Hey everybody, I havent been on here in quite awhile. I ended up selling my '75 GMC(family matters) and just picked up a '78 Chevy C10 w/250. I am looking at rebuilding the engine and adding a bit more go power and still being able to pass smog. My questions are:
1)I am going to have the block decked...how much? I dont know. I want to use 307 pistons to bump up the CR....But how much will it bump it up?
2)I want to enlarge the valve size in the heads,port match and clean up the runners and CC. Will 1.94x1.60 be OK?
3)What cam would be smog friendly? I have a cat and EGR valve. and for the smog test will be running the stock intake and carb. I am looking at putting TBI on it later.
I plan on balancing the entire assembly. I am running a 4spd and 3.73 gears. I plan on towing my BBQ trailer(1000 lbs).
I know there are more details I am missing, but THANKS for any help.
-Lance

Does it have the Integral Head?
I think a Stock Cam or RV cam would work, the smog problem is from if you have too much fuel dumping in there or the spark ain't hot.
What kind of 4 speed will you be running?
The cam needs to match the rest of the drive train. The Stock 250 won't be running much over 4,000 to 4,800 rpm.

That's my two cents. Now can I have my change? ha ha

I'm very far from an expert on this but I've heard using a 1.94" intake valve is pushing the limit for these heads. I've talked to a few people that say it gets touch and go with hitting water jackets or just being very thin and prone to cracking in general.

There's a few guys on here who have done this first hand and for a lot longer so I am sure they'd be happy to chip in.

It's a NON integral head. As for the valve size,what would be a 'good' intake size? I have seen machinist turn the dia. of the valve head down to a specific dia. Should I go...1.90 or 1.88? Thanks
-Lance

As far as a 1.94" valve pushing the limit of valve size in these heads before hitting water, I will have to say that I frequently installed 2.150" intake valves in our 292 race heads, and never once struck water. If people are installing a 1.94" and hitting water, they are sinking the valve seat much to far to form the valve seat. Also, sinking the valves in these heads kills the short side radius and its airflow, and will usually defeat the purpose of putting larger valves in to start with. Don't forget to open the bowl area up also when installing larger valves, if undone, it too will negate any gains you expect to see from installing larger valves. Just putting larger valves in by themselves will not usually show much increase in airflow or performance, and will leave the enthusiast with the impression that there isn't much potential to be had with heads or engines. Also installing some bolt-in "lumps" will greatly enhance the airflow, even at low engine speeds, so that too should be considered as an upgrade....

So this really comes down to the skill and knowledge of the machinist more then anything?

Also what is the theory behind the lumps, is it that they cause more turbulance in the intake and help the fuel atomize? or is that they cause a restriction in the intake to enhanced velocity?

Since most inline engines don't have much of a short side radius to begin with, the intake and exhaust ports enter/exit the head at a straight in shot, the lumps actually create a better radius to direct the airflow into the bowl area. If you did a cross section cut on a V8 head, you would see that the intake port tapers as it gets closer to the valve, creating a short side radius that is real desirable for enhancing the flow thru the port. What the late Kay Sissell did when he developed the "lump" port technology, was create or duplicate, this same tapering of the port to form a short side radius that was comparable to what most V engines already had benefit of. Now, instead of spending hours heating and brazing, and grinding, and then more heating and more brazing, you can just install a bolt-in "lump" that will give similar results for far less $$$$'s. As for the techniques used by others that have made claims of hitting water when only cutting the seats for 1.94" valves, the experience card could be part of it, but it was probably due to other underlying issues like a badly core shifted head casting, or maybe a crack that was undetected prior to beginning this procedure.

OK, so should I remove the boss and install lumps? Remember this has to pass California smog test.
How about the CR? any ideas as to how much it will give me? The head is a stock 250 NON integrated head.
Thanks again.
-Lance

You would need to contact Mike Kirby to see if these heads will come close to passing emission testing for California with mods like this done to them. I would say that anything that makes it run better will probably affect emissions proportionally, and likely fail....

Just got off the phone w/Mike Kirby. I will be taking my engine to him for machine work. Thanks guys!
-Lance

Will lumps affect my low end grunt?
At what RPM range will they be most effective?
Will they change my gas milage and will I have to use premium fuel?
thanks, Al

The "lumps" will be beneficial thru out the entire RPM range of the engine. They will greatly enhance the throttle response at lower RPM, and make the engine operate more efficiently. They possibly could improve MPG because they also create a higher velocity of air/fuel mix thru the bowl into the cylinder causing more complete fuel burning. The premium fuel requirement is mostly a function of your compression ratio, and is not affected by having or not having "lumps" installed in the cylinder head. Feel like I just did an Infomercial(LOL)!

CNC-Dude,
you should get paid for the above post!
LOL

MBHD

I couldn't resist throwing that last humorous comment out there about the infomercial....It almost sounds like I was reading from a script. If Larry will pm me, I'll tell him where he can send a check.

