Inliners International Forums General Discussion Blown and Injected Turbo and Injected 270

Hello Inliners, last year I made a post as I was hoping to begin work on rebuilding, for a second time, my 1946 GMC. I have finally been able to carve out some time and money to work on my project. Here is what I have so far:

270 GMC with small port head
Lakewood bell housing with 10.5" clutch and lightweight flywheel
Saginaw 4 speed with overdrive conversion
GM 10 bolt axle either 3.43 (65mph@ 1900 rpm) or 3.73 (6 mph@ 2050 rpm)
Intercooler

My primary goal is something that is very drive-able with good fuel economy.

I fabricated the manifold below with a 70mm throttle body and planned to run 3 injectors fired in a batch. I understand with the Siamese intake ports there is some difficulty. Would it work to but an injector in every runner and fire them all together or will there still be a lean/rich condition? I would like to use the manifold setup as shown just to make the plumbing much cleaner as the output of the intercooler can plumb directly into the throttle body without additional bends in the pipes. An alternative would be to run two 115 lb injectors right behind the throttle body. I have several questions, but thought I would start with the injection layout first and go from there.

I would use injectors at each runner.A number to remember: at 6000 rpm independent injectors will fire 50 times per second.If you use two injectors below the throttle body, I would think you would want "tuned" runners.

Just my opinion - I'm no expert.

Will Willis

There are several build threads here for siamese heads using EFI on older GMC and Stovebolt engines. You should read them first before you go much farther. Siamese heads and EFI are tricky and react much differently than conventional port cylinders heads do.

Suggest you go to the msefi.com and msextra sites and research the issues with siamese port heads. The mini engines have the same issue and are discussed in detail.

A viable alternative is to use 3 TBI's - TBI's off of 2.8 GM V6 engines would be the right size. Since TBI mimic a carb fuel delivery - just with much better fuel control - the A/F delivered at the port is more uniform - using a plenum would assist.

Would there be any reason the throttle body could not be mounted horizontal instead of vertical and eliminate the 90 degree bend through the intake hat?

It looks like the 5.7L GM TBI's used 55 or 65 pph injectors. I found the big blocks used up to 90 pph. Using an injector sizing calculator it showed a 350hp turbo engine would need 97pph ith only two injectors. What are other people using for injectors in the GM TBI's?

If you switch to a TBI style setup instead of EFI, you'll need to also change your intake design. Something more along the lines of this if you use a 2V TBI unit.

So I have been keeping myself up at nights after browsing some old threads about siamese ports and fuel injection. Today I found an old thread Tom Lowe when he was experimenting with dividing the intake ports on a 292. I am running the small port 270 head. By my math the 1.25" diameter port is 1.22 in^2. I presume, but definitely would like the experts to confirm that I could open up this port to the 1.75" diameter of the 302/270H head. I am also assuming that if I can fit a 1.75" round opening I could machine in a 1.75" square opening.If so the total port area would be 3.06 in^2, subtracting out for an 1/8" divider total area is 2.84 in^2. This would still give me more than twice the area of the 1.25" port, so theoretically I could get similar flow out of this divided port arrangement as the shared port. Please keep in mind my goals are not a 6,000 rpm race engine so the peak horsepower number does not concern me as much as driveability.

I have been running a 270 in my '53 Chevy pickup since 1978. It is a driver being my only pickup for most of that time. While most of the miles have been short trips with moderate loads it has often hauled over a ton on steep rough roads. It has cruzed from here to the coast and from there to Texas. It has never failed get me home (on long trips).

I have run a single 2 bb, 500 cfm 4 bbl, 2X1 bbl and for a few minutes a TBI at a demo that EFI-DIY and I did with a Megasquirt in 2008 at the Inliner picnic at Bonneville. Seeing your original post saying, "My primary goal is something that is very drive-able with good fuel economy." I would suggest the '89 GM TBI we used in '08. It was a total junk yard setup except for the MS. It could be done with the stock controller from a 90% V6. I didn't get to do anything but fire it up in less than an hour but it showed real promise. The truck is ready to try it again whenever I am. Every time someone here wants to build a dependable useable driver the "every squeezable rpm/hp" guys take over. The Siamese port issue has very little to do with non-race engines. If you are not going to be driving the difference between a $2,500 head and a $ 250 head most of the time you are waisting your time and money. A lot of work has been done with Siamese port heads.

Siamese ports are only an issue if your trying to do port injection.

As for mounting the TBI units near horizontal - they should be mounted with the CL of the TB above the CL of the intake port with the runners angling down from the TB to the port.

This will keep the the fuel from puddling out the TB between the intake events when at idle.

I was in the garage last night building some front shock mounts and thinking about all of the things I needed to do. I came to the conclusion that it had been 20 years since I had driven my truck and I wanted to get it going ASAP. Can't say sometime in the future I wouldn't mind experimenting with trying to split the ports and due sequential injection, but for now TBI it will be. I would like someone to double check my match here. It looks like for a 350hp turbo engine I would need 97 lb/hr injectors at 90% duty cycle. The information I found stated the big 7.4L injectors were rated at 90lb/hr @13 psi fuel pressure. Best I can tell if I increase the fuel pressure to ~15psi then I can get 97lb/hr. I could not find any information on converting the GM TBI to reference manifold pressure, but it seems like if I drilled and tapped the top side of the regulator and ran a line to the manifold it would bias the diaphragm to manifold pressure. Anybody have any experience here? If noting else I would remove the built in regulator and install my fuel regulator in the output line. I was having trouble finding the thread on the TBI conversion that Beater and EFI-DIY did at Bonneville. Would is the best suggestion for a manifold to run the TBI, an aftermarket 4 barrel or welding a new flange on a stock 2bbl manifold?

I haven't seen any of your build specs for this engine but you keep throwing the "350 HP" number out here quite often. So are you expecting to get 350 HP out of this 270 build?

I had in my mind targeting something in the 300-350 range. Still need to figure out the specifics of the build, but here are my initial thoughts I will continue to refine as I get feedback.

-270 stock bore and stroke unless the cylinders need cleaned up
-Small port head, will need to decide if the valves need to be enlarged or if the small ports will be the limiting factor and enlarging the valves won't gain much.
-Forged pistons, zero deck 8.5-9.0:1 compression
-Stock lower end
-Boost 8-10 psi
-Looking like will go the TBI route
-Appropriate cam

Does the target of 300-350 and good drive ability work with the rough build outline above?

I think your build would benefit greatly from larger valves and mild porting. But aside from that I think your goal is well within reach.

There are two 270 small port heads. The early domed chamber was used on all GMCs 228-270. There is a later 270 head with a D chamber more like the 302 D chamber head. Mine is the later head I think the valves are stock size but it is shaved .035" and the ports cleaned ups bit. I would go with bigger valves like 1.86 and 160. Small ports work well on the street and is what Bill Fisher recommended for the street.