[Bazooka]

I am thinking about buying a 67 firebird that has a rebuilt 350. The engine has never been fired and I was told it was built by Glenn Gibbons of Apache Junction Az. From what I have been able to dig up hes a pontiac guy and respected engine builder.

The seller told me the 350 is .040 over with #13 Heads. These are Big valves screw in studs and listed by the factory at 72cc. They decided on the Comp Cams XE274 cam.

Using the wallace racing comp. ratio calculator this comes out to 10.4 which is what the seller told me the comp. ratio of the engine was.

The seller told me that the engine was built to run on 91 octane which is what is highest octane pump fuel available here.

I always heard 9.5 compression is about the safe limit for 91 octane.

Is this engine doomed to suffer detonation or can it run fine if tuned properly?

Thanks for your advice

[rtanner]

tough question, that cam is so big it may be alright, id really like to here back from you if you use it, if it were mine id put a summit 2801 in it and call it good, but that my opinion i know it will work w pump gas

[ponyakr]

None of this adds up to me.

The #13 heads would need to be shaved down to about 65-68cc or so to make 10.4 comp on a 350. That would be similar to a '69 350HO with #48 heads. You'd also need thinner head gaskets than the Fel-Pros and either .005 or zero deck height.

If it is actually 10.4 CR, that's well above the safe limit for pump gas, with iron heads. And, if it has cast pistons, that's not good for a high comp pump gas engine.

That cam is way to big for a mild street 350. Plus, it's one of the steep ramp cams which is said to increase cylinder pressure. This will further increase the chance of detonation.

IF you knew all the specs, such as valve relief volume, deck height, head cc, gasket thickness, etc, you could plug the info into the CR calculator and figure the CR.

http://www.wallaceracing.com/cr_test2.php

[77 TRASHCAN]

The compression ratio MIGHT be manageable, but the cam more than likely needs to go, and other things done, such as zero decking the block, and proper degreeing of the cam. Just don't start the thing till you decide which way to go. You could sell the cam/lifters as new, now, after it's it's started the cam is only good for a property line marker post...

Many respected engine builders show their lack of knowledge here, by using X/E cams. Just because someone sells a particular product, doesn't mean that one needs to buy it......Not intending to knock anyone. Your 350 can be made to run nicely, it just won't be much in it's current build...

Hopefully Cliff will give you some advice...

Sounds like an awesome car!!!!

Welcome to Pontiac World!!!!
Jeff

[Bazooka]

I was told it was setup with 0 deck

I used the wallace racing comp ratio calculator and it came up with 10.4 compression ratio as well. below is what I input into the calculator

350 .040 over

0 deck height

72cc heads

[PunchT37]

What gear and trans? I think it`ll be fine as long as you don`t have a stock converter and 2.78 gears.

72cc heads on a 350 should be around 9 to 1 on a 350 with flat tops. 64cc`s or so, to make a 10 to 1 type 350.

[BubbleTops]

The trans is a muncie 4 speed. Not sure on the rear gear ratio.

The pistons are speed pro. I think thats what they are called. They come in a Black box with red lightning bolt. Think they have dual valve reliefs

So is the Wallace racing calculator not correct in this case?


Sent from my iPhone using Tapatalk

[PunchT37]

Thinking back, I think those 13 heads are around 78 to 80cc`s anyway. Not the 72 that the "book" says. 12`s are closer to 72cc`s.

[72blackbird]

Originally Posted by Bazooka

I was told it was setup with 0 deck

I used the wallace racing comp ratio calculator and it came up with 10.4 compression ratio as well. below is what I input into the calculator

350 .040 over

0 deck height

72cc heads

Are you accounting for piston volume (reliefs), gasket thickness and diameter? I get 9.4-9.5:1 SCR, depending on what head gasket you use.

Geno

[ponyakr]

Just for kicks, let's plug in a few numbers and see what we can come up with.

