Benefits of Changing Cam Timing ??

[splicer]

I have a question for any of you that may have played with retarding/advancing cam timing.....

What I have is a big block stroker with 10.75 to1 compression ratio...
Cam is Cmp Cams 23-701 mechanical roller..
Lobe Seperation 110
Int Centerline 106
Duration 280/286
Lift 570/576

Cam is installed staight up at the moment.

Right now , car(3800 lbs) comes out of the hole hard with 0 to foot time of 1.60 but seems a little lazy for the rest of the run....11.42 secs. at 120


Would advancing or retarding the cam do me any good ????


Thanks
Splicer

 

[ArtH]

I have a question for any of you that may have played with retarding/advancing cam timing.....

What I have is a big block stroker with 10.75 to1 compression ratio...
Cam is Cmp Cams 23-701 mechanical roller..
Lobe Seperation 110
Int Centerline 106
Duration 280/286
Lift 570/576

Cam is installed staight up at the moment.

Right now , car(3800 lbs) comes out of the hole hard with 0 to foot time of 1.60 but seems a little lazy for the rest of the run....11.42 secs. at 120


Would advancing or retarding the cam do me any good ????


Thanks
Splicer

Generally, advancing your cam timing 4 to 6 degrees with increase your low end torque as the intake valve is now closing a bit earlier and building greater cylinder pressure. Retarding cam timing works better for high end horsepower. With your cam specs I don't know what you can expect. I do know that during the early 1970's some manufacturers retarded cam timing to lower emissions without using EGR valves. If you use and earlier crankshaft sprocket for the 60's you can advance timing as well. I recall reading and article in Hot Rod mag on taking the retarded sprockets and turning them around on the crank (with minor machining) and using that to restore earlier cam timing. YMMV.

 

[IQ52]

What does "straight up" mean to you, dot to dot? Or 106 or 110 ICL?

What are your cylinder heads? What do they flow cfm wise? What ignition timing, carburetor, intake manifold, headers, gear ratio, fuel pump, fuel line size? RPM shift point?

 

[lewtot184]

the cam already has 4 degrees of advance ground into it.

 

[IQ52]

the cam already has 4 degrees of advance ground into it.

Exactly. So straight up to me, would be installed on 110 degree intake centerline. Straight up to someone else may be where the cam is ground, potentially at 4 degrees advanced or 106 ICL, which would be dot to dot on the timing chain, IF the chain and associated parts (camshaft, gears, crankshaft) were manufactured perfectly.

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I quess what is being said is........I see a lot of red flags here. The simple answer is, retarding the cam might build a little more power in the upper rpms and advancing it in the lower rpms. But unless the cam is WAY off in timing, its not gonna make a lazy engine breathlessly exciting. I think there is potentially another problem.

 

[lewtot184]

Exactly. So straight up to me, would be installed on 110 degree intake centerline. Straight up to someone else may be where the cam is ground, potentially at 4 degrees advanced or 106 ICL, which would be dot to dot on the timing chain, IF the chain and associated parts (camshaft, gears, crankshaft) were manufactured perfectly.

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I quess what is being said is........I see a lot of red flags here. The simple answer is, retarding the cam might build a little more power in the upper rpms and advancing it in the lower rpms. But unless the cam is WAY off in timing, its not gonna make a lazy engine breathlessly exciting. I think there is potentially another problem.

i agree. the OP may think "straight-up" is "as installed, or dot to dot" and it isn't. my comment was to make the OP aware there is already advance ground into the cam. probably not a cam issue but there aren't enough details to give direction. could be that 120mph isn't bad for the combo.

 

[splicer]

What does "straight up" mean to you, dot to dot? Or 106 or 110 ICL?

What are your cylinder heads? What do they flow cfm wise? What ignition timing, carburetor, intake manifold, headers, gear ratio, fuel pump, fuel line size? RPM shift point?

Cam is installed dot to dot and checked with degree wheel...

Heads Eddy Performer RPM, stock
Carb 950 Holley DP
Intake Torker II with 1 inch spacer
Headers 2" TTI
Gears 3.73
Fuel Pump 120 GPH Carter Mechanical with 3/8" line tank to carb
Convertor 2600
Shift Point 6200
Timing 10 degrees initial with 24 degrees mechanical, all in at 2500 RPM

 

[RustyRatRod]

Ok, that still does not tell US where the camshaft is. "Checked with a degree wheel" could mean anything without a number.

 

[splicer]

Ok, that still does not tell US where the camshaft is. "Checked with a degree wheel" could mean anything without a number.

I verified with the degree wheel and dial indicator that the intake centerline is at 106 degrees....

 

[RustyRatRod]

Now THAT Helps.

 

[lewtot184]

Cam is installed dot to dot and checked with degree wheel...

Heads Eddy Performer RPM, stock
Carb 950 Holley DP
Intake Torker II with 1 inch spacer
Headers 2" TTI
Gears 3.73
Fuel Pump 120 GPH Carter Mechanical with 3/8" line tank to carb
Convertor 2600
Shift Point 6200
Timing 10 degrees initial with 24 degrees mechanical, all in at 2500 RPM

probably not the best intake for max performance. there are two carter pumps. a 6psi and a 9psi. from my carter pump tests there is a big difference in free flow. the 6psi pump (6903) won't come close 120 gallons per hour. do a fuel delivery check, psi and volume. those carter pumps have a transfer passage in them that's only 1/4"id; i think its a restriction. might think about an electric pump or at least doing some internal mods to the mechanical pump. i wouldn't dig into the engine until all the external stuff is verified correct. in my opinion, i don't think there is a cam issue.

