[Red Box Rebel]

Need some advice on doing a partial rebuild on my stock '68 400 GTO engine.

Here is the current game plan:

Will be using the following parts:

Original RAM Air factory Exhaust Manifolds and stock '68 duel exhaust system
3:55 rear gears
Roller cam with kit from Lunati in Mississippi (#20510710 1400-5400 RPM Range 507/515 211/219 112lsa)
Edelbrock ProFlo4 EFI
Edelbrock Aluminum Heads (either 72cc or 87cc) designed for use with Roller Cam

The lower block remains stock. Car has factory A/C and Power Steering and has the stock '68 RAM Air induction system with open scoops.

The car will be driven on the street and taken for long cross country trips as well as some in city type driving. I will expect to use regular unleaded fuel with a low of 87 and a high of 93 octane.

Edelbrock can sell me a 72cc head or an 87cc head. If I was able to buy premium leaded fuel from the 60s or 70s, the 72cc head would be compatible with the high octane fuel.

The question is, can the engine run on standard pump gas with a 72cc head? Will the EFI with aluminum intake, roller cam and aluminum heads allow the engine to run on pump gas without the need to use expensive octane boosters in every tank?

Stock GTO compression in 1968 was 10.75:1. I do not know what the compression ratio with 72cc heads will be, but I am sure that is is very close to the original factory 10.75:1 ratio.

I am also not sure what the compression ratio will be with the 87cc heads, but I assume that it will be closer to the 9:1 ratio.

Although this car will see the drag strip once or twice a year, it will be used mainly on the street. Street-ability, therefore, is the goal.

Please vote for the 72cc or 87cc heads and explain why. I would love to have the 72cc heads if they will run on pump gas, but will settle for the 87cc heads if they work best on pump gas driving down the Interstate at 70 to 80 MPH.

Comments and suggestions welcome. Thank you

[mgarblik]

I ran a few numbers based on the information you provided. These are a an educated estimate IMO. Based on your info, 72 CC heads would give you 10.36CR. 87 CC heads would be 9.0:1 . IF your current engine made 400 HP with the 72 CC E-heads, the 87 CC chamber heads would make 385 HP. So approximately a 15 HP loss from compression ratio alone. 10.36 compression would be right on the detonation line even with aluminum heads and the crappy pump gas we get around here. With a careful ignition curve and really good fuel map, you could just get by on pump gas IMO. An expert tuner could get you there for sure. 9:1 would be a no-brainer and you could run on any crap gas. Depends how much aggravation 15 HP is worth to you I guess.

[Baron Von Zeppelin]

Do you plan to rebuild the shortblock ?
Just asking because it may need a fresh bore, which will raise CR a small amount more.

In that case - you could just have new forged pistons with mild dish to come in at 10.25 , or less , and use the 72cc heads

If 93 octane is readily available in your area , 10.5 CR is probably safe
91 octane - idunno for sure

[4zpeed]

Personally I would lean toward the safe side, after all look at how much you are investing... not to mention your time and effort.

Think also of the places you'll likely travel from what you've stated, most likely distant, possibly remote and unfamiliar.

Take into consideration different geographic areas, climates, altitudes, and availability of fuel, that itself will play a large part.

15 HP is nothing compared to being trouble free, less finicky in elevations and will undoubtedly add longevity to your engine.

If you want it back that bad, use some cut outs for the exhaust or throw some Hookers on that bad boy!


Juz my
Frank

"Edit" - (PS. I would also post this question in the street section, you may get a more diversified answer.) Good Luck !

[Red Box Rebel]

Great replies!! Thank you.
The 87cc Heads sounds like the best way to go. I will post the results after all the work is complete sometime next year.

[grandam1979]

I would not use 87cc heads on a stock 400 comp will be about 8.75 with the 72cc it’s just over 10.1

[Red Box Rebel]

Originally Posted by grandam1979

I would not use 87cc heads on a stock 400 comp will be about 8.75 with the 72cc it’s just over 10.1

Okay and thanks for responding.

If you read the previous posts, the consensus is that the 87cc heads will yield a 9:1 compression ratio. You are claiming that the compression ratio will be only 8.75:1 with the 87cc heads. How are you getting those numbers and why are your numbers so different from mgarblik?

