Background: I have an 88 Fiero GT with an 88 Cadillac Seville HT4500 and a 440T4 transaxle. The swap was done before I purchased it.
When I got it, modifications already done included exhaust adapted to Fiero (very nicely), removal of EGR and power steering pump, and it had a Holley 500 CFM 2 barrel instead of the TBI system. The intake looks like this ( manifold ), but I have since used some adapters and installed a Slayer-Series Carburetor 450CFM Black Diamond VS 4 barrel carburetor . I expect the 2 barrel Holley was better than the TBI, and the the current 4 barrel is certainly better than that.
Modifying another manifold like what is on it is another option. I bought one off of ebay already and use it for mock up with my adapters. Either way, I have since purchased a 625 CFM Street Demon Carburetor , which I do intend to at least experiment with...
625 cfm carb is way too big for a 4.5 Cadillac. You might get a few more peak hp, but low rpm toque and part throttle drivability will suffer.
Even if you spin the 4.5 to 6000 rpm, you only need 400 (85%VE) to 500 cfm (100%VE), and that is assuming the intake, heads, and exhaust can actually flow the 400 to 500 cfm.
Carbs needs a certain amount of velocity and pressure drop through them to pull in and atomize the fuel. Putting too large of a carb on, will reduce the velocity and pressure drop and cause issues at low RPM and part throttle.
625 cfm carb is way too big for a 4.5 Cadillac. You might get a few more peak hp, but low rpm toque and part throttle drivability will suffer.
Even if you spin the 4.5 to 6000 rpm, you only need 400 (85%VE) to 500 cfm (100%VE), and that is assuming the intake, heads, and exhaust can actually flow the 400 to 500 cfm.
Carbs needs a certain amount of velocity and pressure drop through them to pull in and atomize the fuel. Putting too large of a carb on, will reduce the velocity and pressure drop and cause issues at low RPM and part throttle.
I certainly get where you are coming from, and appreciate your input. They say the most common mistake with carburetors is using to large of one. That line of logic, and the results using the old carb selector formula is why I ordered the 450 CFM carb that is on it currently. However, the 625 CFM carb in question is similar to a spreadbore in design. The primaries are smaller on it than on the 450 CFM Square bore carb.It also has vacuum secondaries. It should have crisper low and part throttle response than either of the previous carbs. It may need re-jetted though.
Hi, For your 88 4.5 engine you are better off to stick with your 2-barrel than try to upgrade to a 4-barrel for many reasons. The heads on that year 4.5 are very restrictive with small ports and valves. They are also a low compression motor, 8.5 to 1. Unless you are doing some major work to your engine; cam, compression, head work, free flowing exhaust system, you don't need a 4-barrel. If you want more power get a 4.9 or a 1990 4.5. They are the better motors to build in this engine series. They have better heads with larger valves and more compression. Good luck with your build. John
Hi, For your 88 4.5 engine you are better off to stick with your 2-barrel than try to upgrade to a 4-barrel for many reasons. The heads on that year 4.5 are very restrictive with small ports and valves. They are also a low compression motor, 8.5 to 1. Unless you are doing some major work to your engine; cam, compression, head work, free flowing exhaust system, you don't need a 4-barrel. If you want more power get a 4.9 or a 1990 4.5. They are the better motors to build in this engine series. They have better heads with larger valves and more compression. Good luck with your build. John
I feel honored that you replied to my thread! I have looked over your website quite a bit, and you have some excellent work on it! I really like your WB5 4 barrel manifold, and as you may notice it is one of the options I am considering.
I appreciate the tip concerning the heads. I had been wanting to ask someone if later model heads would bolt up.
I hope you are not offended that I am also pointing out some error in your reply. The 1988 TBI HT4500 is 9.0 to 1 CR engine , and has the less restrictive dual plane version of the TBI manifolds on it. Also I have already proven that it performs better with the currently installed 4 barrel carb than it did with the 2 barrel that was on it when I purchased it. Whether or not I can get the small primary/goggle valve vacuum secondary design Street Demon 625 to work well remains to be seen.... I may need some luck with that
Actually, despite that one error I think I could learn a lot from you about these engines if I had the right opportunity to pick your brain... Also there are a number of parts I may order from you!
P.S. Do you sell heads already adapted to roller rockers? Can your Headers be purchased either without the crossover pipe, or with a true dual exhaust crossover pipe? Better still, would you be able to manufacture tri-y style headers for an HT engine in a Fiero?
[This message has been edited by bjm362 (edited 10-07-2018).]
Hey Buddy, I didn't see the part about the 450 cfm carb you are using, I thought we were talking about a 625 cfm. I think you are on the right track with the 450 carb and that's great it is running well for you. I like that 4.5 intake manifold you are referring to. I have one of all the intakes models for these motors and that on the 4.5 is definitely a very well flowing manifold. I have used them for my first 4 barrel intake manifold I made for a 4.9 engine.
As far as the compression I really don't know what's posted out there. When I was building my 4.5 sc motor I measured the combustion chamber volume, the piston dish volume, the head gasket thickness for both the 4.9 and the 89 4.5 engines. When I did the math the 4.9 came out to 9.5:1, the 4.5 came out to 8.5:1 and the 4.5 with the 4.9 heads came out to 7.8:1. I can only speak for my own results.
I am no longer selling parts on my web site. I took the store part down on my site several months ago and I am surprise you were able to access it. I have a couple of local customers that I am working with on some r&d stuff but that's all the time I have right now to apply to the business. Maybe in the future.
Good luck with your build, Cheers!
John
[This message has been edited by Johns 4.9 (edited 10-08-2018).]
Again, fieroguru and John's4.9, I do appreciate all participation. It may turn out that as this engine sits I already got lucky and hit it's sweet spot with the 450cfm. Since I already have the 625, I do have to at least try it out... Also I am experimenting with turning the carb sideways too.... We shall see how it all works out! \
Could try switching to a quadrajet, they are much more versatile than a holley.
But Electronic Fuel Injection is the most efficient and versatile, being able to self adjust for any engine requirement for any driving condition. It will also result in cleaner combustion and better gas mileage. I don't see any advantages in removing it and going with old time technology.
------------------ " THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite. "THE COLUSSUS" 87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H " ON THE LOOSE WITHOUT THE JUICE "
But Electronic Fuel Injection is the most efficient and versatile, being able to self adjust for any engine requirement for any driving condition. It will also result in cleaner combustion and better gas mileage. I don't see any advantages in removing it and going with old time technology.
I guess I thought he was currently running a Holley.
2.5 , I had considered trying an RQJ (actually an Edelbrock RQJ clone). I replaced the Holley 500CFM 2 barrel that was on it with a Holley square bore 4 barrel instead. The second carb I bought (A Street Demon) is like a RQJ spreadbore design, but much improved.
This is what the engine compartment looked like with the Holley 2 barrel it had on it when I got.
