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My engine swap is finally done (I hope) except for adjusting the timing and checking the advance. I wanted to confirm this procedure for a stock points Oldsmobile 455 with a GM HEI distributor. The block is a 73, E heads, shallow recessed head pistons, unknown cam (not super choppy but more than stock). Please let me know if the following steps are correct:
1. Disconnect vacuum advance from distributor and plug port.
2. Adjust timing light to 10 degrees advance so at 20 degree initial timing the indicator plate will read 10 degrees. (Do I set the light so the digital readout says 10 degrees? Never used a timing light before not to mention an adjustable one).
3. Once initial timing is set to 20 degrees, reset the timing light advance to 20 degrees to check mechanical advance. The timing should read 0 on the indicator plate?
4. Rev the engine (gradually?) To determine if and how the mechanical advance responds. It is my understanding mechanical advance should reach 11 degrees if the timing light advance is set to 20 degrees and it should reach 11 degrees at about 3,000 rpm. Please confirm.
5. If the mechanical advance reaches 11 degrees to soon (what rpm?) It needs heavier springs, too late (what rpm?) it needs lighter springs or the weight bushings need to be reamed out.
6. Once the mechanical advance is sorted out, reconnect the vacuum advance to the distributor and to a manifold port.
7. Check vacuum/total timing (setting the timing light to 31 degrees of advance and revving to 3000 rpm?). Not sure at all how this last step is done or what total timing would be.
Hopefully someone can confirm/explain. I was a bit disappointed in the performance of the 455 set at 10 to 12 degrees after all money and energy spent.
If you want 20 set the timing light to 20 and adjust the marks to align up to 0.
yes I want 20 for the initial timing. So you are saying set the advance on the light to 20 and adjust the distributor so the marks align at 0. Then rev to 3k to check mechanical timing advance reading.
yes I want 20 for the initial timing. So you are saying set the advance on the light to 20 and adjust the distributor so the marks align at 0. Then rev to 3k to check mechanical timing advance reading.
Yes to the above except your looking for when the advance stops advancing and at what rpm.
Yes to the above except your looking for when the advance stops advancing and at what rpm.
one of my concerns is that the indicator plate on the motor doesn't go up to 20 which is where at 1100 rpm I understand initial timing should be to achieve 31 degrees of mechanical advance. 20 degrees initial and 11 degrees mechanical advance. So how do I adjust the light to calibrate for initial 20 degree timing at 1100rpm?
one of my concerns is that the indicator plate on the motor doesn't go up to 20 which is where at 1100 rpm I understand initial timing should be to achieve 31 degrees of mechanical advance. 20 degrees initial and 11 degrees mechanical advance. So how do I adjust the light to calibrate for initial 20 degree timing at 1100rpm?
then what is an acceptable rpm to achieve the 11 degree mechanical advance?
20 degrees initial is a lot are you sure your distributor only has 11 degrees mechanical advance?
anyway like old cutlass said if your dial is at 0 thats your actual timing on the engine timing mark. if you set the dialto 20 and your seeing zero on the timing mark on the engine its actually at 20 degrees. Once you set it to 20 for a base timing, You can rev to 4k or so to see how much mech advance you have. If you set the t light to 31 andit reads 0 onthe engine timing mark you have 11 mechanical advance.
20 degrees initial is a lot are you sure your distributor only has 11 degrees mechanical advance?
anyway like old cutlass said if your dial is at 0 thats your actual timing on the engine timing mark. if you set the dialto 20 and your seeing zero on the timing mark on the engine its actually at 20 degrees. Once you set it to 20 for a base timing, You can rev to 4k or so to see how much mech advance you have. If you set the t light to 31 andit reads 0 onthe engine timing mark you have 11 mechanical advance.
so if set to 20 degrees and rev the motor, would the mark advance from 0 to 11 degrees if the advance is working? I thought the idea is to see how it advances as you increase rpms. Or would it just jump to 11 degrees?
