Compression and Vaccuum
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Postmaster
Joined: Dec 2003
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From: Honkytonkville
Compression and Vaccuum
Will bumping the compression ratio of a motor(changing from dish pistons to flattops) help or hurt the vaccuum pulled by the motor. My mud truck is pulling about 15.5 Hg right now. Problem is can't have below 15 Hg for my class, so I'm bouncing right now.
Elder User
Joined: Aug 2006
Posts: 642
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From: Olathe, KS
Going up in CR usually helps vacuum. Partly because, most rebuilt engines are below the advertised/percieved CR and the cam choice was based on it. If you are overcammed for the current compression then yes it can(likely) raise vacuum.
A better option may be to tweek on the timing. Given there is room in the tune. Again, not knowing anything about your setup and what you have already done, this may not be an option.
Sounds like you are not looking for a large change in vacuum. But are you wanting to improve HP as well. How stock is this engine, what is your current set up. Most importantly. How far down is the piston in the bore? Head gasket thickness? (all for quench specs) And of course the general build info. CR ratio/ cam specs. carb/FI, exhaust, head work etc...
Pete
Don't know if I would agree on LSA. Or maybe I misunderstand... Cam timing does provide the core of your vacuum behavour, but a LOT of engine manners can be influenced by tune. A stock mild engine can be made pretty ratty, and what many consider to be a big cam, can be made to behave near stock, all with tune.
A better option may be to tweek on the timing. Given there is room in the tune. Again, not knowing anything about your setup and what you have already done, this may not be an option.
Sounds like you are not looking for a large change in vacuum. But are you wanting to improve HP as well. How stock is this engine, what is your current set up. Most importantly. How far down is the piston in the bore? Head gasket thickness? (all for quench specs) And of course the general build info. CR ratio/ cam specs. carb/FI, exhaust, head work etc...
Pete
Don't know if I would agree on LSA. Or maybe I misunderstand... Cam timing does provide the core of your vacuum behavour, but a LOT of engine manners can be influenced by tune. A stock mild engine can be made pretty ratty, and what many consider to be a big cam, can be made to behave near stock, all with tune.
Senior User
Joined: May 2007
Posts: 149
Likes: 0
From: Overland Park, KS
Combustion efficiency can increase vacuum because it can help exhaust scavenging. If the air/fuel is quickly burned and efficiently turned into kinetic energy in the crankshaft, it won't be trying to expand as the exhaust valve opens. This makes it easier for the piston to force the exhaust out, and more of it is gone once the intake valve tries to open. Especially with a camshaft with a lot of overlap, if the air is still trying to expand when the intake valve opens, the pressure is forced into the intake manifold. This is called reversion. People use this as a tool on ultra high-CR engines by letting the air/fuel "bleed" into the intake, lowering the dynamic compression. That's why they put vacuum limits on engines.
If you can get better combustion efficiency and therefore better exhaust scavenging you'll have better vacuum. Two [free] mods I know of will dramatically increase combustion efficiency. One is called "Edging" the chamber and the other is Singh Grooves.
http://fueleconomytips.com/2006/12/27/%e2%80%9cedging%e2%80%9d-ahead/
For those of you that haven't heard of Singh Grooves, you'll probably have a hard time swallowing the concept. It does work, however, and there have been hundreds if not thousands of people that have tried them and absolutely love the effects they give. I personally know of engines purpose built for the strip that won't idle below 1500rpm before the Grooves are added, but after will idle smooth at 800rpm.
http://fueleconomytips.com/2005/11/24/groovy-heads/
Also, the burn is much more complete and more evenly distributed throughout the chamber. This gives a more even "push" on the piston and makes more torque. Here's some before and after pics of a 23 deg SBC 355 with 12.5:1 compression.
If you want the ultimate in torque without losing any top-end, get a set of Omni Valves. They give you infinitely variable valve timing simply by replacing your intake valves. They eliminate reversion and greatly increase low RPM volumetric efficiency. This would also greatly increase your vacuum.
If you can get better combustion efficiency and therefore better exhaust scavenging you'll have better vacuum. Two [free] mods I know of will dramatically increase combustion efficiency. One is called "Edging" the chamber and the other is Singh Grooves.
http://fueleconomytips.com/2006/12/27/%e2%80%9cedging%e2%80%9d-ahead/
Edging also applies to the combustion chamber, extends the detonation threshold, improves fuel economy, and boosts most all power ranges. For somebody with a simple porting kit or Dremel tool, Edging is a DIY proposal.
What it involves is rounding all the sharp edges in the combustion chambers. “That’s been around for decades,” you say. Well, this is taking things to the utter extreme. What has been around is the practice of just taking the sharp edges to reduce the potential for hot spots. This has been effective in warding off detonation under extreme cylinder pressure conditions. But Edging is more.