Thanks CNC-Dude.
Al

Silly me. Have you checked out Leo Santucci's book about Building Chevy Inline Six motors. It has lots of good stuff. I don't understand it all but it is good. I ordered mine and it arrived in about 3 weeks.

I would guess the lump ports would help it smog out better, because they get rid of the dead zone and disrupted flow thru the intake port. That is just a guess.

As far as the cam, I have not smogged a six in a while but on my 400 v8 Ford (about same cid/cyl as a 292 six), with no cats, only an air pump for smog, I ran a 204-214-112 cam and it smogged out super clean, would have passed mid-1990s smog standards. So if you have cats, I bet you could run as much cam as you wanted for a mild build and it would still pass. Unless you're running headers, there is a 194-204-112 cam out there which is about as big as I'd go for an everyday 250 build, maybe even a step bigger would still pass smog given the cats.

I was considering Ford Windsor valves for my 250 build, they are 1.84 x 1.54, has anyone ever done this?

What's the benefit of using the Ford valves?

If you are after larger valves, there are lots of small block Chevy valves available and you can cut 2.02/1.60 valves down to whatever size you want.

No advantages,just use SBC valves.

A lot of SBF,switch to SBC valves when replacing stock type valves,because SBC's have been bigger than SBF's,stock for stock.


MBHD

First off, this isn't a rant or complaint, just an observation. With the exception of emission controlled vehicles, I have seen a trend where 250/292 enthusiasts tend to limit the intake valve sizes in their engines to the 1.84"-1.94" range. And almost thinking that the later is the absolute largest to go with. I have seen that when most enthusiasts begin a performance build-up with one of these engines, they seemingly compare the mods they will perform, to an engine they are more familiar with, say for instance a 350 Chevy. And think that since a 250 is much smaller in size, they need to make a selection for parts that corresponds by a proportionally smaller amount, usually smaller than what a 350 would use, to get the same or similar results a 350 would see. A comparison of these engines on a cylinder to cylinder basis needs to be seen to show how inaccurate a comparison of this nature really is. A 250 at .060 overbore is only 3/4 of 1 cubic inch smaller per cylinder than a 350 is. Many non-performance,low RPM 350's came from GM with 1.94" intake valves in them, as did many 327 non-performance engines, which is even smaller than a 250 by 2 full cubic inches per cylinder. So it stands to reason, that most mods that will show significant gains in a 350, will also show the same gains in a 250/292, especially in the area of the cylinder heads. With the addition of "lumps" in the 250/292 heads,and modest porting, these heads can even be made to out flow many aftermarket V8 heads and still be very streetable, even when 2.02" or larger intake valves are installed, just as a 350 would have with the same head flow and valve size. A 292 is a whole different story. At .060 overbore, it is cylinder for cylinder, larger than a 400 Small Block Chevy, and a 396 Big Block Chevy, and almost 13% larger than a 350. You definately will choke it down and not see all the potential out of it if you put 1.84" intake valves in it, and still, 1.94" might not be adequate either. I think there is a stigma about big valves being "taboo", and only needed for high RPM, or racing applications. Of course, other mods should be done as well to complement the installation of larger valves, such as better exhaust(headers) and ignition, and carb/intake manifold selection to match the intended use, and camshaft swapping is normally done anyway. You wouldn't put a 1.84" valve in a Dart head for your 350. I think one reason some people are disappointed when they modify these 6's, is because they go the wrong direction when choosing parts, thinking they are making an accurate comparison 250 to a 350, when in fact it is the same basic cubic inch, cylinder for cylinder. They end up being too conservative when there is so much more to be had from these engines. A popular trick before there were good aftermarket affordable valves available for the Windsor Ford engines, was to put SBC valves in the Ford heads. The Chevy valves were longer, so it provided some challenges to get the installed height needed in the Ford heads, but provided an economical alternative for the time to put larger valves in the Ford. So I sure the opposite would be the case for the Ford valve in the Chevy head, but as strokersix pointed out, the availability of aftermarket valves for the Chevy is endless, unless you already have the other valves on hand.

the only problem with big valves in inline heads is shrouding around the valve. a 1.94 valve is very shrouded and should really get unshrowded to benefit the motor. that is alot of work. also with a 1.94 valve there is a ton of port work needed to really let it work well.

it really boils down to how much work/money you want to put into your head.

i have also never seen a 1.94 valve hit water. tom

I agree, you do run into issues with shrouding with larger valves, but most shops hit the chamber with a cutter to unshroud it when they cut the seats for bigger valves(if they are looking out for your best interests), or at least will mention it to you that it needs to be done. It does have the potential to take you to a much higher level of performance, for not a lot of extra money by going to 1.94" and beyond. Most shops also can enlarge the bowl area as well to enhance the flow and maximize the benefits of installing larger valves at the same time they are doing the valve job, so you can sometimes kill two birds with one stone by combining this in one operation. Shops that still use stones to grind the seats might have more difficulty in performing these techniques, but can still be done if the shop is willing to take the time to do it.