[chris malish]

Gents, I have a pretty basic question. All other things being equal, is there a greater chance of detonation from installing the same cam "advanced" as opposed to straight up and down/dot to dot? Does installing the cam "advanced" have the potential to increase dynamic compression (e.g. because the intake valve is open less ATDC as compared to when installed straight up)?

Thanks,

Chris

[Stan Weiss]

Yes advancing the cam increases DCR and also Cranking Compression. In many cases when the cam is installed dot to dot it advanced because of the way it is manufactured.

Stan

["QUICK-SILVER"]

Advanced, the compression goes up because the intake valve closes sooner.
More of the stroke gets compressed.

Do need to know if your cam has any advance ground in before you try to add more. With advance ground in... Your gears will still be dots up.

Clay

[pastry_chef]

Originally Posted by chris malish

All other things being equal, is there a greater chance of detonation from installing the same cam "advanced" as opposed to straight up

That is a debate.. and depends on how far advanced.

Paul Carter has stated on many occasions he has cured engine octane sensitivity by advancing cams. Probably due to reduced reversion.

[chris malish]

Thank you gentlemen.

I put together an engine using more or less the same recipe as a number of others with the main difference that this time I installed the cam with extra advance. And it pings way too much.

I think I will reinstall the timing chain and see whether that fixes things.

[Stan Weiss]

What are the engine and cam specs? Can you do a cranking compression test on the engine?
Stan

[steve25]

Yes, the reversion and the fact that the expansion ratio seen in a low compression motor still contains a lot of heat/ energy that can’t be used in terms of making more power even long before the piston see’s BBC and then considering the exh valve opens means that at some point The closing of the exh valve and the rate of cylinder pressure rise is just not enough to fully pump out the hot exh in each complete 720 degree cycle.

This is partially why EGR was put into use, as in try to burn again what you could not pump out in the first place!

[tom s]

Let me put it this way,I have regarded engine a few time and have reduced pumping compression and have on one engine picked up HP.FWIW,Tom

[android 211]

The purpose of the EGR system is to 'contaminate' the fresh charge so that it does not burn with the same intensity and high temperatures that create NO2 emissions. One partial positive side effect is it reduces part throttle pinging so more spark advance can used and mileage improved.

My '96' head 068 cammed 455 pings with heavy throttle. Not light throttle, driving around normally. I hardly hear it full throttle WOT but when getting into it, say half way down I hear knock. Would retarding the cam possibly help? Its installed straight up right now. The spark advance curve is very much like a 70 GTO 455 stock curve.

[steve25]

Tom that’s simply because breathing in terms of hp is always paramount over compression/ cylinder pressure, and that the cam spec was not all it could have been in terms of that motor combo and the placement of the overlap!

[lust4speed]

Originally Posted by android 211

...Would retarding the cam possibly help? Its installed straight up right now. The spark advance curve is very much like a 70 GTO 455 stock curve.

If the pinging goes away with disconnecting the vacuum advance then fixing that curve is the easy fix. The vacuum advance curve isn't backing off fast enough and there is too much timing as you roll into the throttle. More spring resistance inside of the vacuum pot will work, and for that an adjustable vacuum advance pot would be the answer. Usually an Allen key is inserted in the vacuum hole and the spring can be tightened. Along with this adjustment you might also need a mechanical stop to limit maximum vacuum advance. My range is usually 14 or so total degrees added and advance not starting until around 10" of vacuum.

You might also be feeding the engine more mechanical advance that it can use or bringing in the mechanical advance too soon.

[dragginwagon406]

Originally Posted by chris malish

Gents, I have a pretty basic question. All other things being equal, is there a greater chance of detonation from installing the same cam "advanced" as opposed to straight up and down/dot to dot? Does installing the cam "advanced" have the potential to increase dynamic compression (e.g. because the intake valve is open less ATDC as compared to when installed straight up)?

Thanks,

Chris

Yes, advancing increases DCR and can improve low end. But, why not install it where the cam manufacturer designed it to sit? Often cam manufacturers "grind" the cam advanced already, even if installed straight up. A degree wheel confirms this very quickly (for symmetrical lobes).

