[amcmike]

Originally Posted by amcmike

Actually it is, and today's OEMs do.

Modern engines use much higher compression ratios, enabled by direct injection. The higher SCR increases efficiency even at part throttle, thus increasing fuel economy (in addition to increasing torque under wide open throttle). They can get away with these compression ratios due to the charge cooling as the fuel is injected directly into the combustion chamber (especially with multiple injection strategy at under medium loads and low-mid rpm). This then allows them to downsize the engine displacement (if desired) to further improve fuel economy. This is why OEMs use DI even on small economy cars, not just performance vehicles.

I should have said, one of the reasons. The other of course, being emissions (with the exception of particulate emissions, which is actually higher than port fuel).

[GTOKIDRH]

Originally Posted by amcmike

Actually it is, and today's OEMs do.

Modern engines use much higher compression ratios, enabled by direct injection. The higher SCR increases efficiency even at part throttle, thus increasing fuel economy (in addition to increasing torque under wide open throttle). They can get away with these compression ratios due to the charge cooling as the fuel is injected directly into the combustion chamber (especially with multiple injection strategy at under medium loads and low-mid rpm). This then allows them to downsize the engine displacement (if desired) to further improve fuel economy. This is why OEMs use DI even on small economy cars, not just performance vehicles.

So correct me if I'm wrong. The LS engines before DI were mostly if not all in the 10+ CR range. I understand this was largely due to the use of aluminum as well as the heart shaped combustion chamber with a centrally positioned spark plug tip. Yes DI helps immensely.

Sent from my SM-N950U using Tapatalk

[NeighborsComplaint]

I had a '65 Lemans w. 326 2bbl and single exhaust as a kid. A dual exhaust was a big upgrade in performance. I then added a 4 bbl intake and Carter AFB which also made it even better with no other changes. I was amazed at the transformation which included basic, quality tune up parts. It was a 3 spd w. Hurst linkage and would chirp the tires in 2nd and 3rd with ease. If you're up to a cam swap, a Summit 2801 kit with lifters and springs and a new timing set will give a nice performance return on investment. The 670 heads aren't needed for what is just a nice torquey cruiser. The rear gears would be a better investment than the heads if you had to make a choice.

[Formulabruce]

Originally Posted by amcmike

Actually it is, and today's OEMs do.

Modern engines use much higher compression ratios, enabled by direct injection. The higher SCR increases efficiency even at part throttle, thus increasing fuel economy (in addition to increasing torque under wide open throttle). They can get away with these compression ratios due to the charge cooling as the fuel is injected directly into the combustion chamber (especially with multiple injection strategy at under medium loads and low-mid rpm). This then allows them to downsize the engine displacement (if desired) to further improve fuel economy. This is why OEMs use DI even on small economy cars, not just performance vehicles.

While true on modern cars, The 60's and 70's and most of the 80's were built based on Torque And were built to run on the highway just below their torque peak in most cases, for passing power, and economy.
Would be interesting if ANYBODY started Dyno mapping at 1500 instead of 3K .
My Point is that some things were designed well, and do the job.. well....

[amcmike]

Originally Posted by GTOKIDRH

So correct me if I'm wrong. The LS engines before DI were mostly if not all in the 10+ CR range. I understand this was largely due to the use of aluminum as well as the heart shaped combustion chamber with a centrally positioned spark plug tip. Yes DI helps immensely.

It's a bit complicated, but I'll try to simplify.

The GENIII passenger vehicles (Corvette, Firebird, etc.) were in the low-mid 10s with aluminum block/heads premium. The small engine trucks 4.8/5.3 were mid-9s, the exception was the 5.3L HO which used an aluminum block and 9.9:1. The 6.0 LQ4 was only 9:1 but was meant for HD trucks that weighed more and would be used for heavier loads and 87 octane. There was a 6.0L LQ9 which was a 10:1 variant of the LQ4 but was used in Escalades and required premium.


The GENIV LS (debuting with the LS2) were still PFI, and saw a bump in compression with passenger cars adding about 1/2 point. For the GENIV trucks, the iron blocks (and some aluminum) were typically 9.5:1, but often had VVT which bumped up the dynamic compression below 3500rpm. Some larger aluminum blocks (like the L92) were as high as 10.5:1 but recommended premium.

