Valve shims causing low compression?

[scottish-sr]

Valve shims causing low compression?
Since buying Docwra's 'drop in forged engine' I've not had a chance to drive it the way it's meant to be driven. Yesterday I finally had the ££ to take it to HQ Performance and get it on the rolling road. The mapping was checked over and the boost curve is fine. However once boosted the on WOT and to the redline the car then starts miss firing once back on the throttle, after this the car won't start again. A boost leak test has been done with no problems found.
A compression test was carried out and got 6, 5.5, 5, 4 bar which is very low. The garage suggested they think the valve shims are causing the problem with the car. A leakdown test is getting carried out tomorrow. The engine I was told had only done around 5000 miles since built. Ive not even managed 500 miles in it yet.

Is that most likely the problem and is it a fairly easy fix?

[Mark]

Was the compression test done again with some oil poured down the bore to confirm if it was rings or valves? The shims are a gnats cock different from the thinnest to thickest (0.4mm) so i would be surprised that they would cause an issue like this

[scottish-sr]

No as we had ran out of time. The car cruises fine normally and boosted up fine on the RR. Just as soon as that's done it starts missing. I wouldn't have thought the rings would have gone already unless they were like that when I got the engine.

[sideways14a]

When i rebuilt my sr we did have lower compression on a couple of pots due to shims needing a bit of tlc (they were new ones and needed a tiny bit grinding) just to get a perfect score across the cylinders.
That said i cant see it being your main issue.

[redsx94!]

As sideways says, it's not likely to be shims causing that difference.

I would try a different compression tester to rule out it being faulty first.

[scottish-sr]

Any ideas of what could be causing it?
This is the engine spec from the advert.


Bottom end engineering work done by two companies local to FIGJAM, details can be provided. Dynamic & static balancing done by a local engineering firm with the owner having built Le Mans engines. The other who did the boring, shim grinding, valves etc are a well respected local engineering company used by many of the local rally and race companies.

Greddy high capacity baffled sump
NEW crank from nissan
ARP main studs
Eagle rods with ARP 2000 bolts
ACL race bearings on both main and rods
CP forged pistons
Slight overbore
Full cross hone
All oilways cleaned and block cleaned
All threads chased
All tolerences checked with bore-scope rather than plasti gauge
Rebuilt oil pump
All gaskets, chains, guides, tensioners etc replaced with Nissan parts
ARP head stud kit

Top end is standard but rebuilt by Damian at DI Racing, all measured and torqued after being acid bathed and cleaned, he also installed it all into my S13. IIRC its running a standard HG to keep compression high, obviously only top quality oil has been used throughout the engines life and its been carefully warmed up and cooled down.

Injectors are Deutschworks 550's modified to flow at least 740cc.

Pretty annoyed as it was sold to me as a working drop in package.

[sideways14a]

So no uprated springs and retainers???

[Actual_Ben_Taylor]

Top end is standard

I would throw a decent set of springs in it first

[scottish-sr]

Any link to springs and prices? Sure BC do them?

As from ad top end was standard but rebuilt. I think when Figjam had the engine it had an uprated head, which was sold seperately and Doc fitted the standard one.

[sideways14a]

Imho its the classic "pumped up lifters" issue, on a stock head SR you can get some wacky shit out of them when you go red line hunting.

[Mark]

Yer you need to uprate the top end, even my stage 2 SR used to get serious valve bounce at the top end

[redsx94!]

Yer you need to uprate the top end, even my stage 2 SR used to get serious valve bounce at the top end

My second engine did too after 6500rpm, sounds like its off a cylinder when it happens.

[scottish-sr]

Any links to UK suppliers of what I would need to get?

[scottish-sr]

Leak down carried out. 64%, 84%, 88%, 90%. Should only be 10% roughly.
Cylinder heads coming off for inspection.

[redsx94!]

At that % you should hear it coming out of somewhere to confirm the issue without taking the head off.

If you can here air coming out your breather it's rings, if it's coming out of exhaust or intake it's valve train

[redsx94!]

Are you sure these tests are accurate? How does the engine run driving around town.

[scottish-sr]

Maybe I picked them up wrong and that's how much its holding. I wouldn't have thought the car would even have run with it leaking 90% on one cylinder.

Normal driving is fine around town and also cruising on the motorway.

[redsx94!]

There's no way your compression is 5-4etc then.

I think whoever is doing your compression test is doing something wrong.


Is the engine tappy on the top end and how well does the car start?

[scottish-sr]

I was there when compression test was done. Turned over for 5 secs each cylinder on WOT.

Maybe the readings are slightly out but surely that would have been highlighted during the leakdown test?

Can't really hear if tappy due to exhaust noise. I've never noticed anything.

To start the car you need around 40% throttle on before turning the key. Its very lazy at starting, will catch around 500rpm then judder into life, again needing to work the throttle. Its the same hot or cold.

Fuel & oil pressure are fine, coolant temp fine.

Seems to be confusing everyone as it will boost fine, RR showed fueling fine & boost curve fine, but after that it starts missing. When we tried to start it again they said it sounded uneven when turning over & I agree with that.

[redsx94!]

It would feel like a 1litre off boost with compression like that lol

Sounds like you haven't wired in the crank signal wire to the ecu, this tells the ecu your turning the car over and dumps fuel in to start the car.