For the 1st combo let's assume that the heads have not been shaved=72cc

Forged shelf pistons are no longer available. So it probably has SP 357P40 cast pistons. The valve reliefs in the L2262F 400 pistons are 6.7cc, and come almost to the edge. So I'll leave the 357P reliefs at 6.6cc. It's possible the engine guy may have found a set of NOS SP L2339F40 forged pistons. My engine guy found a .030 over set for me. But they won't affect the CR.

Most people use the Fel-Pro Gaskets, since the Cometics are so high. 1016's=.039 x 4.3

And we'll figure a .005 deck height. Let's plug in these numbers.

http://www.wallaceracing.com/cr_test2.php

I get 9.32 CR. What do ya'll get ?

But most say 9.5 is the max for pump gas. So, if the heads are indeed 72cc, you can use Cometic .036 x 3.95 gaskets and come up with 9.52 CR. With zero deck you can use .040 x 3.95 gaskets. That should be safe for pump gas, IF you don't set the total timing too high, and IF you don't use a steep ramp cam like the CC XE or Lunati Voodoo series. I Think an 068 grind or the 2801 Summit that was mentioned would be a good choice.

This all really depends on the actual head cc, deck height, etc. There's no way to know without measuring.

[BubbleTops]

Thank you guys for all your effort helping me figure this out. Sounds like the compression ratio should be lower then what the seller and calculator was telling me. Sounds promising indeed. I figured the cam was to big for the little 350 but am no expert so wanted to get input from you all. Shouldn't be to hard to sell the comp cam for enough to get the summit cam. I have heard good things about it.

I left out the gasket diameter info on the calculator this made a bigger difference then i expected. Opps. Lesson learned

I seen a pic of the pistons they don't have the black coating on the skirts so I assume they are the cast version. They are flat with dual valve reliefs as i said earlier.


Sent from my iPhone using Tapatalk

[Squidward]

I was racing with Glenn in our Pontiac group last week. He still races his "Pouncin' Poncho" Look him up, and give him a call. He's still active in AJ. He might even remember the specific build, and speak to the choice of the XE274.

http://www.dragzine.com/news/the-pou...n-altered-ego/

http://glennsgarageaz.com/brochure.pdf

[Bazooka]

Sorry guys about the different user name. I made a new account because I wanted to change my user name. My phone is still logged into the BubbleTops account on tapatalk. I dont plan on buying a bubbletop catalina anytime soon

[Squidward]

Bazooka and BubbleTops... they seemed to be connected somehow. I just thought you might have forgot an earlier site profile and made a new one, only to remember your original.

No problem. Someday I might become "Mr. Krabs".

[huddie]

Be sure to call Glenn.Always great to talk to the builder.

[PunchT37]

If ya got some gear, use that 274 and turn that bitch to 6 grand.

I put a 274 in a Mopar 340. I know, shorter stroke, but, cubes are cubes. Thing runs good with a 2500 stall and 3.23 gears. High 12`s at 103 in a `71 Duster. Cast pistons. Pulls to 6000 in that thing. Falls off fast after that. Course, that`s high enough for cast pistons.

[ponchjoe]

I say 068 it or 60916 Crower

[74Grandville]

#13 heads are normally around 78cc. i'm coming up with 9:1 compression. I bet it would run on 87 at the pump.

I hope they checked the clearance on the block for the heads and cam. to bad it's not a 400

[Bazooka]

I asked the seller if they had put the chamfers at the top of the cylinders for valve clearance and he told me that he remembers the builder mentioning that and it was taken care of.

I'm still trying to reach the builder.

I'm aware the 350 is not the best engine to base a performance build on but for a street car seeing no track time and considering it comes with the car. I'm ok with it for now. I had a 67 firebird with a stock low compression 350 and 4 speed when I was in highschool. Wasnt that fast but was still a lot of fun.

[Squidward]

I'm having a blast beating the crap out of mine. A 4 speed would be even more fun. And I have WAY less cam in my 350 than the 274.

Try thrashing on it and see how you like it!