I had a friend who wanted more low end when installing a new Comp Cam 268H in his Olds 455, so he installed the cam 4° advanced...the engine made no power anywhere...and destroyed the engine within about a year (near as I can tell, the intake valves (oversized in his G casting heads) were kissing the stock pistons (which didn't have proper valve reliefs for the oversized valves). Eventually, a valve head broke off (right while I was setting the timing (because it ran like a dog, and not the scolded kind).

Anyhow, it would be interesting to know what it cranks for compression, but there are really too many unknown variables to know which position is best.

[389]

326-350 cubic inch use the Pontiac 066 or 067 cam depending on gear ratio..

389-400-421-428-455 cubic inch 068 or Summit 2801 cam...

Use the stock style Morse timing chain and set it up dot to dot..

This has worked for me and many others for years.. No need to complicate things..

[grivera]

Paul Carter (GTOFREEK) had a different experience re: advancing cams related to Voodoo line.

https://forums.maxperformanceinc.com...ht=advance+cam

https://forums.maxperformanceinc.com...ht=advance+cam

[android 211]

I used the 067 in a 350. Ran great. I had headers and duals, got over 20mpg on trips with stock 2.78 rear gears and car was plenty fast. It was a small valve 2bbl motor I converted to 4bbl. About 9:1 compression.. The Summit 2801 works real well in a low compression 400. (.060 over 400 with 6X4 heads and log manifolds). It was an auto trans convt '67 with 2.78 rear also. Everyone who drove it was impressed with power and it ran fine on 87 octane. Got 17-18mpg on trips. Lm

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[Formulajones]

I've followed Paul's advice on several engine builds now and find that it works well on pump gas, even with 11:1 engines.

Cam dynamics will play a roll in how well this works but the idea behind advancing the cam to help with detonation is keeping the intake valve open further than the exhaust valve at TDC during overlap. Paul likes to shoot for .035-.040" intake lifter rise higher than exhaust. So when the piston travels back down the bore it's sucking more on the cooler intake charge rather than the hot exhaust charge, thereby keeping the the combustion process cooler.

I've found that some camshafts need as much as 6 or 7 degrees of advance to accomplish that intake/exhaust relationship, and the wider the LSA on the cam is, the harder it is to reach that goal.

[70GS455]

Advancing the cam too far results in pushing more hot exhaust gas into the intake tract during overlap, heating up the cool incoming charge. So it kinda defeats the purpose of advancing to reduce detonation

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[Formulajones]

Not really, the exhaust valve is open at BDC after the compression stroke, so as the piston moves back up to TDC on the overlap stroke, all the exhaust is pushed out of the exhaust valve. The intake valve is closed at this point. The intake only begins to open near the top of TDC on the overlap stroke, after most all of the exhaust gasses have been pushed out through the exhaust port.

So as the piston passes TDC during overlap and starts back down, the idea is to have the intake valve open further (sooner) by advancing the camshaft to pull in that cooler charge on the way back to BDC before the compression stroke.

[70GS455]

So if you say the protocol is to have more intake lift at TDC than exhaust lift, then the intake valve is open more at TDC. And so the intake valve opens before TDC as the piston is moving up pushing exhaust gas. At some point before TDC the intake and exhaust valves are open equally (unless its a single pattern cam and not advanced). So there is equal opportunity for exh gasses to be pushed into the intake or out the exhaust (actually since the int valve is bigger and flows more, its more likely that exh gas flows into the intake). This of course diminishes as scavenging takes over.

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[Formulajones]

The exhaust valve is open first at BDC after the compression stroke, and the large percentage of the gasses are pushed out the exhaust as the piston comes up before the overlap process.

I don't believe there is enough left to cause a hot intake track so that becomes less important. The more important part of the process is as the piston starts back down again after TDC overlap, having that intake valve open a pinch further drawing more on the intake track than the exhaust track, that's the key point, since the exhaust track is always hot, and much hotter than the intake track will ever be regardless.