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Help Choose OCG grind for 9:24:1
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Author:  hansbuscher [ Thu Oct 31, 2024 12:15 pm ]
Post subject:  Help Choose OCG grind for 9:24:1
Hello All.
Current setup is a 72 d100, granny gear 4 speed, 3.55 gears, cast crank block, drool tube head and Super Six manifold and carb. The head has a .100" cut, chambers at 44cc, pistons are .175 below deck, doing the math I get a 9.24:1 SCR. I run 91octane, limited advance distributor and the vac pod is set really tight. Other than grinding out/ cleaning up the intake, exhaust runners, ports and chambers there is no other headwork. Unfortunately I chose the 2106 RV cam whose IVC is 31* and the cranking compression is 175psi. Doing the math again and that's an 8.3:1 DCR. The truck runs great until the revs get near 2,800 it pings and even with the timing backed off it wont rev under any load past 3K-ish. Been reading here for acceptable cams with later IVCs many of you use and have positive things to say.
The Erson 270 looks to be really popular with the medium compression crowd but I don't see an OCG grind that looks similar. Don't think I need lots of valve lift but it seems that the longer durations always come with more lift. Fortunately/ unfortunately the OCG cam chart has like 70+ grinds and there's no chance I'd be able to whittle down all of those specs into the exact cam for my truck. I have no aspirations to race, rev past 4K or have a lumpy idle, the truck doesn't tow and rarely hauls anything larger than an A motor.
Thanks for sharing any knowledge you have.

-AHB

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Author:  volaredon [ Thu Oct 31, 2024 3:41 pm ]
Post subject:  Re: Help Choose OCG grind for 9:24:1
My CR isn't quite as much as yours but when I built my engine untold many harped in me that that was "the" cam I wanted. (Same as yours)
I don't remember why but I wound up going with their 819.
I have the engine running, but haven't driven my truck yet since I put the engine back in. I have other plans/work to do on my truck (85 D150) before I start driving it. I've never run a 2106 and I haven't driven my 819 yet so "I can't say yet"
As I remember both are a long the (what used to be known as) "RV cams".
Mine has an automatic, and 3.21 gears, have 2 OD trans options in my lap (1ea, manual and auto) and im trying to decide which one to use or whether to fix the leaks and stick with the non OD auto (727) that's original to my truck and worked fantastic except for leaks. I'm not sure how that compares to your setup.
I also have OS valves, a ported head, a ported exhaust manifold opened up to match a true 2-1/4" outlet, and an NOS BBD originally spec'd for a 70s 318 truck. Did you degree yours or just put it in "dot to dot" ???
Author:  hansbuscher [ Thu Oct 31, 2024 5:19 pm ]
Post subject:  Re: Help Choose OCG grind for 9:24:1
Thanks Don.
Yes the 2106 is degreed to 4* advanced. I had mistakenly installed it at 8* advanced, poor vacuum, idle and it really stank. Luckily the cam break-in was on a stand so not too hard to get the cam timing right.
Author:  DadTruck [ Fri Nov 01, 2024 5:18 pm ]
Post subject:  Re: Help Choose OCG grind for 9:24:1
The head has a .100" cut, chambers at 44cc, pistons are .175 below deck, doing the math I get a 9.24:1 SCR.

A bigger cam would lower the DCR, but a bigger cam would also hurt low RPM torque and HP.
You don't want that in a heavy truck.
I would keep your existing cam, but swap on a different cylinder head that is cut to give the motor a SCR of 8.5

That should fix the preignition issues, get you back to running 87 octane fuel and allow for a fair amount of ignition advance.

All of those are 'Wins" especially adding ignition advance.

If you go the head replacement route, there is nothing wrong with keeping the stock sized valves in an engine that will never spin past 4K,
but do spend a few dollars for a multi angle valve job and a back cut on the intake valves. You can pay for that with the money you save going to 87 octane gas.
Author:  hansbuscher [ Sat Nov 02, 2024 10:18 pm ]
Post subject:  Re: Help Choose OCG grind for 9:24:1
Thanks for the advice Dadtruck.
I have to wonder. The 340 had 10:1 compression. If Chrysler's engineers had built the slant with a 9.25:1 comp ratio what kind of cam would they have used?
Simply retard the factory cam to offer a later IVC? Is that a typical solution?
Author:  Dart270 [ Sun Nov 03, 2024 3:10 am ]
Post subject:  Re: Help Choose OCG grind for 9:24:1
Thanks for the well posed question and all the data. I’m traveling and without much time but I can work up an ocg cam for you later this week to suit you. What total mech timing advance are you running at 2800+ rpm where it is pinging?

