4722 replies to this topic

[siztenboots]

There's a lot more to it than just high compression-low boost or low compression-high boost. Thye will not make the same power with the same peak cyclinder pressure and they will not have the same operating characteristics.

For a given engine displacement a lower compression ratio will require a larger compressed volume at TDC. This means there is more volume to trap air fuel mixture. Combine this with the fact that you can run higher boost and the difference amount of trapped air/fuel mixture can be substancial. Of course now your now squeezing it as tight as the high compression engine so you will lose benifit that it provides.

Lets say you have to choose on 8:1 or 10:1. The difference in power just due to compression is on the order of like 6% (There is a mathmatical equation if you want the actual #). However you may be able to trap 10% or more of fuel with the larger chamber and higher boost of the low compression engine. The low compression-high boost engine also has the advantage that it will have lower peak cylinder pressure and slower heat release rate which both translate to lower tendancy to knock.

The big disadvantage of hte low comp engine is that the turbo response will be worse, the mpg (pro rata per bhp) will be worse, and the off boost power will be down. A street car would probably be more fun with high compression and low boost.

static comp ratio is just part of it too , the inlet cams profile , timing will determine the dynamic compression ratio ( which is always lower than SCR )

can your cooling cope with the higher temperatures with high boost and the larger volume of air required

Edited by siztenboots, 05 December 2009 - 04:01 PM.

[siztenboots]

Long duration cams delay the closing of the intake valve and substantially reduce the running compression ratio of an engine compared to the SCR. The cam spec we are interested in to determine the DCR is the intake closing time (or angle) in degrees. This is determined by the duration of the intake lobe, and the installed Intake CenterLine (ICL) (and indirectly by the Lobe Separation Angle (LSA)). Of these, the builder has direct control of the ICL. The others are ground into the camshaft by the grinder (custom grinds are available so the builder could specify the duration and LSA). Changing the ICL changes the DCR. Retarding the cam delays intake closing and decreases the DCR. Advancing the cam causes the intake valve to close earlier (while the pistons is lower in the cylinder, increasing the sweep volume) which increases the DCR. This can be used to manipulate the DCR as well as moving the torque peak up or down the rpm range.

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

[Nev]

Thanks Steve, your tips are taken on board. I will have to re-read up on the matter as I have forgotton a lot of the engineering reasons and maths from approx 3 months ago when I was researching this.

Anyway, I have just released another large BLOG UPDATE to keep you chaps interested.

[Nev]

Thanks Chris, Can you give me the part number of the Siemens injector please, I would be most appreciative as getting propper info (instead of the usual "this will give you 1000+ BHP" bull-sh*t) on this matter is always hard. I concur when you say power delivery is everything on a turbo car, I will re-evaluate the external wastegate issue soon... I was initially surprised that you have left yours on std CR, but as you have your power capped anyway, throttle response is more important to you, so I understand your reasoning Yea, I have seen the new Cossy pistons, I have to wonder if they are really worth the price... As for excitment, yes I DO get big doses of fun and excitement - it's thrilling to understand everything about my engine! Followed by the worry that I can achieve my goals, remember I am just an amateur, building his first ever engine, on his own, and to top it off I am not rich which leads to parts compromise! Thanks for the invite down for the dyno, I'd love to have a big pow-wow with you but I am now working If you felt like PMing me your tel number I could give you a ring at a convenient time Pip pip.

Edited by Nev, 05 December 2009 - 05:31 PM.

[Nev]

Another update posted up today, just cleaning and painting the block, UPDATE HERE

[Zoobeef]

Another update posted up today, just cleaning and painting the block, UPDATE HERE

I see you've added other sections too, good stuff

[Boss VX]

Nev - I would be careful when it comes to working on your cylinder head. I'm sure you are aware of this but you can do more harm than good by removing material or even polishing. Its all about the speed of the air and not necessarily volume. So much you can do tho. I worked in the cylinder head department at Swindon Racing Engines for 2 years and its a facinating subject. Our development on the works mini cooper head took weeks of flow testing to get right. Everything we tried went against what my boss had experienced over the years. We ended up with great results from just taking out a little material from the valve throats and inlet mouths. What you really want is to take advantage of someone whos put the r&d into it and get it done professionally. I started by own head project while there (not on the vx) but never finished it as i changed jobs. A good place to start is buying a 2nd hand damaged head and section it to see what material you have to play with. Not sure if SRE ever did the Z20LET. I shudder to think what they would charge for the r&d or just doing the work if they have done these heads before. Quite an expensive place. Good luck with it all! Bet you havent let the misses see that spreadsheet

[Nev]

Thanks BossVX. I was only vaguely looking at the head and trying to estimate where/what I could do. To be honest, I think I am going to use Neil Roper to do the porting, machine larger valve seats and install the new larger valve train. He is used to working on X20XEV heads which are virtually the same as the Z20LET heads. He works of Formula 1 teams and other rally/race teams and has been highly recommended to me by a couple of prominent people. Has anyone else had any work done by him ? I sent him a quote to tender for the work by email today. Regards, Neville. PS: the Misses knows about the cost and loves the VX and might even buy one, though I'd recommend a SC'ed one for a girl instead of a hairy turbo car ;o)

Edited by Nev, 06 December 2009 - 07:28 PM.

