Inliners International Forums General Discussion Hi Performance Building a 302 GMC for LSR

This is a cutaway of a 302 GMC head. It doesn't look like their is too much material.

I was surprised that the 983 casting I used on our 321 was thinner than I would have believed. I opened up the intake runner to the size of the alignment ring and broke thru on one of the head bolts. I had to install a sleeve in the head bolt hole to seal the runner.

I also machined a hole in the block so I could measure the thickness of the cylinder wall before we bored the block. It was about the same thickness as a 350 SB Chevy (0.200"). I thought these old engines had a lot more material in them than that. To be safe I filled the block to the bottom of the front water outlet with Hardbloc before it was bored.

Ron

It does make you wonder. I can pick up a 302 block. I can't do that to a Ford flathead.

Actually, that is the main focus of another thread that is currently underway in the Engine section entitled "261 Chevy 848 porting"! I will have before and after flowbench results showing how this technology applied to these earlier heads not only improves the flow capabilities tremendously, but when done in conjunction with modern porting techniques will take the 235/261 and GMC's to a much higher plateau than they have ever gone before.

Hey Panic!! I'm enjoying this. Keep it coming. 1 OL REDNECK I.I.#113

Its been known for many years by those that are in the profession of head porting and other areas, that surfaces with an airplane wing edge, that is blunted or ellipsed, creates much less turbulence than a sharp leading edge does in terms of airflow. That mistake is also many times done with knife-edging crankshafts. When a sharp leading edge is ground on the edge of the counterweights. Crower was one of the first to change to the "airplane" wing technology for their crankshafts leading edge almost 20 years ago. For years, people were taking the term literally when speaking of knife-edging, and finally discovered it created almost as much turbulence as the flat edge of the crank did before it was ground on. Sissell/Kirby was and is the leader in siamese port development,also creating the "lump" port technology for the 250' and 292's. Brian and Glen Self are very close behind, and still currently dominate most racing bodies that involve those engines!

How consistent are these castings from port to port and head to head?

Since those heads weigh so much, you would think there would be more metal in them in some of those areas. I agree about the engineers for some of those crank companies too, I guess they figure its been working for them, so why change it....go figure!

Thanks, there is a lot of useful information there. A great deal is coming together here!

Panic....I've looked at a ton of GMC heads and own more than most guys. In my opinion your cut away appear to be one of an exhaust port. My reason is the divider and a shadowed area behind it and the left edge appears to bet retangular in shape and not circular. The valve area is very small and the chamber appears to be one of the last small port ones made with the shrouded intate and exhaust. Hey I could be wrong and often times I am. JD

Yeah, even the 983 head im doing looks a look larger in that area also, must be an early non-302 head. If its the same head as in his other illustrations with the combustion chamber example with the colored rings, its definately not a 302 head. The 302 actually has a pretty good chamber shape and design, similar to the SBC only larger.

Oh,so those pics aren't from something you have personally done yourself then.

I posted the original cutaway picture. Its of the exhaust of a GMC 302 head. I needed a bad head to cut up before modifying my NOS head and Buffalo gave me the bad 302 head at Bonneville a few years ago. If I can find the intake cutaway I will post it.

I found the intake port cutaway. This was cut through the middle of the intake runner. I was trying to see how much wall their was before hitting water.

So, with the small port heads is there more wall or more water? I'm betting on water!

Speaking of cams, as I recall Jim Hedrick had come up with a cam design for these motors. I heard his car run once and at idle it sounded terrible. But when he was on the gas it sounded like a monster. Maybe CNC-Dude can chime in on this.

Often wondered if any of Vizard's work would translate to these engines.

I dont think Jim ever experimented with different specs on lobes for the siamese ports and non-siamese ports on the same cam. The life expectancy of those engines was so short, that track time was at a real premium. Jim worked very closely with Mark Heffington,former owner and pres. of Cam Dynamics on developing the cam profiles for the 292's. He ended up using some Pro Stock lobe profiles in those engines. Which at that time was insane by many peoples standards for a 6 cylinder. Most of the fastest records and passes were made with valve lifts of over .900 lift. And that was back in the late 1970's and early 80's when Jim finally hit upon the right cam #'s.And they remained the same from then on.Cams in modern Comp Eliminator SBC's are that big today.The strange sound he was referring to was a "churping" sound the engines made when idling. He always thought it was a combo of the huge Pro Stock grinds on the lobes and the possible reversion of the exhaust and "lump" porting. It never was found to be on any other engines that they did for other people, and those two things were the only differences,cam profile and head. I only know of Cotton's engine ever being on a dyno twice....ever. I saw the second and last time it was. I think kinda' along the same lines as panic,in that those types of different specs would be more apparent on the Minis as opposed to the larger bores and different engine parameters of a larger engine. But, that is also an area that could be researched and proven or put to rest. I know of Winston Cup teams doing that to some of their engines to compensate for "good ports"/"bad ports" on some of their R&D programs, and only found tiny gains in that area(2-3HP typically).So many new things are coming about with the 235's and GMC's with the roller billets I will soon be making for each of them, and now being able to supply a camgrinder with actual cylinder head flow #'s and data, that is now so critical for the best cam selection. That data has never been provided for either engine except for the special few that have the capability to do so. I have spoken to a cam grinder that has expressed interest in taking that flow data and optimizing some grinds for me in developing the billets I will be providing. He also thought that applying the "lump"port concept to these early engines was the most innovative idea he had heard in a long time,and was really willing to work with me and others in this area. I think that before long we will see some spectacular advances done with these engines,especially in the cam and head category....