Using the 72cc heads will only yield a 10.1:1 compression ratio according to your post. If your numbers are correct, I would be better off with the 72cc heads. If not, I would have a compression ratio that would not have a high enough octane fuel available at the corner gas station.

The investment in the Edelbrock heads is approximately $2,500 not including tax and shipping costs. If the compression is too high for pump gas, I will have a serious problem on my hands.

Please make the argument and let me know how you arrived at your numbers. Thank you

[grandam1979]

I used Butlers compression calculator I put the pistons .015 in the bore it’s probably more. I used .035 as the head gasket thickness you could find them less and more. I used 8cc for the piston with the stock double valve pockets it’s probably more.

[grandam1979]

So just for fun I did it again with the block .000 piston in hole and a .028 head gasket that’s only 10.52 comp. with a 72cc head.

[grandam1979]

FYI same as above with 87cc head 9.1

[mgarblik]

Grandam1979 may be more accurate. I just used the simple calculator from the Wallace site and didn't change any of the basic parameters provided. I think the piston cc's were 6 and the gasket .041 and not sure about the piston to deck. It was an estimate. Sounds like the Butler calculator has more inputs and may be better.

[Baron Von Zeppelin]

Should wait until you have the old heads off and check/test all your parameters.
You can get 91 octane all across the nation.
Some areas have 93 octane. But i would build for 91 octane - or even 89.

The octane rating system has changed since the 60's/early 70's.
There was a thread/study on this site somewhere - but i forget which section.
I don't have the results exactly memorized , but from the shoulder i think it was that todays 93 is equal to antique 100

100 was recommended for all the old High Comp Ratio GTO's back then.

You can easily go a full point higher with aluminum versus cast iron.
New edition heart shape chambers probably help even more (as far as going even above a full point from cast iron).

I do understand your concern on keeping it civil with readily available pump gas unleaded fuel.

The Ebrocks have more port volume cc's than a 400cid can make use of - especially with a mild cam. Some drawback there.
I'm also not 100% sure if the Ebrocks are true D-Port on exhaust for your original D-Port RA exhaust manifolds.

[johnta1]

The e-heads are round port, so need round port exhaust manifolds.

[Baron Von Zeppelin]

Originally Posted by Baron Von Zeppelin

I'm also not 100% sure if the Ebrocks are true D-Port on exhaust for your original D-Port RA exhaust manifolds.

Originally Posted by johnta1

The e-heads are round port, so need round port exhaust manifolds.

John, is KRE/Kauffman the only ones who make an aluminum head with a true D-Port exhaust arrangement ?

And would you know what size chamber(s) they offer ?

[johnta1]

I think the KRE is the only one to make a d-port aluminum head.
Looked like 64cc, 74 cc and 85 cc chambers.

[grandam1979]

Edelbrock makes D-port heads also.

[johnta1]

Cool. Any part #? CC's?


I don't use d-ports so not up on them.

[Baron Von Zeppelin]

I'm not up on the cutting edge of aftermarket either.
The old EBrocks had a D-shape , but Round Port flange config.

They (ebrock) do make true D-Port now.
Found an article by RockyRotella
https://www.pontiacv8.com/articles/2...-burn-chambers

The 87cc have an exhaust cross-over port / the 72cc do not

per his testing of an 87cc with CNC chamber:
Airflow at 28 inches of Pressure

Valve Lift Int./Exh

0.050” 36/27 cfm

0.100” 66/56 cfm

0.200” 147/115 cfm

0.300” 206/156 cfm

0.400” 235/178 cfm

0.500” 259/186 cfm

0.600” 277/197 cfm

Performer RPM CNC Part Numbers
61515 72 cc with hydraulic roller valvesprings
61525 87 cc with hydraulic roller valvesprings
61519 72 cc with hydraulic flat-tappet valvesprings
61529 87 cc with hydraulic flat-tappet valvesprings

[grandam1979]

I don’t have part numbers right off hand but I believe both 72 and 87

[Baron Von Zeppelin]

Stock GTO compression in 1968 was 10.75:1. I do not know what the compression ratio with 72cc heads will be, but I am sure that is is very close to the original factory 10.75:1 ratio.

The actual CR was around 10.25
10.75 was advertised/approx CR (fluffed a little)
1968 #16 heads are right around 72cc +/-