Summit 2802 cam - what rear end - Pontiac GTO Forum
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post #1 of 7 (permalink) Old 11-19-2017, 11:32 PM Thread Starter
 
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Summit 2802 cam - what rear end

I posted this question on Summit's 2802 cam page, but this forum is probably a better place to ask.

I just rebuilt the motor in my 1965. It is a stroked 389, roughly 461 now. I am considering upgrading the powerglide to a beefed up TH350. I would like to cruise without the engine screaming, so I'm hoping I can use a rear end gear of 3.08 or 3.23. Does anyone know if that is too low for this cam? The Summit description says it should have a high gear, but I see other reviews that say it is a good cruising cam with a bored and stroked motor. If I cannot go that low of a gear ratio, I will probably end up going with a 700R4 and around a 3.73 gear, but I would prefer the TH350 setup.

Any opinions on 3.08 or 3.23 with this cam? Thanks.
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post #2 of 7 (permalink) Old 11-20-2017, 09:40 AM
 
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The 2802 is a decent cam for that size engine. I'd go with 3.08 gears. Your engine will probably have somewhere around 500ft lbs of torque. So, you'll have plenty of power for 3.08 gears. Shouldn't even need a stall converter. But, make sure your TH350 is correctly built with the 36 element sprag/drum/race assembly.

Coan Racing COA-31101 TRANSMISSION,"PRO STREET" AUTOMATIC

If you plan to do any racing, with sticky tires, a BOP 8.2 may not hold up, behind that much torque. Might be just fine with hard street tires. I and many others have wiped out 8.2's, with similar power.
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post #3 of 7 (permalink) Old 11-20-2017, 12:34 PM Thread Starter
 
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bigD thanks for the input. That Coan is actually the transmission I was looking at. I was however planning on a 2500 stall converter because the spec sheet for the 2802 cam suggested that as a minimum. Would 2500 stall be a bad idea with a 3.08?
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post #4 of 7 (permalink) Old 11-20-2017, 01:29 PM
 
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Originally Posted by Pacos65&67 View Post
bigD thanks for the input. That Coan is actually the transmission I was looking at. I was however planning on a 2500 stall converter because the spec sheet for the 2802 cam suggested that as a minimum. Would 2500 stall be a bad idea with a 3.08?

With a smaller cube engine, especially a 400, I'd want Rhoads lifters and a stall converter. But, in a 455 + engine, that cam acts much smaller. We ran the larger 041 clone cam, with Rhoads lifters, in several bracket cars. They idled smooth enuff to run a stock 13" converter. Launched from an 800rpm idle. Cut good lights & ran 60' times in the 1.60's. This was with a properly operating 750cfm Q-jet carb.

That 2802 cam is sort of a higher lift version of the 744 Pontiac cam. A cam with the 744 specs was used in the factory SD455 engines, with both auto & manual trans. They made more power with an 041 spec cam. But Pontiac decided to go with the smaller cam, for better emissions.

http://www.hotrod.com/articles/pon-0...55-super-duty/

Last edited by bigD; 11-20-2017 at 01:44 PM.
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post #5 of 7 (permalink) Old 11-20-2017, 08:54 PM
 
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Most cams you will see are based on the 400CI. So the given expectations and recommendations by the cam manufacturer/seller are based on that.

Bigger cubes will make a radical cam act smaller. Often the cam requirements are based on things like lift, duration, cam overlap, lobe center line, and lobe separation angle. Typically, as the cam gets bigger, the overlap gets bigger (which gives you that rough idle), and it bleeds off compression on the lower end. So, the cam manufacturers generally recommend a specific compression to compensate for the bleed off of compression on the lower RPM scale, then a higher stall converter to get the engine up to speed in the lower RPM range where the cam begins to make power, and this is then coupled with the higher gear to accelerate the car through the power range.

Switching from a Powerglide to a TH-350 will not gain you anything at cruise speed as final gear of either is based on the rear gear ratio. The TH-350 has the advantage of gearing, but a good Powerglide/torque converter can be a pulling combo behind a big torque engine. So if you are looking for a good highway cruise combo, then an overdrive trans would be the way to go.

