Inliners International Forums General Discussion Blown and Injected GMC with boost...vs headgasket...

Great....my pistons seem to have some stress cracks on the under side on a few of them...which confirms my thought that these were not the forged Jahns as promised by the seller of the motor....so new pistons...which means I might as well get a better head than my small port which needed valves anyway....anybody got a decent head they are willing to sell?
Suggestions on pistons?...Venolia makes them I know but never had to buy them before...

Of course, you're looking for a 983 or 417 H head, but you need to explore the choices of what to do with it before ordering pistons. The valve and port sizes are nice, but the full open 417 chamber isn't easy to fill and not with good quench. The 983 at least has a quench surface.
If you have good welding available, the usual practice is to fill in the 417 side opposite the plug so it looks more like the 983.
Either one can also be improved (IMHO) by making the closed side a "heart" by adding a V separator, which both improves flow and reduces chamber volume.
Until you know, you can't specify the dome shape, CD, valve reliefs, etc.

Your exactly right...the hunt is on....guess I have more things to work on now that fancy oil pans huh...or...don't have a head or pistons so might as well build an oil pan...

So I pulled a pistontoday to get a closer look at the crack and check bearings etc and it looks as thought it is just a crack from where the casting flash was removed...so that is good...but they are bone stock 4 ring truck pistons...not the best for performance work. On a positive note the crank has .006 thrust with old bearings, you can see reweld of thrust on crank from last rebuild, and rods and mains are std and with in spec and look great, so just a polish and new nearings there I hope. Bore is very fresh so wondering about just buying some good pistons and hone, new rings, install.....kinda chincy, but...I stuck teh cam back in with a degree wheel to get some approx figures...lift as stated before is .320 at the cam, intake duration opens 57deg, and closes 60deg...57+60+180=297deg 'advertized'....but that 60deg close sure seems late...I was taking .050 as well but forgot to write down if the open of 2 deg was before or after 0 deg and close at 128 deg doesn't seem right. I'll recheck those when i'm not in a hurry. 297 for an old school cam isn't out of the question and lobes are quite 'square' and has full .320 lift for 9-10 degrees.

So now to decide what to do on pistons and if I should just run the small port head for now and be on the hunt for some other better breathing options...

That timing doesn't sound right at all. IVC @ 60° is way early for that duration, I'd expect 40-77 or close, puts the ICL at 108.5° ATDC.

our suspicions are confirmed, I'm not sure how I came up with that 60°...must ahve been the wife yelling to hurry up...OKI redid everything including TDC just to make sure everything is right. I did both int and exh came up with the following:
lift at cam: .325int, .322 exh
Duration Int: Open 55°--Close 84°, comes to 319°duration and 104.5 ICL
Duration EXH:Open 76°--Close 60°, comes to 316° and 108°ECL

Cam LCA would be 106.25°...we'll just call it 106°. this as well means the cam is 2° advanced.
I did each step a couple times to get as close to real duration numbers as possible.

With those intake duration numbers i'm thinking a blower with this cam would give you quite a bit of fuel in your exhaust...

Yes, not a good choice for boost.

I sent out some feelers to quite a few cam companies about re-grinding a NOS cam i have into a blower cam and got back lots of good response. I guess teh only thing is that most don't have much experience with the 270/302 especially with a blower. Any of you guys have specific reccomendations for this motor with a stock type head with minimal upgrades? Of course there is a limit as well to how much they can grind on a stock cam .

Specifically, they can't move the LSA more than a few degrees so you're left with either less duration than you want, or more overlap.

just had a cam done this is from a stock core
advertised lift .442
advertised duration 270
lobe sep. 112
overlap 46
data for degeeing cam at .050 tappet rise
exh opens 41 bbdc
int " 3 btdc
overlap 6
exh closes 3 atdc
int " 41 abdc
exh duration 218
intake " 218
i'm not sure what all the numbers mean but the engine has mucho power on this cam

Badsix, where did you get your cam ground? I assume this is a cast cam?

I'm thinking as well of just going ahead and re-grinding the Howard steel cam. I can't use it as is and don't have any plans for it, so might as well use it, if I build a race motor it will get a new core anyhow. The lifters on that cam look strange to me amost a spiral pattern on the face from machining. Is there a different way the lifter face and cam profile is shaped from a cast to steel cam? It's almost like it isn't ground to have lifter rotation and the lobes on the cam seem amost straight across like a roller, of course this is hard to tell with the eye. the edges of the lobes are relieved though, no sharp like a roller.

yes, my cam is the cast steel type. i have had trouble with cams as i have been running the (milk can) style lifters with my cast steel cams. they are a hardend steel lifter and not compatable with the cast cam.this is probibly what you have. i'm now trying some cast steel lifters to see if that gets rid of the cam problem.as for cam grinding i've had cams done by iski, oregon cam grinding and my last cam was done at redline cams in albany oregon. very happy with redline, their a small company and will spend some time with you to get things right. they did a nice job on my cam, reground the lobes polsihed the bearing surface and parkerized it.