I'm looking into doing the T5 swap to my 39 chevy. Main reason is i want be able to cruise at a decent speed while on the freeway. I've read all the threads but still have questions if someone could help me out I've got 1956 235 motor dual carbs dual exhaust on 165R15 tires. My axle is camaro with 3.36 gears. I found 2 trannys one has 1st-3.97 2nd-2.34 3rd-1.48 4th-1.00 5th-.72 The other 1st-4.03 2nd-2.37 3rd-1.49 4th-1.00 5th-.86 Which should I use in your guys opinion
Either one will work good with your 235. I think you will be happier with the .72 overdrive if you are planning a lot of highway driving. If mostly around town the .86 will be better. A 235 just doesn't have enough torque to pull a .72 overdrive with a 3.36 gear at much under 65 mph.
How come the VW tires? Is this a ‘low rider’ ? Those small diameter tires don’t help much with your concerns. But with the 3.36 rear axle ratio and the .86 OD will yield an effective 2.89 in 5th. A pair of rear tires of a size more common to your car, say 27” or so diameter, coupled with the the 3.36 would eliminate the need for the OD trans. If those tires are ‘keepers’, then the .86 will serve you best in top gear. Even with that said, the 1st gear ratio of either trans is pretty low and you may find 1st useless most of the time. No harm in starting in second gear though.
They're both so close 1-4. 165 15's!? That's a short tire as compared to stock. More math required but for starters. 2500rpm will drop to 1800 with .72 od. 2500rpm will drop to 2150 with .86 od.
Yes, more math required for a complete picture. Starting with the ‘rolling diameter’ (circumference) of your present tires. That is found by marking a spot on the ground and a corresponding mark on the sidewall......move the car one full tire revolution, mark the ground where the side wall mark ends up, measure that distance between the marks on the ground. That is how far your car goes for each revolution on the wheel. Assuming a tire diameter of about 23” (15” wheel + 4”x 2 = 23”) That will end up being about 72” (6 feet) A mile is 5,280 feet. Your wheel will rotate 880 times per mile. At an actual 60 mph, it will do that in one minute. So 880 rpm x 3.36 (axle ratio) =2956.8 rounded to 2957. 2957 x .86 (OD) = 2543 engine rpm in 5th gear @ 60 mph. That is 424 rpm per 10 mph in 5th......so 2543 + 424 = 2967 rpm @ 70 mph. Currently, based on the same assumption of tire diameter/circumference, you are pulling 3450 rpm at 70 mph. With the .72 OD.........2957 x .72 (OD) = 2129 rpm @ 60 mph in 5th. That is 355 rpm per 10 mph in 5th.....so 2129 + 355 = 2484 rpm @ 70 mph So, it’s either ......2967 rpm (.86 OD) @ 70 mph or ......2484 rpm (.72 OD) @ 70 mph Given the wind resistance of your ‘39 Chevy @ 70 mph, I think the 235 engine has a better chance of pulling the 2967 rpm with reasonable fuel economy than it does with the taller geared 2484. But, I would concede it could be just fine. Depends on how much torque it produces at these rpm ranges.
Yep bagged lowrider guys on artillery wheels. Would it be a good idea to change the rear end gear ratio
The answer to that depends on some facts unknown, such as specific characteristics of your engine. You could plug in a different ratio to the math processes shown in my prior post and see what rpm numbers it/they produces, but it’s still somewhat guesswork to fine tune the ratio combination with the engine unknowns. We’re it me in your shoes, I think I would decide how important a change is and either do nothing or choose one of the transmissions proposed......probably the .86 OD unit. But I am only speculating on the resulting performance and I am not you. Your call.
165/60/15 is 23" tall a 165/80/15 is 25" we need to know what profile. 235 max torque is around 2000 rpm for a stocker. Ray's numbers post 7 are for a 23" tire if you have a 80 profile 25" tire you are looking at 2719 and 2276 at 70.
