5 which depending on if it came from a 4 or 6cyl could have a 5th gear of .86(4cyl) or .79(6cyl). with such a overdrive ratio and the mentioned 3.08 rear gears this would roughly put me at 60-70mph at around 1900-2100 rpms. would this be too rough on the 235 or just right?!

thanks again!!!!

Depends on the weight/size of the truck.
If it is a 2 ton then you will need higher RPM to keep it moving.
If it is a 1/2 ton then I think about 2,100 - 2,200 rpm is ideal for a 235.

I put about 8,000 miles on my 235 in a 1 ton, all in about a month, kept it mostly at 2,450 or so, but would have run it a tad lower if I had the gears, I was limited to about 55 as it was.

Grigg

it's a 1/2 ton 1954 3100

Where you will be driving is also a factor..
The two times I have been to Florida I have yet to see any hills, unless they suddenly appeared you should be OK choosing an RPM on the low side of what folks recommend.

If you have a hard time deciding try to find some fellow Stove Bolters close to you who also have 1/2 ton AD trucks and 235's, and take a few test drives to see how different gear ratios feel.

Grigg

The best way to tell is to check the manifold vacuum when you are at cruising speed. If you can keep it above 8 inches of mercury most of the time then you are fine, but if it usually runs lower than that you will be getting into opening the power valve in the carb and it will start using more fuel. It is easy to just pull the vacuum line from the wiper motor and temporarily rig a gauge in the cab.

I'm currently running a 3.42 Camaro diff with a 1985 .72 OD T5, behind my tweaked 235 in a 54 truck with a 28.5" tall rear tire. Having tried a 3.08 behind the .72 OD T5 I can tell you that the 3.08 was too tall a gear for my setup. The 3.42 R&P and the .72 OD T5 is the optimum combo.

With the 3.42, my truck cruises all day in 5th gear at 2000 rpms @ 70 mph and will drop down to 45 mph and accelerate up with the normal traffic flow without a downshift. My area on Florida's Ridge, is quite hilly and at 60-70 requires no downshifts out of 5th gear on the long hills. The optimum range I've found with a 261 grind cam, dual Carter/Webers, and Fentons is between 1600 and 2400 rpms.

Numerically, it appears that you could run a .86 OD T5 with a 3.08 but this presents another problem. My 3.76 1st gear in my .72 OD box has me needing to shift out of 1st gear very quickly. The .86 T5 with the 4.03 1st gear would require shifting even quicker, especially with the 3.08 and with a shorter tire height. In essence, the 3.08 worked well as a 4 speed but was flat unless your ground speed was in excess of 70 mph. Ideally, a little taller 1st gear would be just the ticket in the .72 OD box.

The beauty of the 3.42 setup with the .72 OD T5 is that you can cruise around town in 4th gear at 35 mph or if you need to pull a trailer you can run in 4th gear very comfortably at 60-65 mph and still stay very close to the 1600-2400 power band. My mileage AVERAGES between 18-20 mpg with 50% of my driving over the open road. I run an average of a tank of gas through my truck each week so you can see it is not a barn Queen.

Another thing I observed with the 3.08, was the lack of oil pressure due to the tall R&P keeping my cruising rpms down unless I was running 70-75 mph. With the 3.42 I'm running 45-50 psi of oil pressure at 60-70 mph and it idles at 30 psi when hot with 10-30w oil and my operating temperature stays at the 180 thermostat temp even with the air conditioning operating.

When I set up my truck I took into consideration that the 235 was a 4 main engine and that it was initially designed to make torque at the bottom end. With the 261 grind cam, hydraulic lifters, intake and exhaust setup, I feel I've optimized the engines potential while stretching my fuel dollars without sacrificing performance. I hope this helps.

Dave

thanks allot Dave for your detailed answer. i don't have the trans or 3.08's in the car yet. and i guess i can put the trans in with the current 3.45's and feel it out. the trans i'm hopefully getting is out of a 89 s10 4cyl so 1st gear is a 4.03 and the O.D a .86 which according to the calculator would put me at 67mph with the 3.45's or 75mph with the 3.08's at 2,500rpms. so i'm guessing somewhere in the neighboorhood of 2,000-2,400 would be a good cruising rpm range?!

i plan on putting a 3x1 intake/carb setup, with dual headers and maybe a hotter cam into this 235 sometime in the near future. everything else is bone stock and rebuilt/being rebuilt. the motor is .40 over.

and yes florida is as flat as it gets, and i don't plan on towing anything.

I don't see where the .86 OD, 3.45 R&P with a 27" tall tire would be a problem. In fact, my truck vacuum pops up and it seems to enjoy running at about 2200-2400. However, I'm still running a straight axle and combine that with the wind noise I really enjoy kicking back and cruising from 60-65 mph. I had even tossed around going up to a 3.73. The problem with this is that the current 1st gear shift point comes at you so quickly, and the 235 makes such a nasty racket above 2300 rpms that I'd be looking over my shoulder all the time.

I'm going to do some scouting around and see what the potential is for dropping in a taller set of 1st gears.....second, third and fourth are fine in the NWC. I can see why some of the guys prefer the WC T5 out of the Camaros with the taller 1st and a closer ratio pattern all the way through. Maybe someone who is familiar with stretching 1st gear might chime in and offer some suggestions.

