[Goatracer1]

I have a good running 303hp 1962 389 in my Bonneville that was rebuilt sometime in the past. The compression is too high and I would like to lower it to something more pump gas friendly. I know nothing about what was done when it was rebuilt 2 owners ago. Can the chambers in the heads be opened up? Is there another 1961-1964 head that would work? Could I use D port E-heads and later intake? ( very expensive). I don't know valve lift so might hit pistons with wrong notches. I also don't really want to build new short block. Any suggestions?

[Sirrotica]

Usually the high compression chambers can't be opened enough to make it friendly to todays fuels. The answer is the low compression regular fuel heads that usually came on the regular fuel 2 bbl engines. Those chambers were larger as the roof was taller, and used shorter valves.

If you want to use your current cylinder heads, dished pistons would be the best alternative, although costly, and labor intensive requiring the entire engine be disassembled to change the pistons to dished ones.

[77 TRASHCAN]

It would be nice to know more about the rebuild, just saying.
Especially the cam. Knowing how far down in the cyl the pistons are at TDC.
Most combustion chambers can be opened up at least some . I have done such work. It is tedious and not everyone would want to tackle the task.

I had a 64 Bonneville that I was going to work on, same engine. A camshaft with more duration may be all you need. But, of course you don’t want to go too big on duration, either.
I’m sure your engine in it’s current condition is very octane sensitive? My 64 was, but it had never been apart (I never got to work on my Bonneville engine, a woman parked on the interstate in early morning rush hour traffic took that opportunity from me).

Little details like removing sharp edges in an around the chambers and on piston tops help, some.

You may want to try to degree your cam, either in or out of the car to see where you are now. Out of the car, is obviously easier, plus any so knuou do to the engine, can be accomplished much easier if engine is out...

Hope this helps

[aceaceca]

If you are getting detonation, revamping your timing initial, maximum, and curve can make a huge difference.
Back in the day, I ran my 62 Catalina on regular. We found back then and I believe it is true today that the engines
like 12 degrees initial and 9 in the distributor for a total all in of 30 degrees. We raced with that timing set up and
any variance made no quicker times. No vacuum advance. If you cannot sort it out with those changes my preference is
to find a set of low compression heads. I did that on my 65 GTO and it works a treat. Look here in Mr. Wallace's head
chart to find lower compression heads.http://www.wallaceracing.com/head1.htm

[charlie66]

Easiest way is to change the heads

[steve25]

What is the motors cranking compression now when tested out fully warmed up, Carb wide open and after 5 to 6 hits on the compression gauge?

You say the motor was rebuilt 2 owners ago, but was it fully rebuilt with new valves and 16 new valve guides, as but even just a little oil usage will reduce a given fuels octane level a good amount, so how is the motors oil consumption after let's say 200 Miles of driving?

None of your options are all that great in term of not changing out Pistons , but the one good thing about the stock closed chamber heads is that they are less prone to detonation then the 1968 and later opened chamber heads.

One option is to run a Cam or get a custom Cam ground to bleed off some cylinder pressure and just live with the slight reduction in low end power.

The next option ( despite what some wrongly say can't be done ) is to have the heads chambers opened up like that of the closed chamber 1967 670 casting and to also unshroud the the valves at there chamber sides, this would pick you up some 5 CCs and reduce the motors compression by 1/2 point and also pick you up some Intake airflow in the process of doing such!

This work would have to be done very carefully by someone who has done it before and will make the needed molds to to get the chambers even within 1 1/2 a CC, or go the expensive route of having such work done on CNC or big mill set up.

Your last option is to just stick with the early 389/421 heads, but run a bigger CC version of them like the casting number 77 I believe it is.

If the motor is only lightly pinging and depending on how many miles are on it then it could be a case of a combination or carbon built up and the sharp machined edges of the chamber and valve notches making for hot spots that produce the ping.

It could be that just a full chamber polish and sharp edged roll over session will get you good and of course we can't rule out the current timing curve that the motor is running now.

In short, my money's on a Cam change, a chamber polish / sharp edge clean up seassion and a distributor check out and recurve if needed and we can't leave out the question of jetting in the cause of some of your issue!

[Formulas]

Was the 303 / 389 a premium fuel engine back in the day ?

[steve25]

Not if I recall correctly, the 348 hp tripower motor was the highest compression, highest hp 389 non SD motor in 1960.

[Stuart]

Factory specs were 10.25:1 for the 303hp engine, but that was a common value for most standard equipment engines that year. Economy engines were 8.6:1, the Trophy engines were 10.75:1.

I've owned a number of 1961 cars (with engines essentially the same as in 1962) and I've never had any particular issues with pinging or detonation when running them on regular gas, but your results may vary depending on your car, your location, the octane of gas in your area, and how you drive.