According to what I've been reading, you want about 31 degrees with combined initial and mechanical. There is a very short advance curve on the stock GM HEI distributor. I checked the stock motors that came with gm HEI distributors (307 ) and they are set to 20 degrees. Aftermarket HEI distributors may have more mechanical advance and not require as much initial timing to get the mechanical advance where it needs to be. I remember my friend back in high school tried to set the timing of a '72 SBC 400 my dad put into a '78 HEI Impala using '72 timing settings and it ran like ****. My mechanic did the same thing with my 455.
If all else fails, I could set the timing by incrementally increasing till it knocks them back it off a couple degrees. Seemed to work pretty well with my 350 which sadly enough would beat the poorly tuned 455.
You want to set your total advance at whatever rpm it stops to 34* with the vacuum advance disconnected. Then check what your idle timing settings (initial advance) are, do not readjust. Subtract the idle timing number from 34* and that will tell you how much mechanical advance is built into your distributor. I'm sure its more than 11. Limit the vacuum advance at 10*. If you want to play with fine tuning further, if you add 2* initial, it will add 2* to your total.
You want to set your total advance at whatever rpm it stops to 34* with the vacuum advance disconnected. Then check what your idle timing settings (initial advance) are, do not readjust. Subtract the idle timing number from 34* and that will tell you how much mechanical advance is built into your distributor. I'm sure its more than 11. Limit the vacuum advance at 10*. If you want to play with fine tuning further, if you add 2* initial, it will add 2* to your total.
ok so set the light to 34* advance, rev motor enough to swing weights to full advance and adjust timing to read 0 on the timing indicator. Is it essential to have the rpm to be at the exact point where the advance stops while setting the timing or can rpms be higher?
is there a dial on the vacuum advance or will it be a screw adjustment needing the timing light set to 44* to adjust?
I find its easier to move the distributor at idle and check your setting at the higher rpm. You may have to run the engine up a few times to get it right. It does not matter the rpm as long as its high enough to have stopped advancing. You want to buy a vacuum advance stop.
Looks like this
I find its easier to move the distributor at idle and check your setting at the higher rpm. You may have to run the engine up a few times to get it right. It does not matter the rpm as long as its high enough to have stopped advancing. You want to buy a vacuum advance stop.
Looks like this
thanks. Really helpful. Your essentially setting the timing to the advance and not the idle. It sounds like a good failsafe to prevent preignition. don't know how that item works, but I'll look into it. I assume if the advance stops at excessive rpm, a lighter spring is needed or bushings are too tight. Will the carb idle need readjustment after setting timing?
Again run the rpm up and note the rpm the timing stops advancing and the the amount of timing it advances to. If it stops at say 4000 rpms and your total is 31, then add 3 at idle and you should be at 34. Then double check. I usually adjust my carb last, remember the adjustments on the carb only set your idle air/fuel mixture, nothing more. If you want to really dig into your HEI, then get a crane kit and follow the instructions.
Why do you think the distributor only has 11 deg mechanical advance. I found a stock HEI generally has around 20deg or more. See the HOT ROD article: hotrod.com/articles/back-basics-recurve-hei-distributor.
Also, you might want to set initial to 15 deg. When I've run more than that I have experienced starter hold back when cranking.
You do have 12V to the distributor? You can't use the resistor wire to feed an HEI. You can use it to trigger a relay to supply 12V though.
Last edited by danktx; December 20th, 2020 at 06:46 AM.
What I've seen in HEI's is 18-21* of mechanical advance.
we disconnected the vacuum advance, set the timing to 34* with the advance at max. Hooked the vacuum advance back up, but still has lack of power in the 2nd half of the throttle range of motion. We checked and secondaries are opening properly from gas pedal revving. My 350 never had this issue. With the timing set the motor idles smooth, no lope at all. Otherwise the motor run fine. Before looking into cam or opening up the motor, are there any other things to check that would cause this issue.
What is your timing reading at idle with the vacuum advance plugged?
I think it was at about 18*. My mechanic set to the advance 34* at around 3000 rpm and backed off and checked at idle, but I don't think that was with the vacuum connected. We connected to the non ported connection on the carb.
