drag-200stang":1lmqgolz said:
1) As you fab up different induction set ups......... They need to be together to crash and burn efficiently.
2) Is there some kind of formula for figuring out what the right proportion of carb cfm to engine size is for blow through?
3) I would think if an engine was responsive and ran well NA with a certain size carb and a turbo was added it would be OK.
4) I wouldn't worry about your rods until you cam it up.
5) Your new cam under boost should have about 300 rpm higher usable rev range than what's advertised NA, if everything else is in order.
Hi Jim,
1) I'm using 100LL AvGas as a fuel, and that stuff vaporizes INSTANTANEOUSLY, especially with as much heat as there is in that intake log. I don't think there are any fuel droplets anywhere in the intake as it runs. I can be wrong, but AvGas isn't anyhting like car gas.
2) Formula?? The only guys running 750 carbs on 350ish cube motors are running over 1000 HP., and those carbs are super well prepped by specialty shops for about $800. I figure if a 350 chevy with a stock re-jetted 650 DP Holley can run 8's in the quarter, then my 200 won't ever need anything larger than about 250-300-350 cfm. I am going to try a 1.08 motorcraft 2 bbl and probably also a 1.21 motorcraft 2 bbl. I already have both. According to info from here, a 1.21 is about the same as a Holley 350.
3) The carb set-up running OK on a N/A car, if a little too large, will get super crazy over 8-10 psi.
It has something to do with the pressure drop through the venturis.....or lack of it.
I think it is like this: If the carb is too large, boost builds up in the intake, and above the carb, and even though pressure is high, flow through the venturis is (relatively) low. High pressure but low flow and the metering goes away at the venturi.
One guy (actually running a chevy, lots of them on turbomustangs) on the turbo mustangs board was trying to get (I.I.R.C....) a Holley 850 HP series carb that ran great on his N/A 350 to run under boost. He didn't have the $$$$$ to buy a new carb, so he was trying to use the one he already had.
He kept going WAAAY lean under boost, even with the largest jets in it that Holley makes (I think 120's).
The way he fixed it was to have it sent off to....I think CSU? carb shop, and they installed custom downleg annular venturis that were VERY thick, and brought the CFM's down to around 700. That fixed the problem.
I think it also helps to have more consistent metering when the pressure drop through the venturi is very high.....it makes a lot of suction between the venturi and the float bowl. That extra suction helps cover up any flow inadequacies in the metering circuit.
The venturi in my 1 bbl is I think 1-1/8" at the smallest point. I think I can honestly run a 12.99......through a single 1-1/8" venturi!!
I know I need to re-jet a little higher...N/A cars like to run about 12:1 fuel mixture while turbo cars are supposed to be 11.5:1 - down to around 11:1
I was running like 12.1:1 - 12.2:1 on that run......just a bit lean. That didn't help with the detonation either!! I plan to make up for that with alcohol injection, though.
One other thing while on this topic.....a lot of guys use very small "precision piano wire" to partially block the air bleeds. This trick is only needed on some cars and/or set-ups. If you run into a condition where the car wants to lean out at high boost, partially blocking the main air bleeds with a very tiny piece of wire will helpp richen the fuel mixture. You can also drill and tap the air bleeds to accept really TINY jets. In the past, I have cleaned the brass air bleed (or main jet) well, soldered shut, and drilled to the size I wanted. It kind of redneck way to do it, but it works.
4) I have heard the cast rods in post - '74 engines are pretty weak compared to the forged ones. What makes you think I shouldn't be too worried? because I am keeping the RPM's low right now?
5) Interesting thing about raising the RPM range about 300 RPM.
That sounds about right. I have heard that a cam will still dictate the RPM power range of the engine, whether N/A or boosted, it won't change very much.