As bigD pointed out, the 8.2 probably won't last too long under the extra torque and some hard running/launches - unless you run factory skinny tires that will break loose easily to absorb the torque, but then why have a 461? So a rear-end upgrade should be a consideration and since you will probably wind up doing this, why not plan on the OD transmission and something like 3.55 -3.73's depending on the OD's final ratio.

Can you use a 3.08 or 3.23 - probably because you will have the torque to pull it. As I recall, the factory converter is around an 1,800 stall converter. I have run a big .530" lift solid cam in a Chevy with a TH-400, 3.08 posi & wide tires. No problems pulling - but did eventually blow up the factory TH-400. A higher 2,500 stall converter might be a good investment, but not necessary a requirement. If you do go that route, you want to get one made for a Pontiac and not a 350 Chevy. You also want a "tight" converter as opposed to a "loose" converter. With a higher stall converter, when you hit the gas to the floor, then engine will flare up fast and get intake velocity moving. If you have a low stall converter, it is possible with a bigger cam that when you put the gas to the floor, the intake velocity won't build up fast enough and you could experience a bog or even a backfire - think of it as putting your car in 4th gear at low speed and nailing the gas, it'll bog down, buck, or stall out until RPM's pick up where 4th gear can be used. But if you nail the gas in 1st gear, the engine picks up and the car zips away. Now this is a bit of an exaggerated example, but it demonstrates the principals involved.

The above example relates to the lower gearing, BUT, if you had 3.73's, the gearing would make up for the higher stall converter because it would accelerate the car faster, thus working with the engine/cam to get into a higher RPM range quicker. So in this case, the stock converter would probably be OK - but now you will be really winding up the engine under highway cruising speeds and defeating your purpose in creating a highway cruiser.

So, MY OPINION, if going with th TH-350 try the stock converter first and low gears and see how it works and how you like it. If it seems a little sluggish on take off or you find stumbling issues from the engine, then try a higher stall. My guess is that the 2802 cam with a 461 should be fine with the stock converter. HOWEVER, if I were looking long term and putting money into the drivetrain behind a 461CI, I would bite the bullet and go Overdrive trans & upgrade the rear to the 8.5 10-bolt prepped per member Pinion head to handle the HP/torque and gearing to match the OD where you can cruise at 70 MPH at about 2,200 RPM's thereabouts.

Never used the Summit 2802 cam, but it seems to be a cam used by others and I have not read anything bad or disappointing about the cam. Seems to be a good choice for the 455/461. As bigD again pointed out, the 041 cam with Rhodes lifters is another good choice for the bigger cubes. Just make sure you have the matching valve springs for the cam and no binding of the springs at max lift and that there is plenty of clearance between the valve and the top of the piston.
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post #6 of 7 (permalink) Old 11-22-2017, 10:22 PM Thread Starter
 
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Thanks for the good input. A 700R4 was my original plan, with a 3.73 rear end. I like the low first gear and the OD with that combination.

I have been reconsidering now with the TH350, just because I think it would be much easier. No adapter plate, no changes to cross member or drive shaft, cheaper trans, cheaper converter, with the total package then being about $2 K less money.

I don't plan to race, and I don't need to cruise at 80 mph, although having the capability for both would be nice, hence the 700R4 attraction. But if I can make a decent combination with the TH350 and a 3.08 or 3.23 rear end, that may be good especially now that I learn my rear end should be upgraded to a stronger unit.

I was planning to get a new posi unit and gears anyway - it is originally an open 3.08. But now it sounds like I will be getting a new housing and axles too. That's ok though, I'll just put the original rear end in a bag and on the shelf as is to preserve the original components. As you mention, narrow tires kind of defeats the purpose of the built motor, but I could live with 225 tires. I doubt that's as narrow as what you were suggesting though.

Thanks again for the input.
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post #7 of 7 (permalink) Old 11-23-2017, 10:36 AM
 
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"...But if I can make a decent combination with the TH350 and a 3.08 or 3.23 rear end, that may be good especially now that I learn my rear end should be upgraded to a stronger unit.

I was planning to get a new posi unit and gears anyway - it is originally an open 3.08. But now it sounds like I will be getting a new housing and axles too. That's ok though, I'll just put the original rear end in a bag and on the shelf as is to preserve the original components. As you mention, narrow tires kind of defeats the purpose of the built motor..."


Sounds like a good plan, IMO.
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