Dave

well the 1st gear i got in there now on the s10 4 speed is a 4.03 as well so i'm pretty used to it and don't really see it as much of a problem

Hi I run 2.73 rear gears in my 54 car with a 3spd. Your 3.08s in O/D with a T5 would be a really close final ratio to mine. I thought the 2.73s would be to high and Id swap then later. Well I love it I can cruise 65 all day and get 24 MPG! from a stock 235. Seems the ol motor likes pulling around those tall gears. Your truck should have fine getaway in town with the 4.03 1st gear.

0-1500!!!!!!!!!

well finally did it, my rear gears ended up cracking a few teeth off so i had no choice but to install the 3.08's and i did the 5 speed while i was it. well i can say is i should have done this a loooooong time ago. the truck still does burn outs with the stock 235 and i can cruise at 65mph at 1,800 rpms. i got the truck up to close to 90mph just to see if it would do it. and my MPG doubled!!!!

so 3.08's and a 5 speed are PERFECT!!!!!!!

Just to clarify....a 3.08 is a great R&P ratio for a .86 T5 transmission. That same R&P ratio is way too tall for a .72 S10 T5 transmission......and I've got an extra 3.08 differential as proof. I'm currently running a 3.42 diff which has me turning 2000 rpms at 70 mph. A 3.50-3.73 would give you higher efficiency at a lower ground speed. The 235 makes it's best torque between 1600-2400 and you'll maximize your vacuum reading at 2100-2400 rpms.

Dave

I used to get 3800 rpm out of my 235 for short bursts of "speed". Like, 70 mph for passing (4.56 gears).
But not on the Rochester 1 barrel carb. I changed over to the Holley/Webber 2 bbl and got about another 500 rpm out of it then.

Well you live in Fla, fairly flat, if you do a lot of turnpike driving the 3:08 with either 5 speed overdrive would be ok if you don't pull a load. The 2;73 would cause you to do more back and forth shifting with either 5 speed. 3:08's are perfect for these 1/2 ton trucks under most circumstances. I vote for the 3:08's as they would keep you in the 2100 to 2200 rpm range.

NEGATIVE....the 3.08 will NOT WORK as a 5 speed with a .72 s10 T5. Putting the T5 in 5th gear drops your rpms too low in the torque curve. You have to be cruising at almost 80 mph to get your rpms up where your vacuum comes up to a decent level and you can maintain your ground speed. As I tried to explain.....I went with a 3.08 with a 28" tire and the OD was useless. Even with the 3.42 you are running 70 mph at 2000 rpms and are still below the optimum rpm to maxiximize engine performance. I happen to live on the Ridge....a hilly backbone that runs north and south for about 100 miles in the center of the state and although it's not the Rockies, the 3.42 has to work unless you are running at 70-75 mph. Being that most driving in a straight axle truck, is more comfortable (and legal) at 55-65 mph I would adamantly recommend no lower than a 3.50 with a 3.73 being the best choice.

A 3.08 diff might be a viable alternative if you were running a .86 OD T5 but it will be VIRTUALLY USELESS as an overdrive transmission behind a .72 T5. I have an IROC-Z 3.08 posi/disc gathering dust as proof.

Dave

i storta slow, yall put a t5 in a 235, are you changing the rear-end. if so what do i need to get one out of to make it match. the t5 im getting is out of a standard s10 truck. trying to figure out what i need to do do to get the best setup. on another listing i got the info needed to put the t5 to the 235 engine. thanks

carefull everybody DAVE is the man, he knows it all i was just joking i didnt put 3.08's in with my t5. how could i dare????? honestly i'm just lonely and i have nothing better to do with my time then sit at home and lye about gear ratios in a message board =)

Camshaft plays an important role in optimal rpm range.
A stock solid lifter cam in a 235/261 has peak torque around 2000 rpm, giving best mileage/power around that range.

The late car 235 with stock hydraulic cam has peak torque closer to 3000rpm.
That's what is in my truck with stock 3 speed, torque tube, 3.55 rear and 28" tires.
On the freeways I get plenty power at 2500-3000 rpm at 60-70 mph and about 20 mpg.

My bored 261 in the coupe with the same driveline has a mild solid cam grind with the same higher than stock power band range as my hydraulic cammed 235, giving same speeds and mileage but with 50 hp more power.

When the engine runs at it's smoothest and quietest without vibrations, you know that it gives best performance/mileage with least wear.
With my cams that range is around 2500-3000 rpm on both engines.
Still plenty of hp left peaking at 4000 rpm, however mileage would go down.

i have a 235 with a t-5 and 28" tires and cruises very well at 65-70 i believe it has a .86 overdrive. it is reasonbly flat here. ron

im electrical so im sorry im not too mechanically. lets talk english to me, im slow. What i understand is a t5 out of a s10 will fit a 235 with some slight modifications, the s10 comes with 3 different ratio's. I understand that part. I want to cruise down the road about 60 or so. I have a 3/4 ton rear end. with the standard removeable drive shaft.are we talking about putting the correct transmission in to get the desired application. i will not be hauling anything think on the flat bed. just dont want all that slop and be able to change gears with a lot of grinding.you mention 28" tires, man great day. please help. trying to get it straight in my head.