With the GENV having DI, compression even with VVT, is typically 11:1 on trucks, and higher for passenger cars (but with premium recommended). And we haven't even talked about Super Charged cars, but the LT4 runs 10:1 with 9.4 lbs of boost.

[PunchT37]

One can’t talk about modern engine specs as a reference to old car building without realizing the fast and powerful computers running them.

[PunchT37]

Originally Posted by 25stevem

Scott, I think that it was your lack of experience / knowledge at age 18 that made your 2bbl to 4 bbl adventure a flop, especially if it was Q-Jet swap over you did!

If nothing else assuming having in the ball park jetting conditions and having full throttle available the car should have had better drivability at low speed and at least 10 to 12 more hp.
I know that would have been the case for me!

I agree. Even 301’s came with 4 bbl carbs and 2.41 gears without problems.

[25stevem]

I killed a perfectly good 389 at age 16 when I rebuilt the carb.

The rebuild of my first carb went 100% fine, but then I found that I could get a bunch more idle rpm if a adjusted out the big center screw on the face of the AFB carb.

This I came to find was the idle air adjustment.

I left it that way and the motor ran so lean that the resulting detonation cracked two pistons!

So much for the knowledge of a fledgling 16 year old hot rodder!

[GTOKIDRH]

Originally Posted by Formulabruce

While true on modern cars, The 60's and 70's and most of the 80's were built based on Torque And were built to run on the highway just below their torque peak in most cases, for passing power, and economy.

Would be interesting if ANYBODY started Dyno mapping at 1500 instead of 3K .

My Point is that some things were designed well, and do the job.. well....

I would like to see that as well!

Sent from my SM-N950U using Tapatalk

[GTOKIDRH]

Originally Posted by NeighborsComplaint

I had a '65 Lemans w. 326 2bbl and single exhaust as a kid. A dual exhaust was a big upgrade in performance. I then added a 4 bbl intake and Carter AFB which also made it even better with no other changes. I was amazed at the transformation which included basic, quality tune up parts. It was a 3 spd w. Hurst linkage and would chirp the tires in 2nd and 3rd with ease. If you're up to a cam swap, a Summit 2801 kit with lifters and springs and a new timing set will give a nice performance return on investment. The 670 heads aren't needed for what is just a nice torquey cruiser. The rear gears would be a better investment than the heads if you had to make a choice.

I already have the rear end as well as the 4BBL setup and the 670 heads. So.....

Sent from my SM-N950U using Tapatalk

[Scott Roberts]

Originally Posted by 25stevem

Scott, I think that it was your lack of experience / knowledge at age 18 that made your 2bbl to 4 bbl adventure a flop, especially if it was Q-Jet swap over you did!

If nothing else assuming having in the ball park jetting conditions and having full throttle available the car should have had better drivability at low speed and at least 10 to 12 more hp.
I know that would have been the case for me!

I think I was actually 17 but non the less, you are probably right.. it was a carb and intake off of the original 400 that was in the car I blew up..
Never got it to take full throttle without rolling into it.

[NeighborsComplaint]

Originally Posted by GTOKIDRH

I already have the rear end as well as the 4BBL setup and the 670 heads. So.....

Sent from my SM-N950U using Tapatalk

Go for it. The potential for detonation will be reduced if you step up to an 067, 068 or Summit 2801 cam.

[lust4speed]

Correct me if I'm wrong, but in '67 the only Pontiac engine with dished pistons was the "Export" engine, and all others had flat tops. If so, bolting on a set of 670's is going to raise the compression ratio too high. Sounds like a miserable swap to me unless you go with new dished pistons.

[b-man]

Originally Posted by lust4speed

Correct me if I'm wrong, but in '67 the only Pontiac engine with dished pistons was the "Export" engine, and all others had flat tops. If so, bolting on a set of 670's is going to raise the compression ratio too high. Sounds like a miserable swap to me unless you go with new dished pistons.

Correct.

By that time the factory only used dished pistons on the 428 to maintain the same compression as 400 engines using the same heads.

