What max compression on 93 Octane?
 

Learned Gentlemen,

What is a regularly tolerable compression ratio for a built motor on 93 octane?

I am in the middle of a twin plug engine upgrade from 3.2 to 3.5 (thread in engine rebuilds here: http://forums.pelicanparts.com/911-engine-rebuilding-forum/885657-help-rookie-build-going-3-5l.html)

We are getting to the cam selection stage and are about to measure valve clearance and actual compression ratio's. The spec sheet suggests that the compression ratio on my vehicle with unmolested 3.2 heads could be around 11.5:1. The heads have had 4000" taken off all of them to level them consistently.

Thus the question: What is a tolerable compression ratio for this motor?

Here are the Specs:

• 93 Octane
• Spirited daily driving in Texas (consider heat in Summer)
• Heads twin-plugged & worked gently for flow
• Bottom end balanced
• ARP head studs and conrod bolts
• Extrude honed intake, openned throttle body
• Remapped engine management & ignition with slightly larger injectors

http://forums.pelicanparts.com/uploa...1453046905.jpg

This is largely dependent on timing. Twin plug 993's and 964's claim to be 11.3:1, but many say they actually measure out to 10.8 or so. I built my hot 3.0 twin plug and it ended up at 10.65:1.

Cam selection influences this as dynamic compression is more important. Also, don't lug the engine. Keep it in its sweet spot. My 74 has a 95 993 engine with pmos and stock cams. 93 octane works fine as long as I don't lug the engine. My twin plug 3.0 runs 10.5:1 with DC80 cams and actually runs better than the 3.6 on 93 octane. I think the general view is 10.5 with twin plug and 9.5 with single, again depending on cam choice.

I would not go over 10:1 on pump gas, so you can run a decent timing curve, especially in summertime temperatures.

With aftermarket Engine Management, you can use more CR, due to the use of temp compensation tables (We use Motec).

Steve, this does seem to be the consensus on the forum as a whole. I also note that there seems to often be a difference between the CR specs the manufacturers claim and what the motor actually produces.

I am eager to find out my real numbers.

I'm running a Euro 3.0... 9.8-1 single plug/basically stock on 91 (all we can get in Az) without issue and I run it in the summertime too. It runs cool (Carrera oil cooler unit/fan) so that obviously helps.

I did the tune and MAF on this 3.4L
http://forums.pelicanparts.com/911-engine-rebuilding-forum/871561-1985-3-4-build-using-maf-279-56-hp-237-63-rwhp.html
It's twin plugged and it's at 10.5:1 we then dyno tuned it on a very hot 95F day and it tolerated ignition with no signs of detonation. Max WOT ignition was in the 20-22 deg range. Motor is super strong and pulls amazing.

This same motor will be re-cam this month with DC43 cams from Daugherty. Currently it has DC21 cams.

The twin plug motor is less detonation prone. My MAF system also has IAT WOT ign compensation table, so as Steve pointed out I take out ignition at higher IATs, this can be done in a properly done motronic chip.

I'm running12.1:1 with twin plug and megasquirt with no issues.

I suspect you are also running a long intake duration cam? That intake valve is likely closing very late ABDC? What cam are you using?

11.5:1 should be about the max for a twin-plug 3.5 running on 93 octane, although a little lower wouldn't hurt for a bit more margin and responsive ignition curve. 964s and 993s though spec'd from the factory at 11.3 typically come in at 10.8:1.

Here's an AASE motors built twin plug 3.4 tuned a few months ago that runs on 91 octane pump gas. The car runs the stock factory exhaust with a Fabspeed premuffler and sport muffler, cams, our rebored throttle body, and the stock air flow meter with a modified MSDS cone air filter. Compression is 10.5:1. Result is 254 HP to the rear wheels on a Dynojet, or about 299 at the motor if using a 15% transmission loss factor:

http://forums.pelicanparts.com/uploa...1462395312.gif

What are the cams in this 3.5L? Nice result!

This question about static compression can NOT be answered without knowing the cam. A 300 degree duration cam needs a lot more static compression than a 260 degree cam in the same motor. This is because compression can NOT start till the intake valve closes tightly and in the 260deg cam this may occur at 40degees ABDC (on the up stroke of compression) while the 300degree cam won't close the intake till 70-80degrees ABDC, that's 40degrees later in the longer duration cam! If you use 11.5:1 with the 260 cam you may well be into detonation zone but the 300 cam could easily handle 12.0:1 or possibly more static compression! The real question to ask is what's the safest dynamic compression ratio for a single plug motor or a twin plug motor. Ussually dynamic compression is in the 7.0:1 (mild) to 9.0:1 (performance). You need to very precisely calculate dynamic compression based on IATs and Fuel Octane.

