2306

Mike

Lifetime Supporter
I tend to agree but I'm still trying to ascertain the difference in cam profile designed for use with Webers vs one designed for 4bl.
 
Ive just put a link to a race meeting here in NZ that TVR took part in on 'The Race Track' forum, there is a good 'sound' catch of the exh note @ ~1:22>.
 

Glenn B.

Lifetime Supporter
I tend to agree but I'm still trying to ascertain the difference in cam profile designed for use with Webers vs one designed for 4bl.

Mike,

The primary difference is in the Lobe Separation Angle and final timing. Multiple Weber set-ups require LSA's greater than 112 degrees and final timing advanced to as much as 38 degrees, depending on the cam and engine.

From Weber Guru Jim Inglese:

"Any cam is fine as long as you pay attention to the lobe separation angle (LSA). A 112-114 degree lobe separation is ideal for Webers. I generally discourage the use of anything tighter, like 108, or 106. A 110 LSA is marginal. The problem with a lot of overlap is that it causes "reversion"...which are pulses from the exhaust stroke while the intake valve is still open, that have nowhere to go but up into the carburetor. This disturbs high rpm airflow and causes the engine to think it's too lean at high rpm’s. Without a plenum, the flow is pulsed backward up into the carburetors because there is noplace else for the reversion pulse to go."

I run a cam with a 112 LSA and 38 degrees of timing in my Lola with four 48 IDAs on top. Very clean running with none of the Weber "maladies" so many people document.
 

Mike

Lifetime Supporter
I've heard folks use the airpump analogy for an engine. Not sure that is truly accurate but air does move in and out via the combustion process. If one is trying to replicate a certain tone or sound then I would think the manner in which the valves open and close would affect the tone the engine makes. What would be the effect of of running a cam with that much separation on a carbureted engine?
 

Part of what you are hearing is a very light flywheel. Idle revs are 1500 and they pick up frighteningly fast. You need very light rotational mass to do that and cam.

Then yes carbs and a big part is cams, judging by the high idle, that is one high lift cam, the motor is setup for high power acheived at high revs in a narrowish range, its the only way to get really high hp out of 289. It prbably runs iron heads too which may be a factor.

My motor uses modern tech to try get the best of both. Modern blocks allow you to bore a lot more, and a 3.25 stroke gets you more tq, modern rods and pistions keep it all neat in terms of rod ratios and add lightness. Modern aluminum heads allow Hp number an iron head just caant do, and you still get TQ plus save weight. In theory with all that you get the top end of a pure race bred 289 just more Hp, but also with lots of strong mid range and decent low down goodness, all with good rod ratios and very low rotational mass. Thats the theory anyway.

Whichever way you look at it though, you cant get the same sound from a 351 based mtoor, and you cant get the snappy rev response. One is a shrieker the other a slugger.
 

Steve

Supporter
Cam according to build sheet.
Comp cams solid roller.
2.50-2.56 .675-.673 108 ls whatever that all means


Wow, that's an aggressive cam. I'm assuming it's 250/256 duration at .050. Lots of lift, but your upper end will probably handle it.

Steve
 
Wow, that's an aggressive cam. I'm assuming it's 250/256 duration at .050. Lots of lift, but your upper end will probably handle it.

Steve

Max Tq is 465 at 4900 and max Hp 574 at 7100 but at 7500 its still 560.
I dont have TQ/HP figures for what goes on below 4900 rpm, but apprently it has a decent smooth idle.

Its a roller cam, and we are running jezel shaft rockers.

Heads are afr 195's which were bought as blanks and then worked/built up.
 

Mike

Lifetime Supporter
I'm looking forward to hearing yours. I think it will sound quite a but different than that 289. Curious to see.
 
Cam according to build sheet.
Comp cams solid roller.
2.50-2.56 .675-.673 108 ls whatever that all means

250° in/256° ex duration @ 0.050 camlift- .675"/.673" valve lifts ( depends on rocker ratio-assumed std SBF 1.6/1 ratio.). 108° lobe centres.
 

Mike

Lifetime Supporter
Wondering Jac Mac regarding your theory on high rod ratios leaving the piston around TDC longer and minimizing the effects of lots of overlap then why would Webers on a 289 need that much lobe separation to work well?
 

