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Pontiac 350 build questions

General engine tech -- Drag Racing to Circle Track

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RobZ28
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Pontiac 350 build questions

Post by RobZ28 » Tue Feb 11, 2020 3:18 pm

Hello folks. I am in the middle of figuring out a new build for a 1975 Pontiac Firebird with a 350 PONTIAC engine. The engine has a 3.885" bore with a 3.750" stroke. The heads are 5C's from '75 that flow roughly 200cfm intake at .450-.500" lift.

Now the car is really nice and clean and I DO NOT want to change the engine out to a 400. I would like to keep the engine matching to the car but maximize the car for a weekend cruiser/fun car that'll put out some respectable numbers.

There are 2 different size stroker kits for these engines that require custom pistons. In other words, there are NO off the shelf pieces to do any of these. Butler has a deal going with a few manufacturers to provide the pistons to a few of their kits but I am not going to use them for this build.
MY options are a 4.0" stroke or a 4.25" stroke. The heads will be ported to about 240 - 250cfm. Valve sizes will stay 2.11/1.66
Heads will be milled to achieve 9 to 1 or maybe a touch more...

For such a small bore, I have a question as to what the REAL benifit of using a 4.25" stroke would be? This car is going to use a roller cam and A/C. It must drive VERY well. Rear gearing will be a highway friendly 3.08. Thanks!
I have a ZZ Top keychain and a footprint gas pedal. You do the math!

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Re: Pontiac 350 build questions

Post by GARY C » Tue Feb 11, 2020 4:02 pm

How far can you go on the bore? I think a bore increase if possible would be useful due to valve size?

Here is a 4.00 stroke build.
https://www.hotrod.com/articles/hppp-07 ... ine-build/
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RobZ28
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Re: Pontiac 350 build questions

Post by RobZ28 » Tue Feb 11, 2020 4:13 pm

I can probably go bigger on the bore but I want to leave the block usable for another rebuild. Considering nobody is making Pontiac blocks... This isnt a negotiable topic. :mrgreen:
I have a ZZ Top keychain and a footprint gas pedal. You do the math!

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Re: Pontiac 350 build questions

Post by Stan Weiss » Tue Feb 11, 2020 4:16 pm

Is your engine 0.010" over or is the bore size a typo?

What rods are you going to use?

Will the car ever see the drag strip?

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RobZ28
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Re: Pontiac 350 build questions

Post by RobZ28 » Tue Feb 11, 2020 4:36 pm

Stan Weiss wrote:
Tue Feb 11, 2020 4:16 pm
Is your engine 0.010" over or is the bore size a typo?

What rods are you going to use?

Will the car ever see the drag strip?

Stan
That is a typo. Were not sure where its at now as we have not measured yet. But yes, stock is 3.875" - so it'll be .040 or more over. The plan is to try and not do a one and done deal to where it cannot be rebuild again in the future.

The car will see the nostalgia drags or some Saturday night stuff when I get bored (which happens every now and again) but it wont be a competition car by any means.
I have a ZZ Top keychain and a footprint gas pedal. You do the math!

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Re: Pontiac 350 build questions

Post by DCal » Tue Feb 11, 2020 4:40 pm

You could talk to Bruce Fulper at Rock& Roll Engr..He's built many of those using CP Pistons.

RobZ28
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Re: Pontiac 350 build questions

Post by RobZ28 » Tue Feb 11, 2020 4:46 pm

DCal wrote:
Tue Feb 11, 2020 4:40 pm
You could talk to Bruce Fulper at Rock& Roll Engr..He's built many of those using CP Pistons.
HELL NO. I have heard TOO MANY horror stories of that guy taking money and running. People having to wait until some other sucker pays up just to get their stuff done. He may be talented but I just dont want to worry about my hard earned money being thrown away.
I have a ZZ Top keychain and a footprint gas pedal. You do the math!

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Re: Pontiac 350 build questions

Post by DCal » Tue Feb 11, 2020 4:52 pm

RobZ28 wrote:
Tue Feb 11, 2020 4:46 pm
DCal wrote:
Tue Feb 11, 2020 4:40 pm
You could talk to Bruce Fulper at Rock& Roll Engr..He's built many of those using CP Pistons.
HELL NO. I have heard TOO MANY horror stories of that guy taking money and running. People having to wait until some other sucker pays up just to get their stuff done. He may be talented but I just dont want to worry about my hard earned money being thrown away.
Sorry !! I didn't know things had taken a bad turn over there.

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Re: Pontiac 350 build questions

Post by RobZ28 » Tue Feb 11, 2020 5:06 pm

DCal wrote:
Tue Feb 11, 2020 4:52 pm
RobZ28 wrote:
Tue Feb 11, 2020 4:46 pm
DCal wrote:
Tue Feb 11, 2020 4:40 pm
You could talk to Bruce Fulper at Rock& Roll Engr..He's built many of those using CP Pistons.
HELL NO. I have heard TOO MANY horror stories of that guy taking money and running. People having to wait until some other sucker pays up just to get their stuff done. He may be talented but I just dont want to worry about my hard earned money being thrown away.
Sorry !! I didn't know things had taken a bad turn over there.
Yeah, do a quick search and its bad, which sucks because I wouldve had him do a 350 for me. Whatever ordeal he is dealing with, I pray he gets it righted.

So the question still stands. Is there any issues using a big stroke with a small bore in a street deal?
I have a ZZ Top keychain and a footprint gas pedal. You do the math!