Lou
Author:  volaredon [ Sun Nov 03, 2024 6:03 am ]
Post subject:  Re: Help Choose OCG grind for 9:24:1
Lots of engines are at 9:1 or higher these days without ping. I think you can tune the ping out with your current setup. The difference between 8-1/2:1 and 9-1/4:1 CR isn't that crazy at least when considering ping. Cam timing, ignition timing and advance, carb jetting, even vacuum leaks (or not) all play a part.
But just curious if you did so something to lower compression.what head gasket so you have in there? If you had to lower CR and not have to get a different head.
Nother question/ what part of the country are you from?
Author:  volaredon [ Sun Nov 03, 2024 6:12 am ]
Post subject:  Re: Help Choose OCG grind for 9:24:1
And before I say much more, talking cam numbers always makes my head spin. When I built my last /6, (the one with the 819 Oregon cam in it) it took alot to understand what all the numbers meant and how they related to each other. And I haven't built an engine since so you know what they say "use it or lose it". I go thru the same anguish every time I do get involved in anything about choosing a cam, and have to relearn those relationships every time.
Back in the 80s I built a 318 while in local community college auto shop. And picked a cam with the "help" of the local Super shops. Which was futile because they were so Chevy centric. That motor sure made noise but man was it a doag. Worse than it was originally. So ever since that experience I tend to chicken out and go down a cam size or 2 from the first cam that stands out as "the one" I think I want
Author:  Tim Keith [ Wed Nov 06, 2024 10:15 am ]
Post subject:  Re: Help Choose OCG grind for 9:24:1
This topic was my concern when I posted a question about water injection a few weeks ago. I hope the remedy is timing related.

Tim
Author:  hansbuscher [ Thu Nov 07, 2024 3:58 pm ]
Post subject:  Re: Help Choose OCG grind for 9:24:1
Thanks for the replies guys.
I haven't done anything to the long block since the engine was assembled a couple years ago. It is very snappy at low revs and gets mushier as the revs rise above 2,500. It will go to 3,000 but it feels like going uphill and the hill is getting steeper and steeper and the gas pedal is disconnected.
Base timing is 9*, mechanical is 24*@ 2,600, with vacuum 32* but the can is very tight so all the vac advance disappears as soon as the engine sees load. I would rather change the cam to work with what I have. Re: What cam would Chrysler have designed for a 9.5:1 Slant Six delivering efficiency, smoothness and power.
Truck is my daily in Los Angeles.
Author:  volaredon [ Thu Nov 07, 2024 5:27 pm ]
Post subject:  Re: Help Choose OCG grind for 9:24:1
I don't think a cam change will be necessary.
Author:  Dart270 [ Fri Nov 08, 2024 3:24 am ]
Post subject:  Re: Help Choose OCG grind for 9:24:1
I just realized you said 175 psi cranking compression. This means to me that you can run on 91 octane with suitable timing and fuel. I have run up to about 190 psi on CA pump premium (when I lived in LA in the late 90s thru 2002).

Since you are saying max timing advance at 24 deg with the vac disconnected, that should be safe and not ping. Try running with vac adv disco'ed just to make sure it is not hanging up or somehow still giving advance. If it is still pinging, then it is possible/likely that your damper ring has slipped and you are not actually getting 24 deg max advance but more than that. You should be able to run up to about 28-30 deg with that cranking comp value. You should also have gobs of torque across the RPM range and good HP up to at least 4500 RPM if carb is working right.

If your damper has slipped you can just replace it with a new/rebuilt one, or you can recalibrate it by getting a TDC stop and degree wheel. I would just replace it because if it has slipped then it may slip again or come apart.

Try those tests. If timing is actually correct, then you have a carb/fuel problem. The other option is that your cam timing is too far advanced, as that would explain your soft power above 2500.

Happy to discuss more. I would not do a cam swap at this point.

Lou
Author:  Tim Keith [ Sun Nov 17, 2024 11:17 pm ]
Post subject:  Re: Help Choose OCG grind for 9:24:1
It's worth the effort. I follow the Ford 300 as well. Most build the 300 stock, or for more power as a daily commuter. Those who focus on increased torque and fuel economy are far fewer. I dont know if some claims are true, but there are reports of 25 mpg to 28 mpg in an F150 with the big six with raised CR, that does moderate towing. The same tuning issues of the 225. Other similar attempts only achieve 12 mpg. I don't know what to believe, but I figure Lou can help with the 225. These old trucks are relative light compared to new trucks. I prefer an inline six, I can drive slower if I tow more than 3,500 pounds every other year.

Tim
Author:  hansbuscher [ Tue Nov 19, 2024 9:23 pm ]
Post subject:  Re: Help Choose OCG grind for 9:24:1
Appreciate the replies.
As a "test" I closed down the intake lash from .015 to .008 and it definitely has better manners. I'm sure on a hot day at 65mph with a light load it would still rattle a bit. Maybe I'll retard the cam another 4* which would put it in the middle of overlap or whatever no advance or retard is called.
Author:  Dart270 [ Wed Nov 20, 2024 3:36 am ]
Post subject:  Re: Help Choose OCG grind for 9:24:1
What is your operating temperature when it is rattling? Do you have a good aftermarket temp gauge with reliable reading and sensor in the head near the waterneck?

I can certainly help with a new cam if that is what you want to do. Retarding 4 deg should have a substantial effect on cylinder pressure and thus preignition.

Lou
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