Question regarding a Holley 750 cfm, 4150 carburetor compared to the original Qjet 750. Here's my story, I bought a 1970 Buick GS with the base 455/350hp engine. This car was sold to RK motors by the restorer. It was then purchased by the first owner in 2015. I purchased this car in 2021. During the original restoration, engine was rebuilt and bored .030 over (462ci now). Has Stage 1 cast iron heads. Has been port matched. Has a Comp Cam that is more powerful than the original Stage 1 cam. Has original cast iron exhaust headers. Has the original TH400 BA transmission that is rebuilt. Has a posi 2.93 highway deferential. To me, this didn't make any since if you're building a performance car. The first owner added an F-41 suspension package. He also added 11 to 1 aftermarket pistons, which caused major detonation issues. Which has been resolved (we hope). Installed a complete MSD system. He also added a shift kit in the transmission. Here's where I have a question, the original Qjet was not able to be repaired because it had a wrapped carburetor top that leaked gas. The shop purchased another Qjet but it came in faulty, and they could not find another Qjet, so they decided to purchase a Holley 750, model 4150 that is a double pumper with mechanical secondaries and electric chock. My question is, should they have purchased a Holley with vacuum secondary? Reason I ask, I've read that mechanical secondaries work best on light weight cars (my GS is not light) that are also equipped with like a 3.42 differential. Did a just make a huge mistake purchasing this model, Holley????? Just so you all know, I've run Holley's before on my chevy SB and it ran great. However, this is my "first" Buick GS BB. FYI: The GS has been test driven yet because the new MSD distributor failed and a new one was just purchased and installed and requires a final tune up. I would love to hear from some Buick performance experts regarding my question. Thank you all in advance for your help.
I've never heard of a Holley vacuum secondary carburetor working very well on a Buick 455. Mechanical secondary is the better choice. An 850 would be a better choice. That being said, getting a Quadrajet is no problem at all. Ken Gies (techg8) can build you one specifically for your engine. He is Everyday Performance and he is in the Vendor Section. https://v8buick.com/index.php?forums/everyday-performance-llc.172/ https://www.everyday-performance.com/
First, thank you for helping me with my issue. I have only been in this Buick forum since 2021, it's all new to me. I know you are one of the top engine experts on the forum and I read a lot of your advice. Whatever advise you give me I will run with it. You can't beat the amount of experience you have acuminated over many years of wrenching on Buicks. Again, thank you for taking time to help me and in fact, I've been helped by many Buick guys on this forum. FYI: The original engine rebuild was done by John Chamberlain. John and I have become very good friends. He has helped me a lot for which I'm very appreciative of. I took pictures of the engine and its aftermarket 11-to 1 pistons with a borescope. John never installed these pistons. We believe the person who purchased the car from RK Motors in 2015 did a lot of modifications that he never should have done. 850, wow, really!!! Sorry for this long email...... I was looking at maybe an 800 because of the performance mods done to this engine but didn't want to go overboard with any carb larger than an 800. Unfortunately, the head engine mechanic has not worked on many Buick engines. In fact, in my area in Northern Virginia, it's rare to see a GS 455. I know of only one 1970 GS and it was recently purchased on Barrett/Jackson (nice paint job but the rest is a mess). The engine mechanics at the shop were not sure I should go with an 800 but they did modify the 750 with a upgraded power valve and larger jets. Question, how do they know if this is enough for the engine??? I wanted to have the engine Dyno tuned but the mechanics would not recommend that because the engine build was back in either 2010 or 2012. They think it might blow on the Dyno. However, the engine only has 3,500 since it's rebuilt. I don't know how to measure the full potential of the engine as it stands now. Reason why a 750 was installed, I think the mechanic is guessing. Should I install an Air/Fuel gage on the carb to see if I'm getting the best air/fuel mixture I can get? I don't plan on taking it to the drag strip, but I will most likely do some hot rodding on the street. At this point in time, not sure if I should take a chance and get it Dyno tuned anyway? The engine mechanic still has to do a final tune because the first MSD distributor failed and another one had to be installed, they also decided to install an MSD coil also. I thought this was a good idea.