Mike

Lifetime Supporter
Then yes carbs and a big part is cams, judging by the high idle, that is one high lift cam, the motor is setup for high power acheived at high revs in a narrowish range, its the only way to get really high hp out of 289

I've always heard Webers are stronger in the midrange and not so much high end. Each cylinder is limited to the single small carb cdm which limits them compared to a large CFM 4B carb that can feed each cylinder. That along with the effects of running more lobe separation to me indicates a setup that is more midrange than narrow top end.


EFFECTS OF CHANGING LOBE SEPERATION ANGLE (LSA)

Tighten (smaller LSA number) Widen (larger LSA number)
Moves Torque to Lower RPM Raise Torque to Higher RPM
Increases Maximum Torque Reduces Maximum Torque
Narrow Power band Broadens Power Band
Builds Higher Cylinder Pressure Reduce Maximum Cylinder Pressure
Increase Chance of Engine Knock Decrease Chance of Engine Knock
Increase Cranking Compression Decrease Cranking Compression
Increase Effective Compression Decrease Effective Compression
Idle Vacuum is Reduced Idle Vacuum is Increased
Idle Quality Suffers Idle Quality Improves
Open Valve-Overlap Increases Open Valve-Overlap Decreases
Closed Valve-Overlap Increases Closed Valve-Overlap Decreases
Natural EGR Effect Increases Natural EGR Effect is Reduced
Decreases Piston-to-Valve Clearance Increases Piston-to-Valve Clearance
 
Wondering Jac Mac regarding your theory on high rod ratios leaving the piston around TDC longer and minimizing the effects of lots of overlap then why would Webers on a 289 need that much lobe separation to work well?

I would not word it quite like that Mike, more like this.

The longer effective rod ratio of the 289 allows the engine with webers to tolerate the longer duration better at lower RPM while also being able to benefit from the stronger signal at high RPM induced by the exhaust scavenging during the longer overlap period of the radical cam.

To hopefully help clarify this the car/s I experienced this on were a Holden & Ford saloon with ~200 cu in racing in a 'parity' type class. Holden was first of type with [3.685 bore/3.25 stroke/5.25" rod/ 1.61 to 1 rod ratio], with a 'control camshaft', this had a noticeable lumpy idle and quite a crisp exh note. When we introduced the Ford [3.74" bore/3.13" stroke/6.25" rod/ 2 to 1 rod ratio] and tried to use the same camshaft specs as the Holden the Ford idled very smoothly and quickly got nicknamed the 'Whisper Jet' as exhaust was quiet. Power/Parity was not quite there so I fitted a cam with closer lobe centers plus a bit more lift- idle characteristics and exh note were now similar along with performance.
Now if I was to fit a camshaft spec to the Holden similar to what the Ford was now using it would obviously cause the idle quality to deteriorate. Given that both engines ran same carb, cars were similar weight etc I feel it gave a fairly good example of how the large difference in rod ratio effected performance.
 
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Mike

Lifetime Supporter
Thanks. After doing some searching on here it's clear to me that the the topic of cam design and Webers is one that has been widely discussed and there are many that have a lot of experience. Sorry for the thread drift but I have just one goal in the engine I am seeking to build... sound. I don't really care about HP or drivability. All the originals that run around Goodwood or Spa Historics have the same tone.

Can you achieve that tone with a 4 barrel? Does it require Webers?

Can you achieve that tone with a roller lifter and a longer duration cam? Does it require a flat tappet with 106 degree LSA like the originals and the reversion problems with that much overlap?

Can you achieve that tone with a stroke greater than 2.87"?

Can you achieve that tone with a rod ratio less than 1.79?

I have heard a lot of 302s and stroked 302s that sound very little like that link above. I have to imagine that the further one strays from the original specs the more chance there is to alter and spoil the tone I am after. What can you alter from the original specs that will and will not have a great affect on the tone? I'm sure this is gibberish to most but I'm on a mission and discovery has been a struggle.
 
.......... I have heard a lot of 302s and stroked 302s that sound very little like that link above. I have to imagine that the further one strays from the original specs the more chance there is to alter and spoil the tone I am after..........

