compression ratio vs boost

[Don P]

My engine builder wanted to use 9:1 pistons (flat top). I know I've read about the relationship of boost and C/R but can't remember and can't find the articles. Does using a higher C/R simply mean I'll have to back off the boost or is it more complicated. I told him to hold off on ordering the pistons. I know that the higher compression will give me more static hp and is therefore multipled by the boost but what is the down side ??

[GAboySVO]

Don P,

According to the gas faqs on turboford.org:

7.2 What is the effect of Compression ratio?

Most people know that an increase in Compression Ratio will require an
increase in fuel octane for the same engine design. Increasing the
compression ratio increases the theoretical thermodynamic efficiency of an
engine according to the standard equation

Efficiency = 1 - (1/compression ratio)^gamma-1

where gamma = ratio of specific heats at constant pressure and constant
volume of the working fluid ( for most purposes air is the working fluid,
and is treated as an ideal gas ). There are indications that thermal
efficiency reaches a maximum at a compression ratio of about 17:1 for
gasoline fuels in an SI engine [23].

The efficiency gains are best when the engine is at incipient knock, that's
why knock sensors ( actually vibration sensors ) are used. Low compression
ratio engines are less efficient because they can not deliver as much of the
ideal combustion power to the flywheel. For a typical carburetted engine,
without engine management [27,38]:-

Compression Octane Number Brake Thermal Efficiency
Ratio Requirement ( Full Throttle )
5:1 72 -
6:1 81 25 %
7:1 87 28 %
8:1 92 30 %
9:1 96 32 %
10:1 100 33 %
11:1 104 34 %
12:1 108 35 %

Modern engines have improved significantly on this, and the changing fuel
specifications and engine design should see more improvements, but
significant gains may have to await improved engine materials and fuels.

So if you go to a 9:1 compression ratio you will need essentially race gas to keep from detonating your engine underboost. I'm sure there are guys who have done this before. You might try a google search for more info.

Here is a link to a good answer to a similar question:

http://autos.groups.yahoo.com/group/2literprobes/message/1094

[Don P]

Thanks Mike S. I may be able to live with the 9:1 but if I ever sell my SVO, (not likely) or get tired of autox, (very likely) and then use it as a daily driver, I might be sorry. I know that I don't want to have to use race fuel just to run around town. So you have confirmed my uneasyness.

Don Palm

[sweetwm]

Compression Ratio

Hi Mike,

I am in the process of building a slightly modified engine with big valves with ported and polished stock head and E-6 exhaust. I would appreciate seeing your article as I am sure many others would also.


Thanks,

[Don P]

Thanks again Mike!

[DPDISXR4Ti]

Just passing through tonight, I noticed one of my favorite topics...

Last summer I rebuilt my engine with a 2.5 crank, and purposely choose forged flat-top pistons along with the Dual-Plug head. I believe I actually ended up with a SCR around 9.5:1, which was a bit more than I wanted. In fact, I'm very happy with the results and have had as much as 15 lbs of boost. For grins, I've even been able to create an 87 octane calibration, although clearly I pulled a fair amount of timing to accomplish this.

Bottom line, yes, be afraid of 9:1 but don't rule it out. If you're diligent and careful about your tune you'll more than likely be happy with the results. I would also add, use only forged pistons if you go with flat-tops. You want the extra margin of error that cast pistons of any kind cannot provide.

[DPDISXR4Ti]

Originally posted by MikeFleming
If you're using dual plugs, then much less timing would be correct. Since ht flames start from two sides at near the same time, there's half the distance for the flame front to travel - hence less "advance". THis also makes the engine less sensitive to octane somewhat.

The major reason for going to the dual plug design (in the case of the Nissan NAP-Z and Ford heads) is for higher EGR tolerance.

-Mike Fleming
/***/

No doubt Mike. This was the second DP motor I did, and the dual flame fronts were responsible for less ignition lead requirement than the increased compression. Interesting to note too that the early DP intakes were machined for a KS, but it was never used in production. Probably due to the greater tolerance for octane variation, as you suggest.

I'm not quite sure what you mean by "higher EGR tolerance". Can you further explain?

Thanks,

Brad

[Pat_in_L.A.]

Originally posted by MikeFleming


The major reason for going to the dual plug design (in the case of the Nissan NAP-Z and Ford heads) is for higher EGR tolerance.

-Mike Fleming
/***/

The dual plug Nissan design only had the exhaust side spark plugs working on coast mode.

(Bosch paid contingency, but they were crap plugs. I ran the NGK's on the intake side and showed the officials the Bosch plugs on the exhaust side when it was time to collect the contingency money!)

ROFLMAO!!!

[DPDISXR4Ti]

Originally posted by MikeFleming
"I'm not quite sure what you mean by "higher EGR tolerance". Can you further explain?"

I have a SAE article somewhere that outlines the intake port, combustion chamber, piston, and dual-plug ignition system of the Nissan NAP-Z engine. The original design spec was to introduce a high swirl, high turbulence. fast-burn system that would be less sensitive to octane and high EGR flow rates (charge dilution due to EGR flow and "poor" scavenging).

This resulted in significantly reduced NOx emissions with comparable fuel consumptin figures for light truck engines.

IF someone remonds me pwhen I get back home in two weeks] I'll look up the SAR srticle number and y'alls can research it yourselves. I've not read the article in 15+ years so my memory's not he best for details on that one.

Something to add to my previous EGR/combustion system design article.

-Mike Fleming
/***/

Mike- Are you back home yet? This is your reminder...

Thanks...