head shaving and HP from higher compression - Nissan : Datsun ZCar forum :Nissan Z Forum: 240Z to 370Z
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post #1 of 14 (permalink) Old 01-07-2003, 03:35 PM Thread Starter
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head shaving and HP from higher compression

ok,

i have been reading some threads and i'm getting very confused, why does the cam tower have to be shimmed if you shave the head? i mean, doesn't the timing chain have some sort of tensioner that would compensate for the(what i thought would be a) miniscule amount? i did some calculation and i came up with .0625 in(which is not a small amount to take off a head). is approx 1/16th of an inch, now i haven't taken my head off yet(but i will be here in a couple of weeks) so i'm not fully familiar with the chain tensioning system but surely nissan would have engineered it to have some sort of way to take up some slack. i'd much rather find some way of tightening the chain(and it seems ridiculous for such a miniscule amount) than to shim the cam tower. has anyone had the head shaved a substantial amount without having to shim the cam tower?

one other question, given all other variables being the same(i.e. cam, etc), does anyone know or know where to find how much HP increases with increasing compression ratio? i tried a search but the search came back with info irrelevant to what i'm looking for. any help would be much appreciated.

mike

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post #2 of 14 (permalink) Old 01-07-2003, 03:56 PM
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Re: head shaving and HP from higher compression

Good question
The chain tensioner is designed to keep the chain tight on the factory spec head as it stretches over time. If you modify the head by shaving it without shimming the towers, the chain will begin to slap around in its path which can damage the chain guides and even cause the tensioner to loosen enough to come out. Not good. Keep your motor happy by shimming the towers to maintain the spec distance between cam and crank. Don't be hardheaded and do what we say. We know whats best for you. hehe

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post #3 of 14 (permalink) Old 01-07-2003, 04:21 PM Thread Starter
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Re: head shaving and HP from higher compression

hey fumar,

i completely understand what you are saying. i don't want my chain slapping around either but it would seem that the tensioner would take up the slack of the chain if work was done to the head, maybe even get a new tensioner. i don't think i will shave more than .070 inch(if even that much) off the head anyway(as i will be using an E31) on an N/A L24. surely if someone came up with the idea of shimming the cam towers, someone has also came up with an alternate idea....i.e. more tension on the chain. BLAH!! .

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post #4 of 14 (permalink) Old 01-07-2003, 04:33 PM
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Re: head shaving and HP from higher compression

Here is a quote from Brian Little who is well known for his L-6 Head and Engine Mods
"The easy route was to shave the head. But in case you don't know, OHC engines are not supposed to have their heads shaved.
This is because it leaves slack in the timing chain, and you lose much more power than you gain. Adjusting the cam sprocket removes slack, but it doesn't correct
necessary crank/cam distance on the driver side of the motor. A high performance motor should use "correct" settings and a tight chain.
As a result, racers (and rebuilding companies) commonly shim up the cam towers after shaving the head to restore timing. But doing this means that the lash pads need to be replaced with thicker ones. This can alter the valvetrain geometry and cause "poor valvetrain harmonics"

In the same text he also mentions shimming up the valve spring bases to maintain valvetrain harmonics but you can read about the head mod here

http://www.geocities.com/zgarage2001/head.html
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post #5 of 14 (permalink) Old 01-07-2003, 05:52 PM
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Re: head shaving and HP from higher compression

Mike,

I think you might be comparing a timing chain tensioner with a belt tensioner. They are not the same. Timing chain slack is measured in thousandths of an inch and belt slack is measured in inches. Each tensioner is designed with travel limits appropriate for its designed use.

- John

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Benton Performance, LLC
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post #6 of 14 (permalink) Old 01-07-2003, 07:08 PM
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Re: head shaving and HP from higher compression

You won't see too much hp gain from shaving the head and getting a slightly higher comprssion. Here is a comparision. I have a 77 280z with intake, exhuast, high flow cat, headers, mild cam, MSD ignition with coil and wires, sparks gaped at .050 to allow larger spark, aeromotive adj. FPR set at 45psi. I had a dyno tech adjust fuel pressure and timing to work with my cam to produce the most optimum hp and torque along with a good flat torque curve. Put out 155.6hp at the wheels. Now Bastaad brought his 240z with an L28 with shaved head running about 10:1 compression, carbs, headers, exhuast, and mild cam. At the same dyno place after tunning the carbs to get the best hp and torque he got 165hp at the wheels. As you can see thats only about 10hp gain from high compression. Its not the best comparision since I have fuel injection and he is running carbs but it gives you an idea of what kind of gains your looking at. Good luck and yes you do need to shim the cam towers to make it work

Current: 77 280z, 84 300zx, 12 Vette Grand Sport ---The ex's: 06 WRX - RIP, 09 370z sport - RIP, 10 370z NISMO - sold
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post #7 of 14 (permalink) Old 01-07-2003, 08:26 PM
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Re: head shaving and HP from higher compression

you can shim the tensioner too.

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post #8 of 14 (permalink) Old 01-07-2003, 09:03 PM
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Re: head shaving and HP from higher compression

156 and 165 rwhp with cams and 10:1 compression? Funny... ITS prepared race L24s with stock cams, pistons, carbs, intake, etc. and max legal 9.5:1 CR make 170+ rwhp all the time and ITS prepared L28s with a max legal CR of 9.3:1 regularly make over 175 rwhp. I'm not demeaning or making fun of anyone's engine build (150+ rwhp in a 240Z is fun!) but maybe there are more important areas of engine preparation then just CR.

- John

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post #9 of 14 (permalink) Old 01-07-2003, 09:24 PM
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Re: head shaving and HP from higher compression

Actually those hp numbers were taken at about 3000 feet above sea level. Down at sea level the tech said we would most likely have about 10-12hp more. Also my 156hp is without high compression. What I was trying to say is with almost the same set up except the difference of high compresion there was only a 10hp difference. So basicly he only made 10hp more by having high compresion. Also I'm sure race preped L28's have more than the mods that I listed for both my car and Bastaad's. I'm sure if I port and polished the head and got a larger throttle body and did some other stuff that I would have over 175hp too. My goal is to get 175hp at high elevation with out having to open up the engine. I'm only about 20hp off and with the other little mods I plan to do I think I'll get there or close to it. Then once I save up some good money I'll think about opening the engine to port and polish the head and maybe go high compresion.

Current: 77 280z, 84 300zx, 12 Vette Grand Sport ---The ex's: 06 WRX - RIP, 09 370z sport - RIP, 10 370z NISMO - sold
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post #10 of 14 (permalink) Old 01-07-2003, 09:56 PM
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Re: head shaving and HP from higher compression

What kind of diff. ITS prep car? And max HP @ what rpm? I was running about 164RWHP on my 10:1 motor with 274 cam. (pretty small) No prep. on stock block, shaved E88 head with N47 valves. Not really tuned. Can't complain with the stock 22years block with $15 head.

Had a Z : (
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