An excellent book on this topic in detail is written by David Vizard the book is 'Tunning the A-Series engine' the book has a lot of detail on the topic of dynamic compression and how to obtain the correct amount. The neat thing about the 911 air cooled motors is they can handle a significant amount of Dynamic Compression compared to other engines, especially the twin plugged motors.

This is a 3.4, not a 3.5. Cam is slighly more aggressive than a 964 for streetability.

The 964 cam has a rather late intake valve close event, something like 75deg ABDC compare that to the stock 3.2 cam at 68deg ABDC. Basically the 964 cam was by design in need of more static compression. Would be helpful to know what the intake close event timing is for that cam, just curious.

Since we're on the topic, wide lobe separation angles bleed compression too.

Twin plug compression..........
 

I built a 3.3SS for my SC using Mahle pistons that were supposed to be 10.1:1 compression. However, the Carrera heads that I used had been shaved a bit and the measured compression came out at 10.9:1. This engine is using 964 cams, CIS and XDi ignition and runs great on 93 octane pump gas.

Single plug?
The 964 cam needs that extra compression and generally does well at 10.8:1 to 11.3:1 if everything else is in order.

Since you opened the door to discuss Lobe Separation Angle 'LSA' I'll add my understanding here. LSA are often not very well understood but extremely important, you CAN NOT just guess at LSA! LSA dictates when a valve hits it's max lift centerline. So a LSA of 110deg means that the valve is at it's max lift centerline at 110deg ATDC, keep in mind that 90deg ATDC has the piston half way down the intake stroke, so that 110 is achieved after 1/2 way down.

So why is LSA so important? Simply because one and only one specific range for LSA dictates the max torque a cyl design can achieve! The range is very narrow usually within +/- 1degree! so that 110 can't deviate much more than 109-111 because if it does the torque gains are compromised! Others will disagree with what I'm about to say but here goes: The very first choice made by the cam designers and software tools that really get cams correct is to chose the optimal LSA! regardless of durations! LSA dictates the optimal point at which the valve MUST be at peak lift for best cyl filling regardless of duration. What I mean is, regardless of duration you have a perfect spot for LSA to occur for any given duration the LSA usually is exactly the same because it's dictated by the design of the cyl and the size of the valves. Few simple rules apply here:
1 - The larger the bore the smaller the LSA needed if valve size stays the same. (under valved motors need smaller LSA)
2 - The larger the intake valve the larger the LSA if displacement stays the same.
David Vizard came up with a rule called the '128 rule' for LSA and while not perfect it can help verify if a cam is even in the ballpark for the given cyl/valve. Rules looks like this:

LSA = 128 - (CID/# of Cylinders/In Valve dia. in inches x 0.91)
Let’s apply it to a typical 3.4L motor with 3.2L heads.
111.73 = 128 – (207cid/6/1.93*0.91)
Using David’s 128 rule we calculate best LSA for the 3.4L to be LSA=112

And it's better to error on a smaller LSA than a larger one, meaning that if optimal LSA is say 112 and you go with 114 that will produce worse results than if you went with 110. So don't ever over estimate the LSA it's better under estimate it.

Proper cam selection rules:
1 - Calculate/find the very best LSA first! It dictates the peak torque.
2 - Chose the duration next. It dictates RPM range, short duration low RPM range, long duration hi RPM range.
3 - Determine lift from flow bench numbers
4 - Examine overlap, overlap will simply fall into place 'is what it is' based on LSA and Duration. Never attempt to force overlap! This is because LSA and Duration overrule overlap! But if overlap is to high for the low RPM drivability and idle then you can only shorten duration to fix this. Never attempt to fix overlap by moving LSA!
5 - Once the cam is chosen then consider fuel octane and set static compression properly to achieve the target dynamic compression. This is the biggest mistake often made by rookie builders! Setting the static compression wrong has huge implications on the torque capability of the motor across the ENTIRE RPM range!

Damn Sal, you've really thought LSA through!

I just remember the basic rules of LSA:

1. Wide LSA = broad torque, bleeds compression, good for boosted and nitrous oxide motors
2. Narrow LSA = narrow torque band, peaky, higher hp (like a 2-cycle), maintains compression good for race cars and generally best for naturally aspirated engines

Get the book 'Tuning The A-Series Engine' by David Vizard it has all the details.
The A-Series is also a hemi designed motor and it's under valved, it's similar to the 911 3.2 design and has some of the same basic issues.