 

[Cranky]

Now THAT Helps.

He said 106 ICL in his first post

How many cubes is the engine?

 

[lewtot184]

thinking about something else here. that 950 is an awful puny carb for a big engine. those things don't flow close to 950cfm. there just 750's with a contoured body and 850 throttle bores. they only have about 6sqin's of venturi area. a stock 440 had 7.8sqin's. cfm ratings are usually inflated and shouldn't be used as the only criteria for selecting a carb.

 

[IQ52]

With the most recent information, there are two no cost things to consider first.

The XR280R camshaft is going to want at least 16 degrees initial timing and maybe as much as 20 degrees. The RPM head is a good head but 34 total isn't going to get all the power from it either. So change the total mechanical advance in the distibutor to 18-20 degrees and try 16-18 initial for 36-38 total. Should wake the motor up a bit.

Unported RPM heads, even with the the smaller XR280R cam, on a 451 stroker are going to start wheezing a bit on the top end. Its probably makin' peak horsepower at 5600 with a 451 and 5200 rpm with a 500. I think shifting at 6200 rpm is slowing you down, try 5800 or less.

 

[splicer]

He said 106 ICL in his first post

How many cubes is the engine?

Sorry....505 Cubes

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probably not the best intake for max performance. there are two carter pumps. a 6psi and a 9psi. from my carter pump tests there is a big difference in free flow. the 6psi pump (6903) won't come close 120 gallons per hour. do a fuel delivery check, psi and volume. those carter pumps have a transfer passage in them that's only 1/4"id; i think its a restriction. might think about an electric pump or at least doing some internal mods to the mechanical pump. i wouldn't dig into the engine until all the external stuff is verified correct. in my opinion, i don't think there is a cam issue.

Should have mentioned that car has an Innovate Motorsports air/fuel monitor and data logger installed....engine is not running lean anywhere....

 

[splicer]

Bit of an update....rechecked cam ICL today because timing cover is off for gasket issue(leak)...it is now 108 degrees thanks to chain and gear stretch/wear...not a big difference but I think I'd be less inclined to retard cam timing since it's already doing that without any help....lol

Splicer

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thinking about something else here. that 950 is an awful puny carb for a big engine. those things don't flow close to 950cfm. there just 750's with a contoured body and 850 throttle bores. they only have about 6sqin's of venturi area. a stock 440 had 7.8sqin's. cfm ratings are usually inflated and shouldn't be used as the only criteria for selecting a carb.

Carb on mine is a 950 street HP with 1.5625 venturi's and 1.75 throttle bores so its about 7.7 square inches....still not huge but bigger than a 750....you thinkin I should be running a 1050 Dominator ?

 

[lewtot184]

Bit of an update....rechecked cam ICL today because timing cover is off for gasket issue(leak)...it is now 108 degrees thanks to chain and gear stretch/wear...not a big difference but I think I'd be less inclined to retard cam timing since it's already doing that without any help....lol

Splicer

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Carb on mine is a 950 street HP with 1.5625 venturi's and 1.75 throttle bores so its about 7.7 square inches....still not huge but bigger than a 750....you thinkin I should be running a 1050 Dominator ?

all the 950's in my catalogs have 1.375 venturi's, and the 1000cfm have the bigger venturi, but i don't have the latest catalogs so maybe holley did some changing. 1.56 venturi with 1.75 throttle bores is what i'd prefer. i think iq52 hit on something that i made some wrong assumptions on. i didn't look the cam up at first but now have. it's a small street roller and my brain was thinking 280@.050 (big cam). so much for not looking at details. 6200rpm and 500 cubes with that cam is probably a too much. 5800rpm may be too much. i'd try 5300-5500rpm and gradually work up.

 

[IQ52]

505 cubic inches? She is out of air by 5100-5300 rpm. To get the best out of it I would try shifting no higher than 5500 rpm. What's it taching through the lights, 5400 rpm?

Here is the reality of the whole setup. The RPM head OOTB is only going to flow about 275 cfm at .546" lift (.570"-.024" lash).

I have tested a very mildly ported RPM head with flows of 290 cfm @ .600" lift. With an Edelbrock Torker II and a 1.56 venturi carb, that combination only flows 264 cfm @ .600" lift. With the Edelbrock RPM manifold, 251 cfm. The M1 single plane, 266 cfm. The Edelbrock Victor, 271 cfm.

There isn't enough air there, with 505 CI, to make any more power beyond 5300 rpm.

 

[68 HEMI GTS]

ET doesn't seem far off from the 60ft time. what do you think it should ET?

 

[IQ52]

ET doesn't seem far off from the 60ft time. what do you think it should ET?

Thank you for pointing that out. You are correct. Never said it wasn't running good. I got completely lost in making it pull harder.

But at 120 mph, maybe 11.20