What it involves is rounding all the sharp edges in the combustion chambers. “That’s been around for decades,” you say. Well, this is taking things to the utter extreme. What has been around is the practice of just taking the sharp edges to reduce the potential for hot spots. This has been effective in warding off detonation under extreme cylinder pressure conditions. But Edging is more.
http://fueleconomytips.com/2005/11/24/groovy-heads/
The shape of the piston and cylinder head have their greatest affects on combustion efficiency when the piston is within about 10 mm of Top Dead Center (TDC). When the piston is more than 10 mm away from TDC, the cylinder head and pistons are just distant and innefectual walls bordering a large open expanse. So the grooves do their work within this 10 mm range. On the compression stroke, they force the compressing air/fuel charge into the quench area via higher velocity jet streams to better vaporize and homogenize the fuel. On the power stroke, they guide the explosion through these same jet streams but in reverse to create vorticies, to clean out the ring lands, and to some greater or lesser degree further excite the flame front. Due to the rapid burn rate, higher compression ratios can be used without detonation problems. On turbo and supercharged engines, higher boosts can be run on the same octane fuel. In either case, lower octane fuel can be used than by conventional means.
Well guys my Baron is running with Mikes Ported head, smooth idle, 16 Inches of vacuum when i use to only get 15, Deeper exhaust =). I will be taking pictures and videos of it on Saturday.
If you want the ultimate in torque without losing any top-end, get a set of Omni Valves. They give you infinitely variable valve timing simply by replacing your intake valves. They eliminate reversion and greatly increase low RPM volumetric efficiency. This would also greatly increase your vacuum.
Senior User
Joined: Dec 2008
Posts: 121
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From: In the Airstream
Extreme NewB to the Forums Here -
I have a question I cannot seem to get my head around -
Moderators - Please move this post to a more appropriate thread if warranted.
Question - for years and years (more years than I sometimes care to admit) I driven "larger" vehicles strictly by the tach and manifold pressure while cruising on open roads. Of course, these were all carbuerated engines.
Now, with Fuel Injection forced upon me, does it still make sense to install a manifold vacuum gauge to help determine the most economical throttle depression?
I know the computer chips are controlling a lot of things these days, but I cannot figure out the lack of MP gauges in modern instrument clusters - am I correct in assuming that, since they are not generally available, the MP is not an important parameter to observe now that carbuerators have gone the way of the dinosaurs?
Thanks in advance.
Moderators - Please move this post to a more appropriate thread if warranted.
Question - for years and years (more years than I sometimes care to admit) I driven "larger" vehicles strictly by the tach and manifold pressure while cruising on open roads. Of course, these were all carbuerated engines.
Now, with Fuel Injection forced upon me, does it still make sense to install a manifold vacuum gauge to help determine the most economical throttle depression?
I know the computer chips are controlling a lot of things these days, but I cannot figure out the lack of MP gauges in modern instrument clusters - am I correct in assuming that, since they are not generally available, the MP is not an important parameter to observe now that carbuerators have gone the way of the dinosaurs?
Thanks in advance.
Elder User
Joined: Aug 2006
Posts: 642
Likes: 0
From: Olathe, KS
87mh
This really deserves a new thread.
But since we are here. The manifold gauge can still be usefull , but the best way to gauge EFI workload is monitoring injectors Pulse Width & duty cycle. I like the MP reference with cruise control as to how hard it is working since it is easy to install. And can tell me how close I am to shiftdown when pulling.
Nicety on the EFI. Several sell a monitor gauge package that plugs into the ODBII port. Easy install and can relay just about any reading the EFI holds.
This really deserves a new thread.
But since we are here. The manifold gauge can still be usefull , but the best way to gauge EFI workload is monitoring injectors Pulse Width & duty cycle. I like the MP reference with cruise control as to how hard it is working since it is easy to install. And can tell me how close I am to shiftdown when pulling.
Nicety on the EFI. Several sell a monitor gauge package that plugs into the ODBII port. Easy install and can relay just about any reading the EFI holds.
FTE Legend
Joined: Jun 2006
Posts: 31,930
Likes: 1,499
From: Ottawa, Ontario
On some of the EFI systems in use on these trucks.. particularly the early ones, manifold vacuum is measured and used for engine management. Even if it's isn't directly used by the EFI system it's still a good indication of engine load, and you'll get the best milage at the highest speed that is obtained with the lowest engine rpm and highest vacuum. So yeah a vacuum gauge is still usefull on an EFI vehicle. In newer vehicles this gauge has been replaced by an electronic milage display that calculates fuel consumption based on realtime data from the engine computer.