I think it was at about 18*. My mechanic set to the advance 34* at around 3000 rpm and backed off and checked at idle, but I don't think that was with the vacuum connected. We connected to the non ported connection on the carb.
I doubt your timing stopped advancing at 3000 rpm if it has not been modded.
I doubt your timing stopped advancing at 3000 rpm if it has not been modded.
I may need to have it checked by someone else. My mechanic doesn't think the motor should make more power, but with 323 gears, it can't spin tires. I was there when he set timing. He used the throttle screw to increase rpm, set the timing to 34*, then revved motor beyond screw setting and The timing stayed at 34*. The distributor is GM stock out of a newer cutlass i had installed on my 71 350 about 25 years ago. Don't remember what that motor was. We replaced the advance weights and springs with stock replacement parts and used same gauge springs as before. Pic of distributor.
My mechanic doesn't think the motor should make more power, but with 323 gears, it can't spin tires.
Well that is not right. My car with a 350, 3.23 rear gears, and 245-60-15 tires will spin both rear tires completely through 1st gear and into second.
Back in the 80s I drove a 71 SX, 455 hitting on 7 cylinders, 2.56 rearend, and it would light up the right rear tire from 25 MPH.
So yeah, something ain't right with your setup.
Do you know if the camshaft was degreed during the build?
What is the vacuum reading at idle?
Well that is not right. My car with a 350, 3.23 rear gears, and 245-60-15 tires will spin both rear tires completely through 1st gear and into second.
Back in the 80s I drove a 71 SX, 455 hitting on 7 cylinders, 2.56 rearend, and it would light up the right rear tire from 25 MPH.
So yeah, something ain't right with your setup.
Do you know if the camshaft was degreed during the build?
What is the vacuum reading at idle?
The build happened before I bought the boat it came out of. We only pulled heads and intake to see if anything looked like it needed attention because 2 of the cylinders were at 130 and 125 psi while the rest were around 147 psi. The boat was idling rough and we are thinking the lower compression was probably a vacuum leak since the motor runs smooth now. I suspect the cam may be the cause from what I've been reading, either installation or application. One post I read, the cam was advanced 19* and he had no power, went through much to figure out the problem. I will check with mechanic regarding vacuum. I know there is enough to run the power brakes and steering no problem.
Since everything else appears to be in order (carb, throttle linkage, timing, compression at 145 psi), from the research I've done regarding the symptoms of my motor (no power, hard starts after warming up, hot intake manifold), I think coming out of a jet boat, the cam may be set up for high rpm performance at the cost of low end power. So since I don't know what the cam is, I am going to replace it and would like to know a good street application that will spin tires but not rattle My teeth too much in idle. I know going with too much cam would not be a good idea for performance matched to My engine. My motor is stock with shallow dish pistons (about 0.10" deep) 0.30 over, stock intake, E heads, Edelbrock 750 cfm carb. The compression ratio doing the math, 57.63 cu in÷(77 cc head combustion + 15 cc piston volume? + gasket thickness volume 0.17 cu in) = about 10:1. Feel free to correct my assumptions regarding compression. It's a 71 cutlass supreme, so about 3,600 to 3,700 pounds and 323 posi rear end. Any suggestions would be greatly appreciated. Thanks.
I would say for a next step if your confident the distributor advances are working correctly would be to check the cam timing, before going to another cam.
disclaimer i know nothing about boat cams but when i punch it from a stop my 455 goes from idle to 5k rpms in about the time it took to read that sentance. I would think even a poor running w 1 dead cylinder 455 would break the tires loose
does it have a lot of power at higher rpms ? VS below say 3k rpms?
I assure you with 145#'s compression is nowhere near 10:1.
I agree. And if the cam timing is retarded, the intake valves would still be open into the compression stroke which I would assume would hurt compression psi. My 350 has about 170 psi with similar shallow dish pistons.
I have seen the pistons, know what the heads and gaskets are. The motor doesn't blow smoke and there is no indication of internal wear.
wish I had the cam replaced when it was out of the car.
