Summit vortec heads

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Before I tore the engine down I did replace the stock poppet spider with an acdelco mpfi. Not sure what injectors are on it. Its just one they recommend replacing the stock poppet spider with .So do you guys think its possible it would work with the lunati cam I mentioned ? Or would it just not run correctly? Just wanted to get your thoughts before I spend more money.
That's just what I have - the MPFI upgrade, with an L31, Summit iron heads, and the LT4 hotcam. (You have a little less lift than I do with 1.5 rockers; you would have more with 1.6 and still be OK with the Summit heads.) L31 MaxExpress is right - I have never had any lean fuel issues with the acdelco mpfi unit with the ~0.490 lift cam and Vortec style heads. (For peace of mind, maybe go to nastyZ28.com, hptuners forum, or thirdgen.org and do a search on injector sizing. Beware, though - the equation requires knowing your BSFC, which is more rabbit-hole stuff!) (By the way, the research I did indicated that the MPFI acdelco injectors are rated for 19 lbs at 3 bar (42-43 psi), but GMT 400s with the MPFI run at 4 bar (58 psi), which makes the injectors more like 22 lbs, which should be good for 320 - 340 HP.)

OK - I did have one issue: when warmed up, with larger-than-stock tires (265 vs stock 245), going up a moderate incline, at 45-50 mph, the engine would stumble a bit. But, that was in OD; it never did it in third gear, so I believe the issue was tuning, not injector size.

Edit: AAARRRGHH! - You posted before I was finished responding to your earlier post! My experience is limited to 350/5.7s!
 

98k2500

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Hey thanks for this information RockLake. Keith at PCMforless told me now sells a spider setup with 36lb injectors for $600. He told me if I go above a 218 camshaft then I'll need the 36lb injectors. If this is accurate, Keith seems like he knows his stuff, I'll rethink my cam selection.
 

L31MaxExpress

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Hey thanks for this information RockLake. Keith at PCMforless told me now sells a spider setup with 36lb injectors for $600. He told me if I go above a 218 camshaft then I'll need the 36lb injectors. If this is accurate, Keith seems like he knows his stuff, I'll rethink my cam selection.
Stock spider is good for 400-425 hp and maybe as much as 450 hp on an efficient running combo. I have seen stock LT1 injectors feed 470 hp and they are 24 lb/hr. At 70 psi the stock spider flows close to 25 lb/hr. I have seen Vortec head engines get down as good as 0.40 lb/hr/hp BSFC. At a 0.40 bsfc the spider at 70 psi can support 500 hp. At 0.450 its good for 450 hp. Above 425 hp a 36 lb/hr spider wouod be good insurance. Cast-iron vortecs will be the limit before the spider.

I ran my 383 L31 with very high flowing aluminum heads with the stock MFI upgrade spider at 70 psi waiting on my 48 lb/hr spider. I did not run out of fuel until 5,500 rpm. I went with the bigger spider because my plans are to change the fuel pump and run E85.
 
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98k2500

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Stock spider is good for 400-425 hp and maybe as much as 450 hp on an efficient running combo. I have seen stock LT1 injectors feed 470 hp and they are 24 lb/hr. At 70 psi the stock spider flows close to 25 lb/hr. I have seen Vortec head engines get down as good as 0.40 lb/hr/hp BSFC. At a 0.40 bsfc the spider at 70 psi can support 500 hp. At 0.450 its good for 450 hp. Above 425 hp a 36 lb/hr spider wouod be good insurance. Cast-iron vortecs will be the limit before the spider.

I ran my 383 L31 with very high flowing aluminum heads with the stock MFI upgrade spider at 70 psi waiting on my 48 lb/hr spider. I did not run out of fuel until 5,500 rpm. I went with the bigger spider because my plans are to change the fuel pump and run E85.
Hey L31, thanks for this info. Makes me feel much better. I wast really sure why pcmforless stated above a 218 cam I'd need the 36lb injectors but sounds like I will be ok.
 

98k2500

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That's just what I have - the MPFI upgrade, with an L31, Summit iron heads, and the LT4 hotcam. (You have a little less lift than I do with 1.5 rockers; you would have more with 1.6 and still be OK with the Summit heads.) L31 MaxExpress is right - I have never had any lean fuel issues with the acdelco mpfi unit with the ~0.490 lift cam and Vortec style heads. (For peace of mind, maybe go to nastyZ28.com, hptuners forum, or thirdgen.org and do a search on injector sizing. Beware, though - the equation requires knowing your BSFC, which is more rabbit-hole stuff!) (By the way, the research I did indicated that the MPFI acdelco injectors are rated for 19 lbs at 3 bar (42-43 psi), but GMT 400s with the MPFI run at 4 bar (58 psi), which makes the injectors more like 22 lbs, which should be good for 320 - 340 HP.)

OK - I did have one issue: when warmed up, with larger-than-stock tires (265 vs stock 245), going up a moderate incline, at 45-50 mph, the engine would stumble a bit. But, that was in OD; it never did it in third gear, so I believe the issue was tuning, not injector size.

Edit: AAARRRGHH! - You posted before I was finished responding to your earlier post! My experience is limited to 350/5.7s!
Hey RockLake, just curious, how does your truck run with your setup. How much torque and HP you think you are getting?
 
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It's better than it was stock. In addition to the mods listed already, I also advanced the cam to 5 degrees BTDC. I also have a '98 ECSB Silverado, L31, totally stock, and the Suburban is more fun when I stomp on it to merge with highway traffic. At one time, I got 17mpg highway with it. (Averaging 75 mph.)
 

Hellbilly

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Another question from the noob.I chatted with a Summit rep. The description for these heads says max lift .520 with the springs that come with it. Comps 981-16. In the QAnd A section it says .520 max lift with hydraulic flat tappet.

I asked if the max lift for a hydraulic roller would be the same. The rep said I would need to use different springs and that ..

" Flat tappet and roller springs are totally different and should not be interchanged, used on one or the other"

This dodnt make sense to me so thought I'd see what you all have to say.
Go with the GM beehive spring and the offset retainer. Works well with the GM hot cam
 

tfrear

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I have the SUM-151124 heads on my '97 Suburban. (Made by Dart.) Installed them the way they came. The Comp 981 springs are listed as having a spring rate of 373. I also installed an LT4 Hotcam at the same time. Never any problem with this combo. (Bought/installed early 2014, 25K miles on them now.)(The Comp beehive 26915s only have a spring rate of 313, which is also supposed to be OK for roller cams with valve lift numbers like you're talking about.) No idea why Summit has that note saying, "Springs are set up to run hydraulic flat tappet cam."
how did you tune it? did you do the 411 swap?
 
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