[Zoobeef]

PS: the Misses knows about the cost and loves the VX and might even buy one, though I'd recommend a SC'ed one for a girl instead of a hairy turbo car ;o)

Only recently have i been trying to talk the missus into getting an na so i can fiddle with that haha

[steveboyslim]

Interested to know where and how the 'Cosworth style grooves' will help with under piston cooling, any pictures ? Steve

[cnrandall]

Interested to know where and how the 'Cosworth style grooves' will help with under piston cooling, any pictures ?

Steve

Isn't that reasonably self explanatory? How do the oil spray bars work?

[cheeky_chops]

nev, i might of missed it, but whats your time line for rebuild/install/mapping?

[Zoobeef]

Dont think he has one set in stone, as some things are going to be custom fabricated and prob not at the same time. Guessing he'll have a point in the next year that he'd like to have it done by though

[steveboyslim]

Interested to know where and how the 'Cosworth style grooves' will help with under piston cooling, any pictures ?

Steve

Isn't that reasonably self explanatory? How do the oil spray bars work?

Not to me.
I understand the spray bar/oil jet principle, but not the grooves mentioned, if there is an easier method other than installing oil jets, I am interested.
If the groove is in the rod can you explain where it is machined or post some pictures (or message me with them)
Thanks.

Steve

[Nev]

Cheeky, One of the nice things about building a second engine is that there is no rush at all, as I can still drive Nipper with his current engine. Hence there is no time line or completion date. I am finding that I really enjoy each cautious step that I take, learning stuff, considering everyones ideas, looking up the engineering principles behind why people do/do not do things. This all slows things down I know, but is genuinely part of the fun to me. Is there a specific reason why you asked the question ? Also, just updating the blog takes up 20% of the project time, but I am enjoying that too! Nev.

Edited by Nev, 08 December 2009 - 06:31 PM.

[leevx2.2]

PS: the Misses knows about the cost and loves the VX and might even buy one, though I'd recommend a SC'ed one for a girl instead of a hairy turbo car ;o)

[zaff]

Hi Nev, I see that you are fitting a Quaife lsd. Do you have any plans to fit a traction control unit. I noticed your post in the other crash thread about your hairy moment and that is with your current power. Having a lot more power will make things even more twitchy at the back so just thought I would ask. I had intention to leave mine totally standard but now reading your blog has got me looking at all sorts of silly mods. I noticed in your blog that you have around 305bhp at the moment and your est 0-60 time is 3.9 secs. I would have thought that it would have been quicker than that as a VXR is 4.2 and has a lot less power or am I missing something. cheers Daz

Edited by zaff, 09 December 2009 - 12:43 PM.

[SteveA]

A lot of it will be down to gearing and traction. 0-60 is a silly way to measure power as so many other factors come into play. From a standing start the weak link will be the tyres. IIRC TMS tried to get 4.2 out of a std VXR for ages and failed. Closest they could get was 4.9. Im my stage 2 with r888's (30 more bhp and 30kg lighter than the VXR) the best i've got is 4.5. I'm no launch expert tho. letstorquebhp calculator seems to be quite close.

Edited by SteveA, 09 December 2009 - 02:39 PM.

[Nev]

Hi Zaff,

Traction is the limiting factor for 0-60, it's impossible to get all the torque down on the road. The only way to improve upon this is:

a) Better tyres (softer rubber, less treads).
Wider wheels/tyres.
c) Lighter car
d) beefier clutch may help

Adding any more power to my car wont improve 0-60 at all.

As for TC, I have thought about it and have a search on eBay, but I refuse to pay £800 for what amounts to a chip, an LCD control panel and a few wires!

As for the Quaife LSD, I will install one if I have any money left at the end of the project !

Nev.

Hi Nev,

I see that you are fitting a Quaife lsd. Do you have any plans to fit a traction control unit.

I noticed your post in the other crash thread about your hairy moment and that is with your current power. Having a lot more power will make things even more twitchy at the back so just thought I would ask.

I had intention to leave mine totally standard but now reading your blog has got me looking at all sorts of silly mods.

I noticed in your blog that you have around 305bhp at the moment and your est 0-60 time is 3.9 secs. I would have thought that it would have been quicker than that as a VXR is 4.2 and has a lot less power or am I missing something.

cheers
Daz

Edited by Nev, 09 December 2009 - 03:56 PM.

[zaff]

Thanks guys for the info. So the power will be used in overall driving, overtaking, track days etc rather than sheer acceleration from the traffic lights That calculator is pretty good I have not seen that site before. Is the gearbox up to the job of that sort of power then or will you have to do some tweaking of that too? My mate has a RS200 replica with RWD, Cosworth engine and 440 BHP and he has just blown his 6 grand gearbox after 100 miles. That was built by Quaife so it has been returned for a dose of looking at..