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Re: Pontiac 350 build questions

Post by Walter R. Malik » Tue Feb 11, 2020 6:54 pm

RobZ28 wrote:
Tue Feb 11, 2020 3:18 pm

There are 2 different size stroker kits for these engines that require custom pistons. In other words, there are NO off the shelf pieces to do any of these.
I am curious as to why you are balking at custom pistons yet a custom stroke seems to be fine with you.
Do you have the intention to use the POS Pontiac rods in this engine...?

The longer the stroke means piston speed will increase for the cubic inch size of the finished engine. This mostly will create quicker street response at part throttle driving; wide open throttle is merely all about the maximum breathing potential.

I built a 350 Pontiac recently for a customer ... SCAT 4.250" 4340 steel crankshaft, Scat Chevy 6.700" forged rods and ordered the matching "Custom" pistons; 1.420" compression distance and 3.910" bore.
That was a little over 408 cubic inches and in that 1977 Safari Station Wagon being used for a tow car, it all worked exceptionally well.
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Re: Pontiac 350 build questions

Post by RobZ28 » Tue Feb 11, 2020 8:45 pm

Walter R. Malik wrote:
Tue Feb 11, 2020 6:54 pm
RobZ28 wrote:
Tue Feb 11, 2020 3:18 pm

There are 2 different size stroker kits for these engines that require custom pistons. In other words, there are NO off the shelf pieces to do any of these.
I am curious as to why you are balking at custom pistons yet a custom stroke seems to be fine with you.
Do you have the intention to use the POS Pontiac rods in this engine...?

The longer the stroke means piston speed will increase for the cubic inch size of the finished engine. This mostly will create quicker street response at part throttle driving; wide open throttle is merely all about the maximum breathing potential.

I built a 350 Pontiac recently for a customer ... SCAT 4.250" 4340 steel crankshaft, Scat Chevy 6.700" forged rods and ordered the matching "Custom" pistons; 1.420" compression distance and 3.910" bore.
That was a little over 408 cubic inches and in that 1977 Safari Station Wagon being used for a tow car, it all worked exceptionally well.
I’m not at all concerned about using custom pistons. I’m concerned about cylinder filling Issues with using such a long stroke with a shrouded bore. Like how much cylinder head intake port flow roughly will be needed? What should the cam roughly measure to achieve a 1500-5500ish RPM range?

Not a lot of info out there about stroked Pontiac 350’s
I have a ZZ Top keychain and a footprint gas pedal. You do the math!

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Re: Pontiac 350 build questions

Post by pdq67 » Tue Feb 11, 2020 9:31 pm

Please think big block Olds here due to the smaller bores and longer strokes.

And if not mistaken, don't use the Pontiac rods.

pdq67

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Re: Pontiac 350 build questions

Post by 68post » Wed Feb 12, 2020 1:32 am

pdq67 wrote:
Tue Feb 11, 2020 9:31 pm
Please think big block Olds here due to the smaller bores and longer strokes.

And if not mistaken, don't use the Pontiac rods.

pdq67
His stroker is exactly like a '68 - '69 400 Olds if he uses a 4.250" crank, and I've always thought they would be best used with an OE 2.0" valve vs any larger dia.. No one builds these tiny bore Olds either, and for good reason, Olds 455/425 blocks are everywhere to use (and better smallblocks too), but so are 400 Pontiacs.
My '68 Olds 400 block will go with the car when sold but be driven with a 455 block until then.

O.P. , look at the Buick 350 builds also , 3.xxx bore and 3.85" stroke stock. Good luck with the build !

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Re: Pontiac 350 build questions

Post by mag2555 » Wed Feb 12, 2020 7:19 am

If your going to port the heads to flow above 225 cfm with the 2.11" valve size of the 5C heads then be sure to add a Intake valve chamfer in the top each Bore like most of the 4 bbl 400, 4 bbl 428, and 4 bbl 455 blocks have, other wise your in a big way wasting your money on the level of porting work to get to 240 cfm@ 28".

You would be better off to start with if you had a head with the 1.96" valve size and then ported that, but your passed that point now!

Adding the needed Intake valve chamfer to the Bore will add like 2.5 CCs to lower your compression ratio so be sure to recover that with head or block milling and then shoot for a 9.4 ratio, not 9 to 1.

Seing that even in ported form the heads are a limiting power / rpm factor, I would not go with longer then the 4.00" stroke as you would only be making that power limiting factor worse.

By the way, Pontiac aftermarket blocks have been around for near 20 years now so they are easily had, it's just that you would start off with far more then 350 cid.

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Re: Pontiac 350 build questions

Post by My427stang » Wed Feb 12, 2020 7:38 am

A buddy just did a 326 with a 4.25 stroke and a tri-power. It's no monster, but it runs real nice in a 3 speed 'bird and 3.73s. 383 inches if I remember correctly

I wouldn't stay away from stroke, get all the cubes you can and cam it according to use. Just go with a good valve and valve job and whatever porting you can do to that head without beating it up too badly

Regarding the cam, I did a 400 Poncho +.030 last year, Day 2 kind of resto build, 400 inches (the standard way), 4 speed, 3.26 gear (I think, forgot what GMs factory gear is) Tri-Y headers and a distributor recurve, bone stock otherwise. We went conventional on the cam (or unconventional according to the Poncho guys, they seem to like real wide LSA) and swapped the stock 67 4 barrel cam out for a 280/224 280/230 combination of Thumpr and Magnum lobes, 111 LSA on 106. The car runs real strong in the range you talked about but mild for family driving and power brakes
70 Mustang, 489 FE, TKO-600, Massflo SEFI, 4.11s
71 F100 SB 4x4, 461 FE, 4 speed, port injected EFI, 3.50s

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