Even more static compression?

[dnb]

After a successful run on the rollers at the Growl, I'm thinking like Grover - what next?

The power run starts quite high up the rev range because the car wasn't happy at full load at low RPM. (I am fairly sure this is fixed now) So the graph looks "peakier" than it should prior to 3k RPM.


The fuelling is pretty good for the run. (Power figure shown is at wheels)


One of the things I can think to do is to increase the static compression from 10.8 to 11.5 or even 12:1. I wonder if the benefit of more compression would be more than cancelled out by having to pull loads of advance out at the top end. Peak advance was 25deg on the dyno on Saturday, so already conservative. Maybe it would have stood another couple of degrees as it is now? (I didn't risk it because intake temperature was already quite high)

Does anything else stand out, other than it would be worth tuning the top end properly on a dyno?

[Boosted LS1]

It's not about static, it's about dynamic.

[dnb]

I know that, but static is easier to measure...

Cam is H404 at the moment, and there are no plans for anything milder. So the question is really: What static compression will give a good dynamic compression for the H404?

I think 11.5:1 would be not far wrong, but I don't want to make even more low RPM torque at the expense of top end power through det management.

[rev-erend]

There is always the chance that it will pink like a pig .. at 11.5 and you will end up having to back off the ignition to just make it run.

Been there, done that .. (on another engine)

[350Matt]

Dave

I've used this calculator in the past to come up with compression ratios so its reasonably accurate;

http://www.empirenet.com/pkelley2/DynamicCR.html

Part of the problem is the RV8 combustion chamber shape which is poor (as you'd imagine)

You can improve it by radiusing edges, de-shrouding the valves and ensuring the exhaust manifold is a few mm larger than the port to help prevent flow reversal but I'd consider 9.25:1 (dynamic) as an upper limit.

You'd have to crunch the numbers to confirm what this works out to static with your cam unless you're feeling brave with some sort of active knock control?

I've also asked this question here:

http://www.v8forum.co.uk/forum/viewtopic.php?p=428...

Edited by 350Matt on Thursday 6th August 13:23

[rev-erend]

The engine builder I used advise 11:1 as the upeer limit and that was with a race cam .. with plenty of overlap.

My engine has 10.5:1 again and my heads are not as det sensitive as the RV8 heads. I could have had a bit more but in my mind - it leaves a bit in case I warp a head in the future and need a head skim.

[v8 racing]

I would certainly not be increasing the cr any more than you have got, to be absolutely honest i wont set an engine any higher than 10.5 unless it is going to be purely a race engine, the amount of engines i have seen that have run big ends/bent rods melted pistons etc just trying to get a bit more cr!, the other thing the difference between 10.5 and 11.5 is around 5 bhp on the same given motor, on a n/a engine you can over cr an engine and lose power, "this is even worse on a fi engine". one due to having to retard the ignition and the other which is higly overlooked is the amount of traped air in the cylinder before the valve closes, for instance total random figures and exagerated but if you have a total cylinder volume that measures 1 litre that you are fillingand then you suddenly skim a chunk off the head and you now only have a cylinder volume of .5 litre you have just halved the potential cc in your engine to make power with. i hope this kind of makes sense!
Personally i see you have a 5.2 bottom engine and a h404 cam but i have no idea what stuff you are running on top? to be frank to only be making 320 bhp there is something very a miss, i am presuming you are still running a plenum, 72mm? that is going to be your biggest restriction, with a decent plenum maybe a tripple carbon or the like you should be seeing more like 350 with no other changes.

Edited by v8 racing on Thursday 6th August 18:48

[dnb]

Mine's a standard 5 litre with some bolt-on parts. You're confused 2 different engines with similar dyno results.

The intake is a custom made twin plenum setup. I don't believe it is a significant restriction since the ECU monitored the manifold pressure and it didn't drop below 98.3 kPa when atmospheric pressure was 99.6 on the day. There may be some advantage in changing runner length to cause a peak at max HP...

[rev-erend]

dnb said:

Mine's a standard 5 litre with some bolt-on parts. You're confused 2 different engines with similar dyno results.

I don't believe it is a significant restriction ...

If you are still using the same inlet manifold bottom end .. then you have plenty of restriction. You might have 45mm at the top of each trumpet but each narrows and bends heavily.

Here's a picure of no restriction .. the lighting for no 3 cylinder is great - as you can see the valve stem !

Edited by rev-erend on Thursday 6th August 20:17

[dnb]

I was meaning the plenum is not a restriction as you well know. But any excuse to post those pictures is a good one...

I have something on the drawing board at the moment, but I need to ensure the bonnet will close first.

[v8 racing]

dnb said:

I was meaning the plenum is not a restriction as you well know. But any excuse to post those pictures is a good one...

I have something on the drawing board at the moment, but I need to ensure the bonnet will close first.

Even though you where getting 98kpa i will lay my life on it that the plenum and air intake system is still holding you back, espiecially if its an sd1 type plenum?

[dnb]

Nothing like an SD1 plenum. Like I said, it's a twin plenum (think Cerbera v8), NOT a twin throttlebody single plenum.
Granted, the manifold base will be holding it back compared to throttlebodies. Maybe this is now the biggest restriction to airflow into the cylinders?

[spend]

While you're watching Rob, what are your views on putting tin gaskets in a motor like this?

and secondly would they yield 10.8:1 on TVR 5.0...

[GreenV8S]

v8 racing said:

i see you have a 5.2 bottom engine and a h404 cam but i have no idea what stuff you are running on top? to be frank to only be making 320 bhp there is something very a miss

Is it so bad? It seems pretty good compared to the figures that are typically reported for a standard 500.

[v8 racing]

GreenV8S said:

v8 racing said:

i see you have a 5.2 bottom engine and a h404 cam but i have no idea what stuff you are running on top? to be frank to only be making 320 bhp there is something very a miss

Is it so bad? It seems pretty good compared to the figures that are typically reported for a standard 500.

Well its not dyer but its not brilliant, the majority of engines that have been fitted with 404 cams even 500's that are relitively standard are making over 320 bhp.

[v8 racing]

spend said:

While you're watching Rob, what are your views on putting tin gaskets in a motor like this?

and secondly would they yield 10.8:1 on TVR 5.0...

I quite often run tin gaskets upto 10-1 ie 450's but i have never tried at nearly 11!

[v8 racing]

dnb said:

Nothing like an SD1 plenum. Like I said, it's a twin plenum (think Cerbera v8), NOT a twin throttlebody single plenum.
Granted, the manifold base will be holding it back compared to throttlebodies. Maybe this is now the biggest restriction to airflow into the cylinders?

Ahh i was only looking at your engine today at work via the net obviously! and discussing it at work! am i right in thinking it has a pair of green filters on the end?

[dnb]

The 10.8 was a back-of-envelope calculation, so is subject to a certain amount of error... Specially when someone not too long ago pointed out that the TVR handbook quoting 10:1 as standard may not be entirely accurate!

The tin gaskets seem to work. I only used them as a cheap experiment instead of doing the job properly in case it didn't work and left me with heads too skimmed to be any use.

[rev-erend]

dnb said:

I was meaning the plenum is not a restriction as you well know. But any excuse to post those pictures is a good one...

I have something on the drawing board at the moment, but I need to ensure the bonnet will close first.

Thanks - they do look great.

[dnb]

v8 racing said:

Ahh i was only looking at your engine today at work via the net obviously! and discussing it at work! am i right in thinking it has a pair of green filters on the end?

It does indeed, although to attempt some form of cold air induction, the filters are located where the standard TVR panel filter used to be (soon to be moved to the inner wings if Clive's idea works)

What did you think?