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Pushing the red line on a stock 3.2 and a bit on cams...


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Been a while since i was on here.

I have a fresh DIY built 3.2 on PMO carbs and simple distributor, not a computer in sight.

Gives 260 bhp/230 LbFt and the hillclimb car is 560Kg.

My questions for those who understand:

1

Can I rev this engine further than 6200 red line? The distributor is an RS 2.7 type with a rotor arm fitted with a sprung cut out.

If there is room to stretch this engine, what might be the max? Rod bolts are new Porsche parts etc, rods and pistons etc all stock.

2

Cams: It has stock 3.2 cams timed to mid tolerance and stock valve springs etc.

I'm thinking of changing them to '964' cams, Porsche ones or new from Kent Cams.

Is this a good move and if so, can anyone quantify it?

If the engine revved to 7000 rpm will the 964 cams keep giving?

 

Thanks in advance gents.

 

 

Edited by 911hillclimber
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I have 964 cams and have opened up rev limit to 6900 rpm.    The 3.2 Club Sports were limited as standard at 6850 and other than blueprinting and sodium filled exhaust valves weren't any different to the standard 3.2 engine which incidentally was limited at just over 6500 if I recall?    I really cannot see 7000 rpm as a problem but wouldn't want to go much above this.

My engine was rebuilt 13 years / 25K miles ago and has been exercised spiritedly on road and track  🙂

My car makes identical peak numbers to yours on remapped standard Motronic.   The 964 cams really do make a difference.  Whilst the lower to mid range (upto say 4K) is fairly similar in feel to standard cams then beyond this there is a seat of the pants difference and the car doesn't seem to run out of puff at just over 6K as it used to.  Unlike before max power coincides with near max revs and whether on track or just quick road driving the engine just feels stronger and wanting to carry on.  

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I'm not speaking from personal knowledge but I am told that for a 1980's 4 stroke engine 18 - 20 m/s is a good safe limit for mean piston speed for a road engine and that mean piston speed is linked to RPM by the length of the stroke: the longer the stroke, the lower the RPM at which the motor will reach its safe limit. The formula is: Max RPM = (max piston speed / stroke) x 30000. so if your engine has a stroke of 70 mm the piston will reach the 20 m/s limit at 8570 RPM; if the stroke is 85 mm the limit will be reached at 7060 RPM. In the case of 3.2, with a 74.4mm stroke, 18m/s would equate to 7258rpm.

The Anderson book states that the standard conrod bolts on the 3.2 were reduced in diameter by 1mm versus the 3.0 and are not suitable for racing or high performance applications. So a change or rod bolts might be wise.

Of course all of the above is theoretical and assumes the breathing and ignition works as needed for the application.

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10 hours ago, 911hillclimber said:

I'm thinking of changing them to '964' cams, Porsche ones or new from Kent Cams.

Since you are running on carbs, you could also consider more aggressive cams than 964 ones, especially if this engine is going into an ultralight race car.

I'm not an expert myself and can't give meaningful guidance on cam selection but, if you don't have it, I thoroughly recommend Bruce Anderson's book to get an overview: https://www.amazon.co.uk/Perfomance-Handbook-1963-1998-Motorbooks-Workshop/dp/0760331804

Edited by WP0ZZZ
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5 hours ago, WP0ZZZ said:

Since you are running on carbs, you could also consider more aggressive cams than 964 ones, especially if this engine is going into an ultralight race car.

I'm not an expert myself and can't give meaningful guidance on cam selection but, if you don't have it, I thoroughly recommend Bruce Anderson's book to get an overview: https://www.amazon.co.uk/Perfomance-Handbook-1963-1998-Motorbooks-Workshop/dp/0760331804

+1, Pages 188-189 list all the cam specs for various popular cams and suggests the 911S or GE-70 camshaft for a 3.0 to 3.2L engine on carbs. The Mod S cams seems to be very popular. I'm using a '66 Solex profile on my engine which doesn't extract the maximum power but is very tractable which might be interesting for a hillclimb engine. You should also be looking at the valve springs and retainers for a high revving engine.

Edited by jehatwhitland
typo
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This is all great info and dovetails in with a parallel thread on DDK.

