timing vs compression

[psalt]

It is only my opinion of why his engine only had 8.5:1 compression. In my opinion , in this range, a one point increase in compression is worth it, even if you have to back down a few degrees from the 38 BTDC. The fact is that MBT timing is relatively insensitive to a few degrees, were as the compression increase is proportional. I'm sure there are other opinions.

[BURN-BROS]

Originally Posted by psalt

It is only my opinion of why his engine only had 8.5:1 compression. In my opinion , in this range, a one point increase in compression is worth it, even if you have to back down a few degrees from the 38 BTDC. The fact is that MBT timing is relatively insensitive to a few degrees, were as the compression increase is proportional. I'm sure there are other opinions.

Fair enough!

[gestalt1]

btw, i don't have a rs 2.7 type engine, i was just using it as an example of a high performance factory engine with low compression. many claim that after a re-build involving a compression increase that they notice a quicker throttle response. i have a feeling that throttle response has more to do with the induction system and the setting of the timing than compression. i have no doubt that greater compression does increase hp and fuel mileage, but not as much as many think. if you look at dyno charts comparing a factory S engine to a race engine the 80hp/L and the 100hp/L produce very similer power at 7000rpm. the race engine just has a higher redline so the cams/porting allow power to continue to build from 7k to 8k.

[K24madness]

Originally Posted by PFM

The best solution I have seen was a design by Porsche. They used a full dome piston nested closely against the chamber wall to provide a high squish velocity. The actual combustion chamber was actually a bowl cut into the top of the piston. This combination produce extraordinary high flame speeds, and a compact chamber to prevent detonation. I could see where they could run very high turbo boost pressures and still prevent detonation.

Regards,

PFM

I agree with the above description of the 993NA piston/head combo. The factory was able to run 11.2CR using that design and twin plugs. Having a knock sensor for safety also helped when pushing the edge.

I have spoke to a few turbo kit builders who also run the above combo with turbos at 6psi on pump gas.

I personally don't trust that combo to pump gas but am able to run 10 PSI with the same setup on my car using 100oct fuel.

Surprised nobody does WI. It works great for the turbo guys.

[perfpow]

One thing that nobody has mentioned, or I haven't noticed, is the effect of cam timing on compression. The compression ratio of an engine would only actually be the advertised ratio if the intake valve was fully closed at BDC as the compression stroke began. Of course this isn't how it works. One reason that an engine with a late intake valve closing NEEDS high compression is because the compression is lost when the air being compressed in the cylinder leaks past the still open intake valve. Early valve openings are beneficial in high revving engines, improving their volumetric efficiency, but they cost compression over a mildly cammed engine. When checking compression dynamically, it's not unusual to find a mildy cammed turbo engine pumping 150 psi per cylinder compared to an N/A racing engine with lots of intake duration pumping only 125 psi despite the turbo CR being 7.5:1 and the racing engine 12:1. So static compression ration (needed for engine design and building) is only remotely related to dynamic compression.
Helpful, interesting or just confusing?

Sol

[kenikh]

Originally Posted by perfpow

One thing that nobody has mentioned, or I haven't noticed, is the effect of cam timing on compression. The compression ratio of an engine would only actually be the advertised ratio if the intake valve was fully closed at BDC as the compression stroke began. Of course this isn't how it works. One reason that an engine with a late intake valve closing NEEDS high compression is because the compression is lost when the air being compressed in the cylinder leaks past the still open intake valve. Early valve openings are beneficial in high revving engines, improving their volumetric efficiency, but they cost compression over a mildly cammed engine. When checking compression dynamically, it's not unusual to find a mildy cammed turbo engine pumping 150 psi per cylinder compared to an N/A racing engine with lots of intake duration pumping only 125 psi despite the turbo CR being 7.5:1 and the racing engine 12:1. So static compression ration (needed for engine design and building) is only remotely related to dynamic compression.
Helpful, interesting or just confusing?

Sol

It was mentioned, but is a bit over the target of this thread. That said, you can definitley see the comparisons of dynamic compression below:



You can clearly see that the target DCR based on cam overlap is in a consistent range across all of the cams. You may also notice that the more agreesive race cams tend to err on the lower end of the range - my guess is that this is in response to very high Volumetric Efficiiency of these motors at high revs actually building higher cylinder pressures from improved filling.

These numbers are of course calcualted against resting engine states and do not take into consideration the effects of VE on cylincer filling - in reality, the race motors will have a higher DCR at speed due efficiencies generated high in the rev band.