[steve25]

So was the 303 hp a 2 bbl motor?

[Formulas]

Reason I asked, seems like a non premium fuel engine should run on today's premium fuel if the same parts are used

Today's gas is different and will run leaner in a 60's carb might recalibrate
Check spark curve and ensure it's not to aggressive somebody could of used a crappy aftermarket kit
A step colder plugs
The next heat range lower t'stat
Add a 068 camshaft
If mainly driven in summer block intake heat crossover
Check for balancer timing mark slip may have you thinking good but actually advanced
Alot of seemingly small things add up

[dhcarguy]

Would thicker head gaskets help with this problem?

Dave

[steve25]

When you run a thick enough head gasket ( or even stacking them) to get a compression change which is needed in a situation like this of more then 1/4 point then the increase in deck clearance and the accompanying reduction in turbulence at a TDC can make for the ping or knock situation getting even worse!

With that being said if the head gasket the OP is running now is less then the now normal 4.300" Bore size, then he can get a few CCs picked up by running the common Fel pro 8515 or 1016 head gasket for a start.

[Tom Vaught]

Originally Posted by dhcarguy

Would thicker head gaskets help with this problem?

Dave

Many Posts about this questions.

Stock Pontiac pistons are typically about .023" down in the hole.

Say you decide to install a 455 head gasket that is for easy math is .055"
thick. Say it crushes between the head to .050" and you have a .020"
piston in the hole dimension. Total distance from the head surface to the piston is now .070", and anything between .070" and .120" inches total distance historically may have combustion flame issues and the tendency to cause engine knock. Thick head gaskets to lower the compression ratio might actually be the wrong direction to go with the engine and cause issues.

It might be better to use the .042" thick gasket and run 89 octane gas in the engine.

Tom V.

[Goatracer1]

Steve25 the 303 engine is the basic 4bbl engine. Over the winter I'll try to check out the engine more and try to figure what might have been done to it. Maybe I can incorporate some of your suggestions.

[Goatracer1]

If I were to change the heads what chamber size should be the minimum cc's and what should be the maximum cc's I should look for? Thanks.

[steve25]

What heads does your 389 have, as maybe I need to dig deeper but I can not find the 303 hp motor until 64, not 62?

[Stuart]

Originally Posted by steve25

What heads does your 389 have, as maybe I need to dig deeper but I can not find the 303 hp motor until 64, not 62?

303hp was the hp for a 4 barrel 10.25:1 389 starting in 1961. You can look at factory brochures with the horsepower ratings for each year here: http://oldcarbrochures.org/United%20...iac/index.html

[Stan Weiss]

Originally Posted by steve25

What is the motors cranking compression now when tested out fully warmed up, Carb wide open and after 5 to 6 hits on the compression gauge?

You say the motor was rebuilt 2 owners ago, but was it fully rebuilt with new valves and 16 new valve guides, as but even just a little oil usage will reduce a given fuels octane level a good amount, so how is the motors oil consumption after let's say 200 Miles of driving?

None of your options are all that great in term of not changing out Pistons , but the one good thing about the stock closed chamber heads is that they are less prone to detonation then the 1968 and later opened chamber heads.

One option is to run a Cam or get a custom Cam ground to bleed off some cylinder pressure and just live with the slight reduction in low end power.

The next option ( despite what some wrongly say can't be done ) is to have the heads chambers opened up like that of the closed chamber 1967 670 casting and to also unshroud the the valves at there chamber sides, this would pick you up some 5 CCs and reduce the motors compression by 1/2 point and also pick you up some Intake airflow in the process of doing such!

This work would have to be done very carefully by someone who has done it before and will make the needed molds to to get the chambers even within 1 1/2 a CC, or go the expensive route of having such work done on CNC or big mill set up.

Your last option is to just stick with the early 389/421 heads, but run a bigger CC version of them like the casting number 77 I believe it is.

If the motor is only lightly pinging and depending on how many miles are on it then it could be a case of a combination or carbon built up and the sharp machined edges of the chamber and valve notches making for hot spots that produce the ping.

It could be that just a full chamber polish and sharp edged roll over session will get you good and of course we can't rule out the current timing curve that the motor is running now.

In short, my money's on a Cam change, a chamber polish / sharp edge clean up seassion and a distributor check out and recurve if needed and we can't leave out the question of jetting in the cause of some of your issue!

This would be very helpful information to have.

Stan

[MarkS57]

I had a 62 Bonneville with the 303hp 389. 4 bbl AFB, 4 speed super hydramatic. As an FYI, the factory made a dish piston for that engine, that's where the 8.6 CR economy engine came from. Here are NOS dished pistons on ebay:

https://www.ebay.com/itm/389-PONTIAC...UAAOSwUuFW10Hm