IIRC, the pistons on my stock engine will not take high spec early 911 cams, there are no pockets for the extra valve lift. I think this is where the 964 cams are good, a bit extra without the need for piston change.

Also I have a set rev limit governed by what rotor arms are available for the early 911 distributor I have. Thus I have the current 6500, then there is a 6900 and a 7100 rpm cut-off. I'm not sure just how accurate these cut outs are!

The 6900 looks the best so keeping below the exciting 7000 red line.

There seems to be a number of choices here away from an all-out 8000 red line engine.

1

Fit a 6900 rotor arm and get on with it. I think the current cams will almost stop that happening or being reached. Cost: £40

2

Fit the rotor arm and 964 cams. I am sure the engine will hit 6900 and be giving more at the upper level. Cost £750 ish

3

Fit ARP rod bolts to the existing rods, keep the pistons, 964 cams and 6900 rotor arm. Cost £1250 ish

4

8000 rpm screamer: cost north of £6000

 

Are the rod bolts the weakest link or are the cast piston equally weak?P1060824.JPG.8af3ba48939941b595b20e4f2187fb39.JPG

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Remember the slightly weedy rod bolts on a 3.2 (compared to those on an SC ;)

+1 on the stronger, lighter valve-train mods - TI retainers and race springs 

Edited by GaryH
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Posted (edited)

Seems i have the Permission to build the screamer from The Boss and keep my 'project' fund machine, my freshly restored Honda CB175.

Time to build a shopping list

Thinking Wossner pistons and rods, the rods come with ARP bolts and are steel Need to check if they are forged first. £2K gone

Cams: Maybe early 911 2.4 E or S profile. Have had a 2.4 with E cams which was a real hillclimb engine.

Springs on the valves? Help needed.

Heads, might have them machined for twin spark, but will initially run single plg, just blank off the second plug taping with plugs. Bit of future proofing.

Ignition, keep it analogue or Clewitt, need to talk to a friend who has one such system on his 2.4 on Carbs

Keeping the PMO carbs

In a few very broad brush stokes that is about it.

Comments most welcome.

Graham.

 

Edited by 911hillclimber
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10 hours ago, 911hillclimber said:

Seems i have the Permission to build the screamer from The Boss and keep my 'project' fund machine, my freshly restored Honda CB175.

Time to build a shopping list

Thinking Wossner pistons and rods, the rods come with ARP bolts and are steel Need to check if they are forged first. £2K gone

Cams: Maybe early 911 2.4 E or S profile. Have had a 2.4 with E cams which was a real hillclimb engine.

Springs on the valves? Help needed.

Heads, might have them machined for twin spark, but will initially run single plg, just blank off the second plug taping with plugs. Bit of future proofing.

Ignition, keep it analogue or Clewitt, need to talk to a friend who has one such system on his 2.4 on Carbs

Keeping the PMO carbs

In a few very broad brush stokes that is about it.

Comments most welcome.

Graham.

 

My engine uses the Aase Springs and titanium retainers. If you go twin-plug, the Clewitt style ignition is the most cost-effective option.  I've got a JB Racing twin-plug distributor in combination with 2 x the Classic Retrofy CDI boxes - however, it's a road car and I wanted the engine bay to look the part!  That said, the set up works very well and the engine picks up pretty cleanly despite having PMO 46 carbs.

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Thank you, this engine has 50mm PMO' but picks up well.

not sure about twin plugs at this stage, but prepping the heads for the second plug seems a good idea should it be required due to hi comp crown/wedge Pistons.

 

Anyone with opinions on the Wossner Pistons and rods?

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I think you would notice the difference with twin plugs and as you say machining them into the heads whilst you have the engine apart is a good idea. A back to back test was done on my engine with JE pistons that had large cut outs and showed a 12bhp increase at flywheel when switching on twin plugs.

I have no experience of using Wossner pistons and rods, both Mahle and JE have worked well for me and the Carrilo rods seem good and were recommended by several race engine builders.

I looked at various springs and retainers and was about to order Aasco ones, but there was a significant delay in getting them from the US and I bought a similar set from Schrick through Fvd for around the same price with delivery within a few days.

The parts I have bought from Clewett have been great, but make sure you order all you want first time as shipping costs are fairly high from the US now.