There are a lot of factors that come into play here, Static CR and DCR being the easiest to calculate. Once you get into calculating DCR using VE, it gets very, very involved and not appropriate to consider unless you ahve much more data than cams and pistons can provide. Without flowbench data of heads and intake tract, you are a bit stuck with only being able to generate static DCR.

[BURN-BROS]

Originally Posted by perfpow

One thing that nobody has mentioned, or I haven't noticed, is the effect of cam timing on compression. Helpful, interesting or just confusing?

Sol

The suggestion to change to a ModS cam over the S cam was really the dynamic compression change. The lobe center on the Mod S is greater so you get the bump of the dynamic compression for improved torque especially at the lower rpms and little better top end due to revised ramp rates.

[Bill Verburg]

Bosch has some great reading on these subjects, It tends to be more generalized than what some of you are looking for

I'd start w/ The Bosch manual "Motronic Engine Management" I believe that the reference # is 1 987 722 161

here are a few tidbits

[Bill Verburg]

Another great resource is a Porsche tech manual "P21 Fuel and Ignition System" It covers both general and specific models from Carrera 3.2 to 964 to 993 to 996 & Boxster. It might be hard to get this one

[PFM]

Kenik,

Thanks for the DCR information. I was a little surprised to see the DCR less than 8 to 1. Do you by chance have the required information for the DCR on a 3.6? Or better yet a factory race trim 3.8? I wonder if the 11 plus to 1 static and the wide lobe center on the cams if the DCR is higher than 8 to 1.

Again thanks for the info,

PFM

[PFM]

Kenik,

I found a little 3.8 ltr info. The static compression was 12 to 1, the bore 102 mm the stroke 76.4 mm. I do not have cam specs for this beast.

Does anyone have some input?

Thanks,

PFM

[kenikh]

Originally Posted by PFM

Kenik,

I found a little 3.8 ltr info. The static compression was 12 to 1, the bore 102 mm the stroke 76.4 mm. I do not have cam specs for this beast.

Does anyone have some input?

Thanks,

PFM

I will have to calculate it, but intuitively, since this is a common plenum motor so overlap won't be extreme, thus expect dynamic CR to be at least a point higher than the early motors. Remember the 3.6 motors run knock control, wideband O2 and all of the other tricks that optimize fuel and ignition much more granularly than older engine.

[Steve@Rennsport]

Originally Posted by kenikh

I will have to calculate it, but intuitively, since this is a common plenum motor so overlap won't be extreme, thus expect dynamic CR to be at least a point higher than the early motors.

The 3.8 RSR engines had ITB's under the plenum,....

Cams are:

IVO 80 deg before BTDC
IVC 114 deg after BDC
EVO 104 deg before BDC
EVC 65 deg after TDC

Valve clearance settings are different between I & E.

Lift, I & E .485
Int LC 108 deg
Exh LC 110 deg

For a full race engine, these are very civilized.

[kenikh]

And the number are in!

The 3.8 RSR comes in at a resting DCR of 7.94:1, using Steve's numbers.

Using John Dougherty's specs for the 3.8 RSR cam, DCR comes in even lower at 7.84:1.

Even on those wide lobe centers, overlap comes in at 87 and 85 degrees, respectively, which is just a bit softer than an 'S' cam, which is 91 degrees, but spot on the Solex cam at 84 degrees.

So in the end, it appears possible that knock sensing only gives about a half point in added DCR.

[PFM]

Kenik,

Thanks again for doing the leg work. I guess I am quite surprised that the DCR was that low. Clearly the 3.8 was not meant to run on pump gas. The running compression with intake tuning and dynamics at work would be higher but still less than 8 is quite a surprise.

Thanks again,

PFM

[kenikh]

Originally Posted by PFM

Kenik,

Thanks again for doing the leg work. I guess I am quite surprised that the DCR was that low. Clearly the 3.8 was not meant to run on pump gas. The running compression with intake tuning and dynamics at work would be higher but still less than 8 is quite a surprise.

Thanks again,

PFM

To the contrary, I know of three street driven 3.8RS spec motors in my area. All three are street driven and run on pump gas. Remember, knock control buys a lot of forgiveness.

[Steve@Rennsport]

Just to be clear,.....the 3.8RS engine was intended for 92 or better fuel,.....the 3.8RSR version requires race gas, only.