I have a Model A coupe I'm building, so far I have a bare 9", sbc rebuildable core and a ST10 4spd. The 4spd is the super wide ratio with 3.42, 2.28, 1.46 and 1.00 ratios. I'm running 7.50/16s, which are around a 31.5. I've calculated out that 60 mph if I use a 3.00 ratio I'll be at just under 2000rpm. The tall ratio would make 1st gear usable around town as well. (At 30mph she would be 3280rpm). I think with a light model a the big gear changes shouldn't be that noticable, but the drop in rpm is fairly substantial. Roughly 3200 at 30mph in 1st will be 2200 in 2nd. Higher RPM is worse at 60mph 1st-6566, 2nd 4378, 3rd 2726, 4th 1920. I know the cam will play a big role in making the transition feel better Plans are a pretty mild small block, 300 horse would be plenty. I'm not wanting to run a high rise, maybe a factory aluminum or edelbrock cb3x or similar and some form of cast iron heads. Aftermarket, or rebuilt 1.90s or 2.02 most likely. So any suggestions for a cam and engine components? Not looking for anything radical obviously, a bit of a lumpy idle and tire smoke and I'd be happy. Maybe occasional trip down the strip but much more highway and around town than anything. Input on gear ratio? I haven't bought the rear gears yet so that's all still open for suggestion. Thanks
What you have in mind, all sounds fine to me. 3.0 gears and a mildish cam (210 degrees at .050) and around 9:1 compression, should do the job. There are minor variations on this theme you can play with, but they'll probably all work about the same.
That's what I'm thinking, just want to make sure before a guy pulls the trigger and realizes he over looked something.
The cam that drives best with that gearing, will probably be milder than you have in mind. So....do you sacrifice the "hot rod sound" for the sake of having a pleasant to drive car? A trick I used to do when I built a few cars for folks way back when, was to tell them the cam was bigger than it was...they really liked how they ran, and could brag about what a big cam they had.
I've got mild Comp cam in my 327 (spec to have the torque kick in at 1800 rpm) and works great with a 3.09 rear end. Runs about 2200 rpm for 60-65 mph.
This is based on you winding it out to 30mph in first. You can actually snick it into second earlier. That diff [or slightly higher ratio] is a good combination for that gearbox. The 3rd to 4th spread is the same as an M20, but 1st to 2nd and 2nd to 3rd is spread wider. Changing into 2nd earlier wont hurt because there is greater torque multiplication in the lower gears I would consider even higher rear-end ratio if this is highway cruiser. Your combination is 1920rpm at 60mph [you won't need ear plugs] Use an RV grind cam [Crower Baja Torquemaster ] and a good vac secondary carb and you should get 30mpg with a lightweight model A
...................Ha, Jim, I'm a bit surprised to hear you refer to a cam as "bigger" as opposed to "more radical" or "more aggressive". I assume that is just how you expressed it to the owner to make them feel better. Kind of reminds me of those ads you see for cars for sale and the guy says, " Car has powerful motor, it's got a cam in it". Well no kidding!!!!....All in fun, Jim, as you are one of the most knowledgeable folks on here.......Don.
Thanks for taking the time to reply, I would say that me wanting a bigger cam than needed would be a fair assessment. I had originally thought something like a comp 268h, which is a pretty common cam. http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=73&sb=2 This would probably fall into the wanting too big of a cam category, eh? It would be nice to keep it around a 3.00 gear, but even a 3.5 is only 16% difference, at 60mph is only 320rpm. The problem obviously has you stretch her legs out, that the difference gets up there quickly. It would also limit 1st gears usefulness to a point around town. I don't want to building something too mild, but I absolutely agree that a little more mild, good top rpm, mpg, would serve better in the long term. I'd equate it to chopping the top, I'd really like to hack out 6-7" because it looks so cool, but know it will be a bitch to drive. 2-3" would make it easier, but just not enough attitude, so somewhere between 4-5" is a good compromise.
..................Personally, I think that would be an ideal cam for you, maybe with a 3:50 rear gear, but I will be eager to get Jim's take on this. Just for the record, in case you weren't aware, Jim knows his stuff and whatever he says, take it as gospel.
One other possibility would be to contact the Howard Cam folks (http://www.howardscams.com/) I was looking at their cams, picked one out that I thought would be good but after calling them they suggested a much milder cam. I keep forgetting that I am not a kid in the 60's and will never race the old 38 I will give a plug on Competition Products (http://www.competitionproducts.com/basket.asp) which owns Howard Cams, their prices are very competitive and both the folks at Competition Products and Howard Cams were extremely courteous and helpful when I called with questions. I'd never heard of this company until my neighbor told me about them. He has a very nice 40 Ford Coupe and a 50 Chevy gasser and also likes to get the most bang for his buck.
Don't lower the rear end gearing , it will be a mistake Even though it is only a 16% drop in ratio , it is multiplied at the lower end [where your gearbox of choice already has the advantage ] If you overlaid the ratios against a Muncie M21 [M22] for comparison 1st gear with a 3.0:1 rear is the same as having 4.66:1 gears behind the Muncie. 2nd gear with a 3.0:1 rear is the same as having 4.17:1 gears behind the Muncie. 3rd gear with a 3.0:1 rear is the same as having 3.42:1 gears behind the Muncie. So 1st, 2nd, and 3rd will be fun gears to play with ,and you have a great cruising gear also. Most engines usually only have a usable 2000 rpm power band. 6000 to 8000rpm on a race engine is 25% of the rev range Whereas 3000 to 5000rpm on a street engine is 40% of the rev range That is why close ratios are needed in racing but impractical on the street You need a cam that "pulls like a schoolboy" in the 3 to 5 range [maybe move the range 500 rpm either way]
Kerrynzl and Jim thanks so much for the information. Good to know, I will stick with the 3.0 rear. I'm glad that setup sounds like where it's at for my application. A almost 32" tall tire and 3.00 gears sounds crazy at first, but when I ran through the math with the low 1st, seemed like a good ticket for a poor mans overdrive. If I had a performer or similar low rise type aluminum intake, say camel hump or similar 2.02 valves, and a mild .210ish cam, what size carb would be ideal? I'm assuming around a 600cfm?
assuming it is a 350 you'll need 557 cfm at 5500 Rpm with 100% volumetric efficiency . A 600 vac will be fine but get the cam and make sure it is tuned to the manifold vacuum. Don't go too big in the valves or ports, you want a high velocity "fuel feeder" If you're driving on the primaries most of the time, the engine should be crisp and responsive For coolness I would consider the old Holley street dominator manifold . They're a direct copy of the factory Z28 manifold [cast in the same "Winters" foundry]
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