First, thank you for the compliment, but there are lots of very knowledgeable guys here, I wouldn't say I was the best, I just have the most posts here on V8, mainly because I have been here since 2002. Buick engines like big carburetors. Never mind the formula, but if you use an 850 and a 750 at the track, the car will go faster with the bigger carburetor. Buick guys have known this for years. The 750 might be better on the street, the throttle response will be better, but the bigger carburetor will make more power. The original Q-jet was a 750. Starting in 1971, Buick used the bigger Q-jet (800 CFM) on the 455 engines. If the engine is sound, there is no reason you can't run it on a Dyno. The engine Dyno puts a huge load on the engine, more of a load then anything it would encounter on the street. I would guess a Chassis Dyno would as well. The only thing I would worry about is detonation at high load WOT. The solution to that is to run 110 leaded race fuel and make sure total timing is correct when it is on the Dyno. Also, you'll want to verify that the engine has the proper oil pressure for the RPM you intend to spin it to. That would be 11-12 psi/1000 RPM. Of course you can run it down the track as well. The Trap Speed will give you a good idea how much HP the engine is making. The launch will not effect the trap speed, so you can baby it out of the hole and get it into high gear at WOT through the traps. I would run some race gas mixed in with the pump gas, again, as insurance against detonation.
Thank you, Larry, for your help. Here are some engine specs/tests I had done on my car that might help. (1) Comp Cam - specs in my 1970 Buick GS 455 Gross valve lift: Exh. Lift. .496” Gross Valve lift: Int. Lift. .478” Duration @ .050: Exh. Duration .231 Duration @ .050: Int. Duration .218 Lobe separation: 112 degrees (2) COMPRESSION CHECK– on the 455. Looks like great numbers to me, higher than I expected. •dr/s: #1—180 #3—185 #5—180 #7—175 •pa/s: #2—180 #4—185 #6—180 #8—180 (3) LEAKDOWN TEST, valves look to be in great shape. •dr/s: #1—95/100 #3—92/100 #5—94/100 #7—90/100 •pa/s: #2—90/100 #4—92/100 #6—93/100 #8—90/100 (4) Mechanic found several vacuum leaks, brakes would operate. New vacuum number after fixing leaks and other issues is 15 in. mercury and I drove the car this morning and the brakes feel great honestly. (5) Aftermarket pistons: I never could identify them, not enough info from the borescope picture. I John seems to think they might be Jahns pistons, that is just a guess. Here are a few pictures, I also noticed it has a notch machined into the top of the piston, never seen this before. Hope any of this info helps. VET (Navy)
I'm running a Holley 850cfm vacuum secondary on my 455. Works like a dream sickle. Bought it from deadmanscurve that's on this site
Well that's good to know. I seem to remember a few posts that complained about secondary tuning. As long as the carburetor is seamless, and works for you, that's all that counts.
I currently run an 830 hp on my modified 350. Have used 750 Dbl Pmp Holleys, to an 850 Demon prior. The 850 performed very well. (Shouldn't have sold it). I should have tried my buddies 1050 Dominator before I sold it for him. Like Larry says, 750 is probably too small. Experiment!
Larry, I'm amazed Mart has used an 830 & 850 cfm on a 350 ci engine. I never would have believed it. Larry, what do you think my pistons might be manufactured by???? And what compression they might be??? What do you think of the Compression and leak down numbers? I was thinking about pulling the heads and removing the pistons and doing a value job too. But, I got to thinking, if I'm going to do that, might invest and do a total rebuild. What's your advise? OR, maybe I should just run the car to see how it preforms before I sink a load on money into it (more $$$ I have already spent).
OK, sounds good to me. Sometimes I get ahead of myself, thinking about possible fixes that I may not have to do. Thank you very much. Regards VET
Just make sure you don’t hear it detonating. Higher compression and small cam can cause higher Dynamic compression and a need for higher octane fuel. There is no way to know what your static compression is without measuring everything.
Maybe I should add Octane booster per tank??? The highest Octane at gas stations in my area is 91. I hate to spend money on Octane booster if I don't need it.
X2, 110 leaded race gas is best. I have heard good things about Torco and Boostane though. https://torcoracefuel.net/products/torco-accelerator https://boostane.com/about-boostane/
Larry, question, I have always been a 4 speed guy. This TH400 BA trans is my first go with an automatic. I recently read that the torque converter is a major part I'm the performance of the car. Since my car will primarily be a street performer, do I need to investigate what type converter I should get for my Buick or is current trans is all I need for the street. I must admit torque converters are a science of their own. VET