I would say for a next step if your confident the distributor advances are working correctly would be to check the cam timing, before going to another cam.
disclaimer i know nothing about boat cams but when i punch it from a stop my 455 goes from idle to 5k rpms in about the time it took to read that sentance. I would think even a poor running w 1 dead cylinder 455 would break the tires loose
does it have a lot of power at higher rpms ? VS below say 3k rpms?
it seems to. I blew up the 700r4 shrapnel and smoke and all at about 90mph but that's another story.
Is this a fresh build? Before tearing into the engine, are you sure the timing chain is good, and installed correctly? If the chain is stretched or installed wrong ( pretty easy to do if it’s a timing set with different keyways) your going to have low compression and lousy low end performance.
Any decent 455 should easily destroy tires, especially with 3.23 gears. Olds engines don’t really have a torque curve, if you look at a graph it’s more like a torque plane. The torque starts high, hits its peak, then drops back down to high. You say the cam sounds a little choppy, for the cam to be big enough to noticeably affect performance it would have to be big enough to shake the fenders off.
I’d pull the timing cover off and look at the timing set before pulling the trigger on a cam. Unless your planning to put a roller cam in, I’d avoid messing with the cam if at all possible. Too many horror stories and risks with flat tappet cam break in problems. The cam that’s in it now survived break in, no need to roll the dice unless absolutely needed
Is this a fresh build? Before tearing into the engine, are you sure the timing chain is good, and installed correctly? If the chain is stretched or installed wrong ( pretty easy to do if it’s a timing set with different keyways) your going to have low compression and lousy low end performance.
Any decent 455 should easily destroy tires, especially with 3.23 gears. Olds engines don’t really have a torque curve, if you look at a graph it’s more like a torque plane. The torque starts high, hits its peak, then drops back down to high. You say the cam sounds a little choppy, for the cam to be big enough to noticeably affect performance it would have to be big enough to shake the fenders off.
I’d pull the timing cover off and look at the timing set before pulling the trigger on a cam. Unless your planning to put a roller cam in, I’d avoid messing with the cam if at all possible. Too many horror stories and risks with flat tappet cam break in problems. The cam that’s in it now survived break in, no need to roll the dice unless absolutely needed
judging from the machine marks and lack of carbon in the motor, the rebuild is fresh. I removed the splash cover over the cam when the intake was off and the cam and lifters looked brand new. Wish we had pulled the timing cover off when the motor was on the stand. Based on the condition of the rest of the motor, I bet the chain is in good shape. I'll make sure the chain and cam were installed properly. Properly for a jet boat running constantly at 3500 plus rpm may be different than a street application. If a new cam is needed, I understand CR is important in choosing one. Any idea what my static CR would be with shallow dish pistons (approx 0.1" deep) and E heads with fel pro gaskets. The pistons a flush with deck at TDC. I did the math and got about 10:1. Not looking for exact number, but if the cam timing is off, I don't think the 145# test results would be a good indicator of static CR. The engine looked like it was a fresh rebuild when we pulled the heads and intake.
Last edited by Donaldbabineau; January 8th, 2021 at 06:47 AM.
Why do you think the heads are 77cc? The two sets of BBO heads that I have are in the 80-84cc range.
from the Olds FAQ, heads on this site. The chart show 77cc's for E heads except larger valve Toro heads which are 80cc's. The lowest are D heads, 69 and 75, and the rest are 80cc's except for some J heads 82. Of course I have made no direct measurement.
from the Olds FAQ, heads on this site. The chart show 77cc's for E heads except larger valve Toro heads which are 80cc's. The lowest are D heads, 69 and 75, and the rest are 80cc's except for some J heads 82. Of course I have made no direct measurement.
and I am looking for approximate CR. 3 cc's probably not going to be much of a difference.
yeah, there's all kinds of info out there. I was getting just under 10:1 with felpro gaskets. Looks like maybe 9.84 or 9.64 with 0 deck clearance. The one thing I am reticent about using the cam from the jet boat is we just don't know what it is. It could be ground for marine application. If we decide
to change, do you have recommendation for a cam in THAT CR range with 1800 stall t400 and 323 gears and 26" diameter tires.
December 19th, 2020, 03:02 PM