Ian

 

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Just read through the Dr's engine build thread posted above...not encouraging!

I've rebuilt a 911 engines 3 times, one a 2.4 and the racer's 3.2  twice, the last racer rebuild was after a huge over-rev.

I used parts from Type911 throughout, zero probs.

Dr:

It seems you used the stock pistons, is that right?

If so, what is the red line of the engine?

I can't get a clear approach on this engine,

can the cast 3.2 pistons take 7500 rpm (or even 8K)?

Can the rods with arp bolts take same rpm?

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1 hour ago, 911hillclimber said:

It seems you used the stock pistons, is that right?

If so, what is the red line of the engine?

Yes, stock Mahle B&P's. 

Not sure on the redline - vmax'd at 161mph, revs somewhere off the rev counter....let's say the engine was good for c7k rpm. Have a chat with Chris @ GCR...

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10 hours ago, waddy said:

I think you would notice the difference with twin plugs and as you say machining them into the heads whilst you have the engine apart is a good idea. A back to back test was done on my engine with JE pistons that had large cut outs and showed a 12bhp increase at flywheel when switching on twin plugs.

I have no experience of using Wossner pistons and rods, both Mahle and JE have worked well for me and the Carrilo rods seem good and were recommended by several race engine builders.

I looked at various springs and retainers and was about to order Aasco ones, but there was a significant delay in getting them from the US and I bought a similar set from Schrick through Fvd for around the same price with delivery within a few days.

The parts I have bought from Clewett have been great, but make sure you order all you want first time as shipping costs are fairly high from the US now.

Ian

 

That's very interesting, thanks Ian. Just out of curiosity, what sort of rev limit do you think is reasonable after all those modifications?

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13 hours ago, WP0ZZZ said:

That's very interesting, thanks Ian. Just out of curiosity, what sort of rev limit do you think is reasonable after all those modifications?

The rev limit is determined by the weakest link and with the modifications mentioned it would be the rockers that would become the problem. I understand that the Carrillo rods with H Beam in theory are good for up to 8,500 revs and the Carrillo rods with Carr bolts will go beyond 8,500. I think you should be able to rev to 8,000 revs providing you have a fully modified and balanced engine. There will need to be good clearance for the valves to give you some margin of error. With an engine on standard pistons that is highly revved I have always found some evidence of valves kissing the top of the pistons even when I don't think there has been an over rev.

Hope that helps.

Ian

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Certainly does Ian.

I need an engine that will rev to 7600 or even 8 so to negate the need for some gear changes in this car (915 box/awkward indirect linkage)

 

Plan based on chasing weak links:

forged Wossner pistons with deep pockets for those stock valves

H beam forged steel rods, Wossner/ARP bolts (or similar)

Total balance job with flywheel

S cams (say)

Twin plug heads (single plug use initially)

Springs as you have

Rockers...964?

I think that will nail 7600.

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Add Titanium valve spring retainers to your shopping list. 

You are being massively conservative on cam choice.  Even in a 1000kg race car an S cam in a 3.2 litre motor is conservative.  S cams were aggressive in a 2.0 litre, but not in a 3.2.  Peak power with an S cam is sub 7000rpm - and you are building for 7600rpm.  In a 500kg race car.  For example, I know Ryan is very happy with IIRC DC60 cams in his 2.9 MFI tarmac rally car where tq is pretty important.

For the record, the 964 cam is also a conservative choice for stock 3.2 when you can go up to a GT2 Evo cam.  I don't actually know of a 3.2 build with that cam, but theory says it works (certainly 993SS has been tested).

Anyway - super exciting build.

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The road trip didn't happen - because COVID. I did a shorter trip with Felix in 2019 in our Boxsters that was a bit of a dry run for the 2020 trip.  In 2020, I did a solo trip on more or less that route in September in my CSL.  It was the window post summer before more lockdowns and travel restrictions, but I wasn't comfortable having more people.  It was also my last euro road trip for a while, so I kind just wanted to do it my own way.  

All the trips have been posted on here.  I have done a few:  Alps, Pyrenees and a little further east, as far south as Madrid.  Mainly summer but one in March as well.  Now I am in NZ its a whole different ballgame.

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