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Pontiac 350 build questions

General engine tech -- Drag Racing to Circle Track

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Walter R. Malik
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Re: Pontiac 350 build questions

Post by Walter R. Malik » Wed Feb 12, 2020 9:46 am

RobZ28 wrote:
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
The heads I used were #16 small valve heads which came on a big passenger car. The smaller towing cam was the limitation for RPM here, not the cylinder heads.
http://www.rmcompetition.com
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RobZ28
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Re: Pontiac 350 build questions

Post by RobZ28 » Wed Feb 12, 2020 10:48 am

mag2555 wrote:
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.
Pontiac 350’s have the chamfered block. So you’re saying 250cfm isn’t enough to operate a 409ci to 5500rpm? How much flow do I need? My goal with the 4.25” stroke over the 4” is to provide more cubes which should drive better for the power made. It’s that simple to me. Am I wrong?

What cam tricks as far as LSA etc help a small bore fill for longer strokes??

Where are the affordable blocks at again?
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 mag2555 » Wed Feb 12, 2020 12:13 pm

All I am saying is that with the Cam it sounds like your likely to build this motor with, and a 4.250" stroke just don't expect rising HP numbers above 4500 rpm with the port area that the iron heads will have if they flow only 240 cfm.

Will the motor rev to 5500, sure with the needed spring pressure for the big 2.11" valve.

68post
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Re: Pontiac 350 build questions

Post by 68post » Wed Feb 12, 2020 12:33 pm

[quote=RobZ28 post_id=848582 time=1581522486 user_id=11743

Where are the affordable blocks at again?
[/quote]

In the rust belt normally.
In states where cars actually last the engines are either more worn by a lot or still in a driving car.

I'd sonic check the block and leave just enough for +.020 and call it good, IF there is ever a rebuild wanting the same block and it needs another .030 or more then sleeve that one cylinder.

My opinion is get all your stroke and bore available, but I think 2.11 is huge for the bore.

pdq67
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Re: Pontiac 350 build questions

Post by pdq67 » Wed Feb 12, 2020 2:39 pm

Please remember that Olds made (2), 400" engines back then.

An almost 4" b x 4" s and the other, something like a 3-7/8" b x 4.25" s.

pdq67

RobZ28
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Re: Pontiac 350 build questions

Post by RobZ28 » Wed Feb 12, 2020 6:52 pm

Thank you for your help. I appreciate the feedback.
I have a ZZ Top keychain and a footprint gas pedal. You do the math!

tjs44
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Re: Pontiac 350 build questions

Post by tjs44 » Wed Feb 12, 2020 8:28 pm

350 HO used the big valve 48 heads in 69.With the added CI they should be good.Tom

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Re: Pontiac 350 build questions

Post by LCaverly » Thu Feb 13, 2020 8:01 pm

go with the 4.25 crank and dont look back. the 5C heads have a large combustion chamber so you will have to use a domed piston , cc your heads and go for a 10.0 to 10.25 CR. Be careful porting as later heads are lighter castings . 225 cfm will support up to 500 hp so you don,t need to do a radical porting job. A hyd roller cam with 225 intake 235 exh on a 110 @ .550 lift range will work well.
Len C

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