Rebuilding Motor - Pontiac GTO Forum
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post #1 of 13 (permalink) Old 04-14-2018, 12:23 PM Thread Starter
 
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Rebuilding Motor

I have a 1967 Gran Prix 400 HO Motor that I have had rebuilt by a machine shop and putting in my 1970 GTO and was wondering (rebuilding motor changing cam does it affect timing ? ) By the book it says to set the timing at 6 above TDC with the Vacuum advance unhooked. Thanks in advance
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post #2 of 13 (permalink) Old 04-14-2018, 01:49 PM
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Purty...

I'd be concerned about those 670 heads on pump gas unless you also had dished pistons installed to drop the compression. Forget about the 6-degrees initial timing, what really matters is the total advance that comes in when it's maxed out and no vacuum. 670 heads tend to like more advance than the later open chamber heads do, provided that your compression ratio will allow it without getting into detonation problems. What all can you tell us about how it was built?

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post #3 of 13 (permalink) Old 04-14-2018, 01:55 PM
 
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Originally Posted by Cory Wood View Post
I have a 1967 Gran Prix 400 HO Motor that I have had rebuilt by a machine shop and putting in my 1970 GTO and was wondering (rebuilding motor changing cam does it affect timing ? ) By the book it says to set the timing at 6 above TDC with the Vacuum advance unhooked. Thanks in advance
Nice looking car & pics.

Great question!

It can affect timing if you advanced or retarded the cam - but several factors need to be done to get this number. Many cam grinders build in a 4 degree advance and will be found on your cam card. So the best way to set-up your cam is to dial it in with a cam degree wheel. Of course, many of us don't and just install and line up the cam/crank sprocket dot's and go with it - me included.

Some of the advance ground in the cams are to take into account the eventual stretch of the factory type timing chain. However, if you use a double roller chain, they don't stretch very much and maintain a more accurate timing. The built in advance may even take into account that some engines need to be slightly align honed which can change the distance between the crank and cam gear axis. Align boring can affect this more and should be measured and if too much change, they sell timing chains specifically for this which are a hair smaller to compensate.

Using a degree wheel will confirm the cam position as per your cam manufacturer's recommendations as found on your cam card. This ensures your cam has been ground correctly and there are no parts, like chain & gears which could be off a tad.

So, advancing or retarding the cam more than what the cam grinder has built in is done with either a multi-slotted crank gear which has built in advance or retards and is typically marked on the crank gear. The other option is to use an off-set keyway specific to the amount of degrees you want to advance or retard.

I found this on another website which sums it up perfectly: "Retarding the cam typically decreases cylinder pressure and therefore more timing MAY be needed. Advancing the cam typically increases cylinder pressure and therefore less timing MAY be needed. The advancing or retarding of a cam usually moves the power band up or down in the RPM range as well. Moving the cam will not affect TDC as that is fixed - piston at the top of the cylinder is 0 degrees, but if you advance or retard the cam on an engine with a distributor based timing system the ignition timing will change as the cam drives the distributor and this will affect the reference point of the distributor (and cam) in relation to the crankshaft - so you must recheck timing after the cam is moved - ie advanced or retarded."

That said, the technical answer is "yes." The advance or retard, either added in by the cam manufacturer or by the engine builder will have an affect on your timing at the harmonic balancer. Timing is something you will find that you will have to play with as each engine build is different and many factors will affect what your engine needs with regards to initial timing, timing curve, and full timing. You did not say if your compression is stock or has been lowered to run on pump gas? 7 degrees is probably going to be a little low in general IF you lowered your compression (10 - 14 seems to be the average I see on this forum). If you still have the factory 10.5 compression, then 7 degrees may be right or even too much depending on the gas/octane you use. Listen to the engine as it will "tell you" what it likes. Too much retard and the engine will run hot. Too much timing and you will hear that engine destroying "pinging" sound. Today's gas can also have a big impact on timing.

