The V8 Owners Forum -

Im very close to finalising a spec for my engine rebuild but thought I would get some opinions here before making final decisions.

My TVR engine is a rover based 4.6 cross bolted serpentine with large valve heads.
Last year I damaged 2 of the standard cast pistons due to detonation which was my own fault as I used too much gas in the wrong place at the wrong time with too little retard in the ignition. We live and learn. Its all part of the fun IMO.

The short engine is to be rebuilt by Mike Johnson at Boost Performance UK and will have custom forged pistons with file fit rings and custom pins. Modified chevy rods will be used and the whole engine will be built with ARP fasteners. Kent will be grinding me a mechanical cam with 310 deg inlet and 285 ex with a 112 deg LSA. A megasquirt ECU running wasted spark and a dedicated nitrous map with suitable retard will be employed also.

The main question is the compromise I make on the final compression ratio. How low do I go in order to safely use 300-350 bhp of nitrous (my ultimate aim) and still have the engine perform well on an every day NA basis.
I also need some advice on what type of head gaskets would be best.

As per usual all comments, advice and opinions will be much appreciated at this time as I finally take the step to rebuild my engine at last.

Hi,
An interesting choice of cam,what is the thinking behind having 25 degrees more inlet timing than exhaust especially on a nitrous engine?
Mike

kiwicar wrote:Hi,
An interesting choice of cam,what is the thinking behind having 25 degrees more inlet timing than exhaust especially on a nitrous engine?
Mike

Oops. You spotted the deliberate mistake

The cam is 284 degrees on the inlet with .485 lift and 310/.534 ex.

Hi
That makes alot more sense to me. mechanical spec seems good to me and would make a free reving engine with moderatly high compression. However I think though I am going to have to back out here as at 300 to 350 of nitrous you are in specialist teritory here, I think there is going to be little room for compromise here, I think the engine and particulally the CR are going to be determined by handling that much nitrous with every other consideration being secondary. I would say with that much liquid (petrol and nitrous in an atomised liguid state) CR will be determined by not locally hydro locking the engine, have you calculated how much fuel you will be injecting each cycle? I think the squish band may have to be more open than normally ideal and possably getting it all to burn might be an issue.
Best regards
Mike

No actual calcs on fuel levels. I should have sufficient supply with a seperate dedicated pump. A fuel pressure safety cutout switch will be in the system just in case.

I ran upto 250bhp of gas on top of my 273bhp NA last year ok on the standard engine with a CR of 9.35:1.
Mike at boosted is talking of a CR as low as 8:1 but im not sure thats a little too much. I was originally thinking around 8.7.
This is why ive posted for more opinions and advice

Hi
If you were getting 350 extra BHP on top of 275 by using a supercharger then I recon you would need about 22 to 24lbs boost (allowing for blower efficiency) on that basis I would say you would want a CR in the region of about 8.25:1 to be safe certainly not above 8.5:1.
Best regards
Mike

It does look like i will have to run a lower 8 CR.
It will be interesting to see the NA rolling road graph when its all sorted.
Maybe a decent mapping with the megasuirt will make the best of the set up.

Can you get the heads moded for a second plug (I've seen rover V8s modified for aircraft use modified like this), you could probably push the CR half a point that way. Get you around 8.75:1 I don't think you will be able to get much higher without knowing exactly how much liquid you are shoving into the cylinders.
Best regards
Mike

That sounds like a whole other can of worms.
8.25:1 methinks.

Hi derek
As an engine builder i would seriously be tempted to go with what your engine builder is suggesting, i doubt if warranties are going to be on your engine but if a problem was to occur, you wont have a leg to stand on if you tell him you want something different! I would for the cost of it nowadays ceramic coat the piston crowns and the heads of the exhaust valve at minimum, if you fancy do the lot, exhuast and inlet tracks and combustion chambers, that is a poop load of gas you are going to be running and cylinder temps are going to be a little on the hot side!!, also make sure you go up a few plug numbers, maybe a 9 on the ngk range. ps nice cam specs

Hi Rob
im guessing you are Rob R that kindly gave me the advice for my cam spec.

Ive decided to go with Mikes suggestion and will be running as low as 8:1 CR to allow a 300 shot when the time comes...... 2nd run maybe?

The custom pistons Mike has manufactured feature forced pin oilers, ceramic coated crowns, deep valve cutouts, low friction anti-scuff side skirt coatings, gas accumulator grooves, reinforced ring lands.
A set of Crower SBC rods will be machined to suit. Its not cheap but it should hold together under the abuse.

Trevor from the wizards of nos reckons that the engine will run fine NA at this CR and will ultimately allow for higher levels of nitrous without the concern for det if all the normal nitrous rules are followed. ie timing, plugs, ignition, fuel supply etc
He also recommended the use of the standard tin gaskets as he doesnt rate the composite alternatives.
ARP will be used throughout but there is always a risk of gasket failure at such high pressures.

I can't remember the company but should be searchable.
But one of the TVR suppliers do a slightly thicker than normal composite gasket with solid sealing rings.
Using these would help you get a lower C/R, also with the added option if it too low, you could just go to standard composite gaskets, to get a higher C/R without doing any serious work.

Are you having the block top hatted?

Talk to Mike about having the heads o-ringed, he has had some impressive blown Fords done this way. I like solid copper with rings and just viton bits to seal oil/water.

As Rob said spend some money on coating the head chambers, ports & valves. Theres not a lot of casting left in some areas of the TVR BV heads and I'd want as much heat protection as possible for longevity never mind anything else.

There has been no talk of top hatting. Am i imagining it or was this more a problem with the 96mm bore engines???

I will certainly ask about o rings.
My heads are big valve but arent ported as the 500's were.
Ive pocket ported them myself and blended into the seats and opened up closer to gasket sizes.
I need to find out the costs of coating the heads etc but can definately see the benefits of treating the exhaust valves at least.

yes i am that rob!, TIN GASKETS? they wont last a weekend with the kind of power you are talking about, i would assume mike has allready figured on gaskets so again go with your builder, also the squish band and deck heights need to be taken into consideration, and running a cr as low as 8 you may just need that thickness of gasket to acheive that, but i would be surprised if he doesnt use copper with a ringed head, as for the way your car is going to drive off gas with 8-1 cr, it is probably going to be somewhere close to a standard 450 on bhp.