Compression Testing

Reply Subscribe

Thread Tools

Search this Thread

Aug 4, 2017 | 07:28 PM

0maha

Thread Starter

Seasoned HDF Member

Joined: Nov 2012

Posts: 6,559

Likes: 4,820

From: Omaha

Compression Testing

The various comments and comparison numbers on compression test results had me wondering, so I decided to do a little testing.

(On my stock, 1990 Evo, 32k on the clock, CV carb.)

Filter cover and filter on, throttle at idle: 70 pounds

Filter cover and filter on, throttle at 100%: 130 pounds

Filter and cover off, throttle at idle: 70 pounds

Filter and cover off, throttle at 100%: 135 pounds

Filter and cover off, throttle at 100%, hold the slide up: 150 pounds

Ok. I feel better now.

Aug 4, 2017 | 08:40 PM

Prot

Club Member

Joined: Aug 2008

Posts: 22,880

Likes: 17,998

From: Lafayette, Louisiana

I am surprised that jump is as big as it is holding the slide up.

Aug 5, 2017 | 01:00 AM

timsmcm

Cruiser

Joined: Jul 2013

Posts: 243

Likes: 6

From: Texas

Wow that is a really big difference.

Aug 5, 2017 | 05:55 AM

98hotrodfatboy

Seasoned HDF Member

Joined: Mar 2010

Posts: 21,168

Likes: 7,676

From: Poolville

Why doesn't the throttle hold it up all the way. Is it out of adjustment?

Aug 5, 2017 | 07:33 AM

0maha

Thread Starter

Seasoned HDF Member

Joined: Nov 2012

Posts: 6,559

Likes: 4,820

From: Omaha

Quote:

Originally Posted by 98hotrodfatboy

Why doesn't the throttle hold it up all the way. Is it out of adjustment?

The slide on a CV is operated by vacuum pressure. There is no mechanical linkage to it.

This vid is pretty good at showing how it works.

Aug 5, 2017 | 08:28 AM

98hotrodfatboy

Seasoned HDF Member

Joined: Mar 2010

Posts: 21,168

Likes: 7,676

From: Poolville

Not ever having one, that kinda slipped my mind. So next question is then, Why would you run one if you couldn't get maximum lift to the slide especially when you just proved the performance difference in cfm intake (15 psi is a lot)......

Aug 5, 2017 | 01:02 PM

0maha

Thread Starter

Seasoned HDF Member

Joined: Nov 2012

Posts: 6,559

Likes: 4,820

From: Omaha

Quote:

Originally Posted by 98hotrodfatboy

You could probably write long papers on that (and I'm sure lots of guys have), but in practical terms, I think it comes down to a few things:

- CV carbs give more accurate AFR over the entire power range, which translates into better mileage.

- S&S style carbs give snappier throttle response. Although the accelerator pump helps, a CV carb has a certain delay as the slide moves and the main jet opens up.

- CV carbs self-adjust for altitude variations (at least to a degree)

- S&S style carbs are simpler and quite a bit easier to get set up

Kind of a "pick-em" situation, really. If I was building a bike from scratch, I'd probably get an S&S. Since I've already got a perfectly good CV, I don't see any reason to change.

(THC, that's your cue!!