The Stovebolt Site Forums The Shop Area Engine Shop 261 performance gains

I am putting a late 261 with 848 head into a frequent driver 47 car this winter/spring and looked at the McGurk 261 dyno numbers.

I was curious about the actual percentage gain from doing one upgrade to the next to find out what makes sense for my application.
This car will mostly run on freeways and streets between 2000-3000rpm.

So I crunched the numbers and got some interesting results:

Upgrades ........ 2000rpm ..... 3000rpm

Dual exhaust ..... 4 / 4 ........... 6 / 3

Dual Intake ..... 12 / 6 ......... 10 / 12

Head mods ........ 2 / 2 ........... 7 / 7

3/4 cam ............ 8 / 5 ........... 3 / 2.5

Numbers show % Hp / % Torque gains from stock to dual exhaust and then one upgrade to the next.

With stock tranny, 3:55 rear and stock tires we get 49mph @ 2000rpm and 73mph @ 3000rpm
Remember the gains are all without wind, drivetrain and tire/road resistance factors.


An easy intake and exhaust upgrade can be done by just about anybody with access to a muffler shop and gives 16% total Hp increase over stock.
The extra gains from expensive and time consuming head and cam upgrades may not be worth it for daily or frequent drivers with a running engine.
If building from scratch it seems however a good idea, especially plus boring the cylinders.

Here are the HP/rpm curves from test 1-5 plus test 7 as published by Frank McGurk in the 1955 Article for Hot Rod Magazine


[img]http://www.siegmundguitars.com/images/engine/streetgains.gif[/img]

Thanks for the translation. I'm also building (or helping my neighbor build)a hopped up 261 to go in my truck. I'm pretty much doing all the above with the addition of HEI and increasing the stroke w/292 rods and using caddy pistons. The bore will be .050 over. I'm also using a milled 848 head with powerglide intake valves. The machine shop should be done with the block any time now.

Here are the remaining tests with the high performance upgrades.


[img]http://www.siegmundguitars.com/images/engine/hipogains.gif[/img]


The purpose of creating the charts was to determine how far I want to go with my 261 build for maximum street/freeway performance.
Looks like boring the cylinders, raising compression ratio and a mild cam upgrade is where street upgrades are maxed.
An HEI ignition would be another upgrade.

All upgrades show biggest gains per 1000rpm between the 2500-3500 range.
Gains level off after that with the street upgrades.

Perfect - that's exactly the range I will drive the most.

McGurks article and the dyno numbers are truly a great aid and guide in rodding the 235/261 engines.
The graphs give a helpful visual image of the numbers.

I couldn't agree more. I found an original copy of that May 55 Hot Rod issue. I've read it 20 times.

What are the specs for the 82 Comp cam?
arnie

All the specs are in the article and a lot more info....just follow the link above.

This cam shows some of the notorious low end loss in favor of the gains in the higher rpm range.
Looking at the curve graph helps resisting the urge to install a too hot cam for daily driver application.
The 3/4 super cam is hotter yet, showing poor street performance with more potential from additional hipo upgrades.

I looks like to me that that the dual intake/exhaust combo showed the largest gain in hp and torque measuring by percentage of increase. Definately the easiest and cheapest way to get more without having to disassemble the block and spend a ton of time and money. That's how I started with my 235. And now I can't stop. Gotta have more

Bigmac- - - -how much of a stroke increase are you doing, and how? With the .050" overbore, and the extra stroke, what will the final displacement be? I just acquired a 261, and I'm thinking of hotrodding it a little. Also, which pistons are you using?
Jerry