My opinion. I think a lot of the "tone" you hear is a lot to do with the high revs being used, 6500/7000rpm, which will be hard to achieve with a stroked engine. Also, a lot of the cars at Goodwood did not run silencers/mufflers.
 
My opinion. I think a lot of the "tone" you hear is a lot to do with the high revs being used, 6500/7000rpm, which will be hard to achieve with a stroked engine. Also, a lot of the cars at Goodwood did not run silencers/mufflers.

Depends on what you consider a 'stroked' engine. Seans engine with 3.25" is considered a stroked engine when referring to a 289/302 small block, but the 351 is 3.5' stroke std. The TVR I have mentioned is the same & it runs an 7800 limiter and can touch that with a light tail wind on some circuits.

To answer mikes Q above my suggestion if he is not concerned about mega cubic inch's would be to use an 8.200 block with 4.125"+ bore, a lightweight 3.00" stroke with 5.400 rod and piston with 1.300" compression height. This will give the 289 rod ratio of 1.8/1. [ If I wanted to build a motor like that I would swap to a 5.700 rod to get 1.9/1 rod ratio- this requires a more expensive piston with 1.000" compression height ].

To get more cubic inches use the 8.700" block available from Dart & others with the 4.125" bore/3.400" stroke/6.000" rod/1.000" piston comp height/1.76/1 rod ratio. That will hurt the rod ratio slightly, but the ~360+ cubic inches should make up for that!!

On more 'fact' in favour of the long rod theory..... The 302 Small block Chevs in F5000/Formula A series had 4.000" bore/3.000" stroke/5.700" rod combo that gave a 1.9/1 rod ratio [ a 6.000" rod gets even better @ 2/1 rod ratio]. They all had long duration/large overlap cams and used Webers until the fuel injection guys got up to speed..

Better stop drifting in Seans thread now....
 
All this discussion makes me very interested to see and experience how my motor turns out. Was looking at the dyno sheets, it goes from 575hp at 7100 to 565 at 7500 probbaly not too noiticable but Tq is dropping off fast.

Based on gearing if the motor will pull 7300 or so with the 26.9 avons we will exceed 200mph assuming tires are not expanding with spining which they will so, maybe even 7250. Of course one will need to find a road long enough, which means going to daytona or towing out west to the silver stae classic, one day, lots to do with the car till then..

On track runnign 26 diameter hoosiers and keepign revs to 7200 top speed will be mid 180s, I wonder where in the USA besides elkhart there is a big enough track.

At MMC which will be the home track its 4.2 miles around, a big straight with a kink, shoudl be perfect for a Gt40. We will see starting this spring.

Dennis tells me he can get down to just over 2mins in his car at the Glenn, thats runnign avon groooved slicks. Frankly If I ever break 2.10 I will be thrilled, currently do 2.18 in the 195hp lotus and its right at the edge through the bends.

Hopefully the motor is smooth and durable, and we got the gearing right. Hopefully I have enough maturity at age 50 to learn into the car over time. 2 previous track mishaps have been through lack of maturity, stupidity, and the last one was just 2 years ago, so holding thumbs.

Been debating on another forum the ideal fun street track car. Frankly I cant think of one for under 300k I would rather have, well cant really think of another at any price,maybe in dream world a mclaren F1 and the new ford gt looks really inetresting, for a modern computer car. So all in all havign a spf, the fact that one even eixts to buy feels really lucky. In the absence of an spf you would need what 400k for a gelescoe or 9 mill for an old build, neither of which I have. With the Gt40 I am finaly reaching the pinacle for my tastes anyway. Wife hopes so, the car obsession she does not get.

Its going to be really fun runnign with and hopefully humiliating Gt3's vettes and ferraris in a old school car where everythign is direct and there are no nannies, at least thats the plan. Plus I can do all maintance.

The latest evo track car of the year really loved a new build T70, the only thing faster was a new maclaren gt3> gt40 should by defintion be slower than a t70, but properly driven not much else. Well gotta learn to properly drive it then, should be a few years.
Feels like maybe a lifetime of climbing the car ladder coming to fruition, or maybe I expect too much. Either way will let you all know how it goes.
 

Brian Kissel

Staff member
Admin
Lifetime Supporter
Sounds like a excellent well thought out plan to me. Wish you the very best safely attaining your goals.

Regards Brian
 
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