Inliners International Forums General Discussion Tech Tips 261 performance gains

I am putting a late 261 with 848 head into a frequent driver 48 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


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.


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

RR;

Great presentation!!

That's with stock bore & stroke too.

Mine use +.125 bore, 10-1 pistons & 3 carb. & a better cam. If I ever can get one together,
I'll try to 'dyno' it to compare.

Which part of the desert are you in??

Just basic chart plotting....

Here are the remaining tests with the high performance upgrades.





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.
A pertronix 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.

The higher compression and 'later' cam is what does it. HEI is only helpfull in freezing temps.

I think the curve would be different today. Remember that data is circa 1955.

I/we shifted @ 5,500 when racing in 1962 and we estimated 250 Hp. @ 5,000 +/- "way back when".

Yes, I rather stick with the pertronix ignition, like on my 235. It seems like the simplest, most reliable and longest lasting ignition.

I also wonder what the curves would look like with turbo chargers or the caddy 261.

Or perhaps the longer stroke version 285/292" ones.

What part of the desert are you in??