Keep in mind that if the original balancer is used, the rubber ring that bonds the inner and outer inertia ring deteriorates with age and can move around quite a bit and be slightly off or even be way off - it is wise to replace them for safety, and to get the correct reading when timing the engine.
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post #4 of 13 (permalink) Old 04-14-2018, 11:16 PM Thread Starter
 
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WOW Thank you I am not very smart when it comes to total timing just learning. Wish I knew some old school mechanics I have a long way to go. Here is what the engine shop did for me.
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post #5 of 13 (permalink) Old 04-15-2018, 06:28 PM
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Keep in mind that cam timing and ignition timing are NOT the same thing. Cam timing controls when the valves open and close in relation to crankshaft position. Changing the cam timing requires at least a partial disassembly of the engine, far enough that you can get to and perhaps remove/re-install the timing chain and timing gears. Cam timing is determined when the engine is built. Ignition timing controls when the spark plug fires in relation to crankshaft position. Ignition timing is changed by loosening the distributor hold down clamp and turning the distributor body. The 6-degrees initial Pontiac factory specs are applicable only on a 100% stock engine, stock ignition system, stock distributor, etc. You may have heard the expression "advance curve". There's a system of mechanical springs and weights inside the distributor that adds more and more advance as it starts to spin faster and faster. How 'fast' the advance 'comes in' when this starts to happen is the advance curve. Heavier weights and/or lighter springs make it come in more quickly, lighter weights and/or heavier springs slow it down. There's a mechanical stop inside that controls what the maximum advance can be.

Those '670' heads on your engine have small, closed combustion chambers. Those 670's generally require quite a bit of ignition advance in order to make the best power - often approaching if not surpassing 40 degrees total. Unless your engine builder used dished pistons during the rebuild, then you're likely to have problems trying to run this engine on pump gas. By "problems" I mean detonation - which can destroy the engine in a short time. You may be in a catch-22 situation though because more advance makes an engine more likely to get into detonation, and those 670's "like" lots of advance.

It's really important to know exactly how your engine was built. Which pistons and head gaskets does it have? Has the block been bored any? If it's all "stock", then you're probably running close to 10.2:1 compression or better and are very likely to have issues.

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post #6 of 13 (permalink) Old 04-15-2018, 11:42 PM Thread Starter
 
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Thanks for the info I am going to talk to my engine builder tomorrow. I know he used the same pistons that was in the original motor and he told me that it was already bored 60 over see pic & he redid the heads. Do you have any questions I should ask him Thanks for the help!!!
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post #7 of 13 (permalink) Old 04-18-2018, 10:32 PM
 
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Thanks for the info I am going to talk to my engine builder tomorrow. I know he used the same pistons that was in the original motor and he told me that it was already bored 60 over see pic & he redid the heads. Do you have any questions I should ask him Thanks for the help!!!
Hi Cory , first thing as Bear has stated is to find out what you CR is. Those 670 heads are nice flowing head but have small chambers ( think 72cc)I would ask your builder if he cc the head to see where your at. The 60 bore will help you but over the years the heads could have been made even smaller with the work that has been done to them and deck of motor. Also what are your plans with this car? If your going to drive it a lot you want that CR down . Most Pontiac builder will tell you no more than 9.5 CR with Iron heads. If everything is stock your looking at well into mid. 10 on your CR which will not work with pump gas. It not to late to make some changes but we need to know the cc on those heads. Best luck Doug
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post #8 of 13 (permalink) Old 04-19-2018, 09:21 PM
 
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The pistons are not stock and look to be aftermarket cast pistons perhaps. Trying to match them up, the best I could find with that valve notch configuration was 17 cc's. The 670 heads are rated at 72 cc's, but they are often a few cc's larger, so might be 75 cc's or 72 cc's or something else if they have been cut. The heads, pistons with 17 cc's, stock block (.060" over)/rods/crank, and a .040" head gasket should put you in the under 9.5 compression which should be OK for pump gas. BUT, hopefully your machinist can help you out on the exact numbers to give you an accurate compression.
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post #9 of 13 (permalink) Old 04-20-2018, 07:46 PM
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Yeah, those are the famous/infamous "double eyebrow" pistons that have the valve reliefs on both top and bottom, and at two different angles so that they can be used in both pre-67 and 67-up engines. They are "one size fits all" and not "performance/quality" parts by any stretch of the imagination. They probably have somewhere in the neighborhood of 16-17 cc's volume. *IF* your heads actually have 72-cc chambers (they can vary from the factory and also may have been cut, as has already been pointed out), and *IF* you're running stock thickness head gaskets, and *IF* your deck clearance volume is where Pontiacs tend to be at around .010-0.015 "down", then your compression ratio at +0.060 overbore (which actually raises compression), will be in the vicinity of 9.4:1. That's an awful lot of "if's" to leave to chance for something this important, in my opinion...


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post #10 of 13 (permalink) Old 04-22-2018, 01:57 PM Thread Starter
 
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WOW This site is great and has some pretty knowledgeable people I appreciate all the help. I am going down to have the exhaust done tomorrow 10:00 and then I am going to talk to the builder of the engine and ask those questions thanks again.
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