The 292 roller lobes would be way too radical for either the 235 or GMC. My cam grinder is going to evaluate his lobe library to see what could be used based on the head flow specs and compression and the normal data needed to select an optimum lobe profile. Plus, the 292 heads are much more capable of better airflow than the earlier heads by far. Even though they share many commonalities and are both siamese style engines, I suspect the overall lifts and durations in comparison will be much lower for the 235/GMC's,unless a 12 port head is used that can significantly boost the airflow #'s. I will be getting some 8620 2"bar stock this week to begin roughing in a couple of roller blanks so I can be on "GO" when I reach that point. Making the rockers wont be that much of a challenge, especially with the CAM software I have available and the equipment to machine them with coupled with my experience in this field. The hardest part will be to acquire the pushrod cups and roller tips and pins for the rockers, most cam companies like Crane and Comp wont sell you those pieces individually, because they dont want anyone to make rocker arms but themselves.

I may be able to get a set of BBC roller rockers for parts. They were offered to me for my 292 project but he'll never know!

Ok, put them up for a rainy day,im sure they will come in handy!

Guys,

I built a complete roller rocker assy with stands and a 1.0" shaft, rifle drilled for oiling. The ratio is 1.85:1. After 20 pulls on the dyno the valve lash didn't move.

If someone will explain how I post pictures I'll show what I built.

By the way, I talked to Vizard about headers for the GMC and he didn't think his experience with the 4 cylinder would apply to the inline 6 engines with siamese ports.

Ron

Ron, Here is where I learned to post links to pictures. What you learn there works here. LINK
Or maybe this is better. LINK

you admit it is photoshopped after you cant identify the head

This is so cool, always appreciate the rude personal comments.

Fixed all that offensive stuff for you.

Whats rude about pointing our that you didnt know what cylinder head you displayed? It didnt add anything pertinent to the discussion as you could not identify it - someone else did.

Nothing rude about that. sorry if you found it offensive

Re: You can block off the oil port in the head, and use conventional oil-thru-pushrods to get the oil to the rockers

I'm not sure whether that's best. It saves some plumbing, no lines to fracture. However: it takes pressure from the tappet gallery, yes?
I didn't think the GMC had pressure to the tappets, only the 1954-* Chevy?
If I missed that (and there is pressure) you still have leaks and pressure loss at ever transfer: gallery to tappet, tappet to pushrod ball, pushrod ball to rocker end, rocker body to rocker shaft. I don't know how much is lost, or whether there's a front to rear bias (last tappet from the gallery feed gets lower), but any dirt it picks up will stop the pushrod oiling.
Using a single line to a distribution point allows adjustment to the volume/pressure without taking the motor down (blue rocker? more oil).

Re: cams

Paraphrasing Vizard, the siamese port layout means that there is a maximum practical duration before the exhaust valve of 1 cylinder is open during the intake stroke of the opposite cylinder on those that have both intake and exhaust shared (#2-5), causing the vacuum to be satisfied by exhaust pressure rather than fill the cylinder from the intake manifold. Unlike the usual high overlap problem, this doesn't decay to harmless at higher speed.

Panic,

I agree, and I haven't found a header arrangement that will work on our GMC since the siamese exhaust pulses are 240 degrees apart and my cam is bigger than that. I do know the zoomie headers are hurting the low-mid range TQ.

Ron

Ron,
Have you tried changing the firing order....using a custom ground cam naturally!

Ouch: that's a new crank!!
Just swapping the events on some cylinders can be done with just the cam, but it won't change the number of degrees of split between paired cylinders.

I've been wrestling with the firing order, paired cylinder thing (my head hurts).
That explains why the Mini can use a header: the cyinder pairing has 360 separation.
What pairing of exhaust pipes have you tried?

I suspect that the zoomies are either:
1. too big, and you're getting reversion because of low vel in the primary, and it's backing up into the port
2. the length is creating a wave that arrives at a bad moment and stifles something else

As Smith suggests: the easiest, most trouble-free exhaust system is 8" stubs. No wave can arrive late enough to affect anything, no backpressure, no length at all (they can differ if you have chassis clearance probs). Probs: sounds like gunfire, flames under the hood. Even this can be fixed (as you'll see on really old racers), the stubs empty into a very large can - as long as the cam area immediately inside the stub entrance is 10X the stub area there's no collector effect. If the pipes are 1.75" OD (1.652" ID based on 18 ga.) the area is 2.14", so if the can is at least 5" in all directions from the stub it's the same as exiting in free air (except for the noise, heat, flames). You only need a single exhaust, any size will do, and pointed per rules (down and back).

