Joined
·
224 Posts
Would you change 781's over to 702s for my combo? I got ahold of some 702s and am wondering if the increase in cr would be worth giving up some of the better flow of the 781's . I am so close to giving this heads a try .
702 BBC head over 781s.
1 - 16 of 16 Posts
The 702s have a pretty tight chamber, If your .275 dome pistons are designed for the open chamber heads you will have serious clearance problems. Why are you wanting to change? the 10.2 compression you have in your signature should be plenty for a street pump gas deal.
Joined
·
224 Posts
Still learning on what works on big blocks. I was about to buy some aluminum heads off ebay but thanks to this site I avoided what would of been a costly mistake. I guess I am at a point with all my small problems that I am thinking by combo over and am wanting to do some changes(I think)
. First my hyper pistons were the wrong pistons to go with , yes you guys here told me not to go that route with my intended usage but I didn't listen I should have kept my options open and to whether I would use nitrous.Now I have no choice, can't go with nitrous other than maybe a 75-100shot. I wish I had gone with a little more cam . The other mistake that was done was that I didn't check if my engine builder had installed a compatible dist. gear when he stabbed the dizzy it! . The last oil change has turned out "clean" from dizzy gear material but still it bothers the heck out of me .I'd keep the heads you have, RPM airgap, and 850dp minimum, you'll love a 950HP. My son swapped his 750dp for a ProForm 850 at the track yesterday and picked up 2 tenths and this was on my old .030 over 454 with a small Lunati solid ft cam. He had run a best of 12.oh's with the 750dp last fall, ran [email protected] yesterday in much worse air, bolted on the 850 and went [email protected], jetted up a couple sizes in back and went [email protected] 111mph. That same motor went a best of [email protected] in my car with more suspension work and a 950HP.
Heck...I'm running a Performer RPM and a 2" open spacer on my 505, shifting at 6k. Before I swapped to aluminum heads, 2 different times I lost 60ft and e.t. swapping to a Vic Jr from the Perf. RPM with a slight increase in mph, and I think the Vic Jr is a much better intake the Holley Strip Dominator.
Joined
·
224 Posts
I am going to give the edelbrock rpm a try with and without the spacer. I have always wondered if a dual plane intake would work better with this setup. I have a feeling I will be pleasantl surprised with the results of the edel. rpm. I am also getting a bigger carb on there. What do you think of my shift point of 6700rpms ?
I shift at 6k on the tach, so I'm sure the 1-2 I'm in that neighborhood (6700). Personally I think your shifting at too high of an rpm, you got all those cubes, let em work for you, the heads might be making it gasp for air. The open spacer adds plenum volume for the upper rpms
Joined
·
5,147 Posts
Well if you can wait about 2 weeks youll have atleast some point of reference, I am running the 702 heads with bigger valves installed. I am also running an rpm air gap. Also a 1000hp pro systems 4150.
I like the older closed chamber heads, if you get them worked a bit, they flow nicely...but in your case, if you're already at over 10/1, I think I would leave it alone, like the others are saying. Besides, I havn't looked up the specs on your pistons, but if they are not designed to work with closed chamber heads, you're out of luck anyway. Im sending you a PM also. John
Joined
·
224 Posts
What would be a shift point to come down to ? How about 62-6300rpms. I understand I'd be guessing but its worth a shot and the e.t s wouldn't lie. I just wish my local track would supply more than e.t . The mph would info would have me completely heading in the right direction. One other thing that has got me thinking my combo over is the wifeys' comments regarding the 383 I had before my current combo(just for the record , I'll never regret going with the 496). Comments like " it just doesn't run like the old(383)motor". It sounded better. She means well but she makes the comments at the wrong time , like when I am having ignition problems in the middle of a competition! She understands a little about engines but not enough. I know my 496 has way more than my old 383. I know torque wise my 496 kicks the old 383s' azz and top end it does better to. My best with the 383 was a 12.3 at 109 mph with great traction. This 496 gave me a best so far of 12.4 mph?? with traction issues. I have given up the 702 idea , will stick with the 781s until I get some quality aluminum heads probably some Brodix RRs . I will get the edel. rpm ordered along with the bigger carb , lower shift points and see what she does.
Joined
·
436 Posts
You may run out of manifold volume if you go to the oval rpm. The carb is too small and that is an awfull mild cam to be shifting that high with a 496. with those heads try to keep it under 6000rpm and see if it doesn't go faster.
Crankitup (Trey) on this board has a 496 that had less than 9-1 compression and ran 11.90's, swapped heads from 781's with small valves to the Edelbrock rovals with 100cc chambers and big valves, mainly to get the compression closer to 10-1 and has run a best of 11.66, this with a small hydraulic roller and RPM intake w/ a 850 Demon, not to mention 3.31 gears out back on drag radials. I think if you can get traction, I know the RPM will feed the cubes with a 6k shift point.
My 10:1 502 has run a best of 11.8's at 114 mph with a 226/236 hydro roller (shifting at 5800) with a race weight of 3936 lbs using a RPM Airgap, GM aluminum 290cc heads, and a HP950 carb. And my 60's were horrible at 1.8X. If you can get it to hook, I bet your times will be better with the RPM intake. I'm now running a solid roller with 251/261, .660/.660 and shifting at about 6300 rpm, I have run a best of 11.2 @120.
Joined
·
224 Posts
Believe me guys I am paying attention . Looks like I have my shift points all messed up. What got my attention on one of my runs , I recall having overheating problems on one of my runs so I am keeping an eye on the temp gauge . It turns out that I shift early into second gear and then before the shift light comes on I shift into 3rd once again(not concentrating) earlier than what I had my shift light set to, I still ran a 12.7 and that's with completely letting off the accelerator way before the 1320 mark. Making me think I was on my way to a very nice run , e.t wise by shifting earlier although not on purpose. Hopefully that made sense.
1 - 16 of 16 Posts
Join the discussion
