Inliners International Forums General Discussion Engines Need Assistance - GMC 270 Performance Options

Good day,

I am needing some insights and opinions about a couple options I have available to me that could improve performance of my 1957 GMC 270.

Currently it is in stock configuration and it is very tight with good compression, clean burning, with smooth idling and acceleration characteristics. Therefore a rebuild is not necessary.

I want to improve the hp for pulling and towing. I plan to install a 5 speed overdive transmission (NV4500) to keep up with traffic on the freeway.

However I plan to install this motor in a 3/4 ton truck and I would like to improve its performance and keep the costs within a limited budget.

Modifying the cam and having a mild 3/4 grind done to it is a given. In addition dual Fenton exhaust manifolds are a given.

My options and questions are these:

• Use of a civillian GMC 302 intake manifold wih either a 2 barrel carb or 4 barrel carb ?? - what is gained and what is lost should I choose a 4 barrel carb over the 2 barrel? Is fuel econmy reduced with a 4 bbl and the 2bbl may be somewhat more frugal on gas consumption?

Can someone venture a guess and provide an estimate of performance values between a Dual Carb Intake setup vs. a 2bbl and 4 bbl carb setup should I use my civillian 302 manifold?

The head on the 270 is the standard 819 Head. I cannot afford an H-Head. Am I right that a 302 - large port 983 head would not improve performance on the 270 block? Is my understanding correct that there would be a loss of compression with a 302 head on a 270? The 819 would give me what I want; a smoother idling motor...as I understand. Correct me if I am wrong.

What can I expect the cost to be to install larger intake and exhaust valves on the 819 Head?

Your assistance and knowledge would be greatly appreciated.

Thank you in Advance.

Tom

A GMC 302 head would require 302 pistons because of the combustion chamber configuration. Chevrolet V8 valves -- 1.94 intake, 1.6 exhaust -- can be installed in the 302 head without difficulty. Breathing in the upper rpm range would considerably improve.

In the day, Bill Fisher among others argued that a "small-port" head would be more than adequate for most "street engines." Given the sort of use you envision, you may find that surmise correct. Any improvement in low- and mid-range torque will help. An "RV" cam grind is probably what you want. You probably will not have need for more than 4,000 rpm. i should think that you will not need more than 300 cfm in carburetion. A good two-barrel should be enough. Economy is a concern, and your overdrive should help that, to the extent that it can be helped in a 3/4-ton truck towing a load.

God;'s Peace to you.

d
Inliner #1450

Have you consider changing over the TBI EFI? All the parts needed are readily available in the wreckers with the exception of an HEI distributor which can be had from Langdon's. A 4.3 V6 TBI will be big enough to feed your 302.

Locally $300 is enough to get all the items needed.

Can the 4.3 TBI units work in a blow through turbo design? What kind of engine management system?

Yes for low boost app's, the only kink is the stock fuel pressure reg (FPR)will have to be removed/defeated and an external boost referenced fuel pressure reg. installed. I would tend to use a TBI off of a 350 cop car (bigger injectors) for a boost app. This ensure you have enough fuel capacity. Note that unlike a blow through carb set up where the the pressure drop across the venturi compensate with more fuel flow, an injection system is insensitive to flow and the injector have to be sized correctly to avoid going lean.

use this

(HP * 0.55)/number of injectors == lbs/hour injector size

e.g. 300 HP *.55 /2 == 82.5 lbs/hr << use a 454 TBI injectors in this case. They are about 85 lbs/hr.

As for EFI control lowest cost is megasquirt (MS2. There is an adapter that allow the MS2 board to plug directly into the GM harness for a TBI system with no wire mod's. These days you can buy a ready built MS system. A $100 laptop running windows XP is perfect for use to tune the unit up.

There are other EFI systems such as holley out there but cost more $$$.

Other than fixing the FPR the hardest part is running the fuel line/pump install especially if you choose to run hard line.

Hope this helps.

Tom,
I'd suggest a set of headers (the longest you can get), Cam about 205-210 @ .050 and the Holley 390cfm 4 barrel # 8007. The Holley has small primaries which will give good throttle response and fuel mileage. You don't need to have the bigger 983 head. At the rpm you'll be running your present head will work well.

Experiment with ignition timing and be pretty aggresive since you don't have very much compression to worry about. If you want you can have the head surfaced 0.060" and raise the compression some.

EFI is a good option if your comfortable with doing it.

Ron

Ron good to "see" you.

I like the header idea.

Tom, the way Randy (efi-diy) is talking about with a TBI unit has been demonstrated to the membership here and is documented on this site.

It seems to be the best overall for power and economy for what you are doing.

Good luck.

Erik,

Good to see you also. What have you been up to?

Ron

Trying to get the Berkeley ready for Speedweek, and trying to learn a new trade.

I'm an AFLAC rep now.

CORRECTION


ON GM TBI unit the FPR diaphram is inside the airflow so no external FPR is needed. And as the underside of diaphram is directly exposed to boost pressure - its self compenstating.

If you use a manifold manufactured for a 302 you need to taper the head to fit it. This increases the velocity thru the port and add to torque and performance. I would heat the manifold even if you use headers..good luck.

thanks to all for the insights. A TBI EFI is all new to me and will now need to study this option. I'm running uphill on the learning curve. Thank you for the responses. tom