Ron, just use what JD has done and all will be well. It's not that big of a deal! A 4 into one W/merge style collector is the best overall header system out there. The small losses we have because of the siamese intake and exhaust ports will be more than offset and the gains well worth it. An ounce of reality is worth a pound of theory.


http://www.landracing.com/forum/index.php/topic,4169.0.html

Thanks guys for the info on the Jimmy header. I considered Smith's suggestion on the short stacks (<8") and I also considered dumping directly into the large collection box.

However, from all the engines I've had on the dyno (conventional engines...not this geek) I've seen large TQ gains below peak TQ, and usually a 1-1.5 percent increase in peak HP, with a 4 into 1 header. On one 406 SBC I verified a 96 lb.ft. increase in TQ @ 4500 RPM when I extended the collector to 18 inches.

I'm satisfied (???) with the HP & TQ (346 & 347) but the thing is a complets dog below about 3500 RPM.

I've decided to build an adjustable 4-1, 1 3/4 x 40" with a 3 or 3.5" collector. If that doesn't work I'll try something else.

Thanks, Ron

Even though your piston and rod combo was a nice swap, I think you left a lot on the table as far as torque goes by going with such a long rod length to stroke ratio for such a relatively low RPM engine. I know your goal was to get an extremely lightweight piston out of the deal.

cnc,
what do you mean by "i think you left alot on the table"? i did not take it as anything bad. just want to know what is wrong with his 8" or so rod and a light piston. tom

No,it definately wasn't meant in any kind of derogatory or diminishing way.Explaining how longer rods kill low end torque is difficult, in that it envolves a lot of physics and technical terms and mumbo jumbo. It mostly has to do with how it changes the dwelling of the piston at both ends of it stroke TDC and BDC.And engines are really pretty sensitive to even small changes in their R/L.The only way to try is to compare an engine that you could relate to. Take a 292 with its 4.12 stroke and 6.76 long rod. Now take that same 292 and put a 7.500 rod in it. The engines are built and prepped indentically in every way, except for rod length.As you begin to push the rod length of a given engine upward, and increase its R/L ratio, you also push the torque curve upward as well. You also create a narrower torque band. In the short rod engine as the piston approaches TDC and BDC, you can graph with a degree wheel and dial indicator various points of rotation relative to piston location in the cylinder, as it approaches and passes these TDC and BDC points. Now if you do the same with the long rod engine, and compare the same degree wheel locations relative to the piston as the short rod engine. You will see that with the the long rod engine, the piston slows down or dwells longer as it approaches and passes those TDC and BDC points. That loss of inertia also causes a loss of torque.(Hope this isn't boring) Low RPM engines(up to 7000)seem to have more usable HP and torque with relatively low R/L ratios, around 1.5 to 1.6,provide good low RPM grunt. Ratios in the 1.7 to 1.8 range are mostly used by engines that have a considerably higher peak RPM(up to about 9000)a Winston Cup Chevy engine has 1.81 with a 6.350"rod and 3.500 stroke, and the loss of the torque down low in the RPM is insignificant in these engines. Even though the GMC has a pretty high ratio already, 1.7 ratio, changing a 350 from a 5.7 to a 6 inch rod makes a huge noticeable change in its low end characteristics, and you only are changing the ratio from 1.64 to 1.72. Now, saying all that, to say this, changing his ratio from 1.7 to 2.0, is a very large jump for an engine that probably has a peak of maybe 6500 or so.He made some very good dyno #'s to say the least, but I think he gave up a lot of torque by making that big of a change in rod length, and since HP is also a by product of it, he really would have seen some much higher HP and torque #'s with a stock length rod. The good thing about HP is that it doesn't know if you have a fly weight piston or a 10 lb. piston, it makes the same either way....

I conquer
MBHD

I think Ron used that rod length because a Jimmy engine , from the deck to the centerline of the main bearings is a little over 11.25 inches. A 10 pound piston is a little hard on the rods at 7 grand.

I agree, im also sure the SBC pistons he used were much cheaper, than an application specific GMC piston. And again, im not saying he was wrong to use this combo, just making an analysis on the subject of how R/L affects the characteristics of an engines performance.

I wonder if anyone has done back to back dyno runs changing only the rod length and piston pin height. I think that is the only way to tell what is true and what is theory.

Well it isn't theory, it can be seen on any dyno with any engine, and has been for many years. That is one of the determining factors in engine development programs that I have been involved with in the many realms of racing i've experienced. There are many benefits of increasing the R/L ratio, but the decrease in torque in the low side of the engines power band is a trade off. You just have to determine if it will benefit your needs, and not affect your performance characteristics you are seeking. He might have 1000 more RPM of usable power band on the low side of his range to gain back, and on a engine makes its peaks around 5500(I think thats what his dyno results stated), that is a pretty big lose to consider. He mentioned it was dead below 3500 RPM, so he has a relatively slim power band, if he gained another 700-1000 below 3500, his results could be much different. It would be nice to do a back to back comparison, its probably not cost effective unfortunately.