Otherwise no dished pistons since the late ‘50s early ‘60s export engines with 7.6:1 compression.

[GTOKIDRH]

Originally Posted by b-man

Correct.



By that time the factory only used dished pistons on the 428 to maintain the same compression as 400 engines using the same heads.



Otherwise no dished pistons since the late ‘50s early ‘60s export engines with 7.6:1 compression.

It is my understanding that the lo po base 400 2 BBL (8.6:1 advertised) used in full size Pontiacs used different pistons. Am I misinformed? I see now that they are not dished as I previously stated. Apparently they are different however?

Sent from my SM-N950U using Tapatalk

[b-man]

The 8.6:1 heads have a larger combustion chamber volume than the high compression heads.

Pontiac regulated the compression by modifying the heads, not the pistons at that juncture

What is the casting number on your heads?

[GTOKIDRH]

Originally Posted by b-man

The 8.6:1 heads have a larger combustion chamber volume than the high compression heads.

Pontiac regulated the compression by modifying the heads, not the pistons at that juncture

What is the casting number on your heads?

Stock 142s

Sent from my SM-N950U using Tapatalk

[lust4speed]

Here is a photo showing pistons available in 1967. If your pistons are original then they are flat top. Notice the valve reliefs are different in the piston tops and your pistons would have the eyebrows in the wrong place for the 670 heads.

If you wanted to continue to use regular gas then the current 8.6:1 compression ratio is about perfect.

[Sirrotica]

Originally Posted by lust4speed

Here is a photo showing pistons available in 1967. If your pistons are original then they are flat top. Notice the valve reliefs are different in the piston tops and your pistons would have the eyebrows in the wrong place for the 670 heads.

If you wanted to continue to use regular gas then the current 8.6:1 compression ratio is about perfect.

Basically what I said earlier, but was met with, "I have 670 heads already". 670 heads aren't going to be the answer to the question, IMO.

Throughout the years the 670 head has been given a lot of ink, but there are better choices for the car in question. This is a cruiser, not a racecar. They surely wouldn't be my choice if I were going to try to wring power from a street car. They likely won't run on pump fuel, even premium fuel. I had a 67 engine for my 67 GTO racecar that I took off the 670 heads in favor of 62 heads before installing in my car, because I always felt the open chamber was superior to the closed chamber bathtub.

The regular fuel 400 basically used the 66 389 heads, hence the reason for one year only pistons with the older holdover valve inclination angle. In 68 the regular fuel engine became the same valve inclination angle as the premium fuel engines. I believe they gave up the bathtub combustion chamber also in all 1968 engines. When I worked at a Pontiac dealer in 1970 I remember a running change during the 1967 model year for broken valve springs, so depending on where in the model year the heads fell, they may have the old springs that were recalled.

If the car runs as poorly as the OP says, there has to be something fundamentally wrong with the engine, but I already have mentioned this in my previous posts. Putting hi compression heads on an engine that runs poorly is just a waste of time. Finding out why this engine runs so poorly would be my first priority, before pouring a bunch of money into a unknown quantity.

If the engine is worn out, or has low compression due to a mechanical defect, the OP might be ahead to find a 428/455 in running condition, and come out with more engine, for less money. Win, win

[25stevem]

Here's one thing you can Bank on with factory iron heads!

The open chamber heads with there 14 degree valve inclination angle make for a shallower chamber then previously used,, this makes for for a lesser surface to volume ratio which is less detonation prone then the earlier 326/389/421 heads with there stepper valve angles.

The sharp edge change over ridge in the chamber also acts as a much needed fuel re atomizing ledge, and this too makes for lesser detonation issues for any given level of cylinder pressure and heat that's in play.

You guys who have in the process of cleaning up there open chamber Heads with a grinder loaded with a even just a polishing roll and have worked over that sharp lip a lot, or even as I have seen totally ground it the heck out of there thinking higher levels of Intake flow would be had have done yourself a great disservice!!

You have increased the likelihood of detonation.

Even the sharp edge on the Intake valve side of the plug hole should be left there, but just broken to the tune of .005".

This once again is to re-atomize wet flow .