@Hnstray has great data here. And he says there are still a number of unknowns. Maybe the biggest being the OPs driving habits. Of course the car's weight, frontal area, cg, peak torque rpm, peak hp rpm are all important. If it was my decision, I want to be able to cruise at 80! I would go with the .72 5th to keep the rpm reasonable at high speed. The added benefit of that T5 is that the taller 1st will be very slightly more usable. And necessary starting on hills or with a car full of people. My 2 cents. Yours may be different. Especially those born in 1943! Mark Sent from the red phone
3.97. I have never seen an S10 with 3.97 gear set, either. I have pulled about 30. The have either had the 4.03, or the 3.76 gear set. For perspective, that's: First: 3.76 Second: 2.18 Third: 1.42 Fourth: 1.00 Fifth: 0.72 It is a tighter ratio set than the two transmissions that you listed. The 4.03 gear set is useless, and has a terrible gear spread. First is too low, and the gap between first and second is way too big. Normally, for that engine, I use the 3.76 gear set S10 T5, a 3.73:1 rear gear, and a 6.70-15 tire (~27"). That's ~2200rpm, at 65. That's about as slow as I would turn a 235, at 65, and it would need performance mods to do that. Depending on the diameter of your dual exhaust, you may have actually made your low-end torque situation worse. Your 3.97 T5, 3.36:1 rear gears, and 25.4" tires, you'd be at ~2315, at 65mph. That's 115rpm over my typical target. A 235 will like neither being over-revved, or under-revved.
The slight difference in tire height from my calculation would add only a few rpm, and that's not a bad thing. I am more concerned with what you have done with the exhaust. If it is collectively too large in diameter, you will have lost low-end torque. That would be the antitheses of what you want, when running an overdrive top gear.
That is about what I recommend for a mild 350, or 400. You don't have enough exhaust flow, with this low revving engine to justify much more than a 1-3/4" single exhaust system, not at red line, and especially not at freeway cruise. What you have done by installing dual 2-inch pipes changed your exhaust system from one with compression wave motion (flow and particles move away from the cylinders), to expansion wave motion (flow and particles move TOWARDS the cylinders). You have killed the scavenging effect, which pulls spent combustion gasses out of the cylinder when the exhaust valve opens, and next the fresh intake charge into the combustion chambers during camshaft overlap. This happens because there is a low pressure area that follows a high pressure exhaust pulse. This pulse is only present when the exhaust is the proper size to facilitate high velocity (e.g. no larger than it needs to be, for the desired rpm and engine size). By killing the scavenging effect, you are leaving behind spent combustion charge in the cylinders, where it will foul the fresh intake charge. Remember what happened to vehicle performance in the 1970's? It went to hell. Why was that? In part, one of the things that was tried out, with poor results, was a new emissions control system called Exhaust Gas Recirculation, or EGR. You installed a totally unregulated EGR system, because your exhaust system is doing the same thing that EGR does. In the case of EGR, it was intended to dilute Oxygen in the intake charge, to reduce Nitrogen Oxide (NOx) emissions. Your EGR system has no controls. It just runs, and the slower the engine turns, the worse it is. TL;DR: You killed any hope of low-end torque that this engine is capable of producing with that exhaust system. That's exactly the opposite of what you want if you have a low rev engine, like a 235, and even worse if you are planning on putting an overdrive behind it. If you want the overdrive, put a single 1-3/4 system on it. If you don't, your 235 may be short-lived, as you will be lugging it, with whichever T5 you choose, and possibly even without an overdrive.
Looking at the calculator that was posted my best bet would be to switch to a taller tire in the rear?
Remember how aggravating automatic overdrives were constantly down shifting on hills? Think about having to do that manually. The .72 trans with your short tires comes close to stock 57 up 3 speed over drive numbers.
THAT is what I was trying to get across to you in my posts! But I offered info to give you insight into what the trans swaps had to offer because you asked. With the rear gear you now have (3.36) a larger diameter rear tire would give you the most improvement for the least money and labor. It will change the ‘look’ a bit however. Can you live with that?
The Jalopy Journal
