The Stovebolt Site Forums The Shop Area Engine Shop Question from the Realistic 2brl thread

Purely hypothetical question here.
Assuming one wanted to keep the single carb setup but was looking for improved AF balance...

If one selectively ported/polished the furthest cylinders, maybe not as much on 2 and 5 and left the centers original could that help in 'balancing' the A/F ratios?

I've done a lot of dyno testing of race engines with thermistors in the exhaust ports trying to evaluate the effectiveness of fuel distribution to individual cylinders. (Leaner mixtures burn hotter) The short answer is yes, porting and polishing affect fuel distribution and individual cylinder performance. However, unless a LOT of other factors are completely equal, I doubt if a measureable gain could be achieved on a street-suitable engine. We were shooting for an exhaust port temperature of 2,000 degrees F. on all cylinders under load at wide open throttle on Chevy 355 cubic inch round track engines at RPM's in the 7,000-8,500 range. Several of those engine were making 600+ HP, corrected for temperature and humidity. They also had a projected lifespan OF LESS THAN 1,000 MILES! Not your average street engine!
Jerry

Thanks Jerry,

So you're allowing for that distance difference by graduation in Vol, Velocity or other factors right?
Are the Intake manifolds done with this in mind more so than the heads, or are the heads also graduated in some way?

There have actually been attempts to control mixture flow to individual cylinders by inserting various kinds of deflectors in intake runners by people with a lot more experience and equipment than me. I used a removable-top Edelbrock STR (Street tunnel ram) intake on a MOPAR 440 that had instructions from the factory included with it on custom jetting of a Holley 4-BBL to maximize cylinder balance (4 different-size jets) and some suggestions on the size and position of deflectors inside the manifold for the same effect, taking into account the RPM range and firing order.

On engines as primitive as the old stovebolts, it would be a colossal waste of time to try such tactics. Sort of like putting lipstick on a pig! For one thing, the port shape and geometry of the intake and exhaust would have far more to do with the fuel distribution than anything done with a die grinder.

I've watched my father port, relieve, and polish flathead Ford engines back in the early 1950's, but he was doing the same thing I did in the 70's- - - -working on exotic race engines that weren't expected to survive very long, and producing at least twice the power they were engineered to have. Doing racing stuff on a street engine is an exercise in futility. If it makes you feel good, do it but it's a colossal waste of time and money.
Jerry

600 horse power...very good. Lifetime of 1000 miles...not so good. Thanks Jerry. That is a little better than the drag motors that make 1000 horse power for about 1 minute. But...not much. When I was making hot rod motorcycle engines the first question I would ask was...How much money do you have? Still valid today. I like the idea of balance in the intake. I do. But so many things come to play. Can you make it better? Yes. Will you notice? Maybe. To get an engine to work very efficiently at a single RPM is not so hard. To make it work over a wide range of RPM is much more difficult. So many things change as RPM changes. That is the reason that compressing the incoming charge works so well. Boost makes up for lots of issues. Right up to the point the parts leave the engine. Then out put equals 0. Gotta love Hot Rods!

Interesting question.

I have done virtually NO p and p (other than one import engine), so cannot give an answer other than Jerry's comments seem to be correct.

I have stated many times that there is gain to be had in using multiple carbs IF and ONLY IF the carbs are properly sized and synchronized. Do it right, and there is a positive gain. Do it wrong, and you can lose performance.

Incidently, Jerry, my 121 CID 4 cylinder was producing a bit over 320 HP.

Jon.

Thanks guys, as I said... Hypothetical, (for me anyway)and now I'm sorry I took a line out of my last post that read, "I imagine it would amount to a fart in a hurricane"

I'm sure I've read Jon's comment on multi carbs and volume many times in the past 5 years so it's sort of my take off point for the question.

I have a gut feeling that you might find a big rusty pile of experimental manifold castings out behind the Chevy motor plant. What went on the car was the best one, and even that changed when some variable did.
Very few aftermarket manifolds had any development time in the aimed at part-throttle high vacuum cruising.

Looking at the first post and the 250 - 292 intakes I am used to, no gain could be had with port work. You need to start at the valve seat and work all the way back to the carb base. Then you risk the chance of opening the port to much which will slow down low rpm air flow. Size is just as important as flow, maybe more, bigger is not always better. Limiting the to two center ports would be a better way to go. One could also look at the exhaust ports on the outer cylinders. If you can get the exhaust out faster, then you have more room for fresh air to come in.

I have a ported 250 in the basement with exhaust ports that out flow the intakes. At wide open it ran really good, part throttle was terrible. Pulling hills it would buck and shack so bad you had to let off. It took a year of messing with the engine to figure out the exhaust was pulling all the fuel out when the valves overlapped creating a hugh lean condition. It took a exhaust temp gauge to find the problem. When it acted up, the exhaust temp would spike, then settle right back down when I let off.

Joe

Interesting feedback. I suspected Panic is right and short of reengineering the whole thing the likely outcome would be like Joe's 250 P&P project... a basement dweller.

So it IS feasible... if your Jay Leno and just want to for S&G's, and to spend more money for fun.
But don't make it a business plan.

Interesting observation from Joe H. The idea that things must work in a balanced fashion is a good one to remember. I have looked at the "lump" modifications and thought to my self, a lot of work to change the charge direction. Also looked at "un-shrouding" the intake valve and the loss of compression that results. The recovery is to mill the head and that means re-milling the head bolt seats because the head has to milled at an angle. All of the changes don't "unlock" any hidden vast improvement. At the same time some have made these engines work very well in race mode. Perhaps the trade to a street able engine is out there just not at say 300 HP. Perhaps a careful approach to 200-250 is the best we can do?

One of the sneaky tricks on the "claimer" 350's we built for the (supposedly) low-budget racing class at Nashville was to put in a .100" bigger diameter exhaust valve and pocket-port the exhaust passages right under the valve. BTW both those mods were highly illegal for the class, as well as angle-milling the low-compression 350 heads we were required to run so they had less combustion chamber volume than a set of camel-hump heads. All the casting numbers were "legal" and the tech inspectors were dumber than a box of rocks, so some of us took advantage of the situation. In NASCAR, "It's only cheatin' when ya get caught!"
Jerry