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Cam for Absolute Mileage Build 4.8

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Old 02-13-2014, 10:51 PM   #1
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Default Cam for Absolute Mileage Build 4.8

First, let me apologize for asking a variant of what seems to be a somewhat annoying question. I've appreciated the insight of this forum as a long-time reader, so I certainly hope to make this a productive thread.

I'm building up a pre-OBD Suburban as a family vehicle, and decided to go with a Gen III 4.8 swap, due to better rod/stroke ratio and tolerable peak power numbers. I have experience driving a factory 4.8 pickup, and am confident that it should have plenty of power for what I need. Heck of a lot better than a minivan, anyway.

I'm doing a programmable Megasquirt ECU with the stock Gen III injection hardware. I have fairly extensive experience tuning with MS, so that shouldn't be a problem. The motor is in and wiring is just about done.

My question is in regard to the cam. Can anyone with experience tell me if swapping to one of the aftermarket low-RPM cams out there will net me a significant increase in efficiency? Or is the stock 4.8 profile close enough to not worry about dropping the extra cash?

This is going to be a long-term vehicle, so a little bit of benefit might make it worthwhile. Would like some real world numbers, if possible. Thanks!
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Old 02-15-2014, 08:49 AM   #2
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No aftermarket cam is going to give you more torque off idle than the stock cam, but the mid to upper rpm range would benefit from an aftermarket grind. I feel that you would leave too much on the table by not going with an aftermarket cam IMO.
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Old 02-15-2014, 11:04 AM   #3
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Jim,
Thanks for the tip.
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Old 02-15-2014, 11:07 AM   #4
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For your application the stock cam would be the best it is not worth it to try some aftermarket cam.
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Old 02-15-2014, 12:55 PM   #5
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Performance above 2000 rpm is of ZERO concern here. So you guys don't think I could benefit from dropping the LSA down from 114?
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Old 02-16-2014, 11:32 PM   #6
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For your wishes I'm not sure an aftermarket cam will gain you anything meaningful in the off-idle rpm range you are talking about, especially in an off-the-shelf grind. The stock cam was already designed for low-rpm pulling/mpg application you are talking about.

You could specify a cam that does perform slightly better in the rpm range you are targeting, but you will kill the overall performance of the motor for a few ft-lbs of torque you will barely feel in your heavy truck. Note that 10 ft-lbs of torque at 2000 rpm is a tiny 3.8 hp.

Tightening up your LSA is going to gain you torque, but mostly in the low/mid-rpm range, while slightly hurting your idle quality/low rpm mpg slightly due to the increase overlap which is obviously more important to your application than most.

That being said. I doubt you would actually notice the hit in idle quality or mpg that a more standard aftermarket cam will give you, and the extra mid range torque will make your suburban feel much stronger. I would strongly suggest you reconsider the rpm range you are targeting slightly, sub 2000 rpm is really low for gas motor who's peak torque is about 4000 rpm. But you can decide that while your driving it on the stock cam.

Just my $.02
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Old 02-17-2014, 12:42 AM   #7
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Yes you can get gains from a cam alone
Its going to be a aggressive lobe and installed on a low ICL 100-106

Comp Extreem RPM lobes are made for low end TQ and lobes from others there are out there but gains can be made from a long runner intake like a Fast and a open exhaust system for pumpin losses

Gearing and tire friction are a big part of MPG
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Old 02-17-2014, 07:35 AM   #8
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Quote:
For your wishes I'm not sure an aftermarket cam will gain you anything meaningful in the off-idle rpm range you are talking about
Quote:
No aftermarket cam is going to give you more torque off idle than the stock cam
Surely a small cam with tight LSA and low ICL is going to increase compression and gain some efficiency cruising. Sure it won't make as much power at idle, but cruising on the highway at 2000rpm will be more efficient with a custom aftermarket cam than the stock cam.
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Old 01-02-2016, 02:22 PM   #9
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Minimal overlap, early intake closing, late exhaust opening.
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Old 07-01-2016, 08:09 AM   #10
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There's always going to be a sacrifice. If you don't care about power anywhere over 4500-5000 rpm, then you can get what you're wanting, its just far from the norm around here so many will say it can't be done...more realistically, it can be done, it's probably just not worth it at all

Close the intake early to get high dynamic compression ratio and open the exhaust valve very late to ensure you're using as much of the power stroke as you can in terms of crank rotation. Overlap will suck fuel straight from intake valve through the exhaust valve without ever getting it burned, so avoid overlap when possible.

A small duration, tight LSA, with very early ICL will get the job done for you by doing the things above. Try maybe a 204/200 on a 105 LSA with a 102 ICL.

IVC=49 degrees (stock is around 52)
EVO=53 degrees (stock is around 58)

More lift than stock will also help with cylinder fill.

A long, small volume runner will also help torque as the air reaches peak velocity early. This helps create a complete cylinder fill at low rpm. the trade